Influence of central set on anticipatory and triggered grip-force adjustments

The effects of predictability of load magnitude on anticipatory and trigger ed grip-force adjustments were studied as nine normal subjects used a preci sion grip to lift, hold, and replace an instrumented test object. Experienc e with a predictable stimulus has been shown to enhance magnitude scaling o f triggered postural responses to different amplitudes of perturbations. Ho wever, this phenomenon, known as a central-set effect, has not been tested systematically for grip-force responses in the hand. In our study, predicta bility was manipulated by applying load perturbations of different magnitud es to the test object under conditions in which the upcoming load magnitude was presented repeatedly or under conditions in which the load magnitudes were presented randomly, each with two different pre-load grip conditions ( unconstrained and constrained). In constrained conditions, initial grip for ces were maintained near the minimum level necessary to prevent pre-loaded object slippage, while in unconstrained conditions, no initial grip force r estrictions were imposed. The effect of predictable (blocked) and unpredict able (random) load presentations on scaling of anticipatory and triggered g rip responses was tested by comparing the slopes of linear regressions betw een the imposed load and grip response magnitude. Anticipatory and triggere d grip force responses were scaled to load magnitude in all conditions. How ever, regardless of pre-load grip force constraint, the gains (slopes) of g rip responses relative to load magnitudes were greater when the magnitude o f the upcoming load was predictable than when the load increase was unpredi ctable. In addition, a central-set effect was evidenced by the fewer number of drop trials in the predictable relative to unpredictable load condition s. Pre-load grip forces showed the greatest set effects. However, grip resp onses showed larger set effects, based on prediction, when pre-load grip fo rce was constrained to lower levels. These results suggest that anticipator y processes pertaining to load magnitude permit the response gain of both v oluntary and triggered rapid grip force adjustments to be set, at least par tially, prior to perturbation onset. Comparison of anticipatory set effects for reactive torque and lower extremity EMG postural responses triggered b y Surface translation perturbations suggests a more general rule governing anticipatory processes.