DIAGNOSING, MANAGING, AND RESOLVING LAMENESS IN HORSES


a gait transition and then ask the horse to gradually resume the original speed without changing gaits. Knee adduction (genu valgum) or abduction (genu varum) and the role of muscle interaction as a indi- cator of human ACL injuries, has been debated in the literature.19 However, the current research is trending in the direction that landing with the knee in an abducted position (typically seen in the lateral rotation of the femur), due to a lack of neuromuscu- lar control of the associated thigh muscles, is a risk factor for ACL injury,20 and training programs to reduce abduction of the knee have also reduced ACL injury.21 Reducing abduction by as little as 2° has been shown to be protective.22 Translating these concepts to equine rehabilita-


tion would indicate that to reduce the pathological rotational joint movement and torsion at the stifle, straight lines should be emphasized and turns should look more like an octagon than a curve. A series of straight lines connected by shallow turns would be a better strategy than performing a con- stant arc during movement after injury. Upon landing from a jump, the rider should allow the horse to take a minimum of three straight strides before turning to the next jump to reduce the decel- eration and rotational combination. Human re- search has shown that during walking, people need three strides to reach a steady state of balance.23 Therefore, three strides would be a reasonable num- ber of strides to remain on a straight line before changing directions so the horse will have time to rebalance before the turn.


Muscle/Tendon Complex


Distal limb injuries are frequent in equine athletes. Muscles of the distal limb in humans and horses were once thought to generate the power for move- ment. However, research has shown that these muscles perform little-to-no net mechanical work during level, steady-speed locomotion24–26 while the tendons of these muscles experience high levels of strain. A consensus from these pioneering works and related studies is that when evaluating the function of the tendon, it is essential that the action of the associated muscle must also be evaluated,27 and that muscular imbalances likely affect tendon function.28 Research on horses has shown that in the fore-


limbs, one muscle head of the massive and complex deep digital flexor (DDF) muscle flexes the fetlock in late swing, and although it is largely a joint posi- tional control muscle, its fast-twitch fiber type char- acteristics render it susceptible to fatigue. The DDF muscle/tendon complex stabilizes hyperexten- sion of the fetlock, as well as assists the superficial digital flexor (SDF) muscle to support the horse’s body weight during ground contact of the forelimbs, in particular during running gaits with single fore- limb support.29 In comparison, the SDF muscle is more resistant to fatigue than the DDF and undergoes lengthening


activation, resulting in elastic energy storage and recovery by its tendon.27 Its long, thin tendon (SDFT) complements the short, pennate fibers of the SDF muscle belly and allows this muscle/tendon complex to be the main source of bouncing during running gaits. This “spring” effect is best realized at the trot, and it helps horses save energy, thereby reducing fatigue. The authors noted that when the DDF muscle fatigues, this requires the SDF to bear considerably more load (to the point of overload), especially during fast galloping.27 Looking at the differences in distal tendon loading


across gaits, research found an increase in these parameters on the DDF tendon (DDFT) with a change from walk to trot and trot to canter.27 The strain, stress, and force resisted by the DDFT roughly doubled when the walk was compared to the canter. In the SDFT, the loading parameters were overall greater than those in the DDFT at all gaits.30 However, in the SDFT, the strain, stress, and force decreased by 14% from trot to canter, although it is important to emphasize that the amount of loading of the SDF muscle/tendon complex is still overall large.27 Other research supporting these data found the same 14% reduction in average vertical GRF when horses transitioned in gait from trot to canter.31


Translating these concepts to equine rehabilita-


tion would indicate that a progressive increase be- tween gaits from walk to trot and trot to canter may not be appropriate for all injuries. For example, rehabilitation protocols for injuries to the SDF mus- cle/tendon complex could introduce the slow canter before the trot to reduce muscle and tendon strain. Whereas the addition of the trot into the rehabilita- tion protocol may help to achieve greater strain for greater elastic energy savings by the SDF muscle/ tendon complex. For DDF muscle/tendon complex rehabilitation,


the canter should be utilized at a later time than for SDF muscle and tendon injuries. Activating asso- ciated muscles, which could reduce the load on the flexor tendons could prove useful. In addition, when limb acceleration is reduced, the ability to position the foot for correct foot contact with the ground is increased.32


Triceps Development


Equine research has also shown that the m. biceps brachii (and lacertus fibrosus), is an elbow flexor that has a large joint torque capacity due to its long proximal muscle moment arm, thus making the bi- ceps a strong contributor to shoulder movement. To counter this strong action of the m. biceps brachii, the m. triceps brachii is thought to have an important stabilizing role during stance.33 The de- velopment of the strong lateral head of the triceps brachii to help control the flexion of the elbow joint34,35 could be of assistance to complement the tension in the SDF and DDF muscle/tendon com-


AAEP PROCEEDINGS  Vol. 65  2019 277


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