Motoneurons, muscles and tendons: Factors influencing muscle force production in humans

This thesis describes a series of studies which examined the effects of neural and mechanical factors on active and passive force production in human muscles.

Maximal voluntary neural drive is thought to be inhibited by concurrent contralateral contraction. Using twitch interpolation we found that maximal voluntary force and neural drive were similar in unilateral and bilateral contractions. This suggests that, at least under some conditions, concurrent contralateral contraction has little or no effect on voluntary force or neural drive.

In a subsequent study we extended the use of twitch interpolation to measure neural drive during maximal concentric contractions. Subjects attained high levels of neural drive in maximal concentric contractions and maintained high levels of drive when lifting and lowering a weight to fatigue.

The twitch interpolation method was also used to show that eight weeks of isometric strength training does not increase maximal voluntary neural drive to the elbow flexors in isometric contractions. This finding challenges the hypothesis that neural drive to muscles can be enhanced with training.

To determine what factors determine the amplitude of the interpolated twitch, we developed a computer model of a motor unit pool. Computer simulations suggested that the amplitude of the interpolated twitch is attenuated by antidromic and reflex effects, and that muscle force and the amplitude of the interpolated twitch become insensitive to changes in neural drive at high forces.

The two final studies in this thesis explored properties of muscles and tendons. In the first, ultrasonography was used to map changes in pennation of human brachialis muscle in vivo. This muscle undergoes large increases in pennation even with weak contractions, particularly at short lengths. There was no change in pennation of the relaxed muscle with changes in joint angle, suggesting that the muscle may be slack at rest.

Similar methods were used to measure changes in the length of muscle fascicles in two human lower limb muscles in vivo. Length changes in muscle fascicles were much smaller than the change in origin to insertion distance. This indicates that much of the change in muscle-tendon length is taken up by a change in the length of the tendons.