The effect of an abrupt change in functional surface properties on equine kinematics and neuromuscular activity

Abstract

Arena surfaces used for training and competition are influenced by factors such as weather and maintenance, which can lead to spatial variations in functional surface properties. The ability of the horse to adapt to such changes may have implications for injury prevention. The aim of the PhD was to quantify kinematic and neuromuscular responses of horses to a camouflaged abrupt change in functional surface properties.
Horses (n=7) were trotted in hand at a consistent speed across an arena surface that had been prepared in four ways: continuous firm; continuous soft and when the surface presented a camouflaged, abrupt change from firm to soft and soft to firm. Kinematic data (232Hz) synchronised with surface electromyography (sEMG) (1926Hz) from selected forelimb muscles were recorded. The first trial (no awareness of change) was categorised separately to the subsequent trials (2-8; aware of change). A General Linear Model was used to assess the effect of horse, stride location and awareness on kinematics and sEMG.
There were limited stride to stride changes on the continuous surfaces. When travelling from firm to soft, fore F (2, 125) = 11.55, P <0.0001 and hind F (2, 116) = 12.47, P <0.0001 limb retraction significantly reduced as the horses stepped onto the soft surface. Awareness of the abrupt change also significantly reduced fore F (1, 125) = 7.28, P =0.008 and hind F (1, 116) = 10.16, P =0.002 limb retraction. When travelling from soft to firm, hindlimb stance duration F (1, 99) = 7.3, P =0.008 and duty factor F (1, 61) = 7.82, P =0.007 significantly increased and peak metacarpophalangeal extension significantly F (1, 93) = 7.85, P =0.006 reduced as the horses stepped onto the firm surface. Awareness of the abrupt change significantly increased stance duration F (1, 99) = 14.92, P <0.0001, duty factor F (1, 61) = 8.18, P =0.006 and peak metacarpophalangeal extension F (1, 93) = 3.98, P =0.049. There was some evidence of neuromuscular contributions that helped to stabilise the forelimb and control posture immediately before hoof impact and during stance.
The gait modifications observed demonstrated horses can alter their balancing strategy to cope with a change in surface condition. Reduced limb retraction shifted the COM position relative to the hoof position at lift off more caudal and reduced a falling forward posture as the horses stepped down onto the soft surface and with awareness. When the horses travelled from soft to firm, vertical impulses increased in the hindlimb, which was thought to maintain pitch stability. Vertical impulses showed a more even distribution between the fore and hind limbs with awareness suggesting the fore limbs played a larger role raising the forehand.