Fig. 4.

Motor unit discharge characteristics at recruitment when the elbow flexor muscles supported a mass: discharge characteristics at recruitment for 20 motor units recorded from the biceps brachii muscle from old adults and 20 motor units from young adults. A: mean discharge rate for the first 5 interspike intervals (ISIs) was less for the large difference between target force and recruitment threshold force in young adults only (age × target force difference interaction: P = 0.036). B: coefficient of variation (CV) for ISI did not differ between young and old adults (P = 0.07) or between the small and large differences between target force and recruitment threshold force (P = 0.43).