Mechanisms for age-related changes of fingertip forces during precision gripping and lifting in adults

We investigated changes across the adult life span of the fingertip forces used to grip and lift objects and their possible causes. Grip force, relati ve safety margin (grip force exceeding the minimum to avoid slip, as a frac tion of slip force), and skin slipperiness increased beginning at age 50 ye ars. Skin slipperiness explained relative safety margin increases until age 60 years. Hence, after age 60 years, additional factors must elevate grip force. We argue that one factor is impaired cutaneous afferent encoding of skin-object frictional properties on the basis of three findings. First, on ly subjects 60 years and older increased their relative safety margins when the friction of the gripped surfaces was varied randomly versus experiment s that varied only object weight. Skin slipperiness did not account for thi s behavior. Second, these older subjects scaled the initial portion of thei r force trajectories for the slippery surface during experiments when frict ion was varied. Third, their grip force adjustments to new surfaces were de layed similar to 100 msec as compared with young subjects. Previous researc h has demonstrated that friction is signaled locally by fast-adapting affer ents (FA I afferents), which decrease in number during old age. By contrast , adjustments triggered by object set-down, an event encoded by FA II affer ents throughout the hand and wrist, were not delayed in our old subjects. O ther findings included that anticipatory control of fingertip forces using memory of object weight was unimpaired in old age. Finally, old and young a dults modulated their fingertip forces with equal smoothness and with simil ar relative intertrial variability.