New strategies to maintain paralyzed skeletal muscle force output during repetitive electrical stimulation

During voluntary contractions, our central nervous system (CNS) uses two mechanisms, rate-coding and recruitment, to control skeletal muscle force output. Rate-coding determines the firing frequency of activated motor units; recruitment determines the number of activated motor units. For individuals with upper motoneuron lesions, if the lower motoneuron pathway remains intact, skeletal muscles can still be activated by applying electrical stimulation and muscle forces can be controlled by varying the stimulation frequency (rate-coding) and intensity (recruitment). Functional electrical stimulation (FES), the use of electrical stimulation to activate paralyzed muscles and produce functional movements, has not gained widespread of application nowadays and one of the major reasons is rapid muscle fatigue. To overcome muscle fatigue and prolong the application of FES, clinicians and researchers usually use a constant stimulation frequency and only increase stimulation intensity when needed. Although previous studies have shown the significant contribution of rate-coding to skeletal muscle force production, no studies have reported how rate-coding and recruitment should be utilized during repetitive activation to maximize skeletal muscle performance for functional tasks. Thus, the overall goal of this study was to systemically investigate the effects of frequency modulation, intensity modulation and the combination of the two on skeletal muscle force maintenance during repetitive electrical stimulation.;To be able to precisely control muscle force output during electrical stimulation, it is helpful to understand the relationship between skeletal muscle force and stimulation intensity, and how this relationship changes with stimulation frequency and muscle fatigue. Thus, the first study in this dissertation project investigated this force-intensity relationship for human quadriceps femoris muscles. The results indicated that the normalized force-intensity relationship does not change with stimulation frequency or muscle fatigue, suggesting that the required stimulation intensities for muscle force maintenance during repetitive electrical stimulation can be predicted using a simple exponential equation.;The second study in this dissertation project systemically investigated the effects of frequency modulation, intensity modulation, and the combination of the two on skeletal muscle force maintenance during repetitive electrical stimulation for able-bodied subjects. Our results showed that a stimulation protocol using frequency modulation alone produced more contractions with peak forces above a targeted force level than a protocol using intensity modulation alone. A combined stimulation protocol using intensity modulation followed by frequency modulation with the initial frequency at 30 Hz produced the most contractions with peak forces above a targeted force level.;The third study in this dissertation project investigated the effects of frequency modulation, intensity modulation, and the combination of the two on paralyzed skeletal muscle force maintenance during repetitive electrical stimulation for individuals with spinal cord injury (SCI). The results indicated that, consistent with the findings for able-bodied subjects, the stimulation protocol using intensity modulation followed frequency modulation with the initial frequency at 30 Hz produced more contractions above a targeted force level than stimulation protocol using frequency modulation followed by intensity modulation.;The results of this dissertation work can provide not only information regarding the relationship between skeletal muscle force and stimulation intensity, but also a simple modeling approach for predicting required stimulation intensity during repetitive muscle activation. Most importantly, this dissertation work can provide general guidelines and better muscle stimulation strategies for FES application.