You’ve heard it since your first posting trot: “Rise and fall with the leg on the wall.” Being on the correct diagonal is all but a time-honored tradition of proper equestrianism on any curve or circle—you lift your body out of the saddle when your horse’s outside shoulder is forward and you sit through the second half of the stride, when his inside shoulder is forward.

But why?

“We don’t know,” said Marie Rhodin, PhD, Dipl. ACVSMR, associate professor at the Swedish University of Agricultural Sciences, in Uppsala. “We can’t find any history of where this ‘correct diagonal’ concept comes from, or when it began. But what we do know is, biomechanically, it makes sense!”

Rising on the outside shoulder on a circle or curve helps counterbalance the asymmetry created by the curve itself, Rhodin said. By contrast, rising on the inside shoulder—in the “wrong” diagonal—enhances the asymmetry, to the point of appearing lame. And if the horse really is lame, the wrong diagonal could really enhance that lameness (depending on the site of the pain) and the right diagonal could mask it.

“The rising trot can be a very useful tool for detecting lameness, and especially hind lameness, because it has a significant effect on the horse’s symmetry,” Rhodin said. “On the flip side, it can give a slight appearance of lameness in a sound horse because of the asymmetry created by the rider’s movement. So it’s really important to know the difference.”

In their study, Emma Persson-Sjodin, a PhD student under Rhodin’s supervision at the Swedish University of Agricultural Sciences, and colleagues used inertial sensors to study asymmetry in 26 riding horses. They put the horses under various tests at the trot, with or without a rider, and with the rider in various seat positions, including sitting, rising (or posting), and two-point (or half-seat).

They confirmed that seat position has a significant influence on a horse’s symmetry, Rhodin said, with the most remarkable finding being the asymmetry produced by the rising trot—which makes sense biomechanically, she said. When the rider pushes down in the stirrups to rise, that downward force goes against the force of the horse’s pelvic rise at that moment. In the trot, when the outside forelimb is extending forward (bringing the shoulder forward), the outside hindlimb is extending back (bringing the pelvis up). The rider’s weight in the stirrups prevents that pelvis from moving up as much as it normally would, creating an asymmetry.

In a straight line, a rising trot can make the horse slightly asymmetrical. “When I’d be working my own horse in an arena, I used to catch glimpses of the other horses and think, ‘Oh, that horse is lame,’ but then when I’d look up I’d see that actually the rider was just working in a rising trot,” Rhodin said. “It made me realize the importance of recognizing that phenomenon and studying it scientifically.”

Knowing how the rising trot and other seat positions affect a horse’s symmetry can help more accurately recognize and diagnose subtle lameness, said Rhodin. First, it can explain certain asymmetries, so that horse people can know that a horse’s apparent lameness might just be related to seat position and not actual pain. And secondly, the rising trot can be very useful in a lameness exam, she said. Knowing how the seat position affects the symmetry can lead to useful tests on circles in both directions, on the right and wrong diagonals, to see if the rising trot improves asymmetry or makes it worse.

Other rider-related factors, especially rein tension, can also affect symmetry, Rhodin added. Tighter reins can mask appearances of lameness—which is why their study focused on comparing different seating styles combined with different levels of rein tension.

“We need to be aware that the rider influences the horse so that we’re not mistakenly thinking the horse is lame when he isn’t,” she said. “But we also showed that many of these slight asymmetries can be difficult to see, especially regarding hind-limb lameness. But by putting the horse on a circle and adding the effect of a rider, we could see increase asymmetry, and by comparing left and right directions we could see an even bigger difference. So I think it can help to detect early hind-limb lameness.

“We also saw different effects on horses with either impact or push-off hind lameness and rising trot may be useful to identify the type of lameness during subjective evaluation,” she said. “A push off lameness would be expected to increase when the rider sits down during the lame hind-limb stance whereas an impact lameness would be expected to decrease.”

As an added bonus, their study also resolves the mystery of why we ride the outside diagonal, she said.

“The asymmetry isn’t just measurable; you can actually feel it,” Rhodin said. “I think we know now why it seems so uncomfortable when you sit the wrong one.”

The study, “Influence of seating styles on head and pelvic vertical movement symmetry in horses ridden at trot,” was posted in PLoS One.