THE INFLUENCE OF AGE ON THE ASSESSMENT OF MOTOR UNIT ACTIVATION IN A HUMAN HAND MUSCLE

Two indices of motor unit recruitment, the ramp-force and repetitive-d ischarge thresholds, were compared in the first dorsal interosseus mus cle of ten young and twelve elderly subjects, The purpose was to deter mine the effect of age on the relationship between the two recruitment thresholds and the spike-triggered average force of motor units. Each subject performed three tasks requiring isometric abduction of the le ft index finger: a maximum voluntary contraction (MVC), a ramp-and-hol d contraction, and a repetitive-discharge task. The elderly subjects u sed coactivation of the antagonist muscle (second palmar interosseus) more frequently than the young subjects during the ramp-and-hold contr action. Many elderly subjects expressed difficulty with the controlled : ramp-down phase of the ramp-and-hold contraction and preferred a coa ctivation strategy to a derecruitment strategy for this task. There we re no differences due to age or gender in the ramp-force thresholds be tween the various-groups. However, the normalized repetitive-discharge threshold was significantly less for the younger subjects and for the male subjects. Nonetheless, the two recruitment thresholds were able to predict the spike-triggered average force with similar success for both the young and the elderly subjects. These data suggest that the r ecruitment threshold of a motor unit in first dorsal interosseus was c haracterized equally well by either the ramp-force or repetitive-disch arge measurement for both young and elderly subjects but that coactiva tion was used more frequently by the elderly subjects during the ramp- and-hold task.