Although stirrups may be considered an essential part of equestrian equipment, there is little research describing their use and function. The aim of the present study was to compare stirrup lengths chosen for flatwork by novice and experienced riders and to measure the associated leg position and knee angles. Ten novice and ten experienced riders, with kinematic markers attached to their greater trochanter, lateral femoral epicondyle, and lateral fibular malleolus, mounted three horses and a mechanical horse. The riders selected an appropriate stirrup length for flatwork by adjusting the unnumbered stirrup leathers. Stirrup length was measured and expressed as a percentage of the rider’s leg length measured from the greater trochanter to the floor when standing. Lateral photographs were taken from both sides with the riders mounted on each horse in a standing position. The kinematic markers were digitised to measure knee angle and ankle position relative to the hip in the anteroposterior direction. Within riders, there was no significant difference in stirrup length between the three live horses or between the mechanical horse and live horse. Experienced riders consistently selected a significantly longer stirrup length as a percentage of their leg length compared with novice riders (combined data for live horses and mechanical horse; P=0.005). Experienced riders demonstrated a significantly larger knee angle (combined data for live horses and mechanical horse) compared with novice riders (118±8° and 109±7°, respectively; P=0.016). Novice riders had a significantly larger knee angle on the mechanical horse compared with the live horse (115±9° versus 107±9°, respectively; P=0.003). The relatively longer stirrup length selected by experienced riders is thought to reflect the development of an independent seat, which implies the ability to move the legs independently of the pelvis. The chair seat adopted by novice riders on the mechanical horse could be considered counter to improving their equitation skills.