Ergogenic aids are commonly used by athletes and recreational practitioners in order to improve exercise performance and body composition (Silver, 2001; Goston and Correia, 2010; Senekal et al., 2019). An ergogenic aid is defined as any mechanical, psychological, physiological, pharmacological, or nutritional treatment that enhances energy production and utilization (Williams, 1992) and thus improves exercise performance and recovery (Kerksick et al., 2018). In this context, photobiomodulation (PBM) therapy, also known as low-level laser (light) therapy (LLLT), has emerged as an important non-pharmacological strategy for improving performance (Leal Junior et al., 2008, 2009, 2010; de Almeida et al., 2012; Toma et al., 2018) and recovery (Leal Junior et al., 2010) from exercise in young adults. The theoretical basis underlying the ergogenic effects of PBM on muscle tissue is related to several mechanisms, including (i) increased production of adenosine triphosphate (ATP) via modulation of mitochondrial activity, (ii) stimulation of defenses against oxidative stress, (iii) improvement of regenerative capacity by stimulation of satellite cells, and (iv) possible increased muscle fiber excitability (for more details, see the reviews, de Freitas and Hamblin, 2016; Ferraresi et al., 2016; Hamblin, 2017).

Despite the proposed beneficial mechanisms, few studies to date have investigated the ergogenic effects of PBM on biceps brachii performance and recovery in young adult subjects. While some studies reported an increase in peak force (de Almeida et al., 2012), attenuation in creatine kinase (CK) levels both immediately (Leal Junior et al., 2010) and 72 h after exercise (Felismino et al., 2014), and an increase in the number of repetitions performed at 75% of the maximal voluntary contraction (MVC) (Leal Junior et al., 2008, 2009, 2010), others did not found any beneficial effects on markers of muscle performance (Higashi et al., 2013; Orssatto et al., 2020), fatigue (Leal Junior et al., 2008, 2009; Higashi et al., 2013; Felismino et al., 2014), and recovery [i.e., delayed onset muscle soreness (DOMS) and recovery of strength performance] (Craig et al., 1996, 1999; Felismino et al., 2014). Therefore, conflicting results have been published concerning the effects of PBM.

Such discrepancies might be explained by the methodological differences among the studies, including sex, training status, and laser parameters. For instance, six of these studies included men only (Craig et al., 1996; Leal Junior et al., 2008, 2009, 2010; Felismino et al., 2014), while one included a mix of men and women (Craig et al., 1999), and another included women only (Higashi et al., 2013). Moreover, some studies recruited untrained healthy participants (Craig et al., 1996, 1999; de Almeida et al., 2012; Higashi et al., 2013), while others used volleyball players (Leal Junior et al., 2008, 2009, 2010) or physically active individuals (Felismino et al., 2014). Another point to be considered is that different wavelengths (i.e., 660–950 nm), number of points (i.e., 2–8 points), and total energy (i.e., 4–56 J) were used for the irradiation of biceps brachii in the above-mentioned studies, precluding to establish a consensus on better parameters of laser PBM for inducing ergogenic effects on muscle tissue.

It is also important to note that most studies that found positive results of laser PBM on biceps brachii performance, and recovery parameters included only male participants and used only one set of repetitions until failure (Leal Junior et al., 2008, 2009, 2010) or a single voluntary isometric contraction for 60 s (de Almeida et al., 2012). Considering that multiple sets are frequently applied in a practical context due its greater effect in promoting muscle strength and hypertrophy (ACSM, 2009), it is important to verify the effects of laser in multiple sets exercises. A recent study (Orssatto et al., 2020) found no beneficial effect of PBM on maximum number of repetitions during a resistance exercise session consisting of six sets of repetitions to failure for the standing calf raise exercise in well-trained men and women. In addition, the only study involving exclusively young adult women (Higashi et al., 2013) reported no significant differences between active and placebo laser conditions on electromyography (EMG) fatigue, blood lactate levels, and number of repetitions performed during a 60-s fatigue protocol involving the elbow flexion-extension exercise. The authors did not discuss the influence of sex on the negative findings, but previous studies reported less DOMS after eccentric resistance exercises (Dannecker et al., 2003) and greater resistance to fatigue in women than men during dynamic (Salvador et al., 2005) and isometric (Avin et al., 2010) contractions to failure in the elbow flexion exercise. These findings raise the possibility that a small effect of PBM on biceps brachii muscle performance may be masked by a lower pain intensity and greater muscular endurance in women, particularly under exhaustion conditions (e.g., multiple sets of repetitions to failure). Therefore, further studies are warranted to confirm whether the laser PBM is effective in improving biceps brachii performance to exhaustion and DOMS in young adult women.

The purpose of this study was to investigate the effects of laser PBM at 808 nm on biceps brachii performance to exhaustion and DOMS in untrained young adult women. Based on previous findings in men, we hypothesized that laser PBM would increase the number of repetitions performed, reduce the rating of perceived exertion (RPE), and attenuate DOMS when compared to a placebo laser.

All participants were classified as eutrophic (i.e., BMI range, 18–25 kg/m²), indicating that body composition had no effect on global results. The sequence of treatments was randomized and counterbalanced between visits (T1, active; T2, placebo vs. T1, placebo; T2, active) in order to ensure there was no order effect associated with treatments (placebo or laser). No volunteer reported adverse effects of laser during the study.

The purpose of this investigation was to evaluate the effects of laser PBM at 808 nm on biceps brachii performance to exhaustion and DOMS in untrained young adult women. Considering previous positive findings in men, we hypothesized that laser PBM would increase muscle performance to exhaustion (i.e., increased number of maximum repetitions and reduced RPE) and attenuate DOMS when compared to a placebo laser. Contrary to our hypothesis, our findings showed no significant difference in the number of repetitions to failure, RPE, and DOMS between placebo and active laser conditions, indicating that the effects of laser PBM on biceps brachii performance to exhaustion or attenuation of exercise-induced DOMS may be negligible in young adult women.

Previous studies have shown different endurance capacity (e.g., number of repetitions performed at a given percentage of 1RM) in lower- and upper-body muscles (Hoeger et al., 1987, 1990; Shimano et al., 2006), probably due to the amount of muscle mass (Shimano et al., 2006) and joints (e.g., single- or multijoints exercises) involved as well as the composition of fiber types. These physiological (i.e., endurance capacity), morphological (i.e., amount of muscle mass, and fiber-type composition), and mechanical (i.e., amount of joints) differences between lower- and upper-body limbs may potentially influence the effects of laser PBM on muscle tissue. Furthermore, the larger size of the lower limb muscles (e.g., quadriceps) compared to the upper body muscles (e.g., biceps) allows irradiation to be applied to a greater number of points, resulting in greater total energy delivered to the muscle and consequently stimulus for performance and adaptations. All these points indicate that a non-dissimilar analysis between different muscles (e.g., lower and upper limbs) may complicate the interpreting and understanding of the effects of PBM on the performance of specific muscles. Therefore, to avoid these confounding factors, we conducted a comparative analysis only with performance data from studies that analyzed the effects of laser PBM on biceps brachii muscle.

Similar to our study, Higashi et al. (2013) found no positive effect of laser PBM on markers of endurance performance (i.e., number of repetitions performed, EMG fatigue, and blood lactate levels) on the biceps brachii in untrained healthy women. It is worth mentioning that the doses applied in our studies (28 and 56 J) are within the dose range (20–60 J) recommended for small muscular groups (Vanin et al., 2018), indicating that this “therapeutic window” may not be effective for women. Consistent with this premise, previous studies involving only men showed improvement in muscle performance (Leal Junior et al., 2008, 2009, 2010; de Almeida et al., 2012), but no positive effect was found on muscle performance or DOMS when only women (Higashi et al., 2013) or a mix of men and women (Craig et al., 1999) were recruited. Albeit the exact mechanisms that explain the influence of sex on PBM-induced muscle adaptations remain unknown, a possible explanation may be the difference in the endurance ability of biceps brachii between men and women. Previous studies reported that women have greater resistance to fatigue than men in multiple sets of repetitions to failure (Salvador et al., 2005) and maximum voluntary isometric contraction (MVIC) to failure (Avin et al., 2010) involving elbow flexion exercise. This greater fatigue resistance could mask a small effect magnitude of the laser PBM on biceps brachii muscle performance to exhaustion in women, suggesting that the effects of PBM may be influenced by sex-related muscle differences. Several factors have been postulated to explain the differences in fatigue resistance between sex, including substrate utilization, muscle morphology, and neuromuscular activation (Hicks et al., 2001), but it remains unknown how these factors may affect muscle adaptations induced by laser PBM. In addition, we cannot rule out the possibility that reproductive hormones (e.g., menstrual cycle) may alter the endurance performance of women (Julian et al., 2017) and, consequently, influence the ergogenic effects of PBM. Future studies are needed to elucidate these points.

In this study, we also show the first data evaluating the effects of laser PBM on RPE after exercise to failure for biceps brachii muscle in young adult women. We found no significant difference in RPE at 60 s after each set between active and placebo laser conditions. Consistent with our findings, Felismino et al. (2014) using a laser at 808 nm applied on biceps brachii reported no beneficial effect on RPE after biceps curl exercise, consisting of 10 sets of 10 repetitions at 50% of 1RM in young adult men. This lack of effect of laser PBM on RPE corroborates the non-significant findings of biceps brachii performance found in our study and others (Higashi et al., 2013) involving only young adult women. However, Toma et al. (2018) reported a lower RPE after an isokinetic fatigue test and greater muscle performance in the laser group, compared to control, in young adult women. This lack of effects of laser PBM on RPE and muscle performance in our study and others involving biceps brachii muscle (Higashi et al., 2013) in contrast to the positive effects observed on the quadriceps muscle (Toma et al., 2018) raises the possibility that laser PBM may have different effects on small and large muscles. However, this supposition is not supported by recent studies that have found no positive effect on physical performance and markers of muscle recovery and damage following the PBM irradiation by light-emitting diodes (LEDs) or laser to the larger muscles or the whole body in young adult men (Zagatto et al., 2016, 2020; Malta et al., 2018; Peserico et al., 2019; Dutra et al., 2020; Ghigiarelli et al., 2020). Therefore, in light of our findings and that of others, its seems that the lack of positive effect of PBM is more associated with the inherent ineffectiveness of the therapy itself than with other factors, such as the type of irradiation (laser or LED), sex, and irradiated muscle.

Finally, we have found no significant difference in the DOMS up to 72 h after repetitions-to-failure exercise between active and placebo laser conditions. To our knowledge, no previous study has investigated the effects of laser PBM on DOMS after exercise to failure in a sample including only women. Our results are similar to previous studies involving only men (Craig et al., 1996; Baroni et al., 2010b; Cieśliński et al., 2018) or a mix of men and women (Craig et al., 1999; Kakihata et al., 2015) where no positive effect was found on DOMS following different DOMS induction protocols, including (i) a single session of neuromuscular electrical stimulation of the quadriceps femoris muscle (Cieśliński et al., 2018), (ii) five sets of 15 repetitions for knee extensors exercise at velocity = 60°seg^–1 (Baroni et al., 2010a), (iii) six vertical jumps lasting 60 s (Kakihata et al., 2015), (iv) repeated eccentric contractions of the elbow flexors until exhaustion (Craig et al., 1996, 1999), and (v) three 3-s isometric contractions of the elbow flexors (Kobordo, 2015). In addition, our findings support the evidence from a recent review (Nampo et al., 2016) in which authors reported limited effectiveness of laser PBM on DOMS. Therefore, further studies are needed to determine the effectiveness of the laser PBM on DOMS, especially in women.

This study has some limitations that should be mentioned. First, we have analyzed the acute effects of laser PBM on muscle performance and DOMS, so we cannot rule out the possibility of a positive effect if the laser is applied chronically or in combination with a resistance exercise program. Second, we have only analyzed the performance of the upper-extremity muscle (i.e., the biceps brachii), so we cannot rule out the possibility of a beneficial effect if the laser is applied to lower-extremity muscles (e.g., quadriceps). Third, we did not use physiological fatigue (e.g., lactate and EMG signal) and recovery (e.g., blood CK levels) markers to corroborate functional performance data, but previous studies have shown that laser PBM does not improve the lactate levels and EMG fatigue index in young women (Higashi et al., 2013). Finally, we did not include the analysis of muscle function (e.g., force production) to corroborate the DOMS data during the recovery from intense exercise. Future studies are required to address these issues.

In conclusion, our results indicate that acute laser PBM (808 nm) at a dose of 28 J does not improve biceps brachii performance to exhaustion, RPE, and DOMS in untrained young women. Therefore, it seems premature to consider acute laser PBM therapy as a potential strategy to improve muscular endurance and recovery of DOMS in this population. Further studies are warranted to confirm whether other laser PBM settings (e.g., wavelength, energy density, and total dose) may provide some benefit in this population or in others (e.g., men and elderly) in different muscles (e.g., lower body vs. upper body) under different clinical (e.g., healthy or not) and training conditions (e.g., trained vs. untrained).

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by the Research Ethics Board of the University of Northern Paraná. The patients/participants provided their written informed consent to participate in this study.

RA, JM, JJ, and JB contributed to data collection and analysis. RS, FP, JN, and AR participated in the design of the study, contributed to data analysis, and interpretation of results. AA participated in the conception and initial design of the project, and contributed to data analysis and interpretation of results. All authors contributed to the manuscript writing, read and approved the final version of the manuscript, and agreed with the order of presentation of the authors.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.