摘要： Background: Muscular fatigue is caused by biochemical alterations that modify the mechanics of muscle contraction, resulting in negative changes in the performance of the contraction. Several resources are studied to mitigate this situation among which we can cite low-level laser therapy (LLLT). The effects of LLLT are being studied in healthy subjects with fibromyalgia and who are athletes, and currently the studies are being performed in spastic muscles with poststroke individuals. The aim will be to evaluate the effects of LLLT on the fatigue of the biceps brachii muscle of healthy individuals and individuals with spastic hemiparesis. Methods: A cross-sectional, comparative, randomized, placebo, double-blind clinical trial will be divided into 2 phases: phase I shall consist of 30 healthy subjects and phase II of 30 poststroke individuals. The study will consist of 3 groups (control group, placebo group, and LLLT group), and all individuals will pass through all groups, following the randomization criteria. The protocol consists of the application of LLLT in the biceps brachii muscle on the dominant side in healthy individuals and in the hemiparetic side of poststroke individuals, and, subsequently, 3 maximal isometric voluntary contractions (MIVCs) will be performed for 50 seconds in the dynamometer, with an interval of 50 seconds between them. Pain intensity will be evaluated by means of the visual analog scale, and the myoelectric activity by means of surface electromyography associated with the evaluation of muscular strength by means of the dynamometer. The local temperature will be evaluated by infrared thermography and blood lactate concentration through the lactimeter, which will be measured at 4 different times, before the application of the laser (basal), and 3, 15, and 25 minutes after the MIVC.