LB-QUADRICEPS-X

– The quadriceps are usually treated as a single muscle group that can be trained either with multi-joint hip and knee extension exercises (such as squats and leg presses) or with single-joint knee extension exercises.
– However, the quadriceps comprise one two-joint muscle (the rectus femoris) and three single-joint muscles (the vastus lateralis, vastus medialis, and vastus intermedius), and research suggests that multi-joint exercises are optimal for training the single-joint quadriceps muscles, while the single-joint knee extension is optimal for training the rectus femoris.

#1. Squats and leg presses

– The single-joint quadriceps muscles are all highly active during the back squat exercise, whereas the two-joint rectus femoris is not.
– This is difficult to assess with EMG because of cross-talk between the muscles (which are very close together).
– However, it can be detected by temporary changes in muscle cross-sectional area using magnetic resonance imaging (MRI) scans.
– Exactly why the rectus femoris is not as active during multi-joint hip and knee extension exercises as in single-joint knee extension exercises is not clear. The most likely explanation is that the two-joint muscle has poor leverage during squats and leg presses
– The simple act of adding a hip extension force to a knee extension exercise reduces rectus femoris activation, which suggests that the central nervous system also reduces rectus femoris muscle activation in the presence of a hip extension force being produced. Perhaps the presence of hip extension muscle activation reduces the activation of any hip flexors.
– Either way, it is not surprising that only the single-joint quadriceps muscles grow to a meaningful extent after strength training with the back squat exercise. In contrast, the rectus femoris does not grow at all.

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#2. Knee extensions

– The single-joint quadriceps muscles are much less active during the knee extension exercise in comparison with the two-joint rectus femoris.
– Consequently, the rectus femoris experiences much greater hypertrophy after knee extension strength training than the single-joint quadriceps.

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#3. The rectus femoris as a hip flexor

– The rectus femoris is a two-joint hip flexor and knee extensor. Importantly, the sprinting movement is primarily dependent upon the work done by the hip extensors and hip flexors.
– The hip extensors do positive work in terminal swing (and the amount of this work done increases dramatically with increasing running speed), while the hip flexors do positive work in initial swing (and the amount of this work done similarly increases dramatically with increasing running speed).
– Thus, to achieve very high sprinting speeds requires the hip flexors to be able to produce very high power outputs.
– In line with this, recent research has shown that the most important muscles for sprint running are hip extensors (including the gluteus maximus and hamstrings) and hip flexors (including the sartorius and tensor fasciae latae). Developing these muscles is therefore very important for athletes who sprint.
– Although research has only linked the size of certain hip flexors with higher levels of performance, it is likely that most if not all of the hip flexors (including the rectus femoris) contribute to faster sprinting speeds. Indeed, previous biomechanical research has shown that the rectus femoris does increase its involvement as running speed increases.

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