Multidirectional analysis of maximal voluntary contractions of the thumb.

A multidirectional dynamometer was used to investigate the maximal static force (MSF) exerted at the proximal phalanx of the dominant and nondominant thumbs of 12 normal women subjects. These MSFs were measured in two sessions within a two-week interval. Eight directions covering 360 degrees, at increments of 45 degrees within the transverse plane of the longitudinal axis of the thumb, were examined. A three-way analysis of variance (ANOVA) with repeated measures (factors: direction, session, handedness) was performed. No significant interaction was found between the main factors. The ANOVA showed that MSFs vary according to the direction of effort. No significant difference, however, was observed between the dominant and nondominant thumbs or between the MSFs obtained in the two sessions. Post-hoc analysis revealed that MSFs exerted in directions containing an abduction component were significantly lower than MSFs exerted in the other directions. It is concluded that directional strength of the thumb is maximized for functional grasping activities and reflects the anatomic distribution of muscles acting at the thumb.