Biomechanics of Balance in the Hand

Abstract

The mechanical balance of a hand has nothing to do with having equally strong muscles on opposite sides of each joint. It has to do with the balance of torque at a sequence of joints while the hand is exerting force on external objects that need to be controlled by the hand. The balance may be calculated or seem to be satisfactory when, for example, the tension in finger flexors is sufficient to hold the fingers flexed around an object that is being moved against external force. During such activity balance is not achieved until other muscles such as wrist extensors exert enough tension to hold the wrist stable against the flexion torque generated at the wrist by the same finger flexors. Any attempt to restore muscle balance in the hand must take into account the way in which the body responds by itself to loss of balance after injury or paralysis. It does this by changing the number of sarcomeres in each muscle fiber as well as by trick movements used to circumvent paralysis or other failure of normal balance.