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)
Objective—To assess the net mechanical load on the distal end of the third metacarpal bone in horses during walking and trotting.
Animals—3 Quarter Horses and 1 Thoroughbred.
Procedures—Surface strains measured on the left third metacarpal bone of the Thorough-bred were used with a subject-specific model to calculate loading (axial compression, bending, and torsion) of the structure during walking and trotting. Forelimb kinematics and ground reaction forces measured in the 3 Quarter Horses were used with a musculoskeletal model of the distal portion of the forelimb to determine loading of the distal end of the third metacarpal bone.
Results—Both methods yielded consistent data regarding mechanical loading of the distal end of the third metacarpal bone. During walking and trotting, the distal end of the third metacarpal bone was loaded primarily in axial compression as a result of the sum of forces exerted on the metacarpal condyles by the proximal phalanx and proximal sesamoid bones.
Conclusions and Clinical Relevance—Results of strain gauge and kinematic analyses indicated that the major structures of the distal portion of the forelimb in horses acted to load the distal end of the third metacarpal bone in axial compression throughout the stance phase of the stride.