Rider weight is less important than skill when it comes to protecting your horse’s spine.
If we were sensible, we would be riding cows, not horses. Cattle have an awesome vertebral shape that interlocks for super support. But horses have been our mount of choice for at least 5,000 years. So when you climb on the back of your horse, what impact does the added rider weight have on him?
Understanding equine spinal support
Let’s break it down two ways — standing or moving, or (in biomechanics-speak) “static” or “dynamic”. A standing horse is already supporting a lot of weight — his own! Most of a horse’s body weight is in the trunk and head/neck complex — the legs weigh comparatively little. Unlike the human spinal “column”, the horse’s back is like a horizontal beam. Spinal support comes from the interaction of spinal vertebrae, ligaments and muscles, but also from their liquid gut contents kept pressurized by the abdominal muscles — this is called hydrostatic stabilization. Think of how a pontoon boat is rigid, even though it’s essentially a balloon. The large torso of the horse is suspended on relatively spindly legs, and stays firm, not saggy, by using spinal and rib structure, ligament tension, and hydrostatic support (see Figure 1). So far, so good.
Adding rider weight and movement
So what happens when we add a rider? In the very simplest terms, we are adding a load to the system. Let’s say an average horse weighs 1,100 pounds, and an average rider plus tack weighs 175 pounds. You have just added 16% more weight to the horse’s load. The good news is that the equine center of mass (COM – see sidebar) is located right below the saddle, so in theory, adding weight will not skew the horse’s balance…as long as you stay upright in the saddle.
What exactly does it mean to support body weight? It means to stay upright against gravity. Gravity is the attraction of one body to another. Because the Earth is so much larger than our bodies, gravity seems like a force that makes us fall down. We stiffen our legs to stand up and move our bodies, and with each step we have an interaction with gravity. Weight = body mass + gravity. So when we add a rider, we are adding mass, which means the horse’s back needs to work harder to push back and not fall down.
But it’s not that simple. The rider is not a sack of potatoes. A “dead” weight increases the load on the system and can put more stresses on the bones and muscles that are resisting gravity. But a living weight brings in a whole new dynamic. Imagine a 30-pound toddler nestling up close to you and clinging tight; now imagine that child having a tantrum and trying to fling himself every which way. That same 30 pounds will feel like a wildly different load to carry. In mechanical terms, the wild movements of the screaming tot are changing the COM of the parent-child unit, and the adult doing the carrying needs to dynamically calibrate those changes to stay upright.
Now extend that analogy to a horse carrying a rider. The horse has to not only balance his own weight plus the rider’s, but also accommodate the rider’s sometimes unpredictable movement. An inexperienced rider, though not intentionally, may be like the flailing toddler from the horse’s perspective: an irrational load who doesn’t know where her body is in space. Most horses make the conscious choice to protect the rider at cost to their own equilibrium, which is hard work. That is why school horses are worth their weight in gold!
Becoming a centaur
By contrast, an expert rider adapts her movements to the horse’s. Indeed, a cooperative living weight can even feel lighter than a “dead” weight. For equestrian sports, the ideal is that the horse and rider be one. One way skilled riders become “centaurs” (one with the horse) is by intuitively matching their body parts to the horse’s movements. Look at Figure 2 — see how the rider’s arms and legs are parallel to the fore and hind legs of the horse?
Another way to look at it is that rider weight is not only the single greatest force keeping you in the saddle, but also the single greatest tool for communicating with the horse. The classical aids of rein and legwork allow you to direct the rider-horse dyad: the weight directs, and legs and hands specify where and how.
Can a rider be too heavy for a horse?
Absolutely. Common sense is required. Rider weight increases the vertical ground reaction forces that load the musculoskeletal system: the bones, tendons, ligaments and muscles. These body parts, in general, have a generous safety margin before breakage. However, excessive loading by inappropriate rider size can make traumatic injury worse through those increased forces. When it comes down to it, carrying a human-sized load for the typical hour a day most people are in the saddle is a very intermittent stimulation, so is unlikely to result in much musculoskeletal adaptation. Back in the “bad” old days, when horses pulled carts or carriages all day long, their size was chosen for the load, and their long hours of work made them strong for that particular exercise. So, unless your sport is endurance or competitive trail riding, the influence of rider weight on your horse’s overall athletic condition is negligible.
The bottom line is this: to keep your horse happy and healthy, learn to be a centaur!