Journal of Biomechanics 31 (1998) 565 — 570 Technical Note Mechanical advantage of the thumb muscles W. Paul Smutz, Apichai Kongsayreepong, Richard E. Hughes, Glen Niebur, William P. Cooney, Kai-Nan An* Orthopedic Biomechanics Laboratory, Mayo Clinic and Mayo Foundation, Rochester, MN 55905, U.S.A. Received 15 February 1998 Abstract The purpose of this study was to measure the moment arms of four extrinsic muscles (flexor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus) and four intrinsic muscles (flexor pollicis brevis, abductor pollicis brevis, adductor pollicis, and opponents pollicis) of the thumb at the interphalangeal, the metacarpophalangeal, and the carpometacarpal joints in the same cadaver specimens and to examine the specific role of each muscle. Measurements were made on seven fresh frozen cadaver hands. The moment arms were measured during flexion/extension of the interphalangeal joint, flexion/extension and adduction/abduction of the metacarpophalangeal joint, and flexion/extension and adduction/abduction of the carpometacarpal joint. Moment arms were computed using the slope of the tendon excursion—joint angle relationship. The specific function of each muscle was determined by multiplying the measured moment arms by the maximum force that each muscle can generate. It was found that the flexor pollicis longus was a pure flexor while flexor pollicis brevis was an adductor as well as a flexor, the extensor pollicis longus was an extensor and an adductor, extensor pollicis brevis was an extensor and a mild abductor, the abductor pollicis longus was an extensor as well as an abductor, the abductor pollicis brevis was mainly an abductor, the adductor pollicis was a major flexor as well as an adductor, and the opponents pollicis was a flexor and an abductor.  1998 Elsevier Science Ltd. All rights reserved. Keywords: Moment arms; Thumb; Biomechanics 1. Introduction Moment arms of the thumb muscles have been studied previously (Brand et al., 1985; Cooney et al., 1984; Law et al., 1989) but no single study has measured the mo- ment arms at all three joints using the same cadaver specimens. Data gathered from different experiments is often difficult to compare or combine because of differ- ences in experimental technique, specimens, or in the precise definition of joint motions. The purpose of this study was to measure the moment arms of the thumb muscles at the interphalangeal (IP), the metacarpophalangeal (MP), and the carpometacar- pal (CMC) joints throughout the range of motion of each joint in the same specimens. The muscles studied were flexor pollicis longus (FPL), extensor pollicis longus (EPL), extensor pollicis brevis (EPB), abductor pollicis *Corresponding author. Tel.: 001 507 294-2589; fax: 001 507 284- 5392; e-mail: an.kainan@mayo.edu. longus (APL), flexor pollicis brevis (FPB), abductor pol- licis brevis (APB), the transverse head of the adductor pollicis (ADPt), the oblique head of the adductor pollicis (ADPo) and opponents pollicis (OPP). 2. Materials and methods Four left and three right cadaver hands from four males and two females were used in the study. The protocol was reviewed and approved by the Mayo Clinic Institutional Review Board. Mean specimen age was 77 (range 72—84). The specimens were prepared by first re- moving the skin and exposing the nine muscles to be tested. Nylon lines (30 pound test) were then sutured to the proximal 10 mm of the tendons of the four extrinsic muscles (FPL, EPL, EPB, and APL) and into the ten- dons at the insertion site of the five intrinsic muscles (FPB, APB, ADPt, ADPo, and OPP). Each muscle was then elevated from its origin and a nylon screw with a hole drilled through the head was placed in the bone at the origin. The nylon lines attached to the nine tendons 0021-9290/98/$19.00  1998 Elsevier Science Ltd. All rights reserved. PII S0021-9290(98)00043-8