Using Customized Rate-Coding and Recruitment Strategies to Maintain Forces During Repetitive Activation of Human Muscles

Abstract

Background and Purpose During functional electrical stimulation (FES), clinicians typically increase stimulation intensity to offset fatigue and maintain functional levels of force production. However, recent studies have suggested that increasing the stimulation frequency is an effective strategy for overcoming fatigue during FES. The purpose of this study was to compare the effectiveness of 5 stimulation strategies on maintaining forces during repetitive isometric muscle activation. Subjects and Methods The right quadriceps femoris muscles of 12 subjects with no history of lower-extremity orthopedic, neurological, or vascular problems were tested. The 5 stimulation strategies were: progressively increasing the frequency, progressively increasing the intensity, and 3 combination protocols that first increased the intensity and then increased the frequency. The only difference among the 3 combination protocols was the starting frequency used in each protocol (20, 30, or 40 Hz). For all protocols, the stimulation frequency or intensity was increased progressively every time the peak force declined more than 10% from a targeted force level. The specific step increases in frequency or intensity were customized for each subject. A contraction was defined as successful when its peak force exceeded 90% of the targeted force level. Results The results showed that progressively increasing only the frequency produced 59% more successful contractions than progressively increasing only the intensity. In addition, the combination stimulation protocol that began with 30-Hz trains produced the most successful contractions (mean=1,205 contractions; 35%–74% more than the other 4 protocols tested). Discussion and Conclusions The results suggest that increasing the stimulation intensity and then the frequency is the best strategy to maintain muscle performance and could help clinicians design optimal stimulation protocols to use for each patient during FES.