BIOMECHANICS CAN IMPROVE REHABILITATION PROTOCOLS

By Marie Rosenthal,MS

When designing the most appropriate rehabilitation protocols for a sporting horse, one must consider the biomechanics of the injury, according to Sheila Schils, PhD, owner of EquiNew Therapy in River Falls, Wis. and former Professor of Equine Science at the University of Wisconsin.

Much of what Dr. Schils understands about the role biomechanics plays in injury and recovery comes from studies in human athletes, but much of it is applicable to equine physical rehabilitation, she explained. The equine muscular skeletal system is design to allow the horse to graze for long periods. However, this spinal position must be modified for the horse to perform, regardless of the discipline. “These modifications produce forces and movement patterns that counteract the natural structural mechanics of the horse, and therefore, specific musculoskeletal adaptations must be developed by the horse to perform successfully in sport,” she said.

Images courtesy of Dr. Shiela Schils

The laws of physics are the basis of biomechanics and several factors, not just 1, are typically present to result in injury. Three of these important biomechanical concepts related to injury are concussion, deceleration and individual-limb weight bearing.

Injury from concussion is related to the direction the force is applied and to the reaction tissue has to stress and strain. For example, the tibia can handle a high level of force and not be injured when the force is applied during compression. However, when that same level of force is applied during torsion or rotation, injury is likely to occur. “Tissue can handle a high degree of concussion as long as that concussion is absorbed by a joint that is [correctly] functioning biomechanically,” Dr. Schils explained at the 66th Annual Convention of the American Association of Equine Practitioners.

Newton’s first law of motion basically says that once a body gets moving, it stays moving.And when you have a cyclic motion, such as movement of the horse’s leg, there is a constant cycle of muscle activity to move the leg forward and then slow the leg down so it can hit the ground. “This slowing of the leg requires a lot of energy and postural control and I think we need to really emphasize the potential of this deceleration phase to produce injury.”

Some muscles appear more prone to injury than others. “Muscles that are decelerating a limb (when the limb is in protraction) are in an extended position and we know, through research, that muscles that are in a lengthened state and/or producing deceleration are more prone to injury.” she said. Many human studies have found that an extended joint increases the force on that joint when it hits the ground. “In one study there was a 27% reduction in ground impact forces when falling occurred with elbows bent rather than with the elbows naturally extended,” Dr. Schils said.

“Therefore, exercises that improve joint flexion will help slow the speed of the gait and since these muscles are different muscles than the decelerating muscles, the decelerating muscles are not overworked and this reduces injury.” In addition, this improved deceleration ability will reduce impact forces. “It’s like buying 1 and getting 1 for free.” Think of the human or equine hamstring, this is a common muscle which is used to decelerate the leg.

In addition, the biarticular function of some muscles—for example the coaction of both the hip and knee joint—also increase the risk of injury, Dr. Schils explained. When hip flexion and knee extension occurs, the hamstrings are placed in a maximally lengthened state, which is predisposing them to injury.

Biomechanical studies have shown that for a limb to support the weight of the body on a straight line, the limb must be placed under the center of gravity of the body, which means the limb must swing slightly medial to “find the belly button.” However, if this medial placement is asymmetrical—where 1 limb— is less medially placed than the opposite limb this can indicate a lack of willingness to bear weight on that limb. “For me—during a movement evaluation—this is the first thing I look for as the horse walks and trots on a straight line to me and away from me to determine if the horse has a right or left hind or front limb preference. Then the horse is moved on a circle and if this excessive medial limb placement is seen regardless of the direction of movement, the horse is indicating a strong preferential limb,” she told Modern Equine Vet. And to make matters worse, the limb that is asymmetrically medially placed is almost always the limb accepting more weight, which could increase injuries, she added.

In addition, the angulation of the swing phase of the excessively placed limb requires that limb to move with a faster and longer stride than the opposite limb, unbalancing the gait. This deviation may make this limb appear to step faster and not stay on the ground as long as the opposite hind.

There are 4 possible reasons why horses place one limb further under their body than the opposite limb, she suggested: 1. That limb is the stonger of the 2 limbs. 2. That limb is the more coordinated of the 2 limbs. 3. That limb is both the stronger and the more coordinated limb. 4. The non medially placed limb is painful and therefore the horse resist weightbearing on that limb.

Evaluating the biomechanics of the entire horse cannot only help when developing a rehabilitation plan, but it could help develop recommendations that could prevent injuries in the first place, according to Dr. Schils. “Biomechanics is a complex field of study due to all the interrelationships, you can’t just look at the trees, you have to really sit down and understand the function of the forest first. It can require a different way of looking.”

Translating These Concepts to Equine Rehab

EXERCISES TO LOWER PEAK FORCES INCLUDE:

• slowing the speed of the gait to increase time on the ground and then to increase joint flexion,

• encouraging elevated carpal and hock action during jumping,

• tightening the distances between trotting poles or fences to help slow the horse or

• using 1 strides while choosing the appropriate height of the fence to obtain the desired joint flection.

Exercises to Improve Symmetrical Limb Placement

• Handlers can lead the horse from the side that improves the symmetry of limb weight bear. Teaching the horse to lead from the right side can dramatically improve limb placement.

• Being aware of symmetrical limb placement during dry or water treadmill work.

• Riders can reposition the hindquarters so that the hindquarters step into the prints of the front feet on a straight or curved line.

• Riders can reposition the shoulders so that the hindquarters step into the prints of the front feet on a straight or curved line.

• Repositioning the hindquarter and the forehand are 2 distinctly different muscular activities and both must be evaluated as which is the ideal means to improve the symmetry of weight bearing

• Straight lines should be emphasized and turns should look more like an octagon than a curve.

For more information:

Schils S, Ober TR, Butcher MT. Review of the biomechanics of injury in the equine athlete: From research to clinical practice. AAEP Proceedings 2019;65:273-280.