Grip force adjustments induced by predictable load perturbations during a manipulative task

The experiment examined the anticipatory modulation of grip force with resp ect to load force during a drawer opening task. An impact force was introdu ced by a mechanical stop that arrested movement of the pulling hand. The re sults showed a typical grip force profile which consisted of two evolving p hases, one to control drawer movement onset, and the other to secure grip f orce at the expected impact. Initially, grip force increased with the load force that was developed to overcome the inertia of the drawer. After the f irst peak, a small decline was observed, followed by a proactive grip force increase prior to the time of impact. During this ramp-like increase of gr ip force, load force remained unchanged. In addition, a reactive response w as triggered by the impact. That anticipatory control with respect to an im pact force is not innate but, rather, is learned by experience was evidence d by a comparison of adults and children. Whereas adults made the character istic grip force adjustments to anticipate the impact, children used a prob ing strategy with irregular build-up of force until impact. Furthermore, ad ults calibrated the second phase of the grip force profile in the initial t rials of the task, indicating that grip force was rapidly updated with info rmation related to the impact force. The present results demonstrate that g rip-load force coordination during manipulation is a necessity for dealing with destabilizing load perturbations produced by self-induced movement and impact forces. It is concluded that grip force is adjusted automatically, but in a flexible manner, to secure grip in accordance with the characteris tics of the pulling synergy.