Muscle fibers and their synapses differentially adapt to aging and endurance training.

BACKGROUND: This project aimed to determine the adaptability of the neuromuscular system to the stimuli of exercise training, and aging. METHODS: Young adult, and aged male rats were randomly assigned to either exercise training, or sedentary control groups. Exercise training featured an 8 week program of treadmill running. At the end of the intervention period, neuromuscular function was quantified with ex vivo stimulation procedures on isolated soleus muscles. Morphological adaptations were determined by quantifying myofiber profiles (fiber size and type) of soleus muscles. RESULTS: Ex vivo procedures confirmed that rested (fresh) young muscles were significantly (P < 0.05) stronger than aged ones. By the end of the 5 min stimulation protocol, however, young and aged muscles displayed similar levels of strength. Neuromuscular transmission efficacy as assessed by comparing force produced during indirect (neural) and direct (muscle) stimulation was unaffected by aging, or training, but under both conditions significantly declined over the stimulation protocol mimicking declines in strength. Myofiber size was unaffected by age, but training caused reductions in young, but not aged myofibers. Aged solei displayed a higher percentage of Type I fibers, along with a lower percentage of Type II fibers than young muscles. CONCLUSIONS: The greater strength of young muscles has a neural, rather than a muscular focal point. The loss of strength discerned over the 5 min stimulation protocol was linked to similar fatigue-related impairments in neuromuscular transmission. The two components of the neuromuscular system, i.e. nerves and muscles, do not respond in concert to the stimulus of either aging, or exercise training.