TRAINING-FACTORS-HIP-EXTENSION


TRAINING-FACTORS – HIP EXTENSION


Hip extension moment

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Hip extension is essential to sports performance, being central to sprint running, jumping, and throwing.
Hip extension moments are the products of the gluteus maximus, hamstrings and adductor magnus muscle forces and their associated moment arm lengths. Muscle forces and internal moment arm lengths can both be altered by joint angle; muscle forces are also altered by contraction velocity.
Hip extension moment becomes progressively more important in squats, deadlifts, lunges, running and jumping, as load or speed are increased. Heavier loads, faster speeds or greater jump heights require a larger proportional contribution from the hip than lighter loads, slower speeds, or lower jump heights.
Hip extension movements at large degrees of hip flexion are likely to require greater adductor magnus involvement and less gluteus maximus involvement, while hip extension movements close to full hip extension are more likely to require greater gluteus maximus involvement and less adductor magnus involvement.
Hip extension moments are affected by both hip flexion and knee flexion angles. They are larger in both greater degrees of hip flexion and in smaller degrees of knee flexion (full extension).
In squats, hip extension moment is increased with heavier loads, a wider stance width, a low-bar, powerlifting technique, by sitting back more, by faster bar speed, and by squatting as deep as possible. In deadlifts, hip extension moment is increased with heavier loads, with a conventional rather than a sumo stance or a hex-bar, and by using a straight-leg technique.
Hip extension moments are important for walking, acting to propel the body forwards. Their importance increases with increasing walking speed. Hip extension moment is the best predictor of walking speed in elderly people.
Chair squats require more hip extension moment than free squats, while sit-to-stand movements with more vertical shanks require more hip extension moment than to sit-to-stand movements with a less vertical shank. As with squats, as load increases, hip extension moments increase faster than knee extension moments.
The ability to produce force with the lower body is critical for vertical jumping performance. Jumping direction does not affect the size or relative proportion of the joint moments. A more upright trunk during vertical jumping decreases jump height and reduces the role of the hip relative to the knee. Increasing jump height leads to greater hip involvement relative to the knee and ankle.
Drop jumps from high boxes produce extremely high hip extension moments and increasing box height increases the role of the hip relative to the knee and ankle. Soft drop landings involve more energy absorbed at the hip and knee, while stiff drop landings involve more energy absorbed at the ankle.
Running at constant speed involves similar joint moments at the hip, knee and ankle but the relative proportion of the hip increases with increasing speeds.
Using a cable tensiometer (lying supine in 20 degrees of hip flexion), hip extension moment can be measured with good reliability. A difference of 13Nm can differentiate between individuals and a change of 31Nm can identify a training effect.
With a hand-held dynamometer, hip extension force can be measured with good reliability. A difference of between 1 – 2kg or 10 – 20N can differentiate between individuals and a change of between 3 – 5kg or 30 – 45N can identify a training effect.

PRACTICAL PERSPECTIVE
Heavier loads, faster speeds, and greater jump heights require a larger proportional contribution from the hip than lighter loads, slower speeds, or lower jump heights. Therefore, high performance sports involve the hip extensors (gluteus maximus, hamstrings, and adductor magnus) to a greater extent than sub-maximal efforts in practice. Additional hip extension exercises are therefore recommended to bridge this gap in training.

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CONTENTS
Full table of contents [show]
Background
Increasing role of the hips
Normative values
Effect of joint angles
Effect of joint angular velocity
Normative ratios
Correlations with athletic performance
Correlations with injury, surgery and age
Resistance training exercise
Functional movements
Sporting movements
Reliability
References
Contributors
Provide feedback

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BACKGROUND
PURPOSE
The purpose of this section is to provide a background to the literature exploring hip extension net joint moments during isometric and isokinetic dynamometry, as well as during resistance training exercises, in both trained and untrained subjects.

BACKGROUND
Introduction
The hip extension movement is the joint movement that brings the thigh (femur) from a position in front of the body to a position just behind the body. The hip first flexes and then extends in many lower body movements. When stepping up onto a box or climbing stairs, the hip first flexes in order to lift a foot onto the box or stair and then extends to lift the body upwards. When running, the hip first flexes to send a foot out in front of the body and then extends as the foot lands on the ground to drive the body forwards. The hip extension action is therefore central to most lower body movements.

Peak hip extension angle
[See more: peak hip extension angle]
Peak hip extension angle (also called hip extension range of motion) can be measured in several ways. Although researchers occasionally make use of dynamometry, the most commonly-used test for assessing peak hip extension angle in both research and clinical practice is the modified Thomas test. It is possible that increasing peak hip extension angle may be beneficial for sporting performance, by enhancing the range of motion through which the powerful hip extension movement can be performed.

Anterior pelvic tilt
[See more: pelvic tilt]
The hip joint is an articulation between the thigh (femur) and the pelvis. The pelvis subsequently articulates with the trunk at the sacrum. Consequently, during lower body movement, there is movement at the hip, pelvis and spine. At the point of peak hip extension angle in running, both anterior pelvic tilt1 and lumbar lordosis are observed. It is thought that these pelvic and spinal movements occur in order to enhance stride length.

Anterior pelvic tilt (left), neutral pelvic tilt (middle), posterior pelvic tilt (right)

HIP EXTENSOR JOINT MOMENTS
Introduction
[See more: moments]
Like any joint moment, the hip extensor joint moment is created by the product of the muscle forces acting at the joint and their moment arm lengths. The main hip extensor muscles include the gluteus maximus, the hamstrings, and the adductor magnus. The two main factors affecting hip extensor muscle force are contraction velocity (the force velocity relationship) and joint angle (the length tension relationship). The length of the moment arms of each muscle also changes depending upon the joint angle. The twofold effects of joint angle on joint moments (by altering both moment arm lengths and muscle forces) means that the effect of joint angle on joint moment is difficult to predict.

Hip extensor muscle forces
[See more: gluteus maximus, hamstrings, adductors]
The main hip extensors are the gluteus maximus, hamstrings and adductor magnus. Muscle force can be estimated by reference to muscle activity. The muscle activity of some of the main hip extensors appears to change with changing hip joint angle. In general, the gluteus maximus appears to display its greatest level of muscle activity in full hip extension (0 degrees of hip flexion), the adductor magnus seems to display its greatest level of muscle activity between 0 – 45 degrees of hip flexion, and the hamstrings seem to display a relatively even level of muscle activity throughout 0 – 90 degrees of hip flexion. This may imply that hip extension movements operating at large degrees of hip flexion are more likely to require greater hamstring and adductor magnus involvement and less gluteus maximus involvement, while hip extension movements operating close to full hip extension are more likely to require greater gluteus maximus involvement. However, other factors are also important in identifying the role of the various hip extensors in specific movements, including moment arm lengths.

Hip extensor moment arm lengths
[See more: moments]
The main hip extensors are the gluteus maximus, hamstrings and adductor magnus. Internal muscle moment arm lengths are most commonly calculated by cadaver measurements (Dostal et al. 1986), which tends to underestimate their length in younger, living subjects.

Changes in hip extensor moment arm lengths with joint angle
The moment arm lengths of the three primary hip extensors change with changing hip flexion angle. The moment arm length of the gluteus maximus increases as hip flexion reduces from 90 degrees up to full hip extension, being largest in full hip extension (standing upright). The moment arm length of the adductor magnus increases as hip flexion increases from full hip extension to 90 degrees of hip flexion, being largest in at least 90 degrees of hip flexion (partial squat). The moment arm lengths of the hamstrings do not alter substantially (Nemeth et al. 1984).

The moment arm lengths of the main hip extensors

These findings may imply that hip extension movements operating at large degrees of hip flexion are more likely to require greater adductor magnus involvement and less gluteus maximus involvement, while hip extension movements operating close to full hip extension are more likely to require greater gluteus maximus involvement and less adductor magnus involvement.

SECTION CONCLUSIONS
Hip extension is a key joint action involved in lower body movements, such as walking, running, sprint running and climbing stairs.
Hip extension moments are created by the muscle forces of the gluteus maximus, hamstrings and adductor magnus, multiplied by their moment arm lengths. Muscle forces and moment arm lengths differ with joint angle, and muscle forces also differ with contraction velocity.
Hip extension movements operating at large degrees of hip flexion are likely to require greater adductor magnus involvement and less gluteus maximus involvement, while hip extension movements operating close to full hip extension are more likely to require greater gluteus maximus involvement and less adductor magnus involvement.

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THE INCREASING ROLE OF THE HIPS
PURPOSE
The purpose of this section is to put forward an observation that clarifies the importance of the hip extension moment in sports: the increasing role of the hips.

Back squat
As relative load increases during performance of the back squat exercise, the ratio of the net hip extension moment to the net knee extension moment also increases. Bryanton et al. (2011) found that as relative load increased from 50% of 1RM to 90% of 1RM, the ratio of net hip extension moment to net knee extension moment increased from 1.1 times to 1.5 times, while Flanagan & Salem (2007) also observed a greater increase in net hip extension compared to net knee extension moment, with increasing load.

Deadlift
As relative load increases during performance of the conventional deadlift exercise, the ratio of the net hip extension moment to the net knee extension moment also increases. Swinton et al. (2011) found that as relative load increased from 10% of 1RM to 80% of 1RM, the ratio of net hip extension moment to net knee extension moment increased from 2.8 times to 3.7 times.

Lunge
As relative load increases during performance of the forward lunge exercise, the ratio of the net hip extension impulse to the net knee extension impulse also increases. Riemann et al. (2012) found that as relative load increased from 12.5% of bodyweight to 50% of bodyweight, the ratio of net hip extension impulse to net knee extension impulse increased from 3.4 times to 4.2 times.

Running
As speed increases during running, the ratio of the net hip extension moment to the net knee extension moment also increases. Schache et al. (2011) found that as running speed increased from 3.50 to 8.95m/s, the ratio of net hip extension moment to net knee extension moment during the stance phase increased from 0.65 times to 1.15 times.

Jumping
As height increases during jumping, the ratio of the net hip extension joint work done to the net knee extension joint work done also increases. Lees et al. (2004) found that as jump height increased from 0.35m to 0.53m, the ratio of net hip extension joint work done to net knee extension joint work done increased from 0.64 times to 1.67 times.

Implications for training
The hip extension moment becomes progressively more important during squats, deadlifts, lunges, running and jumping, as either load or speed are increased. Heavier loads, faster speeds or greater jump heights require a larger contribution from the hip joint than lighter loads, slower speeds, or lower jump heights. Athletes performing maximally in sporting situations will therefore find that their performances to involve the hip to a greater extent that their sub-maximal efforts in practice.

SECTION CONCLUSIONS
Hip extension moment becomes progressively more important in squats, deadlifts, lunges, running and jumping, as load or speed are increased.
Heavier loads, faster speeds or greater jump heights require a larger proportional contribution from the hip than lighter loads, slower speeds, or lower jump heights.

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DYNAMOMETRY: NORMATIVE VALUES OF HIP EXTENSION MOMENT
[Read more: moment]
PURPOSE
The purpose of this section is to provide normative values for isometric or isokinetic hip extension moment measured using dynamometry in both trained and untrained subjects.

NORMATIVE VALUES FOR ISOMETRIC HIP EXTENSION MOMENT IN TRAINED SUBJECTS
Selection criteria
Population – any trained adult population
Intervention – any acute study measuring isometric hip extension moment using a dynamometer
Comparison – between sub-groups within the population, between joint angles
Outcomes – hip extension moment

Results
The following relevant studies were identified that met the inclusion criteria: Buśko & Gajewski (2011).

Findings
Buśko & Gajewski (2011) measured the isometric hip extension moment in elite male and female swimmers, in addition to the associated hip flexion, knee flexion and knee extension moments. As might reasonably be expected, at 479 – 494Nm, the maximum hip extension moment was much greater (by 44 – 47%) in male athletes than in female athletes (333 – 335Nm). However, for both male and female athletes, the left leg displayed only a fractionally greater hip extension moment than the right leg (by 0.6 – 3.0%), which is suggestive of little meaningful differences between legs.

NORMATIVE VALUES FOR ISOKINETIC HIP EXTENSION MOMENT IN TRAINED SUBJECTS1
Selection criteria
Population – any trained adult population
Intervention – any acute study measuring isokinetic hip extension moment using a dynamometer
Comparison – between sub-groups within the population, between joint angles
Outcomes – hip extension moment

Results
The following relevant studies were identified that met the inclusion criteria: Smith (1981), Poulmedis (1985) Alexander (1990), Nesser (1996), Dowson (1998), Blazevich (1998a), Blazevich (1998b), Blazevich (2002), Sugiura (2008), Rannama (2013), Brown (2014), De Lacey (2014).

Findings
The studies used a wide range of speeds, at angular velocities ranging from 30 – 480 degrees/s, although the most commonly-measured velocity is 60 degrees/s. The populations studied include various types of athlete, including rugby players, sprinters and recreationally resistance-trained subjects. Even at the most commonly-studied isokinetic speed (60 degrees/s), isokinetic hip extension moment still varies widely, between 228Nm (Dowson et al. 1998) and 471Nm (Blazevich and Jenkins, 1998b).

NORMATIVE VALUES FOR ISOMETRIC HIP EXTENSION MOMENT IN UNTRAINED SUBJECTS
Selection criteria
Population – any untrained adult population
Intervention – any acute study measuring isometric hip extension moment using a dynamometer
Comparison – between sub-groups within the population, between joint angles
Outcomes – hip extension moment

Results
The following relevant studies were identified that met the inclusion criteria: Pohtilla (1969), Waters (1974), Markhede (1979), Nemeth (1983), Cahalan (1989), Hawkins (1999), Worrell (2001), Yerys (2002), Arokoski (2002), Dean (2004), Pua (2008), Perry (2004), Da Silva (2009), Bazett-Jones (2011), Kwon (2013), Stearns (2014).

Findings
The hip extension moments observed in these studies are generally lower than those found in athletes, ranging from 47 – 300Nm.

NORMATIVE VALUES FOR ISOKINETIC HIP EXTENSION MOMENT IN UNTRAINED SUBJECTS
Selection criteria
Population – any untrained adult population
Intervention – any acute study measuring isokinetic hip extension moment using a dynamometer
Comparison – between sub-groups within the population, between joint angles
Outcomes – hip extension moment

Results
The following relevant studies were identified that met the inclusion criteria: Markhede (1979), Burnfield (2000), Arokoski (2002), Tsiokanos (2002), Gribble (2009); Claiborne (2009), Costa (2010).

Findings
The most commonly-studied isokinetic speed in the literature relating to untrained populations is 60 degrees/s, as in trained subjects. At this speed, isokinetic hip extension moments have been reported to vary widely between 57Nm (Costa et al. 2010) in patients with osteoarthritis and 266Nm (Tsiokanos et al. 2002) in young, male physical education students. Despite this large variance, these values are around half the size of those in athletes, suggesting that the ability to generate a large hip extension moment may be a key factor in athletic performance.

SECTION CONCLUSIONS
Hip extension moments measured using isometric or isokinetic dynamometry vary widely even within groups of athletes. However, they are greater in athletic populations than in untrained subjects.

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DYNAMOMETRY: EFFECTS OF JOINT ANGLE ON HIP EXTENSION MOMENT
[Read more: moment]
PURPOSE
The purpose of this section is to discuss how joint angles affect the isometric or isokinetic hip extension moment in both trained and untrained subjects.

INTRODUCTION
[Read more: length-tension relationship]
Joint angle has effects on the joint moment that can be produced at any joint. This is because it can alter both the muscle forces and the internal moment arm lengths. The joint angle affects the amount of force that can be exerted because of the length-tension relationship. The length-tension relationship is the degree to which the sarcomeres within the muscle fibers overlap. Greater overlap means greater forces can be produced. Therefore, there is an optimal length at which muscles can produce the most force, while shorter and longer lengths lead to lower force production. At some joints, the effect of joint angle on the resulting moment that can be generated is not marked. However, the effect of altering hip and knee angles on hip extension moment is substantial. Altering the hip angle alters the length of the gluteus maximus, hamstrings and adductor magnus muscles, while altering knee angle alters the length of the hamstrings muscles only.

EFFECTS OF HIP ANGLE
Within single studies
Isometric
Most researchers directly investigating the effect of hip joint angle on the hip extension moment have concentrated on isometric measures. Almost all of them have routinely found that hip extension moments are smaller in greater degrees of hip flexion than in smaller degrees of hip flexion. Hip extension moments appear to be largest in large hip flexion angles (e.g. 90 degrees) and smallest at small hip flexion angles close to full hip extension (i.e. 0 degrees) (Waters et al. 1974; Nemeth et al. 1983; Cahalan et al. 1989; Worrell et al. 2001; Da Silva et al. 2009). Worrell et al. (2001) found that at 0 degrees of hip flexion, the isometric hip extension moment was 36% of that produced at 90 degrees. Nemeth et al. (1983) found that at 0 degrees of hip flexion, the isometric hip extension moment was 55% of that produced at 90 degrees. Cahalan et al. (1989) found that at 45 degrees of hip flexion, the isometric hip extension moment was 75% of that produced at 90 degrees. Da Silva et al. (2009) found that at 0 degrees of hip flexion, the isometric hip extension moment was 45% of that produced in 40 degrees.

Isokinetic
In contrast to the literature regarding isometric hip extension moments, few studies have been performed assessing the way in which isokinetic hip extension moments change with changing hip flexion angle, making this trend difficult to assess. Pontaga (2004) investigated the isokinetic hip extension moment produced between 40 – 120 degrees of hip flexion at both 100 and 200 degrees/s. They found that isokinetic hip extension torque peaked at 70 degrees of hip flexion at both speeds and was smallest at 40 degrees at 100 degrees/s and at 120 degrees at 200 degrees/s.

Across multiple studies
Hip extension moment appears to alter with changing hip joint angle, when comparing data from studies measuring hip extension moments at different angles (Pohtilla, 1969; Waters et al. 1974; Markhede and Grimby, 1979; Nemeth et al. 1983; Cahalan et al. 1989; Hawkins and Smeulders, 1999; Worrell et al. 2001; Yerys et al. 2002; Arokoski et al. 2002; Dean et al. 2004; Perry et al. 2004; Da Silva et al. 2009; Bazett-Jones et al. 2011; Kwon et al. 2013; Pua et al. 2008; Stearns and Powers, 2014). The hip extension moment appears to be greatest in around 90 degrees of hip flexion and least in full hip extension.

Mechanism
Introduction
Exactly why the hip extension moment is greater in 90 degrees of hip flexion than in smaller degrees of hip flexion is unclear. Since joint moments are the product of muscular force and moment arm length, the differences could arise from either differences in the ability of the hip extensor muscles to generate force, or from changes in the internal muscle moment arm lengths.

Muscle activation
[Read more: EMG]
Muscle activity is one method that can provide an indication of muscle force. Muscle activity is most commonly-measured using electromyography (EMG). Gluteus maximus EMG activity is smaller in 90 degrees of hip flexion than in full hip extension (Worrell et al. 2001). Hamstrings activity does not appear to alter markedly with changing hip flexion angle (Worrell et al. 2001; Mohammed et al. 2002; Guex et al. 2012). Thus, neither the ability of the gluteus maximus nor hamstrings to produce muscular force appears to explain the relationship between the hip extension moment and hip flexion angle. Whether the adductor magnus displays greater EMG activity in greater degrees of hip flexion is unclear.

Internal moment arm lengths
Németh (1986) investigated the internal moment arm lengths of each of the hip extensors at varying degrees of hip flexion. The gluteus maximus was found to have a smaller moment arm length in 90 degrees of hip flexion than in full hip extension. The hamstrings displayed similar moment arm lengths in 90 degrees of hip flexion and full hip extension. Thus, changes in the internal moment arm lengths of the gluteus maximus and hamstrings do not appear to explain the greater hip extension moment in greater degrees of hip flexion angle. However, the adductor magnus displayed a much greater moment arm length in 90 degrees of hip flexion than in full hip extension, which may indicate that this muscle has an important contributory role to hip extension moment in this position. Further research is needed to investigate this matter, as it may have important training implications for athletes.

EFFECT OF KNEE ANGLE
Within single studies
Most researchers investigating the effect of knee joint angle on the hip extension moment have concentrated on isometric measures. There is some disagreement among researchers about the effects of altering the knee angle but overall it seems likely that greater hip extension moments can be observed when the knee is more flexed compared to when it is more extended. Nemeth et al. (1983) did not note any effect of changing knee angle on the isometric hip extension moment. However, when Waters et al. (1974) tested 8 female subjects before and after a sciatic nerve block, they reported a small change in respect of knee flexion in that the isometric hip extension moment was 12 – 18% reduced when the knee was flexed compared to when it was extended when the hip was in 45, 15 and 0 degrees of flexion but was not reduced when the hip was in 90 degrees of flexion. More recently, Kwon et al. (2013) reported similarly that the hip extension moment during prone hip extension with full knee flexion (110 degrees) was 54% of the hip extension moment when the same movement was performed with full knee extension.

Across multiple studies
Hip extension moments appear to be greatest when the knee is in full extension, when studies taking measurements at different joint angles are compared to one another (Arokoski et al. 2002; Bazett-Jones et al. 2011, Cahalan et al. 1989; Da Silva et al. 2009, Dean et al. 2004; Hawkins and Smeulders, 1999; Kwon et al. 2013; Markhede and Grimby, 1979; Nemeth et al. 1983; Perry et al. 2004; Pohtilla, 1969; Pua et al. 2008; Stearns and Powers, 2014; Waters et al. 1974, Worrell et al. 2001, and Yerys et al. 2002).

Mechanism
The hip extension moment is probably greater in full knee extension compared to greater degrees of knee flexion because of greater hamstrings muscle activity (Sakamoto et al. 2009; Kwon et al. 2013), leading to greater hamstrings muscle forces contributing to the hip extension movement. Greater hamstrings activity likely occurs in this combination of joint angles because of a favourable length-tension relationship.

CONCLUSIONS
Hip extension moment is greater when the hip is flexed compared to when it is extended. Why this occurs is unclear but it does not seem to be related to greater gluteus maximus or hamstrings muscle forces or internal moment arm lengths.
Hip extension moment is greater when the knee is extended compared to when it is flexed, most likely because of the advantageous length-tension relationship in the hamstrings causing greater muscle force.

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DYNAMOMETRY: EFFECT OF ANGULAR VELOCITY
[Read more: moment]
PURPOSE
The purpose of this section is to provide a background of the literature in respect of isometric, isokinetic and inverse dynamics measurements of hip extension moment in both trained and untrained subjects and the factors that affect these measurements.

INTRODUCTION
In many sporting movements the hip joint moves with very high angular velocity. Kivi et al. (2002) reported that at 95% of maximum sprint running speed, hip extension angular velocity averaged 666 degree/s. Similarly, in 4 male sprinters, Bezodis et al. (2008) reported a peak hip extension angular velocity of 871 degrees/s. Akkus (2012) recorded peak hip extension angular velocity of 450 degrees/s during the second pull of the snatch lift in elite female weightlifters. Okkonen & Häkkinen (2013) recorded peak hip angular velocity of 604 degrees/s during a block sprint start, 532 degrees/s during sled pulling with 10% body mass, 477 degrees/s during sled pulling with 20% body mass, 758 degrees/s during countermovement jumps, 642 degrees/s during countermovement jumps with 10% body mass, 587 degrees/s during countermovement jumps with 20% body mass, and 435 degrees/s during smith machine half-squats with 70% of 1RM loading. Such very high angular velocities during sporting actions are in contrast to the relatively low angular velocities that are used during testing. As noted above, 60 degrees/s is most commonly used and the upper limit is typically 480 degrees/s.

EFFECT OF ANGULAR VELOCITY
Trained subjects
Isokinetic dynamometry studies performed in trained subjects have found that hip extension moment reduces as expected, in line with the force-velocity relationship. Blazevich and Jenkins (1998a) reported that hip extension moment reduced by 9 – 14% from 60 – 270 degrees/s and by 19 – 25% from 270 – 480 degrees/s in sprinters and recreationally resistance-trained individuals. Similarly, Nesser et al. (1996) reported that hip extension moment reduced by 15% from 60 – 360 degrees/s and by 54% from 360 – 480 degrees/s in team sports athletes. Poulmedis et al. (1985) found that hip extension moment reduced by 23% from 30 – 90 degrees/s and by 22% from 90 – 180 degrees/s in elite soccer players. Rannama et al. (2013) found that hip extension moment reduced by 22% from 60 – 180 degrees/s and by 11% from 180 – 240 degrees/s in high-level road cyclists. Smith et al. (1981) reported that hip extension moment reduced by 25 – 28% from 30 – 180 degrees/s in amateur and professional ice hockey players. Sugiura et al. (2008) reported that hip extension moment reduced by 12% from 60 – 180 degrees/s and by 10% from 180 – 300 degrees/s in sprinters. In trained subjects, only Alexander (1990) has found a greater hip extension moment at a higher angular velocity than at a slower angular velocity. This may be because this study was performed in sprinters and other studies have found that sprinters perform better than the weight-trained non-sprinters at high angular velocities while weight-trained non-sprinters perform better than the sprinters at low angular velocities (Blazevich and Jenkins, 1998a).

Untrained subjects
Similar reductions with increasing joint angular velocity have been observed in untrained individuals (Cahalan et al. 1989; Arokoski et al. 2002; Tsiokanos et al. 2002; Costa et al. 2010).

Effect of muscle action
Reductions in hip extension moment with increasing velocity may be different across concentric and eccentric muscle actions, as Dowson et al. (1998) reported that while concentric hip extension moment decreased with increasing angular velocity, the eccentric hip extension moment did not.

SECTION CONCLUSIONS
Hip extension moment reduces with increasing angular velocity, as expected from the force-velocity relationship. Athletes who require high-velocity hip extension movements for their sport tend to display an ability to perform greater hip extension moments at high velocities.

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DYNAMOMETRY: NORMATIVE RATIOS
[Read more: moment]
PURPOSE
The purpose of this section is to provide normative ratios for hip flexion to hip extension moments, knee flexion to hip extension moments, and knee extension to hip extension moments, in both trained and untrained subjects.

HIP FLEXION TO HIP EXTENSION
Trained subjects
In trained subjects, the ratio of isometric hip flexion to hip extension moment has only been explored by Buśko and Gajewski (2011), who reported a ratio of around 0.20 for both the right and left legs of male and female elite swimmers. In trained subjects, the ratio of isokinetic hip flexion to hip extension moment seems to vary with angular velocity. As isokinetic angular velocity increases from 30 – 480 degrees/s, the hip flexion-to-hip extension moment ratio reduces by >50% from between 0.6 – 0.8 times to between 0.2 – 0.3 times (Smith et al. 1981; Poulmedis et al. 1985; Alexander, 1990; Nesser et al. 1996; Dowson et al. 1998; Blazevich and Jenkins, 1998a; Blazevich and Jenkins, 1998b; Blazevich and Jenkins, 2002; Rannama et al. 2013; Brown et al. 2014; De Lacey et al. 2014). Thus, if the hip flexion to hip extension moment ratio is to be used as a metric to assess muscular balance at the hip in athletes, it is essential to ensure consistency of joint angular velocity when taking measurements. Also, it seems that hip extension moment decreases less than hip flexion moment with increasing joint angular velocity.

Untrained subjects
In untrained subjects, as isokinetic angular velocity increases the hip flexion-to-hip extension moment ratio reduces from 0.7 – 1.1 at an isokinetic angular velocity of 30 degrees/s to 0.5 – 0.7 at an isokinetic angular velocity of 210 degrees/s (Arokoski et al. 2002; Burnfield et al. 2000; Cahalan et al. 1989; Claiborne et al. 2009; Costa et al. 2010; Gribble and Robinson, 2009; Markhede and Grimby, 1979).

KNEE FLEXION TO HIP EXTENSION
Trained subjects
In trained subjects, as isokinetic angular velocity increases, the knee flexion-to-hip extension moment ratio declines only slightly (Smith et al. 1981; Poulmedis et al. 1985; Alexander, 1990; Nesser et al. 1996; Dowson et al. 1998; Sugiura et al. 2008; Rannama et al. 2013; Brown et al. 2014; De Lacey et al. 2014), moving from around 0.5 – 0.6 times at an isokinetic angular velocity of 30 degrees/s to around 0.4 – 0.6 times at angular velocities ranging from 300 – 450 degrees/s. This is in contrast to the larger reduction with increasing angular velocity that occurs in the hip flexion-to-hip extension moment ratio. Consequently, when the knee flexion to hip extension ratio is to be used as a metric to assess muscular balance at the hip in athletes, it is not of great importance to ensure consistency of angular velocity when taking measurements.

Untrained subjects
In untrained subjects, few studies have been performed that have measured both hip extension and knee flexion moments at identical isokinetic angular velocities. Thus, it is currently unclear what trends might exist. However, it is noted that Burnfield et al. (2000) measured a ratio of 0.59 in elderly males aged 60 – 92 years, while Gribble and Robinson (2009) measured a ratio of 0.82 in healthy males and females and a ratio of 0.63 in males and females with chronic ankle instability.

KNEE EXTENSION TO HIP EXTENSION
Trained subjects
As isokinetic angular velocity increases, the knee extension-to-hip extension moment ratio declines only slightly (Smith et al. 1981; Poulmedis et al. 1985; Alexander, 1990; Nesser et al. 1996; Dowson et al. 1998; Sugiura et al. 2008; Rannama et al. 2013; Brown et al. 2014; De Lacey et al. 2014), being around 0.8 – 1.1 times at an isokinetic angular velocity of 30 degrees/s and around 0.7 – 1.0 times at isokinetic angular velocities of 360 – 450 degrees/s. This is similar to the behavior of the knee extension-to-hip extension moment ratio but different to the hip flexion-to-hip extension moment ratio. Consequently, when the knee extension to hip extension ratio is to be used as a metric to assess muscular balance at the hip in athletes, it is not of great importance to ensure consistency of angular velocity when taking measurements.

Untrained subjects
In untrained subjects, few studies have been performed that have measured both hip extension and knee extension moments at identical isokinetic angular velocities. Thus, it is currently unclear what trends might exist. However, it is noted that Gribble and Robinson (2009) reported ratios of 0.98 – 1.15 in young individuals (injured and uninjured) while Burnfield et al. (2000) measured a ratio of 0.86 in elderly males aged 60 – 92 years, and Tsiokanos et al. (2002) measured ratios of 0.62 – 0.83 in young, male physical education students.

SECTION CONCLUSIONS
The hip flexion-to-hip extension ratio appears to be markedly affected by changes in angular velocity, while the knee flexion-to-hip extension and knee extension-to-hip extension ratios do not.
Care should be taken when measuring hip flexion-to-hip extension ratios to ensure standard angular velocities in both measures.

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CORRELATIONS BETWEEN HIP EXTENSION MOMENT AND ATHLETIC PERFORMANCE
[Read more: moment]
PURPOSE
The purpose of this section is to provide a summary of the correlations between hip extension moment and athletic performance measures, such as sprint running and vertical jumping ability.

INTRODUCTION
Studies are conflicting regarding whether isokinetic hip extension moments are correlated with sprint running performance and the research is limited in respect of whether isokinetic hip extension moment is correlated with vertical jumping performance.

Sprint running
[Read more: sprint running]
As regards sprint running, on the one hand, Smith et al. (1981) found no differences between the isokinetic hip extension moments recorded in professional and elite amateur ice hockey players. On the other hand, Blazevich and Jenkins (1998a) found that the sprinters displayed greater hip extension moments at high angular velocities while weight-trained athletes displayed greater hip extension moments at low velocities. Blazevich and Jenkins (1998b) performed a regression analysis in order to assess whether it was possible to predict 20m and flying 20m sprint running times from isokinetic and squat tests. They found that 20m sprint running performance might be predicted from squat and isokinetic hip flexor and extensor moments but noted that the hip flexion moment appeared to be more important than the hip extension moment. Similarly, Nesser et al. (1996) investigated the determinants of 40m sprint running performance in young male athletes and found that the isokinetic hip extension moment correlated significantly with 40m sprint performance. Finally, Alexander (1989) did not find any correlations between the isokinetic hip extension moment or the hip flexion moment and 100m sprint running performance in either male or female athletes.

Vertical jumping
In respect of jumping performance, Tsiokanos et al. (2002) found a moderate-to-strong cross-sectional relationship between the isokinetic moments displayed at the knee, ankle and hip joints and vertical jump performance. More importantly, Marshall & Moran (2015) found a moderate-to-strong longitudinal relationship between the work done at the hip, and vertical jump performance. This suggests that increases in hip work done are the most importance factor for determining improvements in vertical jump height over a long-term training program.

SECTION CONCLUSIONS
There are some indications that hip extension moment is positively associated with measures of athletic performance, such as sprint running ability and vertical jump height.

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CORRELATIONS BETWEEN HIP EXTENSION MOMENT AND INJURY, SURGERY AND AGE
[Read more: moment]
PURPOSE
The purpose of this section is to provide a summary of the correlations between hip extension moment and injury, surgery and age.

INJURY
Introduction
There are some indications that lower body injury is correlated with reduced hip extension moments, although whether this association was present prior to the injury, is caused by disuse atrophy following a lack of training and physical activity, or is caused by some other mechanism is unclear.

Chronic ankle instability
Gribble and Robinson (2009) found that young male and female individuals with chronic ankle instability (CAI) displayed a lower hip extension moment than healthy subjects but they did not find any statistically significant relationships between the average hip extension average moment and the extent of CAI. However, Negahban et al. (2013) found no difference between the eccentric hip extension moment displayed by individuals with CAI in comparison with individuals. They also detected no differences between injured and uninjured limbs in the individuals with CAI.

Patellofemoral pain
Robinson and Nee (2007) found that young females with Patellofemoral Pain Syndrome (PFPS) displayed significant impairments in isometric hip extension moment of their symptomatic legs. Similarly, Rowe et al. (2007) used a hand-held dynamometer and found that females who had been experiencing unilateral knee pain for no more than four weeks displayed lower hip extension strength on the affected limb than on the unaffected limb. However, Boling et al. (2009) found that while young male and female individuals with PFPS displayed significantly lower isokinetic hip abduction and hip external rotation moments, there was no significant difference in the isokinetic hip extension moment between healthy and unhealthy subjects. Knee pain is very common in endurance runners. When Niemuth et al. (2005) compared isometric hip strength between injured and uninjured recreational runners, they found that while the injured runners had weaker isometric hip abduction and hip flexion strength on their injured limb compared to their uninjured limb, the uninjured runners displayed no such side-on-side differences. However, there was no such difference in isometric hip extension strength.

Osteoarthritis
There are some indications that having osteoarthritis is associated with lower hip extension moments, although whether this results from pain inhibition, disuse atrophy, or a pre-existing state of inactivity or weakness that predisposed the individual to osteoarthritis is unclear. Arokoski et al. (2002) found that patients with hip osteoarthritis (mean age 56 years) displayed an isometric and an isokinetic hip extension moment that was 68 – 87% of that found in healthy age-matched controls. Interestingly, they noted that the cross-sectional area of the pelvic and thigh muscles did not differ between groups. Costa et al. (2010) found that male and female patients with knee osteoarthritis displayed a lower isokinetic hip extension moment than control subjects of a similar age. While it may seem that disuse atrophy is likely the cause of such reduced hip extension moments following injury, other factors may be relevant. For example, Yerys et al. (2002) reported that mobilization of the anterior hip capsule using manual therapy led to a significantly improved hip extension moment compared with a sham treatment, suggesting that soft tissue restrictions may be one possible factor.

SURGERY
Hip extension moment appears to be reduced by surgery to the lower body. This can include surgery not performed at the hip, although low hip extension moments are commonly observed after hip replacements (e.g. Lamontagne et al. 2012). Jaramillo et al. (1994) found that hip extension strength was different in individuals who had undergone knee surgery compared with healthy individuals. Hip extension force measured in the surgical group was 220N but in the non-surgical group it was 295N. This difference was larger than the other muscle groups, although whether the deficit was caused by atrophy following surgery or whether it was present prior to surgery is not known. Mizner and Snyder‐Mackler (2005) assessed hip and knee extension moments during sit-to-stand actions in individuals post-total knee replacement. They reported that the peak hip and knee extension moments were significantly lower in the affected limb compared to the unaffected limb. It seems likely that the smaller hip extension moments in these instances are caused by disuse atrophy, although the causal relationship was not assessed in either of these studies.

AGE
Hip extension moment appears to reduce with increasing age. This has a significant bearing on gait and therefore independence in the elderly. Dean et al. (2004) investigated the isometric and isotonic hip extension and flexion moments in young and old subjects and found that maximum isometric hip flexion and extension moments was smaller in the elderly subjects than in in the younger individuals. Burnfield et al. (2000) examined the influence of lower body joint moments on gait characteristics in elderly men and found that hip extension moment was the only significant independent predictor of velocity, stride length, and cadence. They also noted that the ability to produce a higher hip extensor moment was associated with greater stride length, cadence and free walking speed. They proposed that the ability to produce a large hip extension moment is important for being able to tolerate the demands of the larger degree of hip flexion at foot-strike that is associated with a longer stride length.

SECTION CONCLUSIONS
Lower body injury, surgery and age are associated with reduced hip extension moments. Whether such associations are caused by disuse atrophy or another mechanism is unclear.

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RESISTANCE TRAINING
PURPOSE
The purpose of this section is to provide a summary of the literature exploring hip extension moments in various different resistance training exercises.

RESISTANCE TRAINING SECTION CONTENTS
Back squat
Deadlift
Olympic lifts
Lunge

BACK SQUAT
[Read more: squat]
Introduction
Researchers have found that the average hip extension moment measured during the traditional back squat is often as large or larger than that measured during dynamometry. This may suggest that a large hip extension moment is critical to the performance of this exercise. Researchers have found that the hip extension moment recorded in the traditional, shoulder-width stance free-weight back squat ranges from 28 – 595Nm (Wretenberg et al. 1996; Morrissey et al. 1998; Escamilla et al. 2001b; Fry et al. 2003; Kobayashi et al. 2010; Lorenzetti et al. 2012; Lynn and Noffal, 2012; Swinton et al. 2012; Bryanton et al. 2012; Sugisaki et al. 2014).

Factors affecting hip extension moment
Studies have reported that several factors affect the magnitude of the hip extension moment measured during back squats in addition to the type of subjects and the percentage of 1RM used, including stance width, technique, bar speed and squat depth. In general, a wider stance appears to lead to a greater hip extension moment (Escamilla et al. 2001b). A load position that allows individuals to sit back further during the squat also appears to lead to a greater hip extension moment (Fry et al. 2003; Biscarini et al. 2011; Lorenzetti et al. 2012; Gillette & Stevermer, 2012). In general, the powerlifting technique appears to lead to a greater hip extension moment than the traditional or Olympic back squat (Wretenberg et al. 1996; Swinton et al. 2012). Greater bar speed seems to lead to a greater hip extension moment (Morrissey et al. 1998; Manabe et al. 2007; Yoshioka et al. 2009), although the findings of Sugisaki et al. (2014) comparing squat jumps with back squats with moderate loads are difficult to interpret. Finally, with a similar absolute load, greater depth during squats leads to a greater hip extension moment, at least as far as parallel (Bryanton et al. 2012; Yoshioka et al. 2014).

Mechanisms
The changes in the hip extension moment following some of all of these variables may be the result of altered movement patterns. Frost et al. (2013) found that when performing lifting or squatting tasks with a faster movement speed subjects used a less upright posture and shifted to a more hip-dominant pattern involving an increase in trunk inclination towards the horizontal. Such alterations in kinematics might naturally be expected to lead to corresponding changes in kinetics. Indeed, Hay et al. (1983) observed a simultaneous change in trunk inclination along with an increase in hip extension moment during squat exercises with increasing load. Also, Biscarini et al. (2011) found that in a model of Smith machine squats, inclining the trunk closer to the horizontal, moving the tibia closer to the vertical, and placing more weight upon the fore-foot all led to an increase in the hip extension moment and a corresponding reduction in the knee extension moment. It may be the case that the movement pattern can be performed in either a hip-dominant or a knee-dominant way and that greater inclination of the trunk leads to greater hip extension moments (Yoshioka et al. 2007). This may explain the odd findings reported by Sugisaki et al. (2014). They noted that squat jumps produced greater hip extension moment than back squats with a moderate load (272 vs. 149Nm) and that the ratio between hip extension moments in these two exercises was smaller than the ratio between knee extension moments (1.8 vs. 2.3 times). The greater load used in the back squat may have necessitated greater forward lean and hence more hip extension moment in this exercise even though the overall work done (and hence combined joint moments) was smaller.

DEADLIFT
[Read more: deadlift]
Introduction
Studies have reported that the hip extension moment in deadlifts is generally larger than that reported during back squats, ranging from 339 – 671Nm, and increases with increasing load (Nemeth et al. 1984; Brown and Abani, 1985; Cholewicki et al. 1991; Escamilla et al. 2000; Escamilla et al. 2001a; Swinton et al. 2011). Again, the average hip extension moment measured during the deadlift is often as large or larger than that measured during dynamometry. As for the back squat, this again suggests that a large hip extension moment may be critical for the performance of this exercise. Moreover, the hip extension moment measured during deadlifts appears to be slightly greater than that measured in the traditional back squat and this may be a result of the greater peak external moment arm at the hip, (c.f. Escamilla et al. 2000; Escamilla et al. 2001b).

Factors affecting hip extension moment
Studies have reported that several factors affect the magnitude of the hip extension moment measured during deadlifts in addition to the type of subjects and the percentage of 1RM used, including style and technique. Studies have also noted that hip extension moment is greater in the conventional deadlift technique compared to the sumo style (Escamilla et al. 2000) and than in hex-bar deadlifts (Swinton et al. 2011). Such differences in the hip extension moment between lift variations may be the result of differing trunk inclination or differences in other joint angles, as noted above in respect of the back squat (Hay et al. 1983; Frost et al. 2013).

Studies have also noted that hip extension moment is greater when the lift is performed with straight legs rather than with bent legs. Nemeth et al. (1984) compared the peak hip extension moment of straight-leg lifts and bent-leg lifts with and without holding the load far from the body or close to the body. They found that the straight-leg lift displayed the greatest peak hip extension moment (124Nm), followed by the bent-leg lift with the load far from the body (105Nm), and finally the bent-leg lift with the load close to the body (88Nm). In each case, the peak hip extension moment was observed at the point the load left the ground. Again, the difference in peak hip extension moments between these lifting variations may be a function of the peak external moment arm in each case, with the straight-leg position placing the hip joint further from the system center of mass than the bent-leg position.

OLYMPIC LIFTS
[Read more: Olympic lifts]
Hip extension moments have rarely been measured during the Olympic lifts. However, studies exist in which the hip extension moment has been calculated for the snatch (Baumann et al. 1988), clean (Kipp et al. 2011) and for the jerk (Cleather et al. 2012). These studies indicate that hip extension is not maximized when the load is maximal but rather at lower loads. Investigating the snatch at the 1985 World Championships, Baumann et al. (1988) did not perform a study involving a sample of subjects but did report that in weight classes of athletes ranging from 60 – 150kg and in barbell loads ranging from 135 – 202.5kg, the hip extension moment reported ranged from 260 – 660Nm. Investigating the clean, Kipp et al. (2011) reported that the hip extension moment was 248Nm at 65%, 272Nm at 75% and 266Nm at 85% of 1RM, implying that the hip extension moment in the Olympic lifts is maximized at 75% of 1RM. Cleather et al. (2012) reported that the hip extension moment during a jerk with 40kg produced a hip extension moment of 51Nm). The differences in hip extension moments observed in the different load lifts may be the result of differing trunk inclination or differences in other joint angles, as noted above (Hay et al. 1983; Frost et al. 2013).

LUNGES
Hip extension moments have rarely been measured during lunges. However, some useful findings have nevertheless been made. Riemann et al. (2012) measured hip extension moment impulses during lunges with different loads and found that increasing load led to increased ankle and hip joint moment impulses but not increased knee joint moment impulse. Riemann et al. (2012) also noted that the hip extension moment impulse increased more with increasing load than either ankle or knee joint moment impulses.

In a follow-up study, Riemann et al. (2012) found that a standardized step-length for forward lunges makes the exercise more hip dominant by increasing the net hip extension moment impulse and by decreasing the net knee extension moment impulse. It was also noted that the standardized step length led to an increase in the degree of peak hip flexion, which increases hip range-of-motion during the lunge movement. Riemann et al. (2012) proposed that this kinematic alteration is likely the cause of the shift in joint moments from the knee to the hip. Flanagan et al. (2004) reported that in elderly subjects with just bodyweight loading, the forward lunge displayed a greater hip extension moment than the lateral lunge (92Nm vs. 81Nm). In their follow-up study, Riemann et al. (2013) also reported the same finding, that the forward lunge displays greater hip joint moment impulses than the lateral lunge, while the lateral lunge displays greater knee and ankle joint moment impulses than the forward lunge.

SECTION CONCLUSIONS
In squats, the hip extension moment can be increased by using heavier loads, by using a wider stance width, by using a low-bar, powerlifting technique, by sitting back more, by ensuring bar speed is as fast as possible and by squatting as deep as possible without compromising good form.
In deadlifts, the hip extension moment can be increased by using heavier loads, by using a conventional rather than a sumo stance or a hex-bar, or by using a straight-leg technique.
In the Olympic lifts, the hip extension moment is maximized with submaximal loads of approximately 75% of 1RM while rate of hip extension torque development is maximized at 85% of 1RM.
In lunges, the hip extension moment can be maximized by using the forward lunge rather than the lateral lunge and by using greater loads.

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FUNCTIONAL MOVEMENTS
PURPOSE
The purpose of this section is to provide a summary of the literature exploring hip extension moments in various different functional movements, such as walking, climbing stairs and sit-to-stand.

WALKING
Introduction
Hip extension moment appears to be important for walking. Riley et al. (2001) observed that hip and ankle moments contribute primarily to propulsion at all speeds while knee moments do not contribute significantly to propulsion at any speed. They also noted that the magnitude the hip and ankle moments changed with speed and the relative contribution of the hip and ankle joint moments to propulsion also changed with speed, with the relative contribution of the hip moments increasing with increasing speed. Riley et al. (2001) concluded that the hip extensors act primarily in order to propel the body forward during walking while the role of the ankle is predominantly one of supporting the upper-body mass.

Elderly populations
The importance of hip extension moment in walking is most obvious when considering elderly populations. Burnfield et al. (2000) investigated the association between leg joint moments and stride characteristics in elderly males. They found that hip extension moment was the best predictor of walking speed. No other moment variable improved the correlation between joint moment and walking speed in their study. Moreover, researchers exploring hip moments during level walking in older adults have found that the maximum hip extension moment measured is around 70 – 80Nm, while the average maximum hip flexion moment is smaller, at around 53Nm (Kirkwood et al. 1999; Wang et al. 2006). These hip extension moments are surprisingly large, particularly considering that older adults sometimes record values that are substantially less than this during dynamometry testing (e.g. Dean et al. 2004; Inacio et al. 2014). Interestingly, Wang et al. (2006) compared the hip extension moments during walking, stair climbing and sit-to-stand (depth = 43.8cm) movements and found them all to be similar.

STAIR CLIMBING
Stair climbing is most commonly explored in elderly populations, because of the difficulty that this population often has with this task. Researchers exploring hip moments during stair climbing in older adults have found that the maximum hip extension moment measured is 70 – 80Nm, while the average maximum hip flexion moment is very much smaller, at around 23Nm (Kirkwood et al. 1999; Wang et al. 2006). These hip extension moments are large and reflect a genuine challenge to the hip extensor muscles in the elderly, particularly considering that older adults sometimes record values that are substantially less than this during dynamometry testing (e.g. Dean et al. 2004; Inacio et al. 2014). Interestingly, Wang et al. (2006) compared the hip extension moments during walking, stair climbing and sit-to-stand (depth = 43.8cm) movements and found them all to be similar.

SIT-TO-STAND
Introduction
The sit-to-stand action is key for elderly populations, and the loss of the ability to perform a sit-to-stand movement can often lead to a loss of independence. Researchers exploring hip moments during sit-to-stand movements have found that the maximum hip extension moment measured can range widely between 50 – 120Nm (Flanagan et al. 2003; Sibella et al. 2003; Wang et al. 2006; Savelberg et al. 2007; Van der Heijden et al. 2009; Schultz et al. 2009). The sit-to-stand movement does not appear to be identical to a free squat, as Flanagan et al. (2003) found that there was a significant difference in the joint moments between chair squats and free squats. They found that chair squats required much greater hip extension moment than free squats, while free squats required a greater knee extension moment than chair squats. Interestingly, Mathiyakom et al. (2005) compared sit-to-stand movements with varying center of mass placements. They reported that sit-to-stand movements initiated with more vertical shank positions required greater hip extension moments compared to sit-to-stand movements initiated with less vertical shank orientations. Since more vertical shank positions are common to box squat variations (Swinton et al. 2012), it seems likely that the movement pattern during a sit-to-stand bears some similarities to that observed in a box squat. Nevertheless, it appears that there is considerable leeway in terms of how the sit-to-stand movement can be accomplished, as Yoshioka et al. (2007) found that the sum of the knee and hip extension moments was the main predictor of a successful sit-to-stand movement but that this could be accomplished with a range of different combinations of individual hip and knee extension moments.

Effect of increasing effort
Several studies have explored the effects of increasing the effort involved in a sit-to-stand movement, either by increasing load, reducing the height of the chair, or observing weaker and stronger individuals, or individuals of lesser or greater bodyweight. Savelberg et al. (2007) looked at how hip extension moment and rising strategies alter with increasing percentages of bodyweight lifted during a sit-to-stand exercise (with 15%, 30% and 45% of added bodyweight). As expected, they found that increasing loads led to increasing hip moments. Similarly Burdett et al. (1985) compared the hip extension moments between sit-to-stands from chairs of different heights (43cm vs. 64cm). They found that the lower chair involved markedly greater hip extension moments than the higher chair. Similar findings of the effect of chair height were found by Mathiyakom et al. (2005). Van der Heijden et al. (2009) assessed the effects of increasing loads during sit-to-stands led to a strategy shift. Adding load led to a moment transfer strategy that reduced the knee extensor moment (by 6%) and transferred the effort to hip extensor (by 57%) and plantar flexor (by 67%) moments. This indicates that adding load to a sit-to-stand movement may not lead to identical increases in all joint moments, possibly as a result of changes in joint angle movements. In a study exploring a similar question, Schultz et al. (2009) compared the joint moments in young healthy subjects and old healthy subjects during sit-to-stand movements. They found that the old healthy subjects produced lower knee extension moments and more hip extension moments and ankle plantar-flexion moments. Together with the findings reported by Van der Heijden et al. (2009), the findings of this study indicate that as the overall effort of rising from a chair increases, the strategy becomes more hip dominant and less knee dominant, either because of differences in strength between individuals, or because of the addition of external load. Surprisingly, Sibella et al. (2003) reported that obese subjects actually use a more knee dominant movement pattern during sit-to-stands in which the foot is placed closer to the hip joint upon movement initiation. This leads to reduced hip extension moments, greater peak knee flexion angles, and greater knee extension moments. Whether this movement strategy is adopted to reduce lumbar moments (which are associated with hip extension moments) or in response to a difference in the center of mass of obese vs. normal weight individuals is unclear.

SECTION CONCLUSIONS
Hip extension moments are important for walking, acting to propel the body forwards. Their importance increases with increasing walking speed. Hip extension moment is the best predictor of walking speed in elderly people.
Chair squats require more hip extension moment than free squats, while sit-to-stand movements with more vertical shanks require more hip extension moment than to sit-to-stand movements with a less vertical shank. As with squats, as load increases, hip extension moments increase faster than knee extension moments.

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SPORTING MOVEMENT
PURPOSE
The purpose of this section is to provide a summary of the literature exploring hip extension moments in various different sporting movements, such as sprint running and jumping.

JUMPING
Introduction
The vertical jump is a co-ordinated, explosive movement involving hip, knee and ankle extension (triple extension). Although both co-ordination and strength clearly play important roles in determining how high athletes are able to jump, it appears that strength is the more critical factor. Vanezis and Lees (2005) compared two different groups, who were either good performers or poor performers on the vertical jump. They found that good performers displayed greater ankle plantar-flexion, knee and hip extension moments during the vertical jumps than the poor performers, while technique differed less noticeably between the groups. This indicates that improving the ability to produce large lower body joint moments is essential for developing superior jumping performance. Indeed, Tsiokanos et al. (2002) found a moderate-to-strong relationship between isokinetic joint moments at the knee, ankle and hip and vertical jump performance, particularly when bodyweight was controlled for.

Effect of load
Although the ability to produce large joint moments at all lower body joints is clearly important for improving vertical jumping ability, it seems that the ability to produce a large moment at the hip joint might be more important than the knee and ankle joints. This is logical, as it is larger than the joint moments produced at the knee or ankle (Fukashiro and Komi, 1987). Lees et al. (2004) compared maximal and sub-maximal vertical jumps and found that as vertical jump height increased from 0.35m to 0.53m, the ratio of net hip extension joint work done to net knee extension joint work done increased from 0.64 times to 1.67 times. This indicates that as load increases, the role of the hip joint increases relative to the roles of the ankle and knee.

Effect of trunk inclination
Trunk inclination appears to affect the relative contribution of the hip and knee joints to vertical jump performances. Although researchers have not explored joint moments, Vanrenterghem et al. (2008) explored the effect of trunk angle on joint power outputs during maximal vertical countermovement jumps. Joint powers can be taken as proxy measurements for joint moments. They compared joint powers between normal vertical jumps and jumps while holding the trunk as upright as possible. They found that jump heights with an upright trunk were 10% lower than normal jumps, that hip joint power was decreased by 37%, knee joint power was increased by 13%, and ankle joint power did not change. These findings are similar to those reported in slower movements, such as lifting and squatting tasks, in which a more horizontal trunk angle leads to greater hip involvement, as indicated by larger hip extension moments. For example, Biscarini et al. (2011) found that in a model of Smith machine squats, inclining the trunk closer to the horizontal led to an increase in the hip extension moment and a corresponding reduction in the knee extension moment.

Effect of jumping direction
The direction in which jumps are performed does not appear to affect the magnitude of the joint moments, although it may affect the co-ordination pattern of the muscles involved. Jones and Caldwell (2003) studied the ankle, knee and hip moments involved in four different jumps in varying directions: backwards, vertical intermediate-forwards and forwards (i.e. horizontal). They found that the moments at each joint were very similar, irrespective of the direction in which the jump was performed. In contrast, they found that the muscle activity of the primer movers was very different. One trend they noted was that bi-articular muscles, like the hamstrings, tended to increase in activity from backwards to forwards directed jumps, while mono-articular muscles like the gluteals tended to remain constant.

DROP JUMPS
Introduction
Increasing drop jump heights appears to have a very marked effect on the magnitude of the required hip extension moment. Indeed, very high drops as are observed in gymnastics may involve far greater hip extension moments even than those seen in powerlifting. For example, McNitt (1993) monitored the ankle, knee and hip moments during drop landings from low (32cm), medium (72cm) and high (128cm) boxes in both competitive male gymnasts and recreational athletes. From the high box, which is actually not as high as some pieces of gymnastic equipment, the gymnasts produced a hip extension moment of 1,583Nm. This value is 50% greater than the mean hip extension moment reported in a group of elite male, heavyweight powerlifters at the 1989 Canadian Powerlifting Championships of 1,045Nm (Cholewicki et al. 1991).

Effect of drop jump height
In addition, McNitt (1993) found that the gymnasts increased their hip extension moment to a greater extent than their knee or ankle plantar-flexion moments with increasing box height. On the other hand, the recreational athletes increased knee and hip extension torques similarly. This appeared to be related to the gymnasts performing the landings with less hip flexion than the recreational athletes. However, in a study not involving gymnasts Zhang et al. (2000) also found similar superior increases in hip eccentric work done during drop jumps from boxes with increasing box height from 32cm to 103cm, when compared to increases in knee and ankle eccentric work done. Interestingly, in a similar observation to the findings of Jones and Caldwell (2003) in comparing forward and backward jumps, McNitt et al. (2001) observed that the bi-articular hip muscles may play a key role in controlling joint flexion during drop landings, possibly as a result of segmental interactions.

Effect of technique during drop jumps
The way in which landings are performed during drop jumps may affect the relative contribution of the hip, knee and ankle joints. Devita and Skelly (1992) compared stiff and soft landings. They found that the eccentric work done at the hip and knee joints was greater during the soft landings than during the stiff landings, while the eccentric work done at the ankle joint was greater in the stiff landing. Similarly, Zhang et al. (2000) compared stiff, normal and soft landings and found that although the eccentric work done at the knee joint was consistent across all landing types, the ankle joint was more involved in stiff landings, while the hip joint was more involved in soft landings. The difference in the relative roles of the joints in absorbing energy appears to be associated with the movement pattern, as a more vertical trunk position and a more flexed knee position is typically observed in soft landings (Devita and Skelly, 1992). Interestingly, when comparing males and females during single-leg landings, it seems that females display reduced total movement at the hip and knee but greater eccentric work done at the ankle, suggesting that they typically engage in more stiff landings than males (Schmitz et al. 2007). This could be a contributing factor to their greater risk of certain lower body injuries, such as anterior cruciate ligament (ACL) rupture. However, Shimokochi et al. (2009) also noted that increased forward center of mass during single-leg landings seems to be related to reduced knee extension moments and increased hip extension moments, which might be a strategy intended to reduce load on the ACL.

RUNNING
Introduction
Lower body joint moments during running are more difficult to measure than during vertical jumping. This is partly because of the unilateral nature of the movement, partly because running involves two key phases (accelerating and maximum speed), and partly because while one leg is producing force in the stance phase, the other is absorbing energy in the swing phase. Nevertheless, several important studies have explored hip extension moments during running during both accelerating and maximum speed phases and in both stance and swing phases. In general, the hip extension moments seem to be similar to those at the knee and ankle (Belli et al. 2002; Bezodis et al. 2008; Schache et al. 2011).
(Belli et al. 2002; Bezodis et al. 2008; Schache et al. 2011).-
Effect of speed
It seems that increasing running speed leads to a greater role of the hip extensor moment in comparison with the knee extensor and plantar flexor moments (Belli et al. 2002; Bezodis et al. 2008; Schache et al. 2011). For example, Schache et al. (2011) found that as running speed increased from 3.50 to 8.95m/s, the ratio of net hip extension moment to net knee extension moment during the stance phase increased from 0.65 times to 1.15 times. Work done at each joint followed similar pattern. Similarly, Belli et al. (2002) found that as running speed increased from 4.00 to 8.86m/s, the ratio of net hip extension moment to net knee extension moment during the stance phase increased from 0.66 times to 0.88 times.

SIDE STEPS
Although side steps might logically seem to be a function of the abductor musculature, this does not appear to be the case. Inaba et al. (2013) explored the biomechanics of the side step and found that as the side step increased in length, the joint moments in the sagittal plane at the hip, knee and ankle increased but surprisingly the hip abduction moment did not. This may be a function of the greater amount of movement at the hip (Fleischmann et al. 2010). Similarly, Shimokochi et al. (2013) reported that hip abductor function does not appear to be key for lateral movements. Rather, they found that that faster hip extension movements are more important.

SECTION CONCLUSIONS
The ability to produce force with the lower body is critical for vertical jumping performance. Jumping direction does not affect the size or relative proportion of the joint moments.
Using a more upright trunk during vertical jumping decreases jump height and proportionally reduces the role of the hip relative to the knee. Increasing jump height leads to greater hip involvement relative to the knee and ankle.
Drop jumps from high boxes produce extremely high hip extension moments and increasing box height increases the role of the hip relative to the knee and ankle. Soft drop landings involve more energy absorbed at the hip and knee, while stiff drop landings involve more energy absorbed at the ankle.
Running at constant speed involves similar joint moments at the hip, knee and ankle but the relative proportion of the hip increases with increasing speeds.

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RELIABILITY
PURPOSE
The purpose of this section is to detail the literature exploring the reliability of hip extension moment or hip extension force testing in a clinical setting.

RELIABILITY OF ISOMETRIC HIP EXTENSION MOMENT OR FORCE MEASUREMENTS
Selection criteria
Population – any adult population
Intervention – any acute study measuring the reliability of isometric hip extension moment or hip extension force using a clinical method (not a laboratory dynamometer)
Comparison – between tests
Outcomes – Intra-class correlation coefficient (ICC), Standard Mean Difference (SMD), Minimum Difference to be considered real (MD), or Coefficient of Variation (COV), κ statistic

Results
The following relevant studies were identified that met the inclusion criteria: Bohannon (1986), Steultjens (2001), Wang (2002), Perry (2004), Pua (2008), Kelln (2008), Lue (2009), Lu (2011), Tourville (2013).

Findings
MEASUREMENTS OF ISOMETRIC HIP EXTENSION MOMENT
Using a cable tensiometer for straight leg hip extension at 20 degrees to the horizontal in supine, Perry et al. (2004) measured hip extension force, which they multiplied by the test lever arm (from the upper anterior margin of the trochanter to the middle of the ankle cuff) to calculated hip extension moment. They deemed inter-rater reliability to be good (κ = 82%). Using an identical set-up, Pua et al. (2008) reported excellent test-re-test reliability (ICC = 0.97; SEM = 13Nm; MD = 31Nm). Tourville et al. (2013) built a custom stabilisation cage around an examination table that they equipped with a load cell. However, they reported a barely acceptable level of reliability (ICC = 0.78; SEM = 27Nm; MD = 74Nm).
MEASUREMENTS OF ISOMETRIC HIP EXTENSION FORCE
Using a hand-held dynamometer, Bohannon (1986) observed good test-re-test reliability (ICC = 0.87 – 0.95; SEM = 1.2 – 1.9kg; MD = 3.3 – 5.4kg). Kelln et al. (2008) observed good test-re-test reliability (ICC = 0.86 – 0.93; SEM = 0.03 – 0.13kg; MD = 0.1 – 0.4kg). Lu et al. (2011) observed good test-re-test reliability (ICC = 0.92; SEM = 7.6N; MD = 30N). In contrast, Lue et al. (2009) observed only acceptable test-re-test reliability (ICC = 0.92; SEM = 18.3N; MD = 72N).

SECTION CONCLUSIONS
Using a cable tensiometer in supine with 20 degrees of hip flexion, hip extension moment can be measured with relatively good reliability. A difference of 13Nm can differentiate between individuals and an improvement of 31Nm can identify a training effect.
Using a hand-held dynamometer, hip extension force can be measured with relatively good reliability. A difference of 1 – 2kg or 10 – 20N can differentiate between individuals and an improvement of 3 – 5kg or 30 – 45N can identify a training effect.

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