If you’re a runner, you’ve probably noticed that after a 45-minute jog your calf muscles seem a bit swollen or enlarged. They are responding to post-training fluid shifts, fiber hypertrophy (thickening), and the general strain of exercise. Horses’ muscles, particularly along the back, respond to exercise in the same way. Have you ever thought about what effect this might have on your saddle’s fit as your horse works?

Sue Dyson, MA, VetMB, PhD, DEO, FRCVS, and her colleagues at the Animal Health Trust, in Newmarket, U.K., did. They recently studied exercise-induced changes in horses’ back dimensions and presented their results at the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah.

Why are these subtle muscle changes even important? It all has to do with saddle fit, and if a saddle doesn’t fit properly, it can cause a horse pain and impair his performance.

"We know that the saddle needs to fit the horse in motion, but there has been no investigation of whether the thoracolumbar region (lower back, in front of the pelvis) changes in shape in association with exercise or how improper saddle fit may influence potential changes," Dyson began.

Factors she said influence horses’ muscle dimensions include conformation, type of exercise/riding discipline, age, head and neck position, lameness, tack fit, and rider skill and weight. In this study she aimed to also determine the influence of work quality according to the Fédération Equestre Internationale (FEI) scoring system for dressage (e.g., if the horse is working on the bit, swin