Validity of Mechanical advantage hypothesis of human grasping depends on the nature of task difficulty

Abstract

Successful object interaction during daily living involves maintaining the grasped object in a static equilibrium by properly arranging the fingertip contact forces. According to the mechanical advantage hypothesis, during supination or pronation torque production, fingers with longer moment arms would produce greater normal force than those with shorter moment arms. Previous studies have probed this hypothesis by investigating the force contributions of the individual fingers through systematic variations (or perturbations) of properties of the grasped handle. In the current study, we examined the applicability of this hypothesis in a paradigm wherein the thumb tangential force was constrained to a minimal constant magnitude. This was achieved by placing the thumb on a freely movable slider platform. The total mass of the handle was systematically varied by adding external loads directly below the center of mass of the handle. Our findings suggest that in the human hand, the central nervous system (CNS) adopts the principle of mechanical advantage depending on an abstract sense of challenge attached to the task situation.

Evidence to support the mechanical advantage hypothesis of grasping at low force levels

“…Our previous study on the systematic increase in the mass of the handle with the load suspended exactly below COM of the handle could help to understand this situation better 12 . Although external loads of mass ranging from 0.150kg to 0.450kg were suspended exactly below COM of the handle, MA principle was supportive only when an external load of mass 0.450kg was added.…”

“…Since there was no supportive evidence to explain this, we attempted to examine whether the applicability of the mechanical advantage principle is speci c to any particular task and investigate the kind of task that lends support to MAH. To investigate this, our previous study involved systematically increasing the mass of the handle by adding external loads of mass 0.150kg, 0.250kg, 0.350kg, and 0.450kg 12 . With the addition of external loads, the magnitude of supination torque requirement also increased.…”