Purpose Blood flow restriction (BFR) is an innovation in fitness to train muscles with low loads at low oxygen levels. Low-level laser therapy (LLLT) is a bio-energetic approach to alleviate muscle fatigue during resistance training. This study investigated the immediate effect of LLLT pre-conditioning on BFR that accelerates muscle fatigue due to ischemia. Methods Fifteen young adults participated in this study of a crossover randomized design. They completed a low-load contraction with various pre-conditioning (blood flow restriction with low-level laser therapy (LLLT + BFR), blood flow restriction with sham low-level laser therapy (BFR), and control). Force fluctuation dynamics, muscle oxygen saturation of hemoglobin and myoglobin (SmO2), and discharge patterns of motor units (MU) were compared. Results Normalized SmO2 during low-load contractions significantly varied with the pre-contraction protocols (Control (83.6 +/- 3.0%) > LLLT + BFR (70.3 +/- 2.8%) > BFR (55.4 +/- 2.4%). Also, force fluctuations and MU discharge varied with the pre-contraction protocols. Multi-scale entropy and mean frequency of force fluctuations were greater in the LLLT + BFR condition (31.95 +/- 0.67) than in the BFR condition (29.47 +/- 0.73). The mean inter-spike interval of MUs was greater in the LLLT + BFR condition (53.32 +/- 2.70 ms) than in the BFR condition (45.04 +/- 1.08 ms). In particular, MUs with higher recruitment thresholds exhibited greater LLLT-related discharge complexity (LLLT + BFR (0.201 +/- 0.012) > BFR (0.154 +/- 0.006)). Conclusions LLLT pre-conditioning can minimize the BFR-related decline in muscle oxygen saturation, leading to force gradation and MU discharge in a cost-effective and complex manner.
