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1 Center for Activity and
Ageing,
The effects of aging on motoneuron firing rates and muscle
contractile properties were studied in tibialis anterior muscle by
comparing results from six young (20.8 ± 0.8 yr) and six old men
(82.0 ± 1.7 yr). For each subject, data were collected from repeated tests over a 2-wk period. Contractile tests included maximal
voluntary contraction (MVC) with twitch interpolation and stimulated
twitch contractions. The old men had 26% lower MVC torque
(P < 0.01) than did the young men,
but percent activation was not different (99.1 and 99.3%,
respectively). Twitch contraction durations were 23% longer
(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, motor unit firing rates
were recorded. Results from ~950 motor unit 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. Comparisons 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 neuromuscular
age-related remodeling principle.
aging; intramuscular electromyography; neuromuscular remodeling; strength; dorsiflexors
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