Muscle mass and motor unit recruitment

It is often assumed that simply applying a maximal level of effort will allow us to achieve maximal motor unit recruitment, regardless of the exercise that we are performing. Nevertheless, exercises that involve a larger amount of muscle mass generally involve a lower level of motor unit recruitment than those that involve a smaller total amount of muscle mass.

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#1. Greater voluntary activation deficits with greater muscle size

Across the whole body, there is a general trend that larger muscles are much more difficult to activate fully. They display larger voluntary activation deficits due to fewer high-threshold motor units being recruited.

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#2. Bilateral deficit of voluntary activation

The bilateral force deficit is a well-known phenomenon. The sum of the forces generated by two limbs in two separate singe-limb exercises is greater than the force produced by two limbs in one combined two-limb exercise. This seems to be partly caused by a greater level of motor unit recruitment in single-limb exercises.

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#3. Greater peripheral fatigue during exercise with less muscle mass

When we perform exercise that involves smaller total amounts of muscle mass (such as single-joint exercise compared to multi-joint exercise), we can achieve a longer time to task failure and a higher level of peripheral fatigue at task failure. This implies that we can tolerate a higher level of peripheral fatigue (and a higher level of afferent feedback associated with that peripheral fatigue) when the total amount of exercising muscle mass is smaller.

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The same observation can be made when comparing single-limb and two-limb exercises. Greater peripheral fatigue occurs after the single-limb exercises. 

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And the same observation can be made when comparing upper body and lower body exercises. Greater peripheral fatigue occurs after the upper body exercises.

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#4. Greater muscle activation with internal focuses of attention

It is well-known that internal focuses of attention allow greater levels of motor unit recruitment in the target muscle compared to no focus of attention and also compared to an external focus of attention. This may well be because the internal focus of attention spares less central motor command for the synergist muscles that are critical for the optimal movement pattern, which is why they generally involve lower levels of coordination. Yet, this may permit greater concentration of the central motor command upon the target muscle during the exercise.

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Conclusions

It is often assumed that simply applying a maximal level of effort will allow us to achieve maximal motor unit recruitment, regardless of the exercise. However, exercises that involve a larger amount of muscle mass involve a lower level of motor unit recruitment than those that involve a smaller amount of muscle mass. This can be observed in four scenarios. [1] Larger muscles display larger voluntary activation deficits than smaller muscles. [2] The bilateral force deficit appears to be partly caused by a deficit of voluntary activation during two-limb exercises than in single-limb exercises. [3] Exercises that involve less muscle mass (such as single-limb exercises rather than two-limb exercises, single-joint exercises rather than multi-joint exercises, and upper body exercises rather than lower body exercises) experience far more peripheral fatigue at task failure (implying a smaller influence of central nervous system fatigue). [4] Internal focuses of attention involve higher levels of muscle activation in the target muscle, which may be partly due to reduced activation of synergist muscles. Altogether, these four observations indicate that when central motor command is sent to more muscles during exercise, the level of motor unit recruitment in the target muscle is reduced. This likely occurs because the central motor command is limited by the corollary discharge that creates perceived effort in the brain (since we have a maximum tolerable perception of effort). Sending central motor command to more muscles increases the total level of perceived effort for a given level of central motor command to the target muscle.​