Coordination of fingertip forces in object transport during locomotion.

Walking while carrying a hand-held object requires the generation of appropriate grip forces to offset the inertial forces produced during locomotion. The present study examined the interaction between grip forces and locomotion-induced inertial forces across the gait cycle. Eight subjects transported a container under three conditions: self-paced transport with and without accuracy constraints and a velocity-constrained condition. The results showed that the trunk and transported container moved in a synchronized, sinusoidal pattern during all conditions. Grip and inertial forces of the transporting hand were highly coupled in an anticipatory fashion, regardless of task demands. The inertial forces were higher and the coupling was greater in the faster, unconstrained condition. However, grip force modulation was observed even when the inertial forces acting on the container were small and applied indirectly to the container through the locomotor effects originating in the legs and trunk. We suggest that continuous grip force adjustment is used as a generalized strategy to maximize efficiency during object transport regardless of the size or origin of the inertial forces.