Exercises for maximizing gains in motor unit recruitment

What factors determine whether an exercise is effective for increasing the ability to recruit high-threshold motor units (HTMUs)?

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#1. Stimulus for increasing motor unit recruitment

The stimulus for increasing our ability to recruit HTMUs is likely just achieving a maximal level of motor unit recruitment in the trained muscle (read more). So which types of exercise involve the highest levels of motor unit recruitment?

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#2. Level of coordination

Exercises that involve a high coordination demand involve reduced motor unit recruitment because the coordination requirement requires mental effort that increases the perception of effort for a given level of motor unit recruitment. Thus, when effort perception reaches its maximal tolerable limit, levels of motor unit recruitment are lower than we might expect.

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In this way, the presence of an additional coordination requirement functions much like afferent feedback, insofar as it suppresses the maximum level of motor unit recruitment that can be reached during a muscular contraction, despite a maximal level of effort being exerted.

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#3. Level of stability

Exercises that involve a high stability demand also involve reduced motor unit recruitment because the stabilization requirement requires mental effort that increases the perception of effort for a given level of motor unit recruitment. Thus, when effort perception reaches its maximal tolerable limit, levels of motor unit recruitment are again lower than we might expect. (read more).

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#4. Amount of muscle mass

Compared to exercises that involve small amounts of muscle mass, exercises that involve larger amounts of muscle mass involve lower levels of motor unit recruitment in each muscle when performing a maximal effort, most likely because each muscle requires its own central motor command but also probably because of the greater central nervous system fatigue that occurs when the cardiovascular system is taxed to a greater extent. Thus, training with an exercise that involves a larger amount of muscle mass reduces the stimulus for future increases in the ability to recruit HTMUs (read more).

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Conclusions

Strength training is known to stimulate an adaptation in the ability to recruit high-threshold motor units (HTMUs). This adaptation is valuable for athletes because it produces an extremely transferable strength gain in the trained muscle group. It is also helpful for bodybuilders because it allows more muscle fibers to be trained in future workouts, thereby facilitating more hypertrophy. Since the stimulus for increasing the ability to recruit HTMUs is probably hitting a very high level of motor unit recruitment (MUR) during a workout, we can identify the types of exercises that are best at stimulating this adaptation, which are: [1] exercises with low coordination demands (a high coordination demand requires mental effort, which increases the perception of effort for a given level of MUR and thereby reduces the maximum level of MUR that can be achieved at the maximum tolerable perception of effort); [2] exercises with low stability demands (a high stability demand similarly requires mental effort, which increases the perception of effort for a given level of MUR and thereby reduces the maximum level of MUR that can be achieved at the maximum tolerable perception of effort); and [3] exercises involving a small amount of muscle mass (more muscle mass reduces the level of MUR that can be attained in each working muscle since more muscles must share the available central motor command).