Equestrian Rider Core Stability: An IMU Test for Pelvis and Trunk Sway Against Horse Movement

A dressage instructor watches a student post a lovely trot, changing diagonals cleanly on every circle, then asks her to sit the trot for two straight minutes. By the ninety-second mark her shoulders are bouncing a half-beat behind her hips, her lower back has locked into a brace, and the horse has started to hollow away from the contact instead of stepping under. That gap — between how a seat looks when the rider is doing half the work and how the trunk behaves once the horse's back does all of it — is what a seat-mounted IMU test is built to expose, and most lessons never quantify it until the horse starts telling on the rider.

Münz, Eckardt, and Witte (2014) put inertial sensors on both horse and rider and tracked how tightly the two moved together across a range of dressage gaits. Professional riders showed substantially more consistent coupling at sitting trot than the amateur group, a gap that narrowed sharply at canter and collected work, where even skilled riders show more independent trunk movement by design.

This piece lays out a two-sensor protocol for quantifying that coupling and the core's contribution to it: sensor placement, a gait-by-gait test sequence, skill-tier norms, and what Peham et al.'s (2010) work on seat position and back force found comparing sitting trot, rising trot, and a lighter seat.

The most common correction a green rider gets for a bouncy sitting trot is some version of sit deeper or use your core, and the rider usually responds by tightening the lower back and gripping with the thighs. Watched from the rail, the upper body looks quieter for a stride or two. What an IMU on the pelvis and trunk shows instead is that the bounce has not gone anywhere — it has been converted from visible displacement into a held isometric brace, and that brace is exactly what stops the pelvis from rotating freely, which is the actual job of a following seat.

A genuinely stable core does not resist the horse's motion; it filters it. The pelvis needs to rotate through a large excursion at every stride of sitting trot, staying glued to a saddle moving through that same arc, while the trunk above it stays comparatively still, the lumbar spine and deep abdominal wall absorbing the difference. A rider who braces shows the opposite pattern: low pelvis rotation, because the hips have locked against the saddle, and higher trunk motion, because whatever the brace fails to absorb gets passed up the spine. A rider who is simply loose and untrained shows low recruitment in both segments, the trunk swaying almost as much as the pelvis, just out of phase. Both can look calm from the ground; both show up clearly, and differently, on two sensors.

This protocol tracks two linked metrics. Pelvis-Horse Coupling (PHCC) is the cross-correlation between the horse's trunk vertical acceleration, from a girth-mounted IMU, and the rider's pelvis vertical acceleration, from a sacral-mounted IMU, computed stride by stride and averaged. A value near 1.0 means the pelvis is tracking the horse's back closely and on time; a value well below that means the seat is lagging the motion or fighting it. Trunk-Pelvis Dissociation Index (TPDI) is the root-mean-square angular velocity difference, in degrees per second, between a sternum-level IMU and the pelvis IMU. Low TPDI at sitting trot is the number version of a quiet, tall upper body — what an instructor is actually looking at when praising a still seat, expressed as a rate rather than an impression.

The two numbers have to be read together, since either alone is misleading. High PHCC paired with high TPDI usually means the rider is bracing hard enough to move with the horse but transmitting that effort up through a rigid spine rather than absorbing it — acceptable at the walk, failing as tempo and impulsion increase. Low PHCC paired with low TPDI often shows up in riders who are stiff through the hip rather than the back: the pelvis is not following the horse at all, and the trunk stays still mostly because the rider has stopped moving with the gait rather than because the core is doing controlled work.

Mount one IMU on the horse at the girth or just behind the withers, reading dorsoventral movement of the horse's trunk. Mount a second on the rider at the sacrum, on a snug belt or seat pad, and a third on a chest strap at roughly sternum height for the trunk signal — two rider sensors are worth the extra strap, since PHCC alone cannot separate a following pelvis from a stiff one without a trunk reference.

Calibration Sequence

1. Standing-halt reference (10 seconds): horse and rider both stationary and square, establishing the zero-motion baseline.
2. Timestamp sync check: tap or jog each sensor and confirm the spike appears within 0.02 seconds across all three streams — a drifted sync makes PHCC meaningless.
3. Walk familiarization lap: one full lap each rein, unrecorded, so the rider settles into the tack before data collection starts.
4. Neutral seat calibration: 15 seconds of sitting trot on a straight line, confirming sensor placement has not shifted, not analyzed for the benchmark comparison.

Run the test on the rider's regular schoolmaster or a horse the rider already knows well — Münz et al.'s coupling data came from established dressage pairs, and a green or unfamiliar horse adds its own variability into what the test is trying to isolate in the rider.

Protocol Steps

1. Sitting trot, straight line: 60 seconds on a long side, both reins, recording PHCC and TPDI continuously — the primary trial, since a straight line removes bend and turning as confounds.
2. Sitting trot, 20-meter circle: 60 seconds each rein, compared against the straight-line values — a meaningful drop on one rein flags an asymmetry worth addressing before it shows up as a training issue rather than a fitness one.
3. Canter, both leads: 45 seconds per lead, logged separately, since canter's three-beat rhythm and greater vertical excursion produce a different expected PHCC range than trot.
4. Rising trot control trial: 45 seconds, posting normally — not scored against the sitting-trot norms, but a sanity check that sensors and rider are both behaving normally before trusting the harder sitting-trot data.
5. Optional no-stirrup sitting trot: 30 seconds, advanced riders only, removing the leg's ability to compensate for the core.

A rider's first session in an unfamiliar saddle tends to run 10-15% worse on both metrics than the same rider's numbers a week later in their own tack — worth noting before reading a low first-session score as a stability problem.

These bands come from pooling field sessions across skill tiers rather than a single validated laboratory cutoff, and they exist to give an instructor a quick read on where a rider sits before tracking movement across a season.

Canter's own bands sit lower than sitting trot's at every tier, which is expected rather than a red flag — the three-beat gait carries more vertical excursion and a rounder arc through the back, so even elite riders show somewhat looser coupling there than they do at sitting trot. Comparing a rider's canter numbers against the sitting-trot table would flag a false problem.

Münz, Eckardt, and Witte (2014) found that coordination between horse and rider trunk movement was significantly tighter, and far less variable from stride to stride, in professional dressage riders than in amateurs at sitting trot. The effect was clearest at sitting trot and largely disappeared at canter and in collected movements, where the horse's own back mechanics change enough that even top riders show more independent trunk motion by design.

Peham et al. (2010) measured force on the horse's back and seat stability across sitting trot, rising trot, and a lighter forward seat, finding that sitting trot produced the highest and least stable forces on the horse's back of the three positions, while the lighter seat reduced both peak force and its variability — evidence that the position asking most of the rider's independent seat is also the one most likely to load the horse unevenly when that seat is not yet stable.

Two limitations are worth holding onto. Münz et al.'s comparison was cross-sectional across established pairs rather than a training study, so it shows where professionals and amateurs differ, not that closing the coupling gap alone produces a professional-level seat. Peham et al. tested a small number of pairs at trot only, using force plates under the saddle rather than direct rider-side kinematics, so their figures describe what the horse's back experienced more directly than what the rider's core was doing to produce it. Treat both as directional evidence supporting the protocol's logic, not as fixed thresholds for an individual rider.

Run this as a recurring checkpoint every four to six weeks rather than a one-time diagnostic — a single session shows where a rider sits today, but the trend across a season tracks whether off-horse core work is transferring into the tack.

• Early stage: establish PHCC and TPDI at walk and sitting trot on a straight line only, before adding circles or canter, for a clean baseline.
• Build phase: add both-rein circle work and canter trials every four to six weeks, watching whether TPDI on the weaker rein closes the gap toward the stronger rein.
• Pre-competition: run the no-stirrup sitting trot variant to confirm the seat holds up without the leg compensating, since a judge at the rail will not see the difference but a green horse will.
• Off-season: pair low-PHCC riders with hip and pelvis mobility work, and high-TPDI riders with breathing and thoracic mobility work — the two patterns need close to opposite interventions.

Key References

• Münz, A., Eckardt, F., & Witte, K. (2014). Horse-rider interaction in dressage riding. Human Movement Science, 33, 227-237.
• Peham, C., Kotschwar, A. B., Borkenhagen, B., Kuhnke, S., Molsner, J., & Baltacis, A. (2010). A comparison of forces acting on the horse's back and the stability of the rider's seat in different positions at the trot. The Veterinary Journal, 184(1), 56-59.