Development of externally guided grip force modulation in man

We examined the development of externally guided changes of grip force with respect to force rate and direction of force change. Sixty-nine children, 3-6 years of age, and 17 adults produced increasing or decreasing isometric forces on a small cylindrical sensor using a pinch grip with visual feedba ck. Force changes were instructed with a visual tracking task. Ramp-like pa radigms with both force increase and force decrease and two different targe t force rates were used (0.5 N/s, 1.25 N/s). Precision of force tracking sh owed clear age effects and was influenced by the required force rates and d irections of force change. In adults, tracking errors were much more depend ent on target force rate and direction than in children. Up to four years o f age, the children tended to overshoot the target force change in a 'jump and wait' manner in all conditions except for fast target force decreases. Older children tended to overshoot only in the condition with slow target f orce decrease. Adults showed close undershooting in all conditions when fol lowing the target. Adults used either a continuous 'following' strategy or a 'see and catch-up' strategy. The distinct effects on tracking errors sugg est an age-related change of strategies from a feedforward strategy with in termittent use of sensorimotor feedback towards a fairly parallel and well- integrated feedback and feedforward processing. A critical age appears to b e around five to six years. We suggest that these age effects may reflect d istinct developmental velocities of neuronal subpopulations of the cortex a nd of the cerebello-cortical connections. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.