While one must approach the subject of rider size delicately, rider/horse suitability is a topic that veterinarians must occasionally address. Horse/rider mismatch due to a large rider size might compromise gait as well as thoracolumbar muscle function and development. It can also increase the risk of musculoskeletal injury in the horse.
“When we refer to rider size, this term incorporates not only weight but also morphology (shape) of rider, as well as height, including trunk and limb length,” advised Sue Dyson, MA, VetMB, PhD. As there are many horse and rider variables involved, no specific guidelines currently exist regarding the suitability of rider size. Skill, balance, coordination, fitness and saddle fit all play a role. For the horse you must look at age, fitness, muscle development, length of thoracolumbar region, musculoskeletal pain, type of horse, speed/duration of work, and terrain over which he will be asked to work. All of these play a role in determining rider/horse suitability.
Studies Assessing Rider Size
Dyson reviewed the results of two studies assessing rider size. Study A, performed in the UK by Dyson’s group, included four experienced riders with similar abilities. They ranged from 10% to >20% of the horses’ body weights. They each rode six non-lame horses in a cross-over design. The study evaluated gait subjectively and objectively. It assessed behavior using the ridden horse pain ethogram.
In Study B, performed in Denmark, 20 horses were ridden by the “normal” rider. There was added weight strapped to the rider so that the total maximum weight ranged from 15% to 23% of each horse’s body weight.
For the heavy and very heavy riders in study A, adverse effects of gaits and behavior were appreciated. There were no effects noted in study B.
In Study A, the horses were ridden for 30 minutes. But if the horse became lame during the ride, the test was terminated. This happened for all tests for the heavy and very heavy riders. As a result, mean test duration for the very heavy rider was only 8.3 min (range six to 19 min). The mean test duration for the heavy rider was only 16.6 min (range nine to 25 min).
In Study B, the horses were only ridden for five minutes.
Differences in Two Studies About Rider/Horse Suitability
So, why were there differences in the results of the two studies?
- A large, live rider will have a different effect than dead weight added to a rider; the distribution of forces through the saddle will vary.
- There were large differences in the durations of ridden exercise between the two studies, despite some rides in study A being cut short due to tran- sient lameness.
- The spectrum of behaviors observed was less well described in study B.
- There was a high frequency of behaviors (such as head tossing and tail swishing) present at baseline with the normal rider in Study B, suggesting that these horses were already uncomfortable without the extra lead weight. This could conceal the effects of added weight.
No effect on salivary cortisol or heart rate variables—conventional measures of stress—were appreciated in either study A or B. However, in Study A they observed spontaneous blink rate (a measure of stress) did increase after exercise with the heavy rider.
“When video recordings of the riding sessions were reviewed and the RHpE applied, there was a significantly higher RHpE score for the heavy and very heavy riders in Study A,” said Dyson. “In fact, a linear positive correlation between the ratio of rider weight to horse weight and RHpE score existed.”
Icelandic Horse Study
A third study was also reviewed in which eight Icelandic horses were ridden by a single rider. Weight was added to achieve 20%, 25%, 30% and 35% of the horse’s body weight. While no measurable changes in gait symmetry or rhythm were appreciated, there was an increase in “duty factor.”
“Duty factor is the proportion of stride time in the stance phase,” explained Dyson. “In addition, there was decreased stride length and increased stride frequency seen in all four limbs of the Icelandic horses.”
Dyson showed some examples of an increase in duty factor for sporthorses ridden by large riders, both at the trot and the canter. Because of the increased time in the stance phase there was always one limb on the ground when in a trot—or all four limbs on the ground simultaneously during a canter.
Role of Position in Rider/Horse Suitability
Rather than simply focusing on rider size, Dyson said that position of the rider is also important. For optimal rider balance and alignment with the horse’s center of gravity, the rider should sit in the middle one-third of the saddle. The larger riders in study A sat on the caudal aspect of the saddle. As a result, the normal increase in thoracic dimensions that occurs when horses work correctly did not happen. In fact, the dimensions decreased with the heavier riders, which might have adverse consequences for long-term muscle development.
“It is inappropriate to give a categorical rider/horse weight ratio because there are too many variables,” advised Dyson. “It is also inappropriate to increase the horse’s body weight to reduce the rider/horse weight ratio. If we increase the horse’s weight, then the increased load puts him at risk of other problems, including musculoskeletal problems.”
She added: “Overall, I believe the veterinary profession has a moral responsibility to advise riders if they are too big for the horse. To fail to do so would jeopardize the horse’s short- and long-term welfare.”