Functional Electrical Stimulation for Equine Muscle Hypertonicity: Histological Changes in Mitochondrial Density and Distribution

• 22 functional electrical stimulation (FES) treatments on equine epaxial muscles.
• Significant increases found in mitochondrial density and distribution post-FES.
• Clinical reduction in epaxial muscle hypertonicity in all horses post-FES.
• The FES protocol may help muscle tissue function better during exercise.

Functional electrical stimulation (FES) has proven to be advantageous in reducing muscle hypertonicity and reeducating muscle memory in the human population and has recently been used in horses. Six horses ranging in age from 10 to 17 years were selected for the study. Still photographs and clinical evaluations using the Modified Ashworth Scale to characterize muscle hypertonicity were performed on all horses pre- and post-FES. Functional electrical stimulation treatments were performed over a period of 8 weeks, resulting in a total of 22 treatments per horse. Biopsies pre- and post-FES treatments were obtained from the longissimus lumborum muscle of each horse. Mitochondrial density and distribution of the blinded samples pre- and post-FES were determined, and a two-sided Welch's t test was used to analyze the results. The results of the clinical evaluations and of morphometric analyses comparing pre- to post-FES muscle biopsies found: (1) a significant increase (P < .001) in the pooled mean mitochondrial density of both glycolytic and oxidative muscle fibers, (2) a significant increase (P < .001) in the subsarcolemmal mitochondrial high-density patches in oxidative muscle fibers, and (3) a significant increase (P < .001) in the subsarcolemmal mitochondrial high-density area percentages. In summary, the clinical improvements in the reduction of hypertonicity are, conceivably, related to the daily increased muscle contraction and perfusion induced by FES training. Thus, the FES protocol used in this study produced a positive effect on mitochondrial density and distribution, which in turn may help create healthier muscle tissue that is better able to function during exercise.