Motor unit firing rates and contractile properties in tibialis anterior ofyoung and old men

The effects of aging on motoneuron firing rates and muscle contractile prop erties were studied in tibialis anterior muscle by comparing results from s ix young (20.8 +/- 0.8 yr) and six old men (82.0 +/- 1.7 yr). For each subj ect, data were collected from repeated tests over a 2-wk period. Contractil e tests included maximal voluntary contraction (MVC) with twitch interpolat ion and stimulated twitch contractions. The old men had 26% lower MVC torqu e (P < 0.01) than did the young men, but percent activation was not differe nt (99.1 and 99.3%, respectively). Twitch contraction durations were 23% lo nger (P < 0.01) in the old compared with the young men. During a series of repeated brief steady-state contractions at 10, 25, 50, 75, and 100% MVC, m otor unit firing rates were recorded. Results from similar to 950 motor uni t trains in each subject group indicated that at all relative torque levels mean firing rates were 30-35% lower (P < 0.01) in the old subjects. Compar isons between young and old subjects' mean firing rates at each of 10%, 50% , and MVC torques and their corresponding mean twitch contraction duration yielded a range of moderate-to-high correlations (r = -0.67 to -0.84). That lower firing rates were matched to longer twitch contraction durations in the muscle of old men, and relatively higher firing rates were matched with shorter contraction times from the young men, indirectly supports the neur omuscular age-related remodeling principle.