Circumferential force production of the thumb.

We developed an apparatus to measure maximal continuous circumferential forces of a digit in the transverse plane that is perpendicular to the longitudinal axis. Subjects with asymptomatic hands were asked to produce maximal forces in all directions at the proximal phalanx of the thumb. The force components during circumferential trials were used to construct a force envelope for each subject. The force envelope enlarged as the number of trials increased, and converged to a stable pattern after several trials. The shape of the force envelope was asymmetric and oblique, skewed towards the flexion/adduction direction. Maximal force (112.1 +/- 14.8 N) was generated in the direction combining flexion and adduction. The lowest force produced was only 50.3% of the maximal force in a direction combining extension and abduction. The forces in the direction of adduction, extension, abduction, and flexion were 60.1%, 61.9%, 51.5%, and 92.9% of the maximal force, respectively. The forces generated by circumferential exertions were comparable to the forces by focused directional exertions except in the flexion direction. Our methods have the potential to identify deviations from the normative force output pattern and help detect underlying impairments resulting from neuromuscular disorders.