Effect of Electrical Stimulation on Muscle Recruitment as it Relates to Maximal Voluntary Contraction

Regeneration of complex muscle tissue, particularly at the interface between distinct tissue types such as muscle and tendon, remains an intricate and challenging issue. One approach recommended for regeneration is electrical stimulation (ES), which involves delivering an electrical pulse into the body to induce muscle contraction. Despite its well-documented effectiveness in both clinical rehabilitation and tissue engineering, little is known in terms of monitoring direct muscle response to stimulation. The recruitment mechanism for muscular contraction with stimulation is unclear, and to understand the muscle capabilities, it is necessary to filter out the electrical stimulation pulses from electromyographic (EMG) muscle activity, as the frequency spectra often overlap, distorting the muscular signal. The aim of this thesis is to investigate voluntary and ES-induced activation patterns in small and large muscles and to make recommendations for measuring optimal muscle recruitment with ES. In the first part, voluntary recruitment patterns for muscles of different sizes and fiber types are compared. Next, the efficacy of a stimulus-removal technique called empirical mode decomposition (EMD) is evaluated for use with multiple frequencies and contraction levels. This filtering method is then applied to EMG data to determine the optimal stimulation parameters in terms of greatest electrical activity emitted from the muscles. Finally, voluntary and ES-induced activation patterns are compared to evaluate differences in recruitment mechanisms. The results of the investigation show differences between muscle types in terms of electrical activity measured with EMG. For both voluntary and electrically-induced contractions, smaller muscles with smaller fiber types output more electrical signal. EMD was successful in removing stimulus artifact from the signal, and it was possible to compare muscle responses for varying levels of stimulation. Stimulation delivered at higher frequencies appeared to induce greater muscle response measured with EMG. The author recommends further investigation of voluntary and ES-elicited muscle activations, particularly using protocols with greater participants and more varied muscle types.