The function of the forelimb is fundamental to understanding both sound and pathological locomotion. The precise movements of the equine shoulder are hidden by layers of skin and muscle and hence the shoulder is normally modelled as a simple pivot during locomotion which assumes that any translational motion is negligible.To record and quantify the sliding motion of the scapula during locomotion, using a novel imaging technique.Scapula motion during locomotion in the horse was calculated by tracking the ripple of the shoulder blade's movement under an array of markers placed over the soft tissue.Interstride variability was low. Sliding of up to 80 mm in the plane of progression (cranio-caudal) was observed; however, the limits of motion varied by5 mm in the gaits examined, despite variations in stride length. Stride length appeared to be increased by scapula rotation in the plane of progression, and this flexion-extension was largest in trot and was not significantly different between walk and canter. This was in agreement with the distance travelled by the trunk whilst the hoof was on the ground. Substantial sliding in a dorsal-ventral direction was shown and varied with the gait used, both in magnitude and timing, possibly providing a shock absorption mechanism. The sliding did not increase as much as would be expected in canter and this coincided with a more lateral positioning of the scapula and increased impact on the ribcage.It has been assumed that scapula-thoracic sliding increases stride length and hence economically increases locomotor speed. The extra motion of the scapula recorded appeared to absorb shock from forelimb impact and maintain the economy of locomotion, but did not increase with speed and the muscular pretensioning implied could actually impair ventilation.
