Mechanisms for Age-Related Changes of Fingertip Forces during Precision Gripping and Lifting in Adults

Abstract

We investigated changes across the adult life span of the fingertip forces used to grip and lift objects and their possible causes. Grip force, relative safety margin (grip force exceeding the minimum to avoid slip, as a fraction of slip force), and skin slipperiness increased beginning at age 50 years. 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, only subjects 60 years and older increased their relative safety margins when the friction of the gripped surfaces was varied randomly versus experiments that varied only object weight. Skin slipperiness did not account for this behavior. Second, these older subjects scaled the initial portion of their force trajectories for the slippery surface during experiments when friction was varied. Third, their grip force adjustments to new surfaces were delayed approximately 100 msec as compared with young subjects. Previous research has demonstrated that friction is signaled locally by fast-adapting afferents (FA I afferents), which decrease in number during old age. By contrast, adjustments triggered by object set-down, an event encoded by FA II afferents throughout the hand and wrist, were not delayed in our old subjects. Other findings included that anticipatory control of fingertip forces using memory of object weight was unimpaired in old age. Finally, old and young adults modulated their fingertip forces with equal smoothness and with similar relative intertrial variability.

Age-related Differences in Pre- and Post-synaptic Motor Cortex Inhibition are Task Dependent

“…Therefore, despite maintaining the ability to reduce post-synaptic GABAA-mediated inhibition within motor cortex during tonic muscle activation, old adults demonstrate a reduced modulation of inhibitory tone for tasks requiring more complex activation of primary motor cortex. This may contribute to age-related impairments in motor performance during this task, as has been shown previously [43,44]. Alternatively, a recent study by Fujiyama and colleagues [21] showed that greater SICI during the foreperiod of a warned reaction task was associated with faster reaction times in old but not young subjects.…”

Prehension synergies during nonvertical grasping, I: experimental observations

“…The higher SM in the current study might be explained by the nature of the handle used in the present study; it was heavy with expensive force transducers attached but was freely held in space with no safety catch in the event of dropping. This is reminiscent of the "better safe than sorry" strategy documented for special populations (e.g., in elderly: Cole et al 1999;Shim et al 2004, andin cerebellar patients: Babin et al 1999;Fellows et al 2001). …”

“…The prehension literature details many task-dependent instances of mechanically unnecessary force production (e.g., Latash et al 1998a,b;Cole et al 1999;Zatsiorsky et al 2000). Some of these effects were described as "enslaving" (Kilbreath and Gandieva 1994;Zatsiorsky et al 2000;Schieber 2001) and were attributed to a feed-forward controller to physiologically connected effectors (Latash et al 1998a,b).…”

Age-related differences in proprioceptive and visuo-proprioceptive function in relation to fine motor behaviour

“…Proprioception also decreases with age and is closely related to loss of muscle and joint strength. Muscle strength has been found to remain stable until the fifth or sixth decade, but shows a 50 % decrease by age 80 (Cole et al 1999;Sturnieksa et al 2008). The winning performance of Senior Olympians ([50 years) declines approximately 3.4 % per year over 35 years of competition: slowly from age 50 to 75 years and then dramatically after age 75 years (Wright and Perricelli 2008).…”