Why do reining horses start to kick out when changing leads and what can be done to fix it?

The kick-out during a flying lead change — the horse throwing one or both hind legs outward rather than crossing them cleanly through the change — is one of the most common and most discussed problems in reining training, and it has multiple possible causes that need to be distinguished from each other before the appropriate correction is applied. A kick-out that comes from pain looks different and requires a different response than a kick-out that comes from training confusion, anticipation, or evasion. Hind limb or back pain is the cause that must be ruled out first, and it is the cause most commonly missed because kick-outs during lead changes are so frequently assumed to be a training or attitude problem. The flying lead change places significant demand on the horse's hind end — the hind legs must change their leading sequence in a single stride while the horse continues forward at speed, and this specific movement requires the hips, hocks, and stifles to engage in a rapid coordinated sequence that stresses those structures. A horse with hock pain, stifle soreness, or sacroiliac dysfunction will express that pain through exactly the kick-out pattern that trainers most commonly assume is a training deficit. If the kick-out has appeared recently in a horse that was previously changing cleanly, if it is worse on one lead than the other, or if it is accompanied by other signs of hind end discomfort, a veterinary evaluation is the appropriate first response rather than a training correction. Anticipation is the second major cause and the one most specific to the reining training environment. A horse drilled extensively on flying lead changes in the same locations in the arena, on the same size circles, at the same point in the circle's geometry, begins to prepare for the change before the rider's aid is applied — and the preparation includes the physical tensioning of the hind end that produces the kick-out when the change arrives at a slightly different moment than the horse expected. The fix for anticipation-driven kick-outs is variation — changing where in the circle the lead change is asked for, changing the circle size before asking, and frequently performing all of the preparatory aids without then asking for the change so the horse learns to wait for the specific aid rather than triggering on the pattern. Training pressure applied incorrectly at the moment of the change is a third cause. A spurring or leg pressure applied too aggressively or at the wrong moment of the stride produces the kick-out as a reactive escape response from the pressure rather than as a clean organized lead change. The hind leg that kicks out is responding to pressure it received at a moment when it was not in a position to step through cleanly. Reducing the intensity of the aid, improving the timing relative to the footfall sequence, and ensuring that the change is asked from genuine collection and balance rather than from an unbalanced rushing pace reduces the reactive kick-out that pressure-timing errors produce.