DOES PHOTOBIOMODULATION IMPROVE MUSCLE PERFORMANCE AND RECOVERY? A SYSTEMATIC REVIEW

ABSTRACT Photobiomodulation (PBM) appears to limit exercise-induced muscle damage, improve biochemical and functional recovery, and reduce inflammation and oxidative stress. This systematic review aimed to evaluate the effectiveness of photobiomodulation (PBM) in skeletal muscle recovery after exercise, addressing the different types of lasers and parameters used. Randomized clinical trials (RCTs) comparing the effects of PBM were included. The primary outcome evaluated was performance, and the secondary was inflammatory marker expression. The searches were conducted in March 2021. Fifteen RCTs that met the inclusion criteria were included. There was significant variability regarding the doses and wavelengths used, as well as in the types of lasers. However, in most studies, PBM promoted improvement of maximum voluntary contraction, better oxygen consumption, increased time to achieve exhaustion and fatigue, and decreased creatine kinase (CK), oxidative stress, and fatigue markers, mainly when used before exercise. Photobiomodulation applied before exercise, regardless of variations in doses and wavelengths, improves muscle performance and decreases levels of inflammation and fatigue markers. Evidence level II; Systematic review of level II studies.


INTRODUCTION
The practice of physical activity promotes health and quality of life. However, there is a wide range of risks involved according to each sport's physical demand on its practitioners.¹ Thus, many therapeutic modalities have been used after sports activities to improve skeletal muscle recovery, and one of them is Photobiomodulation (PBM). PBM is a non-pharmacological treatment aimed at decreasing the duration of the muscle recovery period. [1][2][3][4] However, the scientific evidence on the efficacy of this treatment is limited. 3,5,6 It is known that exercise increases the flow of mitochondrial oxygen and the production of adenosine triphosphate (ATP) in skeletal muscle. As the main chromophores of PBM are located within the mitochondria, cells with many mitochondria and high metabolic activity are particularly responsive to light. Thus, it is hypothesized that PBM use in sports and exercises increases cytochrome c-oxidase in skeletal muscle fibers, leading to positive mitochondrial regulation, increasing ATP production. 7,8 The increase in ATP production increases energy production and decreases oxidative stress and the production of reactive oxygen species, delaying muscle fatigue and improving the status of biochemical markers related to skeletal muscle recovery. When PBM is applied, an extra amount of Calcium (Ca 2+ ) is transported to the cytoplasm through a process that promotes cell mitosis, RNA and DNA synthesis and cell proliferation.³ In this context, studies have shown that PBM can limit exerciseinduced muscle damage, improving biochemical and functional recovery and reducing inflammation and oxidative stress. 3,4,[9][10][11][12][13][14] However, it is not yet a consensus on the literature since some studies showed no results or even worse results after PBM on muscle recovery after fatigue induction. 2,4,5 Besides, there is great variability in the application parameters (such as power, wavelength, irradiation time and energy) used in the studies, making it challenging to interpret the results and use them in clinical practice, even in sporting environments. 3,15 Therefore, considering the divergence among the available results, a systematic review in this field is extremely important to determine the best criteria to be used so that it is possible to obtain a rapid muscle recovery and the return of sports activities.
Although there are some systematic reviews in this field, [16][17][18] there are many gaps in this knowledge since many studies analyzed different variables or had a limited search strategy. This fact does not allow the conclusion regarding the efficacy of PBM on muscle performance and inflammatory or fatigue markers. Additionally, some new high-quality studies are available, which could contribute to the clinical use of PBM.
Therefore, this systematic review aimed to evaluate the effectiveness of PBM with low-level laser therapy (LLLT) in skeletal muscle recovery after exercise, addressing the different types of lasers and parameters used.

METHODS
This systematic review is based on Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) checklist structure 18 and was conducted following the methodology described in the Cochrane Handbook of Systematic Reviews of Interventions. 19 A specific question was formulated based on population, intervention, control, and outcome (PICO) criteria. The focus question was: "What is the effect of PBM on skeletal muscle function?" According to these criteria, the population consisted of healthy participants, and the comparative intervention was the PBM compared to no treatment or placebo. The primary outcome was the performance (assessed by maximum voluntary contraction, peak strength, time to reach exhaustion, isometric capacity, time on the pitch, time to achieve fatigue/exhaustion), and the secondary outcome was the expression of inflammatory markers.

Identification and selection of studies
Two independent researchers conducted an electronic search of articles published until March 2021, in the databases PubMed/MEDLI-NE, EMBASE, LILACS, Scielo, using the following search strategy: "(laser therapy OR low-level laser therapy OR low-intensity laser therapy OR phototherapy OR photobiomodulation) AND (repair OR regeneration OR rehabilitation) AND (skeletal muscle)".
The studies were selected and classified as "included" or "excluded", based on the reading of the title and summary of the articles by the two reviewers, working separately. A third researcher analyzed all inconsistencies in the choices of the articles by the other two researchers, and a consensus was reached through discussion. The studies selected as "included" were randomized clinical trials, with healthy participants, an experimental group with laser treatment, the presence of a control group without treatment or placebo and in all idioms. Exclusion criteria were: studies in which the intervention group received PBM associated with another therapy; studies that do not specify the intervention protocol; studies that include participants with any kind of disease or pathology; letters, case reports, short communication and studies in animal and in vitro models.

Data extraction
The relevant data extracted from every study included: the author name; year of publication; sample size; characteristics of the participants (gender, age, height, weight, trained or untrained); type of laser used, wavelength, energy, time of application, power, application protocol and outcome measures.

Assessment of quality and risk of bias
After selecting the studies, two reviewers independently evaluated the quality of each study included by using the PEDro Scale (Centre for Evidence-Based Physiotherapy), in which the studies are classified with scores from 0 to 10. The risk of bias in individual studies was evaluated according to the Cochrane Collaboration's tool. 20 This tool comprises seven evaluation domains: random sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessors, incomplete outcome data, selective out-come reporting and other sources of bias. The risk of bias assessment for each of the domains involves classification into three categories: (1) low risk of bias, when the domain described by the study is considered adequate; (2) high risk of bias, when the domain described by the study is considered inadequate; and (3) unclear risk of bias when the study presents insufficient information for assessing the risk of bias.

RESULTS
The combined total number of studies obtained through the electronic search strategy was 4174 references, including 3660 from PubMed/ MEDLINE, 27 from LILACS, seven from Scielo and 480 from EMBASE. A total of 2 publications were obtained in duplicate and eliminated from the analysis. After exclusions based on the title and abstract, 41 studies were selected and evaluated for eligibility. Thus, 15 RCTs 1-15 were included in this systematic review ( Figure 1).

Effects of intervention
There was a great variability regarding the doses and wavelengths used and the type of laser. In each variation, different results were obtained, showing that other PBM parameters may provide distinct effects on muscle tissue. The heterogeneity of the studies, especially on laser parameters and treated muscles, did not allow comparisons of the result, and the meta-analyzes of these data could therefore be questionable due to a possible bias.

Risk of bias
According to the PEDro scale score, most studies showed high quality, with eight of them reaching the maximum score. Only one study had a mean score (5/10) because it did not present the randomization and blinding process of the sample in the internal validity and did not present the variability of the data in the external validity, which can be seen in Table 1. The same can be observed in the Cochrane Collaboration's tool, represented in Figure 2.

Characteristics of the included studies
We included fifteen RCTs published from 2012 to 2018 in English, being all conducted in Brazil. Table 2 shows the characteristics of the participants in the included studies. The studies involved a total of 428 participants, with samples ranging from 6 to 96. The mean age ranged from 18 to 35 years, while the mean height ranged from 169 to 178.8 cm, and the mean weight ranged from 63.58 to 86 kg. The studies involved participants of both sexes; most of them included trained volunteers, while two included only untrained volunteers. The characteristics of the laser can be seen in Table 3, which involves many different types and brands, with wavelength ranging from 640 to 905 nm, energy ranging from 5 to 480 J and power ranging from 0,05 to 400 w. Table 3 also presents the protocols for exercises, laser application and evaluation of the outcomes of strength, fatigue and inflammatory markers.

DISCUSSION
Several studies have been published investigating the effects of PBM on exercise performance and post-exercise recovery, and there is some systematic review on this field available in the literature. Leal-Junior et al., 16 for example, concluded that the number for repetitions and the time until exhaustion increased after phototherapy, mainly when it was applied before exercises, independent of the wavelength used. Corroborating to these findings, Borsa et al., 17 found that exposing skeletal muscle to single-diode and multidiode laser or multidiode LED therapy was shown to positively affect physical performance by delaying the onset of fatigue, reducing the fatigue response, improving postexercise recovery, and protecting cells from exercise-induced damage. The present study updates the knowledge in this field, besides presenting strength outcomes, such as time to fatigue and exhaustion, oxygen consumption, inflammatory and oxidative stress markers and CK activity.

Strength Outcomes
Most studies analyzed the strength outcome, which was evaluated by maximum voluntary contraction or peak torque. Antonialli et al. 9 using 640, 875 and 905 wavelength combination increased maximal voluntary contraction with 10 and 30 J by applying PBM on quadriceps. Similarly, Vanin et al. 6 obtained an increase in the maximum voluntary contraction, using a dose of 10 and 50 J also on quadriceps, but with 810 nm wavelength only.
De Marchi et al. 11 analyzed the isometric capacity with laser application in the biceps brachii, with an increase in the same with wavelength 660 and 850 with dose 41,7 J. Almeida et al., 15 analyzing the peak force, used wavelength of 660 and 830 nm and, unlike other studies, using a lower dose of 5J. However, it also showed an increase in mean peak strength with PBM applications in biceps brachii.
Meanwhile, Dellagrana et al. 7 found no difference in peak torque during maximum isometric contraction with wavelength 670, 850, 880 and 950 and dE 15, 30 and 60 J. This same study used a different protocol, performing four sessions of PBM in quadriceps, hamstring and gastrocnemius regions, with seven days of interval between them.

Time to fatigue and exhaustion
Among the studies that analyzed the time to reach exhaustion, all obtained an increase in time to reach fatigue, suggesting an improvement in performance. De Marchi et al., 10 De Marchi et al. 11 and Pinto et al. 4 used a dose of 30 J, with wavelengths 810 nm, 905 nm and a cluster of 640, 875 and 905 nm, respectively. All of them applied PBM before the exercise protocol in the lower limbs.
In the same way, Zagatto et al. 2 applied PBM in long adductor, using a wavelength of 810 nm and dose 48J, and Larkin Kaiser et al. 14 used higher doses (240 and 480J) with the same wavelength, both of them also demonstrated an increase in time to reach fatigue.

Oxygen consumption
Regarding oxygen consumption, both studies that analyzed this variable obtained an increase in oxygen consumption. 10,13 De Marchi et al. 10 used a single wavelength of 810 nm with dose 30J, PBM was applied in quadriceps, hamstrings and gastrocnemius, while Miranda et al. 13 used wavelengths of 640, 875 and 905 nm, with the same dose of 30 J and regions of application.

CK activity and inflammatory markers
Most studies analyzed the outcome of CK activity, inflammatory and oxidative stress markers. Among all the studies that investigated the outcome of inflammatory and oxidative stress markers, only one found no difference in inflammatory markers but showed a decrease in CK activity, using 810 nm and dE 10 J in quadriceps.
Antonialli et al. 9 applied PBM in quadriceps with wavelengths of 640, 875 and 905 nm and dose 10, 30 and 50J, and also demonstrated a decrease in CK activity only with dose 50J. On the other hand, De Marchi et al. 11 only noted a reduction in CK in one time of analysis, being that in 48 hours, applying PBM with 905 nm and dose 30J in lower limbs.
With this same dose, 30 J, De Marchi et al. 10 and Pinto et al. 4 demonstrated a decrease in oxidative stress (TBARS and PC), CK activity and blood lactate and fatigue markers, using a wavelength of 810 nm and 640, 875 and 905 nm, respectively. De Marchi et al. 1 also analyzed oxidative stress markers (TBARS and PC) and CK activity with PBM applied in biceps brachii using 660 and 850 nm and dose 41,7 J and demonstrated a decrease in these markers.
Zagatto et al., 12 despite performing different application protocols, with six application sessions after exercise, also presented a decrease in CK activity with 810 nm and dose 48J.

Aerobic Training
The study of De Marchi et al. 11 was conducted with six professional athletes, performing phototherapy treatments before matches (40 minutes) and using 905 nm and 30 J. Blood samples were collected before treatments and immediately and 48 h after the end of the matches. The authors showed that PBM significantly increased staying in the pitch and improved all the biochemical markers evaluated. No statistically significant difference was found for the distance covered. The study suggests that pre-exercise PBM can enhance performance and accelerate the recovery of high-level futsal players.

Moment of Application
Regarding the moment of application, ten articles applied PBM before exercise, [4][5][6][7][9][10][11][13][14][15] while two applied after 1,12 and two performed before and after. 3,13 Only one study did not inform the time of application, but also showed no significant differences.² Of the studies that applied PBM before exercise or fatigue protocol, only one reported no significant differences, 14 two others reported less effect on maximal voluntary contraction and time to exhaustion. 5,7 While other studies have reported beneficial effects such as increased maximal voluntary contraction, increased peak strength, neuromuscular economy while running, increased oxygen uptake and time to exhaustion, and decreased CK activity, blood lactate levels. and markers of fatigue. 4,6,7,9,10,15 Analyzing the studies that applied PBM after exercise, both showed a decrease in the concentrations of biochemical markers of oxidative stress (TBARS and PC) and CK levels. De Marchi et al. 1 specifically presented an increase in isometric capacity assessment, while Zagatto et al. 12 reported that there was no significant difference in the 200 meters shot, but there was a moderate increase in 30-second performance after 48 hours.
Only two studies performed irradiation before and after exercise or phage protocols. Oliveira et al. 3 applied PBM 2 minutes pre-exercise and 3 minutes post-exercise and demonstrated decreased CK activity, and improved maximum voluntary isometric contraction at all post-exercise analyzed times (24, 48, 72 and 96 hours). Miranda et al. 13 applied PBM 5 to 10 minutes before and immediately after the treadmill aerobic test. It could be observed that the laser applied before and after aerobic exercise led to a significant increase in the percentage of oxygen consumption and time to exhaustion.

Limitation of study
One limitation of this systematic review is that despite the studies included had good quality, data showed great variance in laser parameters and treated muscle. This heterogeneity of the data made statistical analysis impossible by meta-analysis.

CONCLUSION
These findings demonstrate that PBM showed good results in skeletal muscle recovery after exercise. In most studies, it promoted improvement of maximum voluntary contraction, better oxygen consumption, increased time to achieve exhaustion and fatigue, and decreased the levels of CK and oxidative stress and fatigue markers. Even considering that the red band has a more superficial effect, better results were observed when the PBM was applied before exercise in both wavelengths.
All authors declare no potential conflict of interest related to this article