Modulation of grip force with load force during point-to-point arm movements.

In this paper, we examine grip forces and load forces during point-to-point arm movements with objects grasped with a precision grip. We demonstrate that grip force is finely modulated with load force. Variations in load force arise from inertial forces related to movement; grip force rises as the load force increases and falls as load force decreases. The same finding is observed in vertical and horizontal movements performed at various rates. In vertical movements, maximum grip force coincides in time with maximum load force. The maxima occur early in upward and later in downward movements. In horizontal movements, where peaks in load force are observed during both the acceleratory and deceleratory phases, grip force rises at the beginning of the movement and remains high until the end. The results suggest that when moving an object with the hand the programming of grip force is an integral part of the planning process.