Force- and moment-generating capacities of muscles in the distal forelimb of the horse

Abstract

A detailed musculoskeletal model of the distal equine forelimb was developed to study the influence of musculoskeletal geometry (i.e. muscle paths) and muscle physiology (i.e. force-length properties) on the force- and moment-generating capacities of muscles crossing the carpal and metacarpophalangeal joints. The distal forelimb skeleton was represented as a five degree-of-freedom kinematic linkage comprised of eight bones (humerus, radius and ulna combined, proximal carpus, distal carpus, metacarpus, proximal phalanx, intermediate phalanx and distal phalanx) and seven joints (elbow, radiocarpal, intercarpal, carpometacarpal, metacarpophalangeal (MCP), proximal interphalangeal (pastern) and distal interphalangeal (coffin)). Bone surfaces were reconstructed from computed tomography scans obtained from the left forelimb of a Thoroughbred horse. The model was actuated by nine muscle-tendon units. Each unit was represented as a three-element Hill-type muscle in series with an elastic tendon. Architectural parameters specifying the force-producing properties of each muscle-tendon unit were found by dissecting seven forelimbs from five Thoroughbred horses. Maximum isometric moments were calculated for a wide range of joint angles by fully activating the extensor and flexor muscles crossing the carpus and MCP joint. Peak isometric moments generated by the flexor muscles were an order of magnitude greater than those generated by the extensor muscles at both the carpus and the MCP joint. For each flexor muscle in the model, the shape of the maximum isometric joint moment-angle curve was dominated by the variation in muscle force. By contrast, the moment-angle curves for the muscles that extend the MCP joint were determined mainly by the variation in muscle moment arms. The suspensory and check ligaments contributed more than half of the total support moment developed about the MCP joint in the model. When combined with appropriate in vivo measurements of joint kinematics and ground-reaction forces, the model may be used to determine muscle-tendon and joint-reaction forces generated during gait.

Fig. 1

(A) Seven joints were included in the model of the equine forelimb: elbow, radiocarpal, intercarpal, carpometacarpal, metacarpophalangeal (MCP), proximal interphalangeal (IP) and distal interphalangeal (IP) joint. The reference position for the carpal and MCP joints was taken to be 180° with the limb extended, and flexor moments were defined on the caudal aspect of each joint as shown. (B) Computer-screen shot showing some of the muscles crossing the carpus in the model of the distal forelimb. Shown are the paths, origins, via points and insertions of the extensor carpi radialis, flexor carpi radialis, flexor carpi ulnaris and ulnaris lateralis. The abductor pollicis longus muscle is not shown as it is located behind the radius and ulnar bones. (C) Computer-screen shot showing the muscles and ligamentous structures crossing the MCP joint in the model. Shown are the paths, origins, via points and insertions of the superficial digital flexor, deep digital flexor, the suspensory ligaments, and the superior and inferior check ligaments. The common and lateral digital extensor muscles are also shown, the former obscured partially by its lateral counterpart. Note that the superior check ligament and the inferior check ligament are tendinous extensions of the deep digital flexor (DDF) and the superficial digital flexor (SDF), respectively.

Fig. 2

Joint moments, moment arms and forces calculated for the muscles that flex the MCP joint in the model. Peak net flexor moment occurred when the MCP joint was moved to 240° and coincided with peak muscle force (upper panel). Muscle force dominated the pattern of the joint moment–angle curves for the flexor muscles. The horizontal shaded bar indicates the joint angles associated with the stance phase of walking and trotting (van Weeren et al. 1993; Hodson et al. 2000).

Fig. 3

Joint moments, moment arms and forces calculated for the muscles that extend the MCP joint in the model. Peak net extensor moment coincided with the peaks in the moment arms of the common and lateral digital extensor muscles (upper panel). Muscle moment arms dominated the variation in joint moment over the simulated range of joint movement and for the joint angles associated with the stance phase of gait (horizontal shaded bar).

Fig. 4

Joint moments, moment arms and forces calculated for the muscles that flex the carpal joint in the model. The net flexor moment peaked when the carpal joint was moved to 190° (i.e. 10° of joint extension). This joint angle coincided with the peak muscle force in the superficial and deep digital flexor muscles. The variation in muscle force dominated the joint moment–angle curves over the simulated range of joint movement. The horizontal shaded bar indicates the joint angles associated with the stance phase of walking and trotting (van Weeren et al. 1993; Hodson et al. 2000).

Fig. 5

Joint moments, moment arms and forces calculated for the muscles that extend the carpus in the model. The peak net extensor carpal moment was due to the moment generated by the common digital extensor and extensor carpi radialis muscles. Muscle force and moment arm influenced the pattern of the joint moment–angle relationship. However, for the joint angles associated with the stance phase of gait (horizontal shaded bar), joint moment was influenced mainly by the variation in muscle moment arm.

Fig. 6

Operating ranges of the muscles in the forelimb model corresponding to the range of joint angles commonly reported for the stance phase of walking and trotting (van Weeren et al. 1993; Hodson et al. 2000). Muscle force is normalized to peak isometric force () for each muscle in the model (vertical axis). Note that at a muscle length of , muscle force equals . In all panels, the thin line is the standardized force–length curve used to calculate muscle force. (A) Muscles acting about the MCP joint were predicted to operate slightly below their optimal fibre length at the joint angles associated with the stance phase of gait. Thus, if the MCP joint was moved to joint angles beyond 240°, e.g. as in galloping (Butcher & Ashley-Ross, 2002), the superficial and deep digital flexor muscles would probably remain close to their optimal lengths. The joint range of motion for the MCP joint in walking and trotting is reported to be between 193° and 240° (van Weeren et al. 1993; Hodson et al. 2000). (B) At the carpus, the superficial and deep digital flexor muscles in the model operated at less than 0.74 for the joint angles associated with the stance phase of gait. The other carpal extensor and flexor muscles were estimated to be near their optimal lengths. The joint range of motion for the carpus in walking and trotting is reported to be between 155° and 185° (van Weeren et al. 1993; Hodson et al. 2000). Note that CDE, LDE, SDF and DDF all cross both the carpus and the MCP joint.

Fig. 7

Net isometric flexor muscle moments developed about the MCP joint in the model. Also shown are the joint moments contributed by the superior check ligament (SCL), the inferior check ligament (ICL) and the suspensory ligaments (SL). Note that the ligaments did not produce force when the MCP joint was moved to angles less than 190°. At MCP joint angles greater than 230°, however, the check and suspensory ligaments in the model produced a larger flexor moment at the MCP joint than did the superficial and deep digital flexor muscles. Joint angles greater than 190° are those associated with the stance phase of walking and trotting (horizontal shaded bar).