The effect of full-limb flexion tests on static and dynamic muscle activity and locomotion asymmetry in owner-sound horses

BACKGROUND: Flexion tests are commonly used in equine locomotion examinations to identify underlying locomotor issues, yet their neuromuscular effects remain poorly understood. Response variability raises concerns about their clinical value in lameness assessments and pre-purchase evaluations.

OBJECTIVES: Primarily, to investigate the effect of full-limb flexion tests on static (flexed position) and dynamic (subsequent trot-up) muscle activity. Secondarily, to assess their effect on locomotion asymmetry during trotting.

STUDY DESIGN: In vivo experiments.

METHODS: Sixteen warmblood horses were randomly assigned to one of four groups (n = 4) for surface electromyography of selected limbs. Forelimb region muscles included Splenius muscle, Triceps brachii caput longus muscle, Longissimus dorsi muscle, Extensor digitorum communis muscle, and Ulnaris lateralis muscle, while hindlimb region muscles included Gluteus medius muscle, Biceps femoris muscle, Semitendinosus muscle, Extensor digitorum longus muscle, and Flexor digitorum profundus muscle. Electromyographic data were side-normalised, and kinematic data from inertial measurement units assessed asymmetry in head, withers, and pelvis. Each limb underwent a 60-s full-limb flexion test, followed by straight-line trotting. Muscle activity in static and dynamic phases was compared to baseline using one-way repeated measures ANOVA, and two-way repeated measures ANOVA with statistical parametric mapping, respectively. Kinematic variables were compared using a linear mixed-effect model.

RESULTS: No significant differences in normalised muscle activity were observed in fore- or hindlimb region muscles during flexion or subsequent trotting. However, pelvic displacement increased during the suspension phase of the ipsilateral limb, with significant vertical displacement (MaxDiff) after left (8.4 mm ± 2.4 SE [standard error], p = 0.002) and right (8.1 mm ± 2.4 SE, p = 0.004) hindlimb flexion test.

MAIN LIMITATIONS: Only full-limb flexion tests were investigated, limiting generalisability to distal or carpal/tarsal tests. Flexion force was not standardised.

CONCLUSIONS: We found no evidence that flexion tests affect equine neuromuscular functioning in the muscles we investigated.