Effects of Electrical Stimulation Pattern on Quadriceps Fatigue Development in Individuals with Spinal Cord Injury

Abstract

Functional electrical stimulation (FES) has often been used to facilitate exercise in individuals with spinal cord injury (SCI). Unfortunately, rapid fatigue induced by constant frequency trains (CFT) limits the effectiveness of FES in those with SCI. Variable frequency trains (VFT) have therefore been suggested as useful strategies to offset the rapid fatigue during electrical stimulation. However, most results concern very short duration contractions with limited functional applications. PURPOSE: To test force and fatigue in response to VFT and CFT patterns in training-like conditions (2-s concentric contractions). METHODS: Seven SCI individuals underwent two testing sessions separated by at least 24 hours. During each testing session, muscle torque was measured during concentric leg extension (30°/s) under two sequential electrical stimulation train patterns presented in a randomized order. For the CFT pattern, this corresponded to 2 s of contraction generated by continuous 40 Hz stimulation with a pulse width of 450 μs; for the VFT pattern, this corresponded to 2 s of contraction generated by an 80 Hz doublet followed by continuous 20 Hz stimulation with a pulse width of 450 μs. Stimulation was applied every 5 s until the target torque (fixed at 50% of the maximal evoked force) was no longer produced. RESULTS: For an equivalent torque decrease (VFT: -24.0±0.6 vs CFT: -27.7±3.2 %), more contractions were performed when evoked with VFT as compared with CFT (VFT: 14.3±1.7 vs CFT: 9.3±2.5 contractions, P<0.05). In addition, the total work produced over sessions was significantly greater when VFT pattern was applied (1418.1 ± 115.7 Nm.s) as compared with CFT (879.8 ± 112.4 Nm.s). CONCLUSION: These results showed that paralyzed human skeletal muscle is more fatigue resistant to repetitive dynamic contractions evoked by a VFT stimulation pattern as compared with a CFT pattern. These data are of particular interest for training and rehabilitation after an SCI since they suggest that using VFT, people would be able to perform longer training sessions, potentially resulting in greater improvements.