Journal of motor behavior
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The exceedingly large grip forces that many older adults employ when lifting 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 from grasp. We examined the within-trial variability of isometric force in old (68-85 years, n = 13) and young (n = 11) human subjects (a) when they were asked to produce a constant pinch force at three target levels (0.49, 2.25, and 10.5 N) with external support of the arm, hand, and force transducer and (b) when they were asked to grasp, lift, and hold a small test object with a precision grip. ⋯ Thumb and finger forces for grip are produced through coactivation of many muscles and thus promote smooth force output through temporal summation of twitches. We conclude 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 population. However, force instability may affect other grip configurations (e.g., lateral pinch) or manipulation involving digit abduction or adduction forces.