Finger coordination during moment production on a mechanically fixed object

The moment production by several fingers on a mechanically fixed vertically oriented handle was studied under the systematic variations of task parameters such as (a) moment magnitude (1.0 Nm and 2.0 Nm) and (b) direction of moment production (into pronation and supination), as well as (c) vertical position of the handle from the moment axis, P (0, 2.0, 4.0, and 6.0 cm in both directions). The purpose of this study was twofold: to investigate the dependences between the task parameters and the performance variables and to test the mechanical advantage hypothesis. The performance variables changed symmetrically with P. In particular, magnitudes of the net horizontal and vertical forces both showed an S-shape change. The position of the point of zero free moment (PZFM) was determined. In the intermediate grasp locations (when 0<P magnitude <PZFM), the contribution of M_free (moment produced mainly by pronational or supinational effort) and the moment of the resultant force (moment generated mainly by pushing) into the total moment production scaled linearly with the P. The magnitudes of both agonist and antagonist moments (those acting in and against the direction of the required moment, respectively) of normal forces increased with P magnitudes while the magnitude of agonist moments of tangential forces decreased. For individual fingers, the ratio of finger force to its moment arm was not constant. The mechanical advantage hypothesis was successful in explaining some of the data but could not cope with other findings. We assume, therefore, that this hypothesis is limited in its applicability and may be task and effector specific.