Fig. 3.

Discharge pattern of biceps brachii motor units at recruitment when old and young adults sustained an isometric contraction with the elbow flexor muscles: comparison of motor unit discharge pattern while supporting an inertial mass as observed in the present study (A) and while pulling up against a rigid restraint as reported previously (B). A: a greater proportion of motor units for old adults discharged action potentials repetitively at recruitment for both small and large target force differences when the task was to support a mass (high-compliance load) hung from the wrist. B: motor units of old adults always discharged action potentials repetitively at recruitment for both small and large target force differences when exerting a force against a rigid restraint (Pascoe et al. 2011), whereas the pattern exhibited by young adults depended on the target force difference (Riley et al. 2008a).