THE STABILITY OF PRECISION GRIP FORCE IN OLDER ADULTS

The exceedingly large grip forces that many older adults employ when l ifting objects with a precision pinch grip (Cole, 1991) may compensate for a reduced capability to produce a stable isometric force. That is . their grip force may fluctuate enough from moment to moment to yield grip forces that approach the force at which the object would slip fr om grasp, We examined the within-trial variability of isometric force in old (68-85 years, n = 13) and young (22-35 years. n = 11) human sub jects (a) when they were asked to produce a constant pinch force at th ree target levels (0.49, 2,25. and 10.5 N) with external support of th e arm, hand, and force transducer and (b) when they were asked to gras p. lift, and hold a small test object with a precision grip. Pinch for ce produced in the first task was equally stable across the two subjec t groups during analysis intervals that lasted 4 s. The elderly subjec ts produced grip forces when lifting objects that averaged twice as mu ch as those produced by the young subjects. The force variability duri ng the static (hold) phase of the lift for the old subjects was compar able with that used by the young subjects. after adjusting for the dif ference in grip force. The failure to observe less stable grip force i n older adults contradicts a similar recent study. Differences in task (isometric grip force versus isometric abduction torque of a single d igit) may account for this conflict, however. Thumb and finger forces for grip are produced through coactivation of many muscles and thus pr omote smooth force output through temporal summation of twitches. We c onclude that peripheral reorganization of muscle in older adults does not yield increased instability of precision grip force and therefore does not contribute directly to increased grip forces in this populati on. However, force instability may affect other grip configurations (e .g. lateral pinch) or manipulation involving digit abduction or adduct ion forces.