Force sharing and neutral line during finger extension tasks.

There is general consensus that the minimization of the secondary torque of the hand provides a universal model for explaining the force sharing patterns among the fingers. Since biomechanical secondary axes of the hand are unchanged in extension, it appears relevant to validate this model for finger extension forces. Fifteen subjects performed flexion and extension forces in a four-finger task. Each fingertip force was expressed in percentage of the force produced by an individual finger force over the resultant four-finger force (force sharing), and the point of force application of the resultant force was calculated (neutral line). The force-sharing pattern was different for flexion and extension. The index and ring fingers were equally involved, regardless of the task. The neutral line was located differently in flexion and extension, and for proximal and distal force application in extension. The mode of control of the finger redundancy was specific to the force production in flexion and extension. In flexion, the principle of minimization of secondary torque was confirmed. This was not observed in extension. We concluded that the minimization of the secondary torque is not a universal mode of control of the finger redundancy.