Electromyographic and ultrasonographic evaluation of neuromuscular electrical stimulation training on the equine rectus abdominis muscle

Abstract

The current study aimed to determine the effects of neuromuscular electrical stimulation (NMES) on equine rectus abdominis using surface electromyography (sEMG) and ultrasonographic muscle thickness evaluation. Five horses were trained with NMES for 12 weeks; muscle thickness and sEMG evaluations were obtained before and after the training period. Three different tests were carried out for sEMG evaluations: Test A tried to elicit a voluntary maximal isometric contraction (VMIC); Test B used NMES to elicit a muscular contraction; and Test C used 1 ms repetitive electrical impulses to elicit a series of M-waves. Muscle strength was evaluated from the root mean square (RMS; Tests A and B) and peak to peak (PP; Test C) values obtained from the sEMG data. Measures related to amplitude (RMSa; RMSm; RMSa; PP) were normalised with their pre-training values for every muscle prior to statistical analysis, leading to values as a proportion of the initial value. The evaluation of muscle fatigue was performed using a Fourier test analysis of the frequency range, obtaining the median frequency (MF) for all tests. Muscle thickness (MT) was measured by ultrasound of left and right sides of the rectus abdominis. Data were analysed using non parametric test of Wilcoxon (Test A RMSa; Test A RMSm; Test B RMSa; Test B RMSm; Test C PP; MT; P<0.05) and t-test (Test A MF; Test B MF; Test C MF; P<0.05). Statistical differences were observed between baseline and trained horses. Muscle force increase following NMES training in Test B (RMSa 2.50±0.69; RMSm 2.59±0.76) and Test C (PP 5.20±1.76). Fatigue of the rectus abdominis decreased in Test A (168.33±55.19 vs 232.63±44.15 Hz) and Test C MF (187.93±20.76 vs 236.98±52.39 Hz), but not in Test B (363.98±45.48 vs 327.95±50.84 Hz). The difference in muscle thickness between the two groups was not significant (10.96±0.64; 11.78±0.79 mm). The results suggest that NMES training could be used as an effective method to increase muscle force and fatigue resistance of the rectus abdominis muscle in the horse.