Effects of Photobiomodulation therapy on relieving peripheral and central fatigue in the grip exercise

This study aimed to investigate the effect of photobiomodulation therapy (PBMT) on muscle fatigue (i.e., peripheral and central fatigue) after performing the grip exercise. Eleven healthy college students were recruited to participate in a crossover, randomized, double-blind, placebo-controlled experiment. To induce muscle fatigue, all subjects performed the first grip exercise with a load of the 50% of maximum voluntary contraction force until exhaustion, and then they received PBMT or placebo on the hand immediately to perform the same second grip exercise. Surface electromyography (sEMG) signals were recorded from the first dorsal interossei of right hand to evaluate peripheral fatigue using muscle fatigue index (MFI), and functional Near-Infrared Spectroscopy (fNIRS) signals were recorded from cortical regions to evaluate central fatigue using functional connectivity (FC) analysis. Results showed that muscle fatigue index (MFI) was significantly lower in the PBMT group (p < 0.05) in the second grip exercise, when compared to the placebo group, but there was no significant difference in grip strength duration between the two groups. The PBMT group and the placebo group showed different FC patterns in the second grip exercise, which was manifested by the range of FC between the seed point left primary motor cortex (lM1) and other regions of interest (ROIs) in the placebo group was wider than that in the PBMT group. We concluded that using PBMT immediately after performing the grip exercise had a relieving effect on muscle fatigue in the second grip exercise.