Advertisement

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

View More View Less
  • 1 Department of Biomedical Engineering, College of Engineering, University of Texas, Austin, TX 78712.
  • | 2 Department of Biomedical Engineering, College of Engineering, University of Texas, Austin, TX 78712.
  • | 3 Orthopaedics Biomechanics Laboratory, Department of Orthopaedics and Rehabilitation, School of Medicine, University of Texas Medical Branch, Galveston, TX 77555.
  • | 4 Orthopedic Research Laboratory, Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.
  • | 5 Orthopedic Research Laboratory, Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

Abstract

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° to 70°) and metacarpophalangeal (220° to 110°) joints. Extension of the metacarpophalangeal joint (180° and 220°) 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 ≤ 38% during metacarpophalangeal joint extension. Extensor muscle moment arms at the carpal and metacarpophalangeal joints also varied (≤ 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. (Am J Vet Res 2003;64:351–357)