Age-associated impairement in endpoint accuracy of goal-directed contractions performed with two fingers is due to altered activation of the synergistic muscles

The purpose of this study was to determine whether older adults compared with young adults exhibit impaired end-point accuracy during a two-finger task due to altered activation of the contributing synergistic muscles. Nine young (21.3 years +/- 1.6 years, 4 men) and 9 older (73.1 years +/- 6.4 years, 5 men) were instructed to accurately match the center of a target with concurrent abduction of the index and little fingers (synergistic two-finger task). The target comprised of 20% MVC and 200 ms. Visual feedback of the force trajectory and target was provided 1 s after each trial. Subjects completed 40 trials and the last 10 were used for analysis. Endpoint accuracy was quantified as the normalized deviation from the target in terms of peak force (peak force error), time-to-peak force (time-to-peak force error), and a combination of the two (overall error). Motor output variability was quantified as the standard deviation and coefficient of variation (CV) of peak force and time to peak force. The neural activation of the involved synergist muscles (first dorsal interosseus (FDI) and abductor digiti minimi (ADM)) was quantified with the electromyography (EMG) amplitude (root mean square) and its frequency structure (wavelet analysis). Older adults exhibited significantly greater peak force (46.7 +/- 10% vs. 24.9 +/- 3.2%) and overall endpoint error (68.5 +/- 9.7% vs. 41.7 +/- 4.3%), whereas the time to peak force error was similar for the two age groups. Older adults also exerted greater peak force variability than young adults, as quantified by the CV of peak force (34.3 +/- 3.5% vs. 24.1 +/- 2.3%). The greater peak force error in older adults was associated with changes in the activation of the ADM muscle but not the FDI. Specifically, greater peak force error was associated with greater power from 13-30 Hz and lesser power from 30-60 Hz. These results, therefore, suggest that older adults compared with young adults exhibit impaired endpoint force accuracy during a two finger task because of altered activation of one of the synergist muscles. (C) 2012 Elsevier Inc. All rights reserved.