Differential rotational and translational movements along the thoraco-lumbo-sacral spine of load and non-load pulling military and police horses walking and trotting on hard and soft surfaces.

To improve detection of equine back-related disorders, it is important to establish how movement dynamics in horses' thoraco-lumbo-sacral spine are normally affected by intrinsic and extrinsic factors. This cross-sectional study aimed to quantify how discipline, gait, and surface impact differential rotational and translational movements along the backs of 107 horses from the military and police forces. The horses were walked and trotted in-hand in a straight-line over hard and soft surfaces. During the exercises, nine inertial sensors (XSens MTw) were mounted to the poll, withers, thoracic vertebrae-13 (T13) and 18 (T18), lumbar vertebra-3 (L3), sacrum (between tubera sacrale), left and right tubera coxae, and caudal sacrum ('tail'). Linear mixed models assessed whether horse discipline (load pulling or non-load pulling), surface (soft or hard), age, or stride time influenced rotational and translational range of motion differences between adjacent sensors at either walk or trot (significance at p < 0.05). When compared to non-load pullers, it was apparent that the load pullers had reductions in differential rotational movements (0.5-3.1º in walk, p ≤ 0.039; and 0.5-2.3º in trot, p ≤ 0.049) and reductions in differential cranio-caudal, medio-lateral and dorso-ventral translations (2.2-15.0 mm in walk, p ≤ 0.039; and 1.2-7.6 mm, p ≤ 0.045 in trot), plus an increase in differential cranio-caudal translation at the Poll-Withers (12.8 mm in trot, p < 0.001). Soft surfaces increased differential rotational movements (0.1-1.2º in walk, p ≤ 0.048; and 0.2-2.9º in trot, p ≤ 0.007) and differential translational movements (1.4-16.3 mm in walk, p ≤ 0.002; and 0.5-2 mm in trot, p ≤ 0.007) in most regions of the thoraco-lumbo-sacral spine compared to hard surfaces. The amplitude of differential rotational and translational movements did not show systematic patterns with stride time and age. Further work is needed to understand whether the observed changes to differential rotational and translational movements along the equine thoraco-lumbo-sacral spine, imposed by working discipline and surface conditions, impact long-term equine health, well-being, or susceptibility to injury. This work will lay foundations for the development of an equine back dynamics reference framework, to which individual data may be compared.