Movement asymmetry in apparently sound horses in active service: effects of warm-up and lungeing

<jats:p>&lt;b&gt;Background: &lt;/b&gt;a high proportion of sport horses considered sound show movement asymmetries measurable with inertial sensors, but their magnitude in working horses of the State security forces is unknown. &lt;b&gt;Objectives: &lt;/b&gt;to estimate the prevalence of subclinical movement asymmetry in apparently sound service horses and to characterise how warm-up, circular trajectory and surface type modify its magnitude and detection. &lt;b&gt;Study design: &lt;/b&gt;cross-sectional observational study with convenience sampling. &lt;b&gt;Methods: &lt;/b&gt;ninety-eight horses from four security forces were assessed with an inertial sensor system (Equinosis® Q, Lameness Locator). The protocol comprised straight-line trot when cold, warm-up, straight-line trot when warm, lungeing on both reins on soft and hard surfaces, and distal flexion tests. Asymmetry was quantified as vertical head difference (Vector Sum) and pelvic difference (impact and push-off). Prevalence was expressed with 95% confidence intervals using the Wilson method. The effect of scenario was analysed with linear mixed models and the ipsilateral McNemar test, with Bonferroni correction and false discovery rate control. &lt;b&gt;Results: &lt;/b&gt;eighty-seven per cent (95% CI: 78-92) of horses showed subclinical asymmetry during warm straight-line trot. Lungeing increased head and pelvic asymmetry relative to the straight line (all straight-versus-circle contrasts P ≤ 0.06); no difference was detected between soft and hard surfaces, although the hard-surface comparison was underpowered (n = 17). Lungeing detected left hindlimb asymmetry not evident on the straight line (McNemar P &lt; 0.001; the only contrast surviving correction for multiplicity). Warm-up did not reduce asymmetry. &lt;b&gt;Main limitations: &lt;/b&gt;convenience sampling; unbalanced hard surface design (n = 17); absence of diagnostic analgesia or imaging to confirm aetiology. &lt;b&gt;Conclusions: &lt;/b&gt;subclinical movement asymmetry is highly prevalent in apparently sound service horses and is systematically exacerbated on the circle, potentially revealing hindlimb asymmetry. These sensor-detectable asymmetries represent the limit of detection rather than diagnosis, and their pathological relevance remains unclear.</jats:p>