Reasons for performing study: Skeletal muscle activity can be concentric or eccentric, anisometric or isometric and correlation of the equine splenius muscle activity with the movement of its effector joints at walk and trot has not yet been fully characterised.
Objective: Investigating activity of the splenius muscle together with kinematics of head and cranial neck at walk and trot.
Materials and methods: Kinematics and surface electromyography were measured in 6 horses (8–20-years-old, 450–700 kg) without signs of neck pain. Markers were placed on left and right crista facialis, and on left and right cervical vertebrae 1 and 3. Head and neck angle was calculated in sagittal and horizontal planes. Electrodes were placed over both splenius muscles at the level of C2. Left and right muscle activity was compared using Student t test for paired samples and correlations calculated using Pearson correlation coefficient. Significance was set at P<0.05.
Results: In all horses, maximum surface electromyography (sEMG) values at the trot were higher than at the walk. The intraindividual differences between maximum and minimum values of the EMG ranged from 45–127 mV in walk and from 154–524 mV in trot. Flexion-extension C1 angle changed by 43° in walk and 27° in trot. For each motion cycle, 2 EMG maxima were found in both gaits, occurring just prior to maximum extension of the C1 angle. Lateral bending at C1 angle changed by 16° in walk and 17° in trot and EMG reached maximum values bilaterally during maximum lateral bending at walk.
Conclusions: The splenius muscle reaches maximum activity at the beginning of the forelimb stance phases in trot, indicating functional stabilisation against flexion of the head and neck. Unilateral activity of the splenius muscle representing stabilisation against lateral movement was not found.