How do we produce the tremendous ranges in muscular force that we need to control movement? The acts of talking and playing the violin require incredible subtleties in motor output - too much or too little force produces the wrong musical expression. At the other end of the force continuum, an experienced and trained weight lifter can lift hundreds of pounds. How do we achieve these skills and what adaptations in the motor control system accompany both skill acquisition and the means to produce great feats of muscular strength?
Our laboratory studies the adaptations in motor control that accompany these kinds of tasks. We are especially interested in understanding how motor unit discharge behavior is altered to achieve fine motor precision and great muscular strength. A major mechanism we have available to modulate force output is to vary the number of active motor units. The order in which motor units are activated is largely fixed, but we have shown that certain conditions like increased input from skin receptors can produce an earlier activation of larger human motoneurons. We're not sure if everyone is capable of activating all motoneurons to produce maximal strength and this is one of our current research areas.
![[Photo of Gary Kamen]](kamen.gif)
Gary Kamen
Adaptations in the Human Neuromuscular Control
System
Of particular interest is the firing behavior of human motoneurons. Until fairly recently, technical limitations have precluded an in-depth study of human motor unit activity at high force levels. We use a unique multi-channel recording technique and customized discrimination software to capture motor unit activity at the highest possible force levels. Using this procedure we have demonstrated that maximal motor unit discharge rates can be as much as 50% larger in young adults than in older individuals. Thus, one factor that limits the ability of older persons to produce maximal force may be the rate at which the motoneuron can be driven by the central nervous system. The idea that chronic exercise might produce some adaptation in motor unit discharge has also been little-studied.
Our laboratory has demonstrated that motor unit firing rates can be significantly greater in older adults who regularly train and compete in weight lifting activities than in persons of the same age who do not exercise. We have also shown that a period of regular exercise can increase motor unit discharge rates. This on-going study of human motor unit activity as a function of the aging process and adaptations to stimuli like long-term exercise may ultimately help us understand the tremendous capacity we have for regulating muscular force.
Muscular force is also modulated by a type of cooperation among active motor units. Pairs of motor units fire simultaneously more often than one would expect by chance alone, a behavior we call motor unit synchronization. Synchronization and other joint firing behavior strategies may represent an additional means to grade muscular force. Our current studies are also geared at understanding the link between changes in motor unit firing behavior and muscle morphology. We use a modified macro-EMG technique to study the electrophysiological size of the motor unit. The aging process produces rather profound changes in morphological features and we hope the joint analysis of motor unit discharge behavior and motor unit architecture will help us understand how the central nervous system adapts to constraints imposed by the peripheral musculature.
For further information please visit:
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Dr. Kamen is also a member of the Department
of Exercise Science.
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