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Article

# Musculoskeletal Modeling and Dynamic Simulation of the Thoroughbred Equine Forelimb During Stance Phase of the Gallop

[+] Author and Article Information
Michael D. Swanstrom, Laura Zarucco, Mont Hubbard, Susan M. Stover, David A. Hawkins

Biomedical Engineering Graduate Group, University of California - Davis, One Shields Avenue, Davis, CA 95616 JD Wheat Veterinary Orthopedic Research Laboratory, School of Veterinary Medicine, University of California - Davis, One Shields Avenue, Davis, CA 95616 Biomedical Engineering Graduate Group, University of California - Davis, One Shields Avenue, Davis, CA 95616 Biomedical Engineering Graduate Group, University of California - Davis, One Shields Avenue, Davis, CA 95616 and  JD Wheat Veterinary Orthopedic Research Laboratory, School of Veterinary Medicine, University of California - Davis, One Shields Avenue, Davis, CA 95616 Biomedical Engineering Graduate Group, University of California - Davis, One Shields Avenue, Davis, CA 95616

J Biomech Eng 127(2), 318-328 (Oct 23, 2004) (11 pages) doi:10.1115/1.1865196 History: Received March 04, 2004; Revised October 23, 2004

## Abstract

Because thoroughbred racehorses have a high incidence of forelimb musculoskeletal injuries, a model was desired to screen potential risk factors for injuries. This paper describes the development of a musculoskeletal model of the thoroughbred forelimb and a dynamic simulation of the motion of the distal segments during the stance phase of high-speed $(18m∕s)$ gallop. The musculoskeletal model is comprised of segment, joint, muscle-tendon, and ligament information. The dynamic simulation incorporates a proximal forward-driving force, a distal ground reaction force model, muscle activations, and initial positions and velocities. A simulation of the gallop after transection of an accessory ligament demonstrated increased soft tissue strains in the remaining support structures of the distal forelimb. These data were consistent with those previously reported from in vitro experimental data and supported usefulness of the model for the study of distal forelimb soft tissue mechanics during the stance phase of the gallop.

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## Figures

Figure 1

Segments and joints of the thoroughbred forelimb (medial view of right forelimb). Segments are listed to the right of the limb and joints are listed (in italics) to the left of the limb. The following segments and joints/articulations are denoted by abbreviations: accessory carpal bone (ACB), metacarpus (MC), proximal sesamoid bones (PSB), proximal phalanx (P1), middle phalanx (P2), distal phalanx (P3), navicular bone (NB) segments, and antebrachiocarpal joint (ABC), radial-accessory carpal articulation (Rad-AC), middle carpal joint (Midcarp), metacarpophalangeal joint (Fetlock), metacarpal-sesamoidean articulation (MC-Ses), proximal interphalangeal (PIP), distal interphalangeal (DIP), and middle phalangeal-navicular articulation (P2-Nav). For each segment, the reference frame axes are oriented with the y-axis directed distally along the long axis of the segment, the z-axis perpendicular to the sagittal plane directed medially from the limb, and the x-axis directed parallel to the sagittal plane in a right-handed coordinate system (the x and y axes are depicted, but not labeled). For the ACB, PSB, and NB, the y-axes are directed perpendicular to their articular surfaces with the corresponding long bones.

Figure 2

Muscles and accessory ligaments of the thoroughbred forelimb (medial, caudal, and lateral views). Flexor muscles in the forelimb model are the superficial digital flexor (SDF), deep digital flexor (DDF), flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), and ulnaris lateralis (UL). Extensor muscles in the model are the common digital extensor (CDE), lateral digital extensor (LDE), extensor carpi radialis (ECR), and extensor carpi obliquus (ECO). The LDE and ECO had broad origins that are distributed to three and two points, respectively. The accessory ligaments of the SDF (ALSDF) and DDF (ALDDF) muscles are also included.

Figure 3

Ligaments of the distal aspect of the thoroughbred forelimb (medial and palmar views). Ligaments in the forelimb model include the suspensory ligament (SL); the extensor branches of the suspensory ligament (EB); the straight (DSLS), oblique (DSLO), cruciate (DSLCr), and short (DSLSh) distal ligaments of the proximal sesamoid bones; the collateral ligaments of the fetlock joint (superficial (CLFetS) and deep (CLFetD)), PIP joint (CLPIP), DIP joint (CLDIP), proximal sesamoid bones (CLPSB), and navicular bone (CLNB); the axial, superficial abaxial, and deep abaxial palmar ligaments of the PIP joint (PLPIP); and the distal ligament of the navicular bone (DLNB). The (DSLSh) and (DLNB) are obscured from view in the figure.

Figure 4

Format for defining the joints (two-dimensional). The joint angle θ is the angle between the y-axis of the distal segment and the y-axis of the proximal segment. The x and y joint translations (tx,ty) are the coordinates of the distal segment’s origin in the proximal segment’s reference frame.

Figure 5

Stress-strain curves for the suspensory ligament (SL), straight distal ligament of the proximal sesamoid bones (DSLS), and a collateral ligament (measured from the (CLFetS)). Ligaments were clamped (a cryoclamp was used for the SL and (DSLS) and a bone-ligament-bone prep was used for the CLFetS), instrumented with extensometers, and tested at 2 Hz between 0.5 and 2.5 kN. Ligament forces were divided by their cross-sectional area (203mm2 for the SL, 73mm2 for the DSLS and 32mm2 for the CLFetS) to obtain stresses and curves were generated from quadratic fits to the measured data.

Figure 6

“Artificial” rigid body segments (trunk in black, hindquarters in gray) added proximal to the forelimb for dynamic simulations. The jagged line between the forelimb and the trunk represents a spring/damper. τshldr is a torque applied between the forelimb and trunk about point A. τHQ is a torque applied between the trunk and hindquarters about point B.

Figure 7

Force-time profile of a 4 kg weight dropped 40 cm onto a track surface. Hollow circles depict the measured forces (44), and the solid line shows the forces predicted by the model given in Eq. 5.

Figure 8

Baseline simulation results. All graphs are plotted against the percentage of stance phase (depicted schematically at the bottom of the figure). Plot A shows tendon/ligament strain, plot B shows muscle-tendon/ligament force, plot C shows joint angles, and plot D shows the net ground reaction forces. DIP and PIP flexion (palmar flexion) is reflected by a positive joint angle and fetlock hyperextension (dorsiflexion) is reflected by a negative joint angle. For the fetlock joint angles in plot C, the x markers indicate measured kinematics and the solid line indicates simulation results. The legend between plots A and B identifies the structures in those plots.

Figure 9

Transected ALSDF simulation results. All graphs depict the transected ALSDF results (dashed lines) versus the baseline simulation results (solid lines) and are plotted against the % of stance phase (distal limb depicted schematically at the bottom of the figure). Transecting the ALSDF results in increased peak strains in the remaining soft tissue structures, increased fetlock extension (hyperextension) and DIP flexion (palmar flexion) and decreased PIP flexion (palmar flexion). Plots A and B show tendon/ligament strains and plot C shows joint angles. DIP and PIP palmar flexion is reflected by a positive joint angle and fetlock hyperextension (dorsiflexion) is reflected by a negative joint angle.

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