Do Muscle Activities of M. Splenius and M. Brachiocephalicus Decrease Because of Exercise-Induced Fatigue in Thoroughbred Horses?

Muscle activities of the major hindlimb muscles have been reported to decrease with fatigue in horses. However, those in other muscles have been scarcely reported. We aimed to quantify fatigue-induced electromyographic changes in head and neck muscles and muscles around the shoulder joints in horses. Surface electromyographic recording of the splenius, brachiocephalicus, infraspinatus, and deltoid muscles was performed on a total of nine healthy Thoroughbred horses. Horses galloped on a treadmill inclined to 3% at a constant speed (12.7-14.6 m/second) to make them fatigued after approximately 5 minutes. They trotted at 3.5 m/second before and after this exercise. Stride frequency, integrated electromyographic values for a stride, and median frequency of the muscle discharge were calculated every 30 seconds. These parameters were compared at the start and end of the gallop exercise for the lead and trailing limbs and while trotting before and after the exercise using a paired t-test. The stride frequency significantly decreased at the end of the gallop (P < .001), whereas it did not change while trotting. Integrated electromyographic values of the splenius and brachiocephalicus muscles in both lead and trailing limbs at the gallop and those of both left and right sides at the trot significantly decreased with fatigue (P < .05), whereas those of infraspinatus and deltoid muscles did not change at either gallop or trot. No changes were observed in median frequency in any muscles with fatigue. These results suggest that splenius and brachiocephalicus muscle activities can be associated with stride frequency and speed.