Horses buck as a natural defensive reflex, and the behavior can be triggered intentionally or unintentionally through pressure on specific areas of the body, pain responses, or even diet. Understanding what causes bucking is useful whether you’re involved in rodeo, trying to diagnose why your horse suddenly started bucking under saddle, or simply curious about equine biomechanics.
The Basic Mechanics of a Buck
A buck starts with a neural signal. When something stimulates the sensitive hair follicles and nerve endings along a horse’s flank or lower back, the signal travels to the lumbar spinal cord and activates the large extensor muscles in the hindquarters. The result is an explosive hind-leg extension, often combined with a lowered head and arched back. This is hardwired survival behavior: in the wild, a horse that could throw a predator off its back lived longer. Every domestic horse still carries that reflex.
How Rodeo Flank Straps Work
In professional rodeo, bucking is encouraged through a flank strap, a wide band placed loosely around the horse’s flank area (roughly equivalent to the human waist). The strap doesn’t cause pain. It creates a mild, persistent tactile sensation, something like a loose belt that itches, which triggers the horse’s natural urge to kick out behind itself. The pressure is detected by flank hair follicles, and the resulting neural signal fires up the same spinal reflex that produces the classic rodeo buck.
The strap has to be adjusted carefully. PRCA rules require flank straps to be lined with fleece or neoprene, and they’re tightened only enough to encourage kicking. Overtightening actually causes the opposite of what you’d expect: a horse that’s squeezed too hard will freeze and refuse to move at all. The straps also sit well forward of the genitals and don’t contact them. When the ride ends, the strap is released immediately, and most horses stop bucking within a stride or two, which tells you the behavior is tied directly to the sensation rather than to pain or panic.
The forces involved are significant. Research at the University of Texas Medical Branch measured accelerations during rough stock events and found that a bareback rider experienced peak forces of roughly 46 g’s, far greater than what fighter pilots endure. Even bull riders recorded 26 g’s during dismount. These numbers reflect just how much power a bucking horse generates through that hindquarter extension.
Pressure Points That Trigger Bucking
A horse’s back has four to six reflex points running along each side of the spine within the saddle support area. These points are directly connected to the nervous system, and pressure on them can produce involuntary movement responses. The one most relevant to bucking is located over the connective tissue (fascia) behind the 18th lumbar vertebra, near the back of the saddle area. When something presses on this spot, the horse reflexively tries to throw off whatever is causing the pressure.
This is why an ill-fitting saddle is one of the most common accidental causes of bucking. A saddle that’s too long will sit back over that rear reflex zone and press into the fascia over the transverse processes with every stride. A saddle with a tree angle that’s too wide, a gullet channel that’s too narrow, or a tree that twists during movement can also trigger defensive responses. The horse drops its back, locks its shoulders, rotates its pelvis, and if the pressure is bad enough, bucks to escape it.
In the wither area, a pinching gullet plate can activate a cranial nerve that controls the upper arm and shoulder blade muscles, blocking forward movement entirely. A horse that can’t go forward and is simultaneously being pressured to move has limited options, and bucking is one of them.
Pain-Related Bucking
If a previously calm horse starts bucking, the first thing to investigate is pain. One of the most well-documented causes is a condition called kissing spines, where the bony projections on top of the vertebrae grow close enough to touch or overlap. This most commonly occurs between thoracic vertebrae 13 and 18, with T15 being the most frequently affected. That’s directly under the saddle and the rider’s seat.
Horses with kissing spines often show a constellation of behaviors beyond bucking: hypersensitivity when being brushed, resistance to girth tightening, head tossing, kicking out, hollowing the back, cross-cantering, and refusing or rushing fences. The bucking isn’t defiance. It’s the horse trying to escape pressure on an area that genuinely hurts. The condition can affect the lumbar vertebrae too, though that’s less common.
Diet and Reactive Behavior
What a horse eats can meaningfully change how likely it is to buck. Horses fed high-starch, grain-based concentrates show measurably higher reactivity compared to horses on high-fiber diets. The mechanism appears to involve the blood sugar spike that follows a starchy meal: the high glycemic response creates what riders often call a “fizzy” or hot temperament, with more nervous and excitable behavior overall.
Research using crossover designs (the same horses tested on both diets) found that animals on starch-rich feed had higher heart-rate responses during handling, were less settled, and showed more agitation-related movement. Ponies appear especially sensitive to this effect. Beyond behavior, high-starch diets also alter the microbial balance in the hindgut and raise the risk of colic, laminitis, and metabolic disorders, so the behavioral changes may partly reflect physical discomfort as well.
If you’re feeding a horse significant amounts of grain and noticing reactive or explosive behavior under saddle, switching to a lower-starch, higher-fiber feed is one of the simplest changes you can make. Many owners report a noticeable difference in temperament within a few weeks.
Putting It Together
In a rodeo context, bucking is produced by combining a horse’s natural athletic tendency to buck with the mild tactile stimulus of a properly fitted flank strap. Stock contractors selectively breed for horses that buck enthusiastically, so the strap is amplifying an existing instinct rather than creating one from scratch.
Outside of rodeo, bucking is almost always a signal that something is wrong. A saddle pressing on rear reflex points, spinal pain from kissing spines or muscle soreness, or a diet that’s pushing the horse into a reactive state can all produce the same explosive hind-leg extension. The fix depends on the cause: a saddle fitting evaluation, a veterinary exam of the back, or a dietary adjustment. Horses that buck under saddle without an obvious trigger deserve a thorough physical workup before anyone assumes it’s a training problem.