Figure 1.
Top, a: Axial push force can be modeled as a sum of friction forces at the hand-handle contact, where friction force at each hand-handle contact is a product of the friction coefficient and normal force at the contact. The directions of inward and outward torque exertions and illustration of grip force (Fg) and normal force (∑ Fni) are shown. Middle, b: The handle was connected to a load cell via universal joints. Axial push force results in reaction force in the opposite direction which generates a moment about the wrist in the ulnar direction. Bottom, c: Moment about the wrist joint is reduced when there is downward force applied by the hand to a fixed handle (segmented lines). The downward force results in an upward reaction force which generates a moment about the wrist joint in the radial direction. This radially-directed moment can alleviate the ulnar-directed moment resulting from the axial force (solid lines).