Quantitative motor unit action potential analysis of supraspinatus, infraspinatus, deltoideus and biceps femoris muscles in adult Royal Dutch sport horses

REASONS FOR PERFORMING STUDY: Reference values for quantitative electromyography (QEMG) in shoulder and hindlimb muscles of horses are limited. OBJECTIVES: To determine normative data on QEMG analysis of supraspinatus (SS), infraspinatus (IS), deltoideus (DT) and biceps femoris (BF) muscles. STUDY DESIGN: Experimental observational study and retrospective case series. METHODS: Seven adult healthy Royal Dutch sport horses underwent quantitative motor unit action potential analysis of each muscle using commercial electromyography equipment. Measurements were made according to published methods. One‐way ANOVA was used to compare quantitative motor unit action potential variables between muscles, with post hoc testing according to Bonferroni, with significance set at P<0.05. The QEMG and clinical information from horses with lower motor neuron disorders (n = 7) or myopathy (n = 4) were summarised retrospectively. RESULTS: The 95% confidence intervals of duration, amplitude, phases, turns, area and size index of quantitative motor unit action potential were 8.7–10.4 ms, 651–867 μV, 3.2–3.7, 3.7–4.7, 1054–1457 μV·ms and 1.1–1.5 for SS, 9.6–11.0 ms, 779–1082 μV, 3.3–3.7, 3.8–4.7, 1349–2204 μV·ms and 1.4–1.9 for IS, 6.0–9.1 ms, 370–691 μV, 2.9–3.7, 2.8–4.5, 380–1374 μV·ms and 0.3–1.3 for DT and 5.7–7.8 ms, 265–385 μV, 2.7–3.2, 2.6–3.1, 296–484 μV·ms and 0.2–0.5 for BF, respectively. Mean duration, amplitude, number of phases and turns, area and size index were significantly (P<0.01) higher in SS and IS than in DT and BF muscles. In addition, 4 of 7 normal horses had >15% polyphasic motor unit action potentials in SS and IS muscles. CONCLUSIONS: Differences between muscles should be taken into account when performing QEMG in order to be able to distinguish normal horses from horses with suspected neurogenic or myogenic disorders. These normal data provide the basis for objective QEMG assessment of shoulder and hindlimb muscles. Quantitative electromyography appears to be helpful in diagnosing neuropathies and discriminating these from myopathies.