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, swinging through his back, etc.), saddle fit, and rider skill on these muscle dimensions.

Dyson and her colleagues assessed 63 sound sport horses (mostly dressage horses) ridden by their usual riders in their normal tack. Using a flexible curve ruler, the team measured each horse’s back dimensions at four sites before and immediately after 30 minutes of exercise. They also assessed saddle fit, rider skill (good, moderate, or poor), gait abnormalities or lameness, and trot and canter grades on the 1-10 FEI dressage scale. They divided the horses into two groups based on total work quality score for the trot and canter: Group 1 scored a cumulative 11 or higher, and Group 2 scored 10 or lower.

The following are some of their observations and results:

  • Average back dimension changes (1-1.5 cm) were greater in Group 1 than Group 2;
  • 41 horses were sound, and 22 had gait abnormalities;
  • Average work quality was higher in sound horses;
  • Sound horses had greater average back changes;
  • In regard to rider skill level, the researchers determined that, of the 28 riders, four were poor, six were moderate, and 18 were good;
  • Horses with good riders had the greatest back changes, and horses with poorly skilled riders had the least;
  • Saddles were balanced in 39 horses, had even contact in 35, were unbalanced in 24, slipped in four, oscillated in five, and slid forward in two; and
  • Back changes were greater with well-fit saddles.

Based on these results, Dyson said horses that don’t work correctly, due to either work quality or rider skill, or have a poorly fitting saddle will suffer from a lack of long-term muscle development. In other words, you’ll have an undermuscled horse.

To remedy this, the easiest thing to correct is often the saddle. Dyson provided a few saddle fit reminders:

  • A lack of withers clearance by the saddle and/or pad affects movement;
  • A saddle that tips forward or back results in a focal increase in pressure; and
  • Saddle that fits at rest might not fit during work, which can affect movement.

In conclusion, Dyson said, “Back dimensions do increase with work. This is particularly important to recognize in young horses that have not yet developed topline muscles.”