A systematic review and meta-analysis of the acute effects of photobiomodulation therapy on the maximum number of repetitions in resistance exercise in young adults.

Photobiomodulation therapy (PBMT) has reportedly improved muscle endurance in healthy individuals. However, the influence of possible moderating factors (light source, body limb, sex, and irradiation dose) on muscle endurance remains unknown. Therefore, this meta-analysis aimed to determine whether potential moderating factors (light source [laser vs. light-emitting diodes (LED)], body limb [upper vs. lower], sex [men vs. women], and irradiated dose]) influence the effects of PBMT on muscular endurance in terms of the maximum number of repetitions in resistance exercise in healthy young adults. The databases, PubMed, Central, Embase, Scopus, SPORTDiscus, and Web of Science (last updated February 5, 2025), were systematically searched for randomized controlled trials. The eligibility criteria were determined using the population, intervention, comparison, and outcome (PICO) method. The main outcome was the maximum number of repetitions, and the data were pooled using the random-effects model and expressed as the mean difference (MD) and 95% confidence interval (CI). The risk of bias was assessed using the RoB 2 tool, and evidence certainty was evaluated using the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) guidelines. Twelve studies (n = 346 participants) were included in the analyses. There was an overall effect in favor of PBMT (MD = 3.87 [95% CI: 1.06; 6.69], p = 0.01), compared with the placebo. Compared with the placebo, there was an effect in favor of laser (MD = 3.68 [95% CI: -0.07; 7.44], p = 0.05) and LED (MD = 4.12 [95% CI: -0.13; 8.37], p = 0.057), without difference between the light sources. There was a greater effect of PBMT for the upper limb compared with the lower limb (p = 0.02). Compared with the placebo, there was a significant effect of PBMT for the lower (MD = 1.17 [95% CI: 0.03; 2.32], p = 0.04) and upper limbs (MD = 5.87 [95% CI: 3.11; 8.63], p < 0.001). Compared with the placebo, PBMT had an effect for men (MD = 5.24 [95% CI: 1.61; 8.87], p = 0.005) but not for women (MD = 2.37 [95% CI: -1.78; 6.53], p = 0.26), without difference between sexes. The slope of the meta-regression showed a reduction in effect with increasing dose for the upper limb (slope; p = 0.05; Fig. 7A), but not for the lower limb (slope; p = 0.44; Fig. 7B). In conclusion, PBMT improved muscular endurance in terms of the maximum number of repetitions in healthy young adults. This effect was similar between laser and LED, and to a greater extent in upper limb muscles than in lower limb muscles. Furthermore, PBMT appears to promote greater benefits in men than in women. However, these results should be cautiously interpreted because literature still presents a limited number of studies, and the certainty of the evidence was rated as low or very low.