MECHANICAL ADVANTAGE OF THE THUMB MUSCLES

The purpose of this study was to measure the moment arms of four extri nsic muscles (flexor pollicis longus, extensor pollicis longus, extens or pollicis brevis, and abductor pollicis longus) and four intrinsic m uscles (flexor pollicis brevis, abductor pollicis brevis, adductor pol licis, and opponents pollicis) of the thumb at the interphalangeal, th e metacarpophalangeal, and the carpometacarpal joints in the same cada ver specimens and to examine the specific role of each muscle. Measure ments were made on seven fresh frozen cadaver hands. The moment arms w ere measured during flexion/extension of the interphalangeal joint, fl exion/extension and adduction/abduction of the metacarpophalangeal joi nt, and flexion/extension and adduction/abduction of the carpometacarp al joint. Moment arms were computed using the slope of the tendon excu rsion-joint angle relationship. The specific function of each muscle w as determined by multiplying the measured moment arms by the maximum f orce that each muscle can generate. It was found that the flexor polli cis longus was a pure flexor while flexor pollicis brevis was an adduc tor as well as a flexor, the extensor pollicis longus was an extensor and an adductor, extensor pollicis brevis was an extensor and a mild a bductor, the abductor pollicis longus was an extensor as well as an ab ductor, the abductor pollicis brevis was mainly an abductor, the adduc tor pollicis was a major flexor as well as an adductor, and the oppone nts pollicis was a flexor and an abductor. (C) 1998 Elsevier Science L td. All rights reserved.