Gait analysis for shoeing

Computational model of the equine limb By Lawson, Chateau, Pourcelot, Crevier-Denoix and Denoix.

Biomechanics is “The science that examines the forces acting on or within a biological structure and the effects produced by such forces. ” ~Hay, 1973

However equine biomechanics is generally used to mean the study of equine movement and the equine musculo-skeletal system, ignoring most of the field. It is becoming widespread, and “experts” are springing up all over.

Motion capture systems are now available which will allow the performance horse to have “optimised” shoeing,  examining the before and after effects of any changes. So far these have been used intelligently, but they are not sophisticated enough to distinguish between primary and secondary deviations – cause and compensation – and so it would be easy to overlook the effects of trying to change joint angles.

The equine distal limb joints are shock absorbers, particularly the fetlock (metacarpo-phalangeal joint). It flexes in proportion to the vertical ground reaction force of the limb loading (McGuigan and Wilson, 2003). Changes to the distal joint angles affect strain in the deep digital flexor tendon, the superficial digital flexor tendon and the suspensory ligaments (Lawson et al., 2007a), common sites of injury in the horse. Changes in hoof medio-lateral balance and width affects strain in the collateral ligaments (Lawson et al., 2008). Changes in shoeing affect both tendon and ligament strains (Lawson et al., 2007b) and inter-articular pressure in the distal interphalangeal (coffin) joint (Viitanen et al., 2003). For example an elevated heel increases distal interphalangeal joint pressure, increases superficial digital flexor tendon strain, unloads (and therefore shortens) the deep digital flexor tendon, and yet is often employed in showjumpers ”to help the hock”.

Some gait analysis systems now are being marketed as easy enough for any vet or farrier to use. Let’s hope that they come with a lot of training and promote rather than discredit the field.

Why Study Horses?

 As cursorial (adapted for running) locomotion goes, bottom of the food chain is really the lizards. They’re ahead of the snake, they’ve got limbs and even elbows to lift themselves off the ground, further increasing efficiency. However their shoulder design is a little primitive and they resort to side-to-side (medio-lateral) wiggling to move forward, wasting a lot of energy in the process.

At the other end of the scale is the cheetah. The wiggle has gone and the spine dorsi-flexes (up-down) like a caterpillar. This increases stride length and hence speed by so much that the cheetah could do six mph even if it didn’t use its legs (Hildebrand, 1959). The cheetah is a sprinter, fast over short distances.

The horse however, is not just adapted for speed but efficiency.

- It has a fairly rigid spine, handy if you want to carry a rider.

The stride length is increased by running on the toenails (unguligrade), with the heels (hock) and wrist (knee halfway up the leg), lengthening the legs. 

- Extra bones have been lost, leaving just the middle toe, and all the muscles are at the proximal (top) end of the leg, with long tendons running down the limb. This makes the distal (bottom) ends light, increasing stride frequency.

- Elastic energy is stored and released by the tendons, via the “extra” shock-absorbing joints that the horse has gained by running on its toes, particularly the fetlock, with the small proximal muscles acting as dampers (Wilson et al., 2001, Lawson et al., 2007). This makes the horse efficient.

- Joint constraints keep the limb motion parasagittal. Stable and efficient.

 - The horse has no clavicle (collar bone). The scapula (shoulder blade) is held on by a musclar sling and hence can slide along the thorax increasing stride length and efficiency (Lawson and Marlin, in press).

In fact the horse is an engineering marvel. If you want to understand the musculo-skeletal system, study the horse.