Is posting easier on a horse’s back?

Researchers explain the force exerted by a rider on the horse's back during the trot.
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Q:In “Old Masters” (EQUUS 441), a rider “rode standing in the stirrups to ease stresses on his horse’s back.” I have also heard other riders say they post or stand in the stirrups to take weight off their horses’ backs.

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As someone who’s studied engineering and physics, I wonder if standing really benefits the horse. The stirrups are connected to the saddle and, apparently, the saddle still distributes a rider’s weight onto the same area of a horse’s back regardless of whether the rider is sitting, standing or posting. Sounds to me like standing or posting is for the comfort of the rider and not the horse! Have any studies or experiments borne out this thought? (I enjoy relaxing on my Quarter Horse, who is very comfortable at various trotting speeds, with or without a saddle.)
Carl Stephanus
Berryville, Virginia

A: This is a great question and one that researchers have been studying using pressure mats beneath the saddle and mathematical modeling. Saddle pressure mats measure the force applied to the horse’s back and the pressure distribution pattern. Using pressure mats, researchers have shown that the force exerted by the rider on the horse’s back changes at different stages of the stride; in other words, it increases and decreases in rhythm with the horse’s movements.

At the trot, for example, the horse’s body descends when the diagonal limbs are grounded then rises as the horse pushes off into the suspension phases. When sitting the trot, the rider follows the horse’s movements---the rider sinks into the saddle during the diagonal stance, and the force on the horse’s back increases. As the horse’s body starts to rise, the rider feels a push from the saddle and the force on the horse’s back is at its highest. During the suspension phase the force decreases. So the force on the horse’s back increases and decreases in a regular pattern.

When the rider stands in the stirrups, the hip, knee and ankle joints flex as the horse’s body rises in the suspension phases, which compresses the rider’s leg length. Then these joints extend to lengthen the rider’s legs as the horse descends in the diagonal stance phase. As a result of these changes in leg length, the rider undergoes less vertical motion than the horse. Thus the rider glides along at a fairly constant height above the ground while the horse is bouncing up and down. Saddle pressure recordings show that the peaks in the force curve are smoothed out---the minimal values are higher and the maximal values are lower.

In a rising (posting) trot, the feet push down against the stirrups to provide the force that raises the rider out of the saddle. This results in an increased force on the horse’s back toward the end of the sitting phase. Therefore, the heights of the peaks and troughs become asymmetrical on the left and right diagonals. The peak force is higher for the sitting diagonal than for the rising diagonal, and both diagonals have lower peak forces than at the sitting trot, as shown by the red line on the graph.

So the answer to the question of whether the force or weight on the horse’s back differs between the three options for riding the trot is “yes and no.” The total force of the rider’s weight on the horse’s back over the entire stride is the same for the three riding techniques. In mathematical terms, the integration of the force curves for sitting trot, posting trot and standing in the stirrups would yield the same value.

However, the force is distributed differently over time for the three techniques---the sitting trot has the highest force peaks. We assume that when the rider posts or stands in the stirrups, the lower force peaks are “easier” on the horse’s back.

Hilary Clayton, BVMS, PhD,
DACVSMR, MRCVS
Sport Horse Science
Mason, Michigan

This Month's Expert: As president of Sport Horse Science, Hilary Clayton, BVMS, PhD, DACVSMR, MRCVS, is a consultant, author and speaker with the goal of educating people about the “biomechanical and physiological basis of equine performance and the interaction between the rider, the tack and the horse.” Previously she was Mary Anne McPhail Dressage Chair in Equine Sports Medicine at Michigan State University, where her research focused on equine locomotion and biomechanics. She received her veterinary degree and doctorate from the University of Glasgow in Scotland. In her spare time, she trains her horses and competes in dressage.

This article first appeared in EQUUS issue #459, December 2015.