Adaptations in human motor unit control properties: Influences of aging and training

This research involved systematic investigation of mechanisms which underlie neural adaptations to training. Observations were made of human motor unit control properties. Considerable evidence suggests impairment of neural, neuromuscular, and musculoskeletal function with aging, thus the process of adaptation to training was compared between healthy young and older adults. Experiment I examined whether early strength gains result from changes in central drive, defined as maximal motor unit discharge rate (MUDR). MUDR was obtained at four points over a six week strength training program. Both young and older adults demonstrated significantly increased maximal MUDR on Day 2 which returned to baseline by the end of the training period. Despite significantly lower maximal MUDR at baseline, older adults produced central drive comparable to young individuals on Days 2 and 14. Experiment II addressed the role of variability of sustained motor unit discharge (MUV) and its effect on the generation of muscular force. MUV was significantly redistributed with aging, punctuating the frequency content between 0-4 Hz. With training, only young individuals demonstrated significant adaptation, reducing the 0-4 Hz contribution and increasing the 5-8 Hz and 9-12 Hz contributions to MUV. These training-related adaptations suggest the role of supraspinal influences in damping oscillatory activity of musculoskeletal dynamics. Experiment III examined the range over which motor unit discharge is modulated for adaptation with strength training. MUDR range was similar between young and older individuals when examined with respect to absolute force differences but was significantly lower in older adults when examined with respect to relative effort. Importantly, significant adaptations to training were not evidenced in MUDR range by young adults, while a transient increase, paralleling the pattern of change in maximal MUDR, was demonstrated by elders. Experiment IV examined adaptation and modulation of antagonist motoneuron excitability. Hoffman reflex (H-reflex) responses were obtained from the soleus muscle while a concurrent force modulation task was performed with tibialis anterior. Over six weeks subjects trained to improve control of dorsiflexion force. H-reflex amplitudes were significantly smaller in older adults suggesting alteration in motoneuron pool activation due, potentially, to remodeling of spinal pathways in favor of antagonist muscle co-contraction