In recent years social media and online forums have sparked increased discussion and debate about rider size and its effect on horse welfare and performance. However, we don’t currently have specific guidelines for suitable riding size. So, during the 2021 American Association of Equine Practitioners (AAEP) Convention, held Dec. 4-8 in Nashville, Tennessee, Sue Dyson, VetMB, PhD, an independent consultant based in Suffolk, U.K., addressed the topic by sharing recent study results.
She explained she would be using the term “rider size” rather than weight, “because the suitability of a rider for a horse is influenced not only by their weight but also by their height and the morphology of the rider’s body. Moreover, rider size is potentially a less sensitive and emotive term, particularly when body mass index of the rider is not in itself a problem.”
Relevant (and Conflicting) Study Results
Dyson presented results from existing studies assessing the effects of rider size on horse gait and behavior.
In Study A (Dyson et al.) four experienced riders of similar ability but variable size (ranging from 10% to >20% of the horse’s body weight) each rode six horses in random order. The researchers noted adverse effects on horses’ gaits and behavior with the heavy and very heavy riders.
In Study B (Christensen et al.) 20 riders with lead weights strapped to their torsos (so the horses were carrying up to 23% of their body weight) rode their usual horses. This research team saw no effects on the horses’ gaits or behavior.
Dyson attributed these differences to weight distribution and duration of riding time. “The total weight distribution of a live rider is more difficult to control than dead weight added to a rider,” she said. In Study B the ridden exercise lasted 5 minutes versus 30 minutes in Study A.
In a third study a single professional rider rode eight Icelandic horses at the tölt while wearing lead weights so the horses were carrying up to 35% of their body weight. “They showed no measurable changes in gait symmetry or in rhythm,” said Dyson. “However, there was an increase in duty factor (the proportion of stride time in the stance phase), and this was seen in all four limbs. In addition, there was decreased stride length and increased stride frequency.”
She described a fourth study of 148 sport and leisure horses (Dyson et al.) to emphasize the importance of rider position. In it, the researchers found a significant positive association between ridden horse pain ethogram scores and riders sitting on the caudal (back) third of the saddle compared with riders who sat in the middle of the saddle.
“We believe that the position of the rider in the saddle and force distribution is very important with respect to rider size,” Dyson said. “For optimal weight and force distribution, the rider should sit in the middle third of the saddle for their ideal balance and alignment with the horse’s center of gravity.”
Rider Size’s Effects on Equine Muscles
In Study A Dyson’s team measured horses’ thoracic dimensions before and after ridden exercise to understand the effects of rider size on the back muscles in that area.
“We know that in clinically normal horses with a well-fitting saddle, there is an increase in thoracic dimensions measured at 5 and 15 centimeters distal to the dorsal midline after 30 minutes of ridden flatwork,” she explained.
In her study thoracic dimensions increased after exercise with light and moderate-sized riders but decreased with heavy and very heavy riders.
“The failure of the muscles to expand with exercise has potentially adverse consequences for muscle development and function and may result in long-term muscle atrophy (wasting),” she said. “This effect may be compounded if the rider sits crookedly or is not in synchrony with the horse’s movement.”
In Study A the researchers also evaluated the presence or absence of epaxial muscle hypertonicity (tension) or muscle pain. Hypertonicity scores increased post-exercise for the moderate and heavy riders, and muscle pain scores increased for the very heavy rider, Dyson said.
Guidelines for Suitability of Horse and Rider Size
When assessing rider suitability based on size, too many variables exist to provide a categorical rider-horse weight ratio, Dyson said. These variables include:
- Rider skill, balance, fitness, and coordination;
- Saddle fit;
- Horse age, type, fitness, muscle development, and back length;
- The presence or absence of underlying musculoskeletal pain;
- The speed and duration of work; and
- The terrain over which the horse works.
Keeping all these considerations in mind, Dyson offered tips for assessing rider suitability and communicating it with the rider:
- Evaluate saddle fit for both horse and rider, “bearing in mind that the tree of the saddle should not extend beyond T18 and the panels of the saddle should not extend beyond L1,” she said. “The horse’s thoracolumbar length must be able to accommodate a saddle, which allows the rider to sit in the middle of the saddle.”
- Evaluate the horse’s epaxial muscle development, tone, and pain before and after exercise.
- “Understand the basic concepts of rider position, straightness, and balance, bearing in mind that a crooked rider will compound the problem by asymmetrical weight and force distribution,” Dyson said.
- Consider enlisting the help of another professional, an independent person who is honest about the rider’s vs. the horse’s size.
- Observe and understand the significance of the horse’s behavior (using the ridden horse pain ethogram) when tacked up, mounted, and ridden.
- Compare the quality of the horse’s gait while being longed without a rider and when ridden, and video this to show the rider, she said.
- Consider the horse’s step length, limb flight, suspension, step frequency, rhythm, duration of stance phase, and the presence or absence of lameness when ridden.
“I believe that knowledge of the scientific evidence supporting the potentially adverse effects of a rider provides a foundation on which to base discussion with the rider,” Dyson said. “Inform the rider about the potential effects on thoracolumbar muscle function during ridden exercise, epaxial muscle development or atrophy, and gait.”