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Effects of Photobiomodulation/Laser Therapy Combined With Resistance Training on Quadriceps Hypertrophy and Strength, and Postural Balance in Older Women: A Randomized, Triple-Blinded, Placebo-Controlled Study

Rodrigues, Claudiane Pedro PT1; Jacinto, Jeferson Lucas MSc1; Roveratti, Mirela Casonato PT1; Merlo, Jeanne Karlette PT1; Soares-Caldeira, Lúcio Flávio PhD1; Silva Ribeiro, Alex PhD1; Nunes, João Pedro MSc2; Junior, Eros de Oliveira PhD1; Aguiar, Andreo Fernando PhD1

Author Information
Journal of Geriatric Physical Therapy: July/September 2022 - Volume 45 - Issue 3 - p 125-133
doi: 10.1519/JPT.0000000000000313
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Abstract

CLINICAL HIGHLIGHTS

  • Does the addition of laser therapy to a resistance training program yield greater improvements in quadriceps muscle hypertrophy and strength, and single leg stance performance, than resistance training with sham therapy?
  • Both groups improved in all outcome variables, but there was no statistically significant difference between groups.
  • The results of this small sample study do not support the addition of laser therapy to resistance training programs.

INTRODUCTION

Age-associated declines in muscle strength1,2 and mass,3–5 called sarcopenia, are significantly associated with an increased risk of all-cause mortality.1–5 Sarcopenia is positively associated with risk of falls and fractures,6 postural balance decline,7 functional decline to perform the activities of daily living,8 and increased rate of hospitalization,9 resulting in poor health conditions and quality of life, and increased health care costs in older adults. Therefore, prophylactic measures to enhance or maintain muscle mass and strength in aging may have important clinical, functional, and economic implications for the aged population.

While resistance training (RT) is widely recognized as the main intervention to mitigate the negative effects of aging on skeletal muscle,10,11 photobiomodulation (PBM) therapy has recently emerged as an important adjunctive strategy to improve muscle function in older adults. Photobiomodulation, also known as low-level laser therapy and light-emitting diode therapy, involves the application of red (λ= 400–700 nm) or near-infrared (λ= 700–1100 nm) light12 on muscle tissue before (preconditioning) or after exercise. Photobiomodulation is posited to increase the production of adenosine triphosphate (ATP) via modulation of mitochondrial activity and to improve protein synthesis via activation of transcription factors.13,14

These cellular effects have been shown to improve fatigue,15–17 recovery,18,19 and acute strength performance,19,20 as verified in recent meta-analysis studies.21,22 Besides, only a few studies have investigated the chronic effects of PBM combined with long-term RT on muscle adaptations in older adults, and no additional benefit has been observed for 1-repetition maximum (1RM) strength gains,12,23–25 fatigue index,12,23 markers of inflammation,12 and muscle function.23–25 Similarly, the only one study that investigated the effects of PBM combined with a 12-week RT program on muscle thickness (MT) in older men reported no further effects of PBM.25

In addition, no previous study investigated the effects of PBM plus RT on muscle hypertrophy in older women. Compared with men, older women tend to present a higher proportion of satellite cells in response to RT,26 maintain higher protein synthesis across the lifespan,27 and have a superior mitochondrial activity.28,29 Due to this, PBM may be more effective in older women; however, further studies in older women are needed to test such a hypothesis. Moreover, no study has investigated the effects of PBM during an RT program on increases in postural balance, an important variable associated with reduced risk of falls in older adults.30 Therefore, there is still a limited amount of evidence to confirm the effectiveness of PBM as an adjunctive strategy to RT for improving muscle adaptations in older adults, mainly the hypertrophic effects in older women.

In addition, it is important to note that all aforementioned studies used a parallel-group design. This type of methodological approach does not provide sufficient control over intersubject variability (eg, genetic, food intake, motivation, training level, and lifestyle) during the intervention period, and may therefore influence the results of the study. In particular, the aforementioned studies presented small sample sizes ranging from 11 to 14 participants per group, which increases the likelihood of a type II error skewing the results. Therefore, new studies using well-controlled designs that consider individual variability and larger sample size are needed to confirm the effectiveness of PBM on muscle adaptations during long-term RT programs in older women.

Therefore, the purpose of the present study was to investigate the effects of PBM combined with an RT program on muscle hypertrophy and strength, and postural balance in older women. We used a contralateral control design in which each leg of the participants received active or placebo PBM, eliminating any confounding factors associated with intersubject variability. We hypothesized that active PBM in combination with RT would be more effective than RT alone (placebo PBM).

METHODS

Participants

Twenty-four women were recruited from a group of older adults (n = 30) enrolled in a physical activity and rehabilitation center from the university, and 22 (age 66.6 ± 5.2 years) of them completed the study (Table 1). A CONSORT flowchart is shown in Figure 1. The inclusion criteria were (1) aged 60 to 80 years, (2) classified as eutrophic (ie, body mass index [BMI] ≤ 27 kg/m2), and (3) classified as physically active (ie, performing at least 150 minutes/week of moderate physical activity) according to criteria of the International Physical Activity Questionnaire for older adults.31 We chose to recruit nonobese and physically active participants to avoid any possible influence of the higher thicker subcutaneous fat layer on the absorption of light energy and to ensure high performance in RT sessions. Participants were excluded if they (i) were tobacco product users; (ii) used any ergogenic aid within 1 year prior to the start of the study; (iii) were taking any medication that could affect the somatic and/or cognitive functions (eg, muscle relaxants, antidepressants, sleeping pills, allergy medicines, and stimulants); (iv) had any physiological (eg, cardiorespiratory and metabolic diseases, uncontrolled hypertension, or diabetes) or physical (eg, orthopedic or rheumatic diseases, muscular injury, fibromyalgia, or pain) limitation that could affect the ability to perform the training and physical tests, or (v) were unable to understand the informed consent document and provide a detailed description of their lifestyle. All participants were informed of the procedures, risks, and benefits of the investigation and signed an informed consent document approved by the local research ethics committee (protocol # 2.893.464). All procedures were carried out in accordance with the ethical standards as laid down in the Declaration of Helsinki.

F1
Figure 1.:
Experimental flowchart.
Table 1. - Baseline Characteristics (n = 22)
Characteristics Mean (SD)
Age, y 66.6 (5.2)
Height, cm 156.3 (8.3)
Weight, kg 62.7 (8.6)
BMI, kg/m2 25.6 (2.2)
SBP, mm Hg 121.4 (11.3)
DBP, mm Hg 72.3 (8.7)
Spo 2, % 96.4 (1.4)
Abbreviations: BMI, body mass index; DBP, diastolic blood pressure; SBP, systolic blood pressure; Spo2, blood oxygen saturation.

Sample Size Calculation

The sample size was calculated (G*Power software, version 3.0.1; Dusseldorf, Germany) for repeated-measures analysis of variance (ANOVA), within-between interaction (effects size = 0.45, α= 0.05, β= 0.80, groups = 2, measurements = 2, correlation among repeated measures = 0.5, nonsphericity correction = 1). The ES was estimated by within-group differences from previous studies17,25,32 that found a large ES (>0.8) for knee extension 1RM and a small ES (0.45) for vastus lateralis MT25 following an RT program in older adults. A minimum of 21 participants in each condition were required for an 82% chance (actual power) of rejecting the null hypothesis.

Experimental Design

This study was a randomized, triple-blinded, and placebo-controlled trial. Initially, all participants completed 3 sessions of familiarization to practice the unilateral leg extension exercise (3 sets of 10 repetitions with a self-selected load), and 3 nonconsecutive sessions of unilateral 1RM tests to eliminate learning effects33 and determine the precise basal 1RM load. Thereafter, participants were engaged in a systematized 10-wk RT training program involving unilateral leg extension exercise (2 times/week), in which both legs of the same participant were randomly (via a computer-generated sequence in the Web site https://www.random.org) assigned to receive active or placebo laser irradiation immediately before each workout. In the randomization sequence, the individuals who received even numbers were irradiated with active laser on the right leg and placebo laser on the left leg, and the opposite irradiation was applied to the participants who received odd numbers. In this way, 11 participants received the active laser in the right leg and 11 in the left leg, while the contralateral leg received placebo, thus avoiding any influence of right and left limbs on the workout and testing. All participants had the right leg as dominant. We chose a contralateral control design (crossover-like) to eliminate any confounding factors associated with intersubject variability (eg, genetic, food intake, motivation, training level, and lifestyle) that could affect the results. Systemic arterial blood pressure and blood oxygen saturation were assessed at baseline. Maximal dynamic strength by the unilateral leg extension 1RM, muscle hypertrophy by the vastus lateralis MT, and postural balance test were assessed before and after the RT program.

Interventions

Resistance training program

Participants engaged in a supervised 10-week RT program (2 days/week; 2 sets of 8-12 repetitions, with 1- to 2-minute rest between sets) for quadriceps muscle in a unilateral leg extension machine (Nakagym equipment, São Paulo, Brazil). The first session started with the dominant leg and the second session with the nondominant leg, and so on, so that the leg that started the workout was alternated during the training program. Each training session began with a warm-up that consisted of moderate walking on treadmill for 10 minutes and then 1 set of 10 repetitions with an estimated load based on the training performance (∼50% of the training load). An experienced researcher supervised individually each participant during every workout and the unilateral training load was adjusted every 2 weeks by 5% to 10% increase in load when the participant completed 2 additional repetitions (exceeding 12 repetitions) at the end of the second set. The sessions were performed between 8 am and 10 am.

Photobiomodulation

Laser PBM or placebo treatments were applied using an infrared AsGaAl laser (λ= 808 nm) equipment (Therapy XT; DMC São Carlos, São Paulo, Brazil). The irradiation parameters are shown in Table 2. Active or placebo laser irradiations were applied on 6 points of the quadriceps muscle (3 points on rectus femoris and 3 points on vastus lateralis) of both legs immediately before training sessions. The points were placed at 25%, 35%, and 45% of the total distance between the superior border of the patella and anterior superior iliac spine, bilaterally, with 6 cm of distance between each point (Figure 2). Active/placebo irradiation was based on the randomization list, with one leg receiving the active laser and the other receiving the placebo simultaneously with 2 probes. The irradiation parameters were chosen based on previous studies that showed improvements in muscle function in older women15,23,24 and a review study14 that suggested a total energy range of 18 to 240 J to produce positive adaptations on the quadriceps muscle during training programs.

Table 2. - Laser Parameters
Wavelength 808 nm
Frequency Continuous output
Optical output 100 mW
Irradiance 35.7 W/cm2
Energy 7 J each point
Spot size 0.028 cm2
Fluency 250 J/cm2
Time per point 70 s
Number of points 6
Total energy 42 J
Application mode probe Stationary in skin contact mode

F2
Figure 2.:
Laser irradiation points (black circles)

Blinding Procedures

Participants were blinded to the treatments using a dark blindfold during the PBM procedures. The application of PBM was performed simultaneously in both legs (active or placebo) by 2 registered physical therapists, who used opaque glasses to protect the eyes against irradiation. An experienced coach (>5 years of experience), blinded to the allocation of PBM treatments, was responsible for evaluations and training sessions. Finally, the researcher responsible for statistical analysis was also blinded to PBM treatments.

Outcomes Measures

Blood pressure and blood oxygen saturation measurements

Systolic (SBP) and diastolic (DBP) blood pressure were registered with an automatic monitor (Omron MX3 Plus, Bannockburn, Illinois) that was previously validated.34 Participants were placed in a seated position, and the measurement was performed in the left arm according to recommendations of the American Heart Association.35 The blood oxygen saturation (Spo2) was measured using a pulse oximeter (Nonin GO2 Achieve 9570, Plymouth, Minnesota).

One-repetition maximum

Unilateral leg extension 1RM was assessed before and after the 10-wk RT program, using a standard protocol previously documented elsewhere.36 The test was preceded by a specific warm-up exercise consisting of 2 sets of 10 and 5 repetitions at approximately 50% and approximately 60% of an estimated 1RM load, respectively, with a 3-minute rest between sets. Thereafter, participants had up to 3 attempts to achieve the 1RM with a progressive increase in load (0.5-10.0 kg) between each attempt and 5-minute rest intervals to allow sufficient recovery. Verbal encouragement was provided during each attempt, and the test was standardized and continuously monitored by the same experienced rater to ensure data quality and determine the load within 3 attempts. The intraclass correlation coefficient for test-retest reliability was 0.99 or more for the 1RM test in older women.37

Muscle thickness

The vastus lateralis MT was measured using an ultrasound (model MyLab30, Esaote, Florence, Italy) equipped with a 10-MHz linear array probe. Briefly, axial images of the vastus lateralis belly were obtained at 35% of the total distance between the superior border of the patella and the anterior superior iliac spine, with the probe placed perpendicular to the tissue interface, without depressing the skin, under a thick layer of water-soluble transmission gel. Images were taken of the both legs with the participants placed in the supine position on a bed. To avoid any erroneous influence of muscle swelling, images were obtained at least 48 hours before starting the training program and 48 hours after the last training session. Finally, the images were transferred to an image analysis software (ImageJ, model 1.48v), in which MT was measured by an experienced evaluator, blinded to treatments (active and placebo laser) and time point (pre- and posttraining). Three measures were taken, and the MT was determined as the average of the 3 measures. Ultrasound has been described as a reliable and valid tool for the assessment of muscle size38 and architecture39 in older adults, with intrarater reliability (intraclass correlation coefficient) for vastus lateralis ranging from 0.852 to 0.999.38

Postural balance

All participants performed 3 trials of the one-legged stance test maintained for 30 seconds on a force platform (BIOMEC400-412, EMG System do Brasil, Ltda São Paulo, Brazil), with a 30-second rest interval between each trial. The mean of 3 trials of each leg was used for analysis. During all trials, the participants were instructed to stand on the leg of their preference under the following standardized conditions: barefoot, eyes opened and looking at a target (circle) placed on a wall at eye level 2.5 m away, arms parallel to the trunk. To prevent falls in testing, an investigator stood close by the participant during all experimental tasks. The vertical ground reaction force data from the force platform were sampled at 100 Hz. All force signals were filtered with a 35-Hz low-pass filter (Butterworth filter). The signals from the 4 force platform sensors were converted into center-of-pressure (COP) data using computerized stabilography, which was compiled with MATLAB routines (The Mathworks, Natick, Massachusetts). Stabilographic analysis of COP was used to calculation of the following balance parameters: area of center of pressure (A-COP), velocity anteroposterior (Vel AP), velocity mediolateral (Vel ML), frequency anteroposterior (Freq AP), and frequency mediolateral (Freq ML). There is an inverse relationship between these variables and postural balance; that is, a lower value indicates greater balance.

We chose to assess the postural balance test because there is a relationship between quadriceps strength and balance in older adults, in which 18% of falls were attributed to quadriceps muscle weakness.40,41 A prospective 3-year longitudinal study also showed that quadriceps strength was a predictor of incident falls between community-dwelling older women.42 In addition, previous studies have shown that older adults with lower quadriceps strength have a greater risk of disability and falls,43 and a positive association has been observed between the quadriceps muscle activity and balance.44 Therefore, in our study, a significant increase in quadriceps muscle strength with RT alone (placebo laser) or training combined with PBM (active laser) should improve postural balance in the older women.

Statistical Analysis

The 2-way multivariate analysis of variance (MANOVA) was used to identify possible main effects considering the dependent variables (strength, MT, and postural balance parameters) and factors as time (pre- and posttraining), group (active laser and placebo), and interaction group×time. The assumption for the multivariate approach on the homogeneity of variances-covariance according to study design factors was preceded using Box's M test (M = 100.47; F(84,16004.671)= 1.015; P = .442). When MANOVA identified statistical significance, 2-way repeated-measures ANOVA was subsequent performed for each dependent variable. The significance level was set at P ≤ .05. The percent change (% delta) in the vastus lateralis MT between active and placebo laser conditions was analyzed using an unpaired t test. Statistical analyses were performed using SPSS Statistics for Windows version 20.0 (IBM Corp, Armonk, New York).

Effect size and confidence intervals (CIs) have been recommended as a more appropriate analysis for evidence of the magnitude of an intervention (treatment effect)45,46 due to their biological importance to make clinical decisions,46 and elimination of confounding factors such as sample size and measure variability.47–49 Thus, we also analyzed ES (95% CI) for mean difference (mean diff) between groups and the “minimal clinically important differences” (MCID)50 to ensure a more realistic biological interpretation of data, and to provide practical information on the magnitude or direction of the difference (clinical effect). Minimal clinically important difference was calculated by multiplying the pooled baseline standard deviation scores by 0.2, which corresponds to the smallest ES.46,50 Moreover, Cohen's d (ES)51 was calculated to quantify the magnitude of difference from pre- to posttraining within each group, considering an ES of 0.19 or less as trivial, 0.20 to 0.49 as small, 0.50 to 0.79 as moderate, and 0.80 or more as large.51

RESULTS

Baseline Characteristics and Flowchart of the Participants

Participants were classified as eutrophic (mean BMI = 26.5 kg/m2), nonhypertensive (mean SBP/DBP = 121.4/72.3 mm Hg), and had typical blood oxygen saturation (Spo2= 96.4%) (Table 1), indicating that body composition and hemodynamic conditions were normal at baseline. Thirty older women were evaluated for eligibility. Among these, 6 were excluded for the reasons described in Figure 1. Among the 24 remaining participants, 1 discontinued intervention and 1 did not adhere to 80% of the training sessions, and thus 22 were analyzed (Figure 1).

Statistical Significance for Dependent Variables

The MANOVA showed a significant main effect of time (pre- and posttraining), as indicated by the statistical criterion of Wilks' lambda (λ= 0.76; F(7,78)= 3.47; P = .003; η2= 0.238; β= 0.954) (Table 3). However, no significance was identified for group (λ= 0.95; F(7,78)= 0.52; P = .82; η2= 0.045; β= 0.213) and interaction group×time factors (λ= 0.987; F(7,78)= 0.149; P = .994; η2= 0.013; β= 0.088) for all dependent variables (Table 3). The repeated-measures ANOVA showed significant main effect of time for vastus lateralis MT (F = 416.45; P < .01), knee extension 1RM (F = 158.2; P < .01), A-COP (F = 13.9; P < .01), Vel AP (F = 12.2; P < .01), Vel ML (F = 33.0; P < .01), Freq AP (F = 7.7; P < .01), and Freq ML (F = 4.6; P < .05).

Editor's Note: This highly multivariate repeated measures study with a sample size of 44 (22 legs per group) was underpowered to detect between-group differences (post hoc power = .46) and within-between interactions (.36). It is thus not possible to know if there is truly no difference between groups, or if there is a true difference but the study was underpowered to find it.

Table 3. - Mean, Mean Difference, CI, Percent Change Mean, and Effect Size for All Dependent Variables
Preintervention
Mean (SD)
Postintervention
Mean (SD)
Mean Difference
(SD)
95% CI for Mean
Difference
Percent Change
Mean (SD)
ES Cohen's d
Knee extension 1RM, kg
Active laser 28.0 (10.7) 37.0 (13.5)a 9.09 (5.10) 6.83 to 11.35 35.4 (22.6) 0.74
Placebo laser 27.1 (11.5) 36.3 (13.3)a 9.18 (4.52) 7.18 to 11.18 38.0 (23.5) 0.75
VL thickness, cm
Active laser 1.86 (0.25) 2.01 (0.27)a 0.15 (0.04) 0.13 to 0.17 8.1 (2.2) 0.58
Placebo laser 1.84 (0.30) 1.96 (0.33)a 0.12 (0.04) 0.10 to 0.14 6.3 (1.9) 0.38
A-COP, cm2
Active laser 23.56 (9.55) 17.38 (7.75)a −6.2 (10.7) −10.93 to −1.43 −20.5 (37.2) −0.71
Placebo laser 21.77 (12.14) 16.57 (6.78)a −5.2 (9.5) −9.42 to −0.98 −17.2 (29.1) −0.53
Vel AP, cm/s
Active laser 4.79 (1.73) 3.94 (1.20)a −0.85 (1.19) −1.38 to −0.32 −14.2 (21.4) −0.57
Placebo laser 4.57 (1.88) 3.81 (1.70)a −0.76 (1.45) −1.40 to −0.12 −13.2 (26.4) −0.42
Vel ML, cm/s
Active laser 5.61 (1.62) 4.62 (1.14)a −1.00 (1.35) −1.60 to −0.40 −14.8 (22.8) −0.74
Placebo laser 5.67 (1.57) 4.75 (1.19)a −0.92 (0.80) −1.27 to −0.57 −14.9 (11.7) −0.66
Freq AP, Hz
Active laser 0.58 (0.21) 0.48 (0.20)a −0.10 (0.22) −0.20 to 0.00 −10.4 (41.5) −0.49
Placebo laser 0.64 (0.24) 0.54 (0.18)a −0.10 (0.23) −0.20 to 0.00 −5.2 (38.1) −0.47
Freq ML, Hz
Active laser 0.72 (0.26) 0.62 (0.18)a −0.09 (0.21) −0.18 to 0.00 −6.3 (39.4) −0.45
Placebo laser 0.75 (0.22) 0.68 (0.20)a −0.07 (0.23) −0.17 to 0.03 −3.3 (38.7) −0.33
Abbreviations: A-COP, area of center of pressure; CI, confidence interval; ES, effect size; Freq AP, frequency anteroposterior; Freq ML, frequency mediolateral; RM, repetition maximum; SD, standard deviation; Vel AP, velocity anteroposterior; Vel ML, velocity mediolateral; VL, vastus lateralis.
aP < .05 compared with pretest (repeated-measure multivariate analysis of variance).

Clinical Interpretation for Dependent Variables

Clinical interpretation data—mean difference ES (95% CI) and MCID—are shown in Table 3, and Figures 3 and 4A, B, C, and D. A small clinical effect was observed in favor of the active laser for vastus lateralis MT (Figure 3B), as evidenced by a greater ES (active = 0.58 [mod] vs placebo = 0.38 [small]) and a mean difference between groups (mean diff = 0.03) close to the MCID (0.05). No other clinical effects in favor of the active laser were observed for 1RM strength (Figure 3A), and postural balance variables (A-COP, Vel ML, Vel AP, Freq ML, and Freq AP) (Figure 4A, B, C, and D, respectively).

F3
Figure 3.:
Mean differences (95% CI) between groups (active and placebo, n = 22/group) and minimal clinically important differences (MCID) for knee extension 1RM (A) and vastus lateralis MT (B). ES = effect size from pre- to posttraining within each group. There was a small clinical effect in favor of the active laser for vastus lateralis MT, as evidenced by a greater ES (active = 0.58 vs placebo = 0.38) and a mean difference between groups (mean difference = 0.03) close to the MCID (0.05). CI indicates confidence interval; MT, muscle thickness; RM, repetition maximum.
F4
Figure 4.:
Mean differences (95% CI) between groups (active and placebo, n = 22/group) and minimal clinically important differences for postural balance variables: A-COP (A), Vel ML (B), Vel AP (C), Freq ML (D), and Freq AP (E). ES = effect size from pre- to posttraining within each group. There were no clinical effects in favor of the active laser for postural balance variables. A-COP, area of center of pressure; CI, confidence interval; Freq AP, frequency anteroposterior; Freq ML, frequency mediolateral; Vel AP, velocity anteroposterior; Vel ML, velocity mediolateral.

DISCUSSION

The purpose of this study was to investigate the effects of laser PBM during a 10-wk RT program on muscle strength and hypertrophy, and postural balance in older women. We hypothesized that PBM in combination with RT would be more effective than RT alone (placebo PBM). The main findings of this study were that (1) the RT alone (RT + placebo laser) improved vastus lateralis MT, 1RM strength, and postural balance from pre- to posttraining, and (2) clinical interpretation revealed a small effect in favor of the active laser (RT + active laser) compared with RT alone (RT + placebo laser) from pre- to posttraining for vastus lateralis MT. No other clinical effects were observed in favor of active laser for 1RM and postural balance variables (A-COP, Vel ML, Vel AP, Freq ML, and Freq AP).

Our results are similar to previous studies12,23,24 that demonstrated no additional effect of the active PBM compared with placebo PBM on 1RM strength after an RT program for quadriceps muscle in older women. It is important to mention that these aforementioned studies applied a shorter training period of 8 weeks, which could be insufficient to detect a small effect of PBM. However, our results revealed that increasing the training period to 10 weeks did not result in additional effects of the laser PBM on 1RM in older women, like that observed in older adult men engaged in a 12-week RT program.25 These results support the lack of positive effect of laser PBM alone (not combined with an RT program) on isokinetic peak torque in older women,17 but are contradictory to the positive findings on muscle strength performance in acute (without training) and chronic (with training) studies involving young men and women.16,52,53 Taken together, these findings indicate that PBM may be effective for young, but not for older adults, regardless of sex or presence of a long-term RT program. Given that the total energy range (young: 50-240 J vs older adults: 56-240 J) was similar between studies with young16,52,53 and older12,17,23–25 individuals, it seems reasonable to assume that the lack of a further effect of PBM on muscle strength in the older adults may be more associated with intrinsic biological factors than with external parameters of irradiation. This point requires further investigation.

In relation to muscle hypertrophy (vastus lateralis MT), we observed a similar pre- to-posttraining improvement for both active and placebo laser conditions. However, clinical interpretation showed a small clinical effect in favor of the active laser (ES = 0.58) compared with the placebo laser (ES = 0.38). In contrast to our study, Fritsch et al25 observed that laser PBM had no magnitude-based effects on vastus lateralis MT (ES, active: 0.41 vs placebo: 0.50) and rectus femoris MT (ES, active: 0.19 vs placebo: 0.10) following a 12-wk RT program in older men. A possible explanation for the conflicting results may be the different samples engaged in the studies (women vs men). Previous studies have shown that women have a higher intrinsic mitochondrial respiration29 and relative mitochondrial content28 compared with men, suggesting a superior mitochondrial activity. In addition, it has been shown that older women have a higher basal rate of protein synthesis,54 and a higher proportion of satellite cells than older men in response to high-intensity RT.26 Given that the ergogenic and regenerative effects of PBM are typically associated with better mitochondrial activity (eg, increased ATP production)14,55 and activation of satellite cell,56,57 it is plausible to assume that PBM may be more effective in older women than in men. Another point to be considered is that the study sample by Fritsch et al25 also included overweight men (BMI: 26.8 ± 4.1 kg/cm2), which does not exclude the possibility that a higher thicker subcutaneous fat layer and content of intramuscular fat could have impaired the absorption of light energy. However, this discussion is only speculative due to the vast number of mechanisms associated with muscle hypertrophy and regeneration, and the lack of evidence about how PBM acts on muscle growth in response to RT sessions. Therefore, further studies are warranted to demonstrate the mechanisms responsible for our findings.

We also found similar improvements in the postural balance variables (i.e., A-COP, Vel AP, Vel ML, Freq AP, and Freq ML) from pre- to posttraining between active and placebo laser conditions. It is well established that RT can improve postural balance in older adults,58,59 but only a previous study examined the effects of PBM combined on an RT program on postural balance in older women.24 The authors applied the active laser PBM (λ= 808 nm, 100 mW, total energy: 56 J) or placebo PBM immediately after each session of an 8-week RT program (2 times per week) involving knee flexion-extension exercise at 60% to 80% of 1RM, and found an improvement in the mediolateral stability index in right leg (MLSI—right) in the active PBM group, but not in the placebo PBM group. However, it is noteworthy that the RT program combined with active PBM improved only the MLSI—right, but no other variable related to postural balance improved, such as the overall stability index (both and right), the anteroposterior stability index (both and right), and the mediolateral stability index (MLSI—both) in both or right leg. In concordance with our study, such findings suggest that PBM may not be effective in improving many functional aspects related to postural balance in older adults. Given the importance of postural balance in reducing the risk of falling30 and the inverse relationship between aging and postural balance,60 additional studies are warranted to confirm whether PBM may be an effective strategy to improve postural balance in older adults in interventions of longer length.

This study has some limitations. First, we investigated the effects of PBM on the quadriceps muscle using only one type of single-joint exercise. This type of exercise guarantees greater safety and control of confounding variables. However, we cannot exclude the possibility that the application of PBM on other muscles involving multiarticular exercises could result in better results. Second, our sample was composed only of physically active older women, who have high muscle fitness to respond to the effects of RT and PBM. Further investigations are needed to expand our findings in other clinical populations related to aging (eg, prefrail and frail older adults, and older adults in muscle rehabilitation programs). Finally, our purpose was to investigate the effects of PBM during an RT program; therefore, we did not include a group treated with PBM alone (without training). However, positive results of isolated PBM could be clinically important for the rehabilitation of older adults unable to perform physical exercises. This point should be addressed in future investigations.

In conclusion, RT alone (placebo laser) was able to improve muscle size and strength, and postural balance, but when RT was associated with PBM therapy (active laser), the magnitude of hypertrophy effect was slightly greater (small clinical effect) than RT alone (placebo laser). This indicates that RT alone can be clinically important for counteracting the deleterious effects of aging on muscle mass, strength, and balance, and that laser PBM therapy can be used as a safe, low-cost, easy-to-apply intervention and complementary to RT for maintenance and rehabilitation of muscle mass in older women. Given the importance of muscle strength and postural balance for maintaining functional capacity and quality of life in older adults, researchers are encouraged to perform further investigations to examine the effectiveness of PBM on these variables.

ACKNOWLEDGMENT

We extend special thanks to all the participants for their engagement in this study.

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Keywords:

aged; aging; laser; phototherapy; resistance exercise; skeletal muscle

© 2021 APTA Geriatrics, An Academy of the American Physical Therapy Association.