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
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
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)