Voluntary activation of maximal single and all finger power grip contractions

When all four fingers are engaged together during a grip strength contraction, the force produced by an individual finger is less than the force produced when it acts in isolation. The purpose of this study was to evaluate whether the reduced force output of a digit during an all-finger grip contraction is due to a decline in voluntary activation. Fifteen young adults (n = 7 females) completed voluntary contractions of the index finger in isolation and all fingers together in a dynamometer capable of separately recording forces from each finger during voluntary and electrically evoked contractions. The median and ulnar nerves were electrically stimulated simultaneously at the elbow to record individual finger flexion forces from doublet (100 Hz) pulses. Doublet stimulations were applied during and immediately following contractions at 50, 65, 85, and 100% maximal voluntary contraction (MVC) forces. Two-way ANOVAs were used to compare the effects of sex and finger (single vs. all) on flexion forces and voluntary activation. The index finger produced ∼25% more force when engaged in isolation compared with the all-finger contraction; however, there were no differences in voluntary activation between the single and all-finger MVCs (P = 0.344). The index finger force deficit was larger in females compared with males (34 vs. 18%, P = 0.030), but this was not explained by sex-related differences in voluntary activation. These data indicate that the additional force produced during single-finger contractions is not due to an alteration in voluntary activation, as all-finger contractions display near-maximal activation of each digit.NEW & NOTEWORTHY The deficit in finger flexion force when multiple digits are engaged is well characterized and often attributed to reduced motor unit activation. Using a custom-built, multifinger dynamometer, we report that the index-finger force deficit may not be related to a deficit in voluntary activation. Furthermore, the index-finger force deficit was greater in females compared with males, but this was also not related to sex-related differences in voluntary activation.