Force-generating capacity of thumb adductor muscles in the parallel and perpendicular plane of adduction

The description of thumb movements, especially for adduction at the carpometacarpal joint, still gives rise to a confusion among clinicians and researchers. The purpose of this study was to elucidate this confusion by comparing twitch, tetanic, and voluntary isometric force measurements of thumb adductors with the thumb positioned perpendicular and parallel to the palm of the hand. Fourteen healthy adult subjects, aged between 22 and 40 years, volunteered their participation. Maximum twitch, tetanic, and voluntary contractions were recorded using a strain-gauge mounted on a wooden platform. Twitch and tetanic contractions were evoked by an electrical stimulator that passed current through surface electrodes to the skin overlying the ulnar nerve at the wrist. The results show no statistical difference for the tetanic and voluntary force measurements between the two planes of adduction (p > 0.5). The significant changes observed for the twitch responses between the two planes were attributed to a change in the compliance of the muscle-transducer unit. To conclude, the present findings provide clinicians and researchers with an objective and quantitative basis for assessing the force-generating capacity of thumb adductor muscles in either the parallel or the perpendicular plane of adduction.