Human muscle fatigue: responsiveness of motoneurons and central contributions to the loss of muscle power

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My research program uses innovative techniques to advance our understanding of human neuromuscular physiology, with emphasis on fatigue and aging. Fatigue produced by activity is a common experience for healthy people and is magnified in many conditions such that the performance of even day-to-day tasks can be limited, particularly in the elderly. One theme of experiments is to investigate the mechanism(s) of the reduction in spinal motoneuron excitability caused by fatigue which is described in a recent series of papers (McNeil et al. 2009, 2011a,b,d). By combining multiple stimulation techniques (transcranial magnetic stimulation, corticospinal tract stimulation, peripheral nerve stimulation), I will investigate measures of motoneuron excitability in an effort to localize the mechanism(s). Muscle contraction is controlled by motoneuron output so an improved understanding of motoneuron properties is critical to advance our knowledge of the motor system. The second theme of experiments is dedicated to the investigation of muscle power generated by shortening contractions with a fixed-load. Muscle fatigue has been studied sparingly using this most functionally-relevant contraction type. My earlier work (McNeil & Rice 2007) showed that muscles of elderly men are more fatigable in this situation, which is the opposite finding to numerous studies that used static tasks where muscle length was fixed. The work also suggested indirectly that fatigue occurs almost exclusively within the periphery (the muscle itself). A similar view was held about static tasks for many years but, in the last 15 years, it has been shown that 25-66% of force loss can be due to failure within the central nervous system. Using novel techniques and models such as aging and sex, I intend to explore central and peripheral contributions to fatigue developed by shortening contractions with a fixed-load. An improved understanding of limits to muscle power has implications across the full spectrum of activity, from simple daily tasks to voluntary exercise or training.