Longissimus dorsi muscle activity in sound horses during in-hand trot on a straight line and circle

The objective was to examine and compare muscle activity of the longissimus dorsi muscle in sound horses during in-hand trot on a straight line and circle to understand muscle activity distribution under normal conditions. Five sound horses performed three trials in trot on a hard surface: straight-line in-hand (4×25 m) and lunging on the left and right circle. Bipolar surface electromyography (2,000 Hz) described bilateral longissimus dorsi muscle activity at T16. Wireless inertial sensors (200 Hz) were used to detect locomotion asymmetry and perform stride normalization (hind foot impact of the corresponding side). Twenty strides were extracted after frequency filtering; 40 Hz high-pass, 450 Hz low-pass, 25 Hz low-pass over linear-envelope. Signal amplitudes were normalized to the median peak value during the straight-line trial. Statistical parametric mapping was used (two-way ANOVA) to analyse the level of muscle activity between conditions (straight-line, inside-circle, outside-circle) and anatomical side (left, right) within individual horses. Two distinct bursts of normalized muscle activity occurred in all horses: the first burst between ±20-45% and the second between ±70-95% of the stride cycle. On the straight line, muscle activity was significantly higher compared to the inside- and outside-circle. Muscle activity on the inside-circle was significantly higher compared to the outside-circle during the first burst. During the second burst, muscle activity on the straight-line was significantly higher compared to the outside-circle. Different levels of muscle activity of the longissimus dorsi occurred between straight-line and circles during trot. This might have clinical implications for the aetiology, rehabilitation and quantification of equine back pain and lameness.