KINESIOLOGY-REGIONAL-HYPERTROPHY


REGIONAL HYPERTROPHY


Chris Beardsley – January 2021


– While muscles grow after strength training, they do not always grow evenly across the whole muscle
– The precise regions within a muscle where that growth occurs seems to differ, depending on the exercise that is used during training.
– Such differences can apparently occur either because of differences between heads of a muscle or because of differences within regions of single muscles.


#1. The principle of neuromechanical matching

– During a movement, the recruitment of motor units is determined by Henneman’s size principle as well as the principle of neuromechanical matching.
Henneman’s size principle states that motor units are recruited in size order, from small to large.
– The principle of neuromechanical matching states that the leverage of the muscle fibers controlled by a motor unit also influences whether it is recruited for a given task.
– These two principles work together, insofar as motor units are generally recruited in order of size, but the exact order is altered depending on what movement pattern is performed (since this alters which motor units have the best leverage)
– Importantly, the principle of neuromechanical matching provides a primary mechanism by which regional muscle activation can differ between exercises.
– This is is likely a key reason why certain exercises develop certain regions of a muscle, while other exercises develop other regions of the same muscle.


#2. Regional hypertrophy (when comparing heads of a muscle)

In programs involving certain exercises or exercise variations, there tends to be greater hypertrophy of certain heads of a muscle relative to other heads
– In the literature, this has been observed to occur in three different situations.
(1) it happens when the direction of force is altered, which likely alters which of the heads has the best leverage during the exercise being performed
EX 1: foot position in calf raises affects whether the lateral or medial gastrocnemius head is more developed after training. Altering the foot position alters which of the two heads has better leverage during the movement. Whichever head has the better leverage is activated to a greater extent
EX 2: bench incline used during bench press training can affect which region of the pectoralis major muscle is developed to the greatest extent. Compared to a flat bench, an incline bench produces greater hypertrophy of the superior region of the muscle (which corresponds to the clavicular head). When using the incline bench, it is likely that the superior region of the muscle has greater leverage, and therefore it is activated to a greater extent.
(2) it happens when we compare multi-joint vs. single-joint exercises. Multi-joint exercises seem to involve lower levels of activation (and therefore hypertrophy) in those heads of the muscle that are two-joint, at least when compared to single-joint exercises. The two-joint muscles typically work at a constant length in such exercises, since they are agonists at one joint and antagonists at the other. Whether the lack of activation is produced by the principle of neuromechanical matching due to a lack of leverage (as is observed for the rectus femoris muscle in the squat), by deliberate inhibition by the central nervous system, or by another mechanism is not yet clear
EX 1: bench press training tends not to develop the proximal parts of the triceps brachii (where the long head is the largest element), while single-joint triceps extension training does. This may be due to greater activation of the long head during the single-joint triceps extension compared to in the bench press.
(3) it (probably) happens when the exercise range of motion is altered, or the point at which peak force in the exercise range of motion is altered. These changes almost certainly alter which of the heads of the muscle has the best leverage during the exercise being performed
EX 1: range of motion during calf raises affects whether the lateral or medial gastrocnemius head is most strongly activated.


#2. Regional hypertrophy (within a muscle)

In programs involving certain exercises or exercise variations, there tends to be greater hypertrophy of certain parts of a single muscle relative to other parts
– Again, this has been documented in the literature in two different situations.
(1) it happens when the amount of passive tension experienced during training is altered, which likely alters the amount of stretch-mediated hypertrophy that happens
EX 1: training with a full range of motion (ROM) during the squat causes proportionally greater hypertrophy of the distal region of a single muscle, while training with a partial ROM causes proportionally greater hypertrophy in the proximal regions. The proportionally greater distal growth after full ROM training is often explained by distal regions growing more due to increased fascicle lengths. This sarcomerogenesis is stimulated by the stretching of titin, which is a mechanoreceptor that detects the tension that it produces.
EX 2: eccentric leg press training causes proportionally greater hypertrophy of the distal region of the quadriceps, while concentric leg press training causes proportionally more hypertrophy of the middle region. Again, this likely occurs because the distal region is where much of the sarcomerogenesis occurs. Nevertheless, rodent studies have shown that sarcomerogenesis differs between regions of a muscle and is not necessarily always located in the distal region. Rather, it develops according to the amount of muscle fiber strain that the fibers in each region experience. This suggests that the reason that the distal regions of a muscle grow more after full range of motion strength training and eccentric training (compared to after other types of strength training) is because the distal region is where the most strain is experienced by working muscle fibers.
(2) differences in regional hypertrophy within a single muscle occur when a two-joint muscle is trained by one exercise at one joint and another exercise at a different joint
EX 1: knee extension training for the rectus femoris preferentially develops the distal region of the muscle, while hip flexion training for the same muscle causes similar growth of the distal and proximal regions. Previous research has identified that knee extension training tends to cause greater activation of the distal region of the rectus femoris, while hip flexion training tends to cause preferentially greater activation of the proximal region (although these effects are influenced by muscle lengths during the exercise being used). Therefore, it seems likely that the regional muscle hypertrophy that results from training the rectus femoris with either hip flexion or knee extension exercises is the result of regional muscle activation, due to the principle of neuromechanical matching.


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