Seems like a simple question, right? Unfortunately, there is no ‘one-size fits all’ answer. Unlike a ladder, horses don’t come with load limit instructions. No scientific studies have been completed under real-life conditions to determine a formula for what is - or isn’t - too heavy for a horse to comfortably carry.
Research is now underway in Newmarket, England. Led by distinguished veterinarian Dr. Sue Dyson, head of clinical orthopaedics at the Animal Health Trust for Equine Studies, the study aims to provide evidence-based guidelines for the equine industry as to what constitutes excessive rider size, and how this may change under different circumstances.
Pilot studies have already demonstrated that a high rider-to-horse body weight ratio can induce temporary lameness. Chronic back pain and lameness, together with the horse negatively associating being ridden with pain, are foreseeable long-term outcomes.
When they asked local horsemen, the consensus seems to reflect the longstanding unofficial rule of thumb; a horse should carry no more than 20% of its body weight, including both rider and tack. This, however, does not mean that an overweight horse can automatically carry a heavier rider; nor that a heavier person should not ride - rather, there are ‘horses for courses.’
The average height and weight of humans are on the increase with two-thirds of Australian adults classified as overweight or obese. ‘Too heavy’ for a given horse does not necessarily mean a rider is obese, the converse is also true in that a rider can be obese without being unduly heavy.
The fact remains the load and how this weight is distributed affects the horse. Dr. Dyson’s research is not about finding the ‘correct’ size or weight a rider should be; rather investigating the effects of different rider-to-horse weight ratios.
This may be influenced by many factors, an obvious one being height. The height of a rider affects the rider’s position in the saddle, previous experiments have shown that any adverse influence of poorly fitting tack was markedly accentuated by heavier riders.
Extra weight can also alter posture which in turn results in imbalance. Consider carrying a toddler, both when they are awake and when they are asleep. This will give you a feel for how a horse may react when carrying a rider who holds themselves up versus an unbalanced rider. Immediate changes in the way a horse moves can be seen when ridden sequentially by riders with different weight distribution.
So, size alone does not dictate the weight burden a horse can tolerate. There are many other factors to consider, including the age, breed, conformation, fitness, strength, the injury history of the horse, the duration and intensity of work, and the terrain on which the work takes place. As suggested above, rider ability, coordination, balance and the fit of the saddle also come into play.
We think the determination of optimal body-weight ratios could be a welcome guide in assisting the selection of suitable rider-horse partnerships. If such data became available and was embraced throughout the equine industry, unnecessary pain or injury sustained as a direct result of rider-mount mismatch would be substantially minimised.