The rider's posture and muscle tension communicate to the horse under saddle through a mechanism that is direct, continuous, and impossible to turn off: the horse's back and the rider's seat are in constant physical contact, and the horse's musculature responds to and mirrors the tension patterns it feels through that contact. A rider who carries tension in specific muscle groups — most commonly the lower back, thighs, seat, and shoulders — transmits that tension directly to the horse's back, which produces corresponding tension in the horse's musculature and movement.
The most impactful tension pattern is in the rider's lower back. A rider with a braced, stiff lower back prevents the horse's back from swinging freely through its stride — the horse must move against a rigid, non-following surface rather than carrying a relaxed, following seat. This produces a choppy, restricted stride in the horse even when the horse itself has no physical issue causing restriction. Conversely, a rider with a supple, following lower back that moves with the horse's motion allows and even encourages the horse's back to swing, which produces looser, more fluid movement.
Thigh and seat tension are similarly impactful. Gripped thighs push the rider's seat up and out of the saddle, reducing the rider's ability to feel and follow the horse's movement and transmitting a bracing signal through the saddle. Soft thighs that wrap around the horse without gripping allow the seat to sink into the saddle and follow the movement, which produces a feeling of connection and ease that most horses find more comfortable and move more freely under.
Mary Wanless's work on rider biomechanics is the most systematic treatment of these relationships — her analysis of how specific rider tension patterns produce specific horse movement patterns provides both the diagnostic framework and the corrective exercises for riders who want to improve their communication through posture and muscle tone.