Activity of the equine rectus abdominis and oblique external abdominal muscles measured by surface EMG during walk and trot on the treadmill



Reason for performing study: The rectus abdominis (RA) and oblique external abdominal (OEA) muscles are both part of the construction of the equine trunk and thought to be essential for the function of the spine during locomotion. Although RA activity at trot has previously been investigated, the relationship between OEA and RA at walk and trot has not yet been described.

Objectives: To document abdominal muscle activities during walk and trot, and test the hypothesis that muscle activity at walk would be smaller than at trot.

Materials and methods: Six horses (8–20 years old, 450–700 kg) were used for surface electromyography (EMG) measurements, with EMG electrodes placed caudal to the sternum (RA) and at the level of the 16th rib (OEA). On all hooves, the withers and the sacrum reflective markers were placed to determine motion cycles. Normal distribution of data was tested using a Kolmogorov-Smirnov test and Student's t test was used to compare left-right and walk-trot differences (P<0.05).

Results: Minimum, maximum and mean EMG values recorded at walk were significantly higher at trot than at walk in all horses for OEA and in 5/6 horses for RA. At walk, EMG activity ranged from 8–44 mV (RA) and 7–54 mV (OEA). At trot, EMG activity ranged from 18–150 mV (RA) and 27–239 mV (OEA). There were statistically significant differences between maximum activities of left and right OEA and RA muscles at walk in all horses, and in 4/6 horses at trot.

Conclusions: Muscle activities of OEA and RA are smaller at walk than at trot. At walk, the OEA/RA ratio is lower than at trot. There are more significant correlations between muscle activities of both RA and OEA and limb movements at walk than at the trot.