Electromyography of the multifidus muscle in horses trotting over firm and soft surfaces

Equine sports medicine has developed a focus on the multifidus muscle with little reported knowledge of its activity in normal horses. Our main aim and objective was to use in-dwelling electromyography (EMG) to measure and compare the average and peak activity of the multifidus muscle at the level of the 12th (T12) and 18th thoracic (T18) and 5th lumbar (L5) vertebra bilaterally. We hypothesized that trotting horses in hand over a soft deformable surface would cause an increase in both average and peak activity when compared to trotting on a non-deformable asphalt surface. The EMG signals from four horses each with 25 observable muscle contractions at each location were filtered and normalized to the maximum observed signals. The effect of two surface conditions on the average and peak muscle activity within each muscle section of four horses was assessed using unpaired t-tests. The average muscle activity was significantly higher while trotting over a soft surface when compared to the hard asphalt surface in the right T12 (mean difference [MD]=0.13 p < 0.001), right L5 (MD=0.12 p < 0.001) and left L5 (MD=0.18 p < 0.001) regions, although the left T12 location showed significantly higher average activity on the hard surface (MD=0.13 p < 0.001). The peak activity was significantly higher on soft footing in the left T18 (MD=0.10 p < 0.05), left L5 (MD=0.18 p < 0.001), right T12 (MD= 0.40 p < 0.001), and right L5 (MD=0.10 p < 0.001). Therefore, when compared to trotting on a hard surface, the softer surface induced higher levels of muscle activity in most of the multifidi locations.