Static dynamometer for the measurement of multidirectional forces exerted by the thumb

The functioning of a static dynamometer designed to measure simultaneous forces exerted by the thumb in the vertical and horizontal axes is described. The analysis of the output signals by a desktop computer program provides information regarding the forces generated in eight directions covering a plane transverse to the thumb by 45° increments. In 12 normal female subjects, the maximum voluntary torques exerted at the trapezo-metacarpal joint of the thumb were examined and the muscle activation patterns of the interosseus, flexor pollicis brevis and adductor pollicis brevis muscles were recorded in one subject. Torques and muscle activation patterns were depicted using polar plots. Dynamometric data indicate that strength varies with direction and that higher torques are obtained in directions that bring the thumb towards the palm, i.e. flexion, adduction, combined flexion-adduction and extension-adduction. Patterns of muscle activity vary according to the direction evaluated suggesting that strength depends on the number of activated muscles as well as the relative force contribution of each muscle.