Moment arms about the carpal and metacarpophalangeal joints for flexor and extensor muscles in equine forelimbs

OBJECTIVE: To determine whether muscle moment arms at the carpal and metacarpophalangeal joints can be modeled as fixed-radius pulleys for the range of motion associated with the stance phase of the gait in equine forelimbs. SAMPLE POPULATION: 4 cadaveric forelimbs from 2 healthy Thoroughbreds. PROCEDURE: Thin wire cables were sutured at the musculotendinous junction of 9 forelimb muscles. The cables passed through eyelets at each muscle's origin, wrapped around single-turn potentiometers, and were loaded. Tendon excursions, measured as the changes in lengths of the cables, were recorded during manual rotation of the carpal (180 degrees to 70 degrees) and metacarpophalangeal (220 degrees to 110 degrees) joints. Extension of the metacarpophalangeal joint (180 degrees and 220 degrees) was forced with an independent loading frame. Joint angle was monitored with a calibrated potentiometer. Moment arms were calculated from the slopes of the muscle length versus joint angle curves. RESULTS: At the metacarpophalangeal joint, digital flexor muscle moment arms changed in magnitude by < or = 38% during metacarpophalangeal joint extension. Extensor muscle moment arms at the carpal and metacarpophalangeal joints also varied (< or = 41% at the carpus) over the range of joint motion associated with the stance phase of the gait. CONCLUSIONS AND CLINICAL RELEVANCE: Our findings suggest that, apart from the carpal flexor muscles, muscle moment arms in equine forelimbs cannot be modeled as fixed-radius pulleys. Assuming that muscle moment arms at the carpal and metacarpophalangeal joints have constant magnitudes may lead to erroneous estimates of muscle forces in equine forelimbs.