Electromyographic activity of equine abdominal muscles during single and double riding in hippotherapy

Background Hippotherapy is a therapeutic intervention that uses the horse as a biomechanical and sensorimotor agent to promote rehabilitation in individuals with physical and neurological disabilities. Depending on patient needs, riding may be performed with a single rider or with double riding, in which a therapist accompanies the practitioner on the horse. However, the effects of these riding configurations on equine trunk muscle activity remain poorly understood, despite their relevance for both horse welfare and the quality of therapeutic stimuli delivered to the patient. Methods Nine clinically healthy adult horses (mean body mass: 388 kg) routinely employed in hippotherapy were evaluated. Two healthy female physiotherapists trained in hippotherapy (26 and 27 years; 50.9 and 51.3 kg) served as riders in all experimental conditions. Surface electromyography (sEMG) was recorded bilaterally from the Rectus Abdominis (RA) and External Oblique Abdominis (EOA) muscles during walking at a constant speed of 1.43 m/s under three randomized conditions: without rider (WR), single rider (R1), and double riding (R2). Each trial consisted of a 10-s stable walking period. Data were analyzed using randomized block ANOVA with Tukey’s post hoc tests and effect size estimates (Cohen’s d and Hedges’ g ). Results Compared with WR, double riding (R2) resulted in significantly greater activation of both RA and EOA muscles bilaterally ( p < 0.05), with large effect sizes ( g = 1.50–2.52). No significant differences were observed between R1 and WR when rider load did not exceed 14.75% of the horse’s body weight. Comparisons between R2 and R1 revealed asymmetric activation patterns, particularly increased activation of the left EOA and right RA muscles ( p < 0.05; large effect sizes), suggesting compensatory neuromuscular strategies to maintain trunk stability under increased load and movement. Conclusions Single riding with light loads does not substantially alter equine abdominal muscle activity during walking. In contrast, double riding increases muscular demand and induces asymmetric activation patterns, which may reflect compensatory mechanisms related to load magnitude, rider movement, and handling configuration. These findings have important implications for equine welfare and for optimizing therapeutic decision-making in hippotherapy practice.