Immediate effects of photobiomodulation with low-level laser therapy on muscle performance: an integrative literature review

ABSTRACT

Purpose:

to analyze the influence of low-level laser on muscle performance and to identify the most used dosimetric parameters.

Methods:

the search for articles was carried out on the PubMed, BVS, Web of Science and SciELO platforms. The articles selected were original ones, with available abstracts and that evaluated the use of photobiomodulation on muscular performance. The data were analyzed according to the author, year of publication, sample, place of application, parameters evaluated, wavelength, dosimetry used and results found.

Results:

the final sample consisted of 27 articles published between 2008 and 2017. The sample size in the studies ranged from 8 to 60 individuals, aged from 17 to 70 years. A greater use of infrared wavelength, with punctual applications carried out in the path of the muscle, was observed. Regarding the dose, there was a variation from 0.24 to 50 joules per point. Of the total, only 5 (18.5%) studies had not found significant answers for the considered variables.

Conclusion:

most of the studies pointed out that low-level laser can improve muscle performance. The methodology used in the work was diversified, rendering data compilation difficult, being impossible to set the ideal parameters for this purpose.

Keywords:
Low-Level Light Therapy; Muscle Strength; Stomatognathic System; Speech, Language and Hearing Sciences

RESUMO

Objetivo:

analisar a influência do laser de baixa intensidade no desempenho muscular e identificar os parâmetros dosimétricos mais utilizados.

Métodos:

a busca de artigos foi realizada nas plataformas PubMed, BVS, Web of Science e SciELO. Foram selecionados artigos originais, com resumo disponível e que avaliaram o uso da fotobiomodulação sobre o desempenho muscular. Os dados foram analisados de acordo com o autor, ano de publicação, amostra, local de aplicação, parâmetros avaliados, comprimento de onda, dosimetria utilizada e resultados encontrados.

Resultados:

a amostra final consistiu de 27 artigos publicados entre os anos de 2008 e 2017. O tamanho das amostras nos estudos variou entre oito e 60 indivíduos, com faixa etária entre 17 e 70 anos. Observou-se maior uso do comprimento de onda infravermelho, com aplicações pontuais no trajeto do músculo. Em relação à dose, houve variação de 0,24 a 50 joules por ponto. Do total, apenas cinco (18,5%) trabalhos não encontraram respostas significativas para as variáveis pesquisadas.

Conclusão:

a maioria dos estudos apontou que o laser de baixa intensidade pode promover a melhora do desempenho muscular. A metodologia utilizada nos trabalhos foi diversa, o que dificultou a compilação dos dados e impossibilitou estabelecer os parâmetros dosimétricos ideais para esse objetivo.

Descritores:
Terapia com Luz de Baixa Intensidade; Força Muscular; Sistema Estomatognático; Fonoaudiologia

Introduction

The effects of phototherapy on the muscular system have been the object of study of professionals dedicated to muscle rehabilitation and sports performance¹. Among the main findings reported in the literature are the performance improvement¹, fatigue reduction², greater strength gain³ and relaxation^4,5.

These results are believed to occur due to the biomodulatory action that light can exert on the body. Through the so-called photochemical effect, the light energy absorbed by chromophores is transformed into chemical energy, which produces local and/or systemic biological effects in the organism⁶. For red and infrared wavelengths, the absorption occurs mainly in the mitochondria and directly intervened in the cellular respiration process, allowing the immediate influx of oxygen, the resumption of the respiratory chain and, consequently, the acceleration of intracellular adenosine triphosphate (ATP) synthesis^7,8. Given that muscle activity requires high energy expenditure, it is believed that resources which optimize ATP synthesis may also positively intervene in the functional performance¹.

The effects of laser on the organism are mainly determined by the dosimetric parameters selected by the therapist. Dosimetry is currently the biggest challenge in laser therapy and, in the scientific literature, the data about the best irradiation parameters are still quite controversial, with no specific well established protocols for each objective. Proper selection of physical variables, such as power, dose, energy density, irradiance, energy by points, emission type, application mode and wavelength are fundamental to achieve the desired results⁹.

Until now, only one study¹⁰ has evaluated the effects of photobiomodulation on the performance of an orofacial muscle. Despite the lack of evidence, clinical practice has shown that the association of this treatment tool with the orofacial motricity therapy improves the performance during the execution of the myofunctional and myotherapy exercises, and it is a field with great potential for future research.

Therefore, the objective of this work was to analyze studies on the influence of low-level laser on muscle performance and to identify the most commonly used dosimetric parameters (wavelength, dose, number of points, place of application).

Methods

This is an integrative literature review, which involved the following stages: elaboration of the guiding questions, establishment of the keywords and the inclusion and exclusion criteria of the articles, search, selection and critical analysis of the studies.

The guiding questions were: Does the application of low-level laser influence the muscle performance? What is the most used wavelength? What is the most commonly used dose and application modes? Search expressions were created with descriptors and keywords related to photobiomodulation and muscle performance and are described in Figure 1. Searches were performed on the PubMed, BVS, Web of Science and SciELO platforms.

Figure 1:
Data search strategy

The selection included articles that met the following criteria: be original; have available abstract and have, among its objectives, the one to evaluate the effects of low-level laser on muscle performance through parameters related to endurance, strength and fatigue.

Exclusion criteria were the exclusive use of other therapeutic light sources, such as the Light Emitting Diode (LED), the pulsed light and the high power laser. Articles that investigated the secondary improvement of muscle performance in relation to laser analgesic or relaxing action were also not considered in the analysis.

The selection of articles was made independently by two speech-language therapists by reading the abstracts. Data management was performed in a spreadsheet prepared in Microsoft Excel 2016 that allowed the evaluators two answers for selection: yes or no. The articles that received “yes” from both evaluators were included for full reading. Those who got a “no” answer from both researchers were excluded from the study. It was established that, if there were divergences of answers between the two evaluators, a consensus meeting would be held and, if the impasse remained, a third evaluator would be consulted.

The material analysis was performed in two stages. In the first, the duplicate references in the consulted databases were eliminated and, by reading the titles and abstracts, the articles that did not meet the established objectives were excluded. In the second stage, the previously selected articles were obtained and read in full, being discarded those that met the exclusion criteria. The data analysis stages are shown in Figure 2.

Figure 2:
Articles selection stages

From the articles selected for analysis of the results and discussion of the findings, author and year of publication, objectives, sample and age range, wavelength used, dose, number of points, place of application and results were recorded.

Literature Review

A total of 1,255 articles were initially found. After consensus between the two evaluators, 36 articles were included for full reading, of which nine were excluded because they had used other light sources or had muscle performance related to the laser analgesic effect. Figure 3 presents a summary of the analyzed information from the articles.

Figure 3:
Main findings of the literature on the application of low-level laser for the improvement of muscle performance

The sample of this study consisted, then, of 27 articles published in the years 2008¹¹,2009^12-14, 2010^2,15,16, 2011¹⁷, 2012^18-20, 2013^10,21,22, 2014^23-26, 2015^27,28, 2016^29-34 and 2017³⁵. Of these, only three (11.1%) were published in Portuguese^10,16,28. The other ones are in English, but all of them (100.0%) were produced by Brazilian authors.

The size of the samples in the studies ranged from 8¹³ to 60³⁰ individuals, age group from 17¹³ to 70 years^22,33. Regarding the participants' characteristics, 26 (96.3%) articles were conducted with healthy individuals and only 1 (3.7%) article evaluated the influence of the laser on the functional capacity of hospitalized patients with a history of heart failure²⁹. Still regarding the sample characterization, 3 (11.1%) studies had as specific objectives the evaluation of the performance of elderly women^22,31,33. Of the total, 3 (11.1%) presented skin tone as exclusion criterion, not including in the study black people or those with darker skin phototype^25,32,35.

Among the objectives of the studies, the main investigated aspects were the action of the laser on muscle fatigue^{2,11-13,15,16,18,20-23,25,26,28,29,33,35} and on the gain of strength and/or the performance in a given activity^{2,10,14,16,17,19,20,24,25,27,28,30-35}. The comparison of the laser effects with other therapeutic light sources was also the objective of 3 (11.1%) studies^10,13,35, and 1 (3.7%) work compared the effects of the red and infrared wavelengths on the muscle fatigue¹⁸.

The most used wavelength was infrared, being the choice of 23 (85.2%) ^{2,10,12-15,17,19-27,29-35} surveys. Only 3 (11.1%) ^11,16,28 used the red one. Besides these, 1 (3.7%) author used the two wavelengths separately to compare them¹⁸. No study used red and infrared wavelengths simultaneously.

The total doses used were calculated according to the amount of joules (J) per application point multiplied by the number of points, and it ranged from 1.92 J²⁸ to 300 J³². The minimum value used in one point was 0.24 J²⁸ and the maximum was 50 J³². The number of points was between 2^2,13 and “approximately” 29 points²⁵.

Regarding the application points, in all studies the application was carried out with contact and along the extension of the evaluated muscle or the one responsible for the requested function.

Concerning the results, 5 (18.5%) studies did not find significant answers in at least one of the researched variables^{13,21,28,34,35}. The others found statistically significant responses in at least one of the evaluated variables, with improvement in fatigue levels, strength gain and endurance, showing that low-level laser can be a capable therapy of optimizing muscle performance.

Muscle performance was assessed by the best performance of the proposed task, by cardiorespiratory assessment, by increasing the exercise load or also by the number of repetitions. Regarding fatigue, this factor was considered in the studies with reference to the execution time, the biochemical markers (such as lactate and creatine kinase protein levels) and the electromyographic signal. As these are not the objectives of this research, the assessment methods and instruments and the proposed physical tests were not discussed here. Such analyzes would be hampered mainly because they are specific knowledge of areas beyond the field of competence of Speech-language pathology.

The investigation of laser as a therapy to improve muscle performance is quite recent, as evidenced by the oldest article published, from 2008¹¹. This justifies the difficulties in finding ideal dosimetric parameters to achieve the different proposed objectives, being this limitation discussed by the authors of all analyzed works. It is believed that the fact that all works have Brazilian authorship may be related to the authors' research lines. It was noted that the same authors participated in several works, which led to very similar methodological designs.

In regard to the characterization of the sample, most studies included healthy individuals, and only one investigated the effects on hospitalized patients²⁹. This finding was expected because, when investigating new approaches or therapeutic tools, it is interesting to understand their functioning in healthy subjects and then evaluate their effectiveness in different clinical conditions. Regarding the age, most studies adopted some range between 17 and 36 years. One study presented intervals from 35 to 65 years²⁹ and only three, which intended to evaluate the effects in the elderly, adopted as inclusion criteria the age between 60 and 70 years^22,31,33.

None of the articles approached possible contraindications for laser therapy as exclusion criteria, not even those described in the manufacturers' manuals. Only factors that could influence the researched variables were taken into consideration and three studies excluded dark skinned participants from the sample^25,32,35 claiming that, because melanin is a chromophore, these subjects could have greater sensitivity to light.

Concerning the sample size, N was very reduced in all studies, with number of participants ranging between 8²⁶ and 60³⁰ subjects. All the authors mentioned that this amount may have intervened in the achieved results.

As for the dosimetric parameters, a great variation was observed. The most used wavelength was the infrared one, being the option of most studies^{2,10,12-15,17,20-27,29-35}. The main justification was the reference to previous studies and the fact that this wavelength presents greater penetrability in the human tissue.

Just three studies^11,16,28 used only the red wavelength. One of them did not obtain significant responses, but it was also the one that used the lowest dose²⁸. In another study¹¹, the authors reported that the choice for red was due to the availability of the equipment, recognizing that infrared would have been the ideal choice, although they found a significant increase in the number of repetitions. The third study that used the visible wavelength¹⁶ also observed an improvement in the peak torque (maximum functional muscle strength), but without effects on muscle fatigue. According to the authors, this may be due to the light's range depth, which allowed an energetic input for better contraction performance. However, its limited range was not enough for the accumulated energy to influence the fatigue index. The work that used the two wavelengths separately, for the purpose of comparing them¹⁸, found an improvement in the peak torque for both, without statistical difference between them. Given the justification based on the light range and considering the superficiality of the facial muscles, it is expected that there is no significant difference between these wavelengths when the effects on facial muscles are investigated.

The dose was a parameter that had great variation between the studies, especially considering the number of points and the total dose. The most used doses per point were 7 J^21,22,31,33 and 30 J^{2,15,19,27,32,35}. However, it was not possible to establish a correlation between the dose used and the results obtained, because, for the same dose, different results were found. This parameter is still the biggest challenge for the elaboration of protocols for laser use not only in muscle performance but also in other areas.

Regarding the application techniques, all were performed with tip contact on the skin and on the target muscle. The number of points varied, but in all studies there was concern that irradiation were carried out to the full extent of the target muscle. As there was a wide variety of equipment models and, consequently, in the area of light output, the number of points was also quite diverse. In 4 (14.8%) studies^17,20,31,34 the application was performed after the exercise protocols. In 2 (7.4%) studies^24,26 the irradiation occurred between the series and, in the other ones (77,8%), the laser was applied before the activity. The most used justification for the application after the exercise was based on the fact that the laser assists in muscle recovery after exertion. However, it was observed that this parameter was not determinant for therapeutic success, since it did not guarantee significant responses in all studies. The application before the exercises seems to be related to the increase of ATP synthesis provided by the photobiomodulatory action, which favors the energetic contribution to the muscular work during the activity.

The other parameters, such as power, energy density and intensity density were not discussed here because they are measures related to equipment models and not programmed by the researcher.

In general, it was observed by the results found that low-level laser intervenes in the muscle performance, improving the fatigue index, increasing strength gain, improving chemical markers and also increasing muscle endurance. However, due to the methodological diversity, it becomes difficult to identify effective parameters for obtaining these results.

Conclusion

This review allowed to identify the main dosimetric parameters for the application of low-level laser in the muscle performance. A predominance of infrared wavelength in the studies, with application in the extension of the muscle in equidistant points, was observed. It was not possible to correlate the results obtained with the dose used, but it was observed that low-level laser photobiomodulation is a potential tool for optimizing muscle performance and reducing fatigue levels, following intense activities.

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Publication Dates

• Publication in this collection
  28 Oct 2019
• Date of issue
  2019

History

• Received
  02 Aug 2019
• Accepted
  17 Sept 2019