Functional relevance of regional hypertrophy

It is currently quite popular for strength training experts to dismiss the functional importance of regional hypertrophy. Frequently, they suggest that a small selection of (their favourite) exercises is sufficient to maximize muscle growth and achieve maximum gains in strength. However, there is good evidence to suggest that regional hypertrophy does produce differences in terms of functional outcomes.

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#1. Hypertrophy occurs in different parts of a muscle or muscle group after training

Over the last few years, many studies have reported that hypertrophy can occur in different parts of a muscle or of a muscle group, depending on the exact training variables used in a program. For example, multi-joint exercises tend not to develop the two-joint muscles within a group (like the rectus femoris) while single-joint exercises tend to develop those muscles very effectively.

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The exact same phenomenon occurs in the two-joint triceps brachii long head, which is not developed by multi-joint exercises but is very successfully trained by single-joint exercises.

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Similarly, different multi-joint exercises can also target different regions within a muscle group even when this does not involve the exclusion of two-joint muscles, as we can see by studying the differences in pectoralis hypertrophy caused by incline bench presses and flat bench presses.

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Similarly, different single-joint exercises can also target different regions within a muscle group, as we can see by studying the differences in hamstrings hypertrophy caused by leg curls that are done with an extended hip position and leg curls that are done with a flexed hip position.

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In addition to exercise selection, contraction mode also plays a role in altering the location of the hypertrophy that is produced after a strength training program. Eccentric contractions tend to cause more distal region growth while concentric contractions cause more proximal hypertrophy.

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Similarly, range of motion also plays a role in altering the location of the hypertrophy that is produced after a strength training program. Stretched positions tend to cause more distal region growth while concentric contractions cause only proximal region hypertrophy.

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Overall, a wider variety of exercises and training variables tend to produce more hypertrophy in comparison with less variety within the context of a well-written strength training program.

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#2. Greater exercise variety is better for strength gains and regional hypertrophy very likely contributes to that effect

Importantly, studies have shown that greater exercise variety contributes positively to strength gains, which is a finding that is generally ignored by commentators who do not really understand the principle of specificity.

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It seems likely that differences in regional hypertrophy are in part responsible for this superior effect of greater exercise variety on strength gains in a marker exercise. Indeed, research has shown that specific areas of a muscle can contribute disproportionately to exercise strength.

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#3. Differences in regional hypertrophy cause changes in where future muscle growth will happen

When a muscle has a longer internal moment arm, it will be called upon to contribute during an exercise to a greater extent in accordance with the principle of neuromechanical matching. Greater muscle sizes are associated with longer internal moment arms. Therefore, once a muscle region experiences hypertrophy, it is more likely to experience more hypertrophy in the future.

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#4. Differences in regional hypertrophy cause changes in muscle center of mass

In addition to the differences in strength gains caused by differences in regional hypertrophy, there are changes in the muscle center of mass. During fast movements like sprint running, the location of the muscle center of mass is very important for performance. A more proximal center of mass will be associated with faster running speeds because the angular momentum of the rotating limb is smaller. Conversely, a more distal center of mass will be associated with slower running speeds because the angular momentum of the rotating limb is larger. These differences will have an important effect regardless of the level of force that the muscle can generate.

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Conclusions

Muscles do not grow uniformly after strength training. Various factors have been shown to affect the exact location where hypertrophy happens. Differences in regional hypertrophy occur when training with: [1] multi-joint exercises and single-joint exercises for the same muscle group (e.g. bench presses and triceps extensions), [2] exercises for two-joint muscles at different muscle lengths (e.g. lying leg curls and seated leg curls), [3] exercises using different ranges of motion (i.e. partial and full), and [4] exercises with differing contraction modes (i.e. eccentric and concentric). It is likely that these differences in regional hypertrophy contribute to differences in strength gains, athletic performance, and the potential for future muscle growth. Indeed, research has shown that not all muscle size gains after training contribute equally to the gains in strength in a tested exercise. The hypertrophy of some regions contributes more to gains in an exercise 1RM than the hypertrophy of other regions. Also, when the center of mass of a muscle is altered by regional hypertrophy, this will change the ratio of its force-producing ability to its angular momentum in fast movements like sprinting. Greater angular momentum is disadvantageous for both accelerating and decelerating in the sprinting gait cycle. Finally, when a muscle increases in size, its internal moment arm length increases, which means that the central nervous system will allocate more central motor command to it during movements in the future, in accordance with the principle of neuromechanical matching, thereby enhancing its capacity for hypertrophy.