Effect of Electrical Stimulation Frequency and Pattern on Muscle Fatigue in Spinal Cord Injured Individuals

Abstract

Spinal cord injury (SCI) leads to an extensive skeletal muscle atrophy in the regions below the level of injury. Moreover, the paralyzed skeletal muscles shift towards predominantly Type II (fast‐twitch and highly fatigable) fibers. Functional electrical stimulation (FES) has been used to produce purposeful and controlled muscle contractions to restore functional movements, but rapid muscle fatigue remains a problem. Both pattern and frequency of stimulation can influence muscle fatigue rate, but optimization of stimulation parameters has rarely been investigated for those with SCI. FES has traditionally used constant‐frequency trains (CFT; brief stimulation pulses separated by regular interpulse intervals). However, it has been suggested that doublet‐frequency trains (DFT; close‐spaced pulse trains separated by longer intervals) can result in lesser fatigue. Also, stimulation frequency may determine muscle fatigue, with high‐frequency trains resulting in lesser fatigue compared to low‐frequency trains. However, most studies comparing muscle fatigue across stimulation parameters have been only examined in able‐bodied individuals and it is unknown whether paralyzed muscles respond similarly. There are a few studies suggesting that low‐frequency trains produce less fatigue than high‐frequency trains in those with SCI, in direct contrast to the able‐bodied. Therefore, we examined differences in muscle fatigue across and within able‐bodied individuals (N=8) and those with SCI (N=6) during CFT and DFT stimulation patterns at high‐ (40 Hz) and low‐ (20 Hz) frequencies. Fatigue was defined as a sustained reduction in force of >20% at a constant stimulation intensity. Differences were considered significant at p<0.05 and values are mean ± SEM. For both groups, low‐frequency trains delayed fatigue onset compared to high‐frequency trains, on average by 30 ± 3 %. In the able‐bodied, CFT resulted in slower fatigue onset (#contractions to fatigue CFT vs DFT: 20 Hz 39 ± 4 vs. 34 ± 4, 40 Hz 26 ± 4 vs. 23 ± 3). In those with SCI, DFT resulted in slower fatigue onset (#contractions to fatigue CFT vs DFT: 20 Hz 26 ± 2 vs. 35 ± 4, 40 Hz 20 ± 2 vs. 25 ± 2). In fact, DFT resulted in similar #contractions to fatigue in both populations, whereas CFT induced muscle fatigue significantly faster in those with SCI. These results may indicate that DFT preferentially recruit fast‐twitch fibers, leading to similar fatigue in both able‐bodied and those with SCI. DFT has been previously shown to limit fatigue potentially by increasing calcium release from the sarcoplasmic reticulum and/or by increasing muscle stiffness. These data indicate that DFT trains appear to be preferable for fatigue reduction in the SCI population.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.