Abstract
Objectives
Although photobiomodulation therapy combined with static magnetic field (PBMT-sMF) has demonstrated benefits for enhancing performance and recovery when applied before or after exercise sessions, its effects on adaptations during periods without training after strength training protocols remain unexplored. Therefore, we aimed to evaluate the effects of PBMT-sMF on the maintenance of muscle strength and structural properties of the quadriceps during a 4-week detraining period following a 12-week resistance strength training programme.
Methods
In this triple-blind, randomised, placebo-controlled trial, 48 healthy men were randomised to one of four groups: PBMT-sMF during both training and detraining, PBMT-sMF during training and placebo during detraining, placebo during training and PBMT-sMF during detraining or placebo throughout. All participants completed 12 weeks of unilateral resistance training (leg press and leg extension, twice weekly), followed by 4 weeks of detraining. PBMT-sMF or placebo was applied bilaterally to the anterior thigh prior to each session of exercise and two times per week during detraining. Outcomes included maximal voluntary contraction (MVC, primary outcome), as well as one-repetition maximum (1RM), muscle volume and anatomical cross-sectional area, measured at baseline and weeks 4, 8, 12 and 16.
Results
PBMT-sMF significantly preserved muscle strength (MVC and 1RM) and structural features (volume and anatomical cross-sectional area) during detraining compared with placebo (p<0.05). The group that received PBMT-sMF during both the training and detraining phases demonstrated the greatest preservation across all outcomes.
Conclusion
PBMT-sMF significantly attenuated strength loss and structural muscle changes during the detraining period after a resistance training programme.
Trial registration number
Keywords: Exercise, Adaptations of skeletal muscle to exercise and altered neuromuscular activity, Assessing physical training modalities in enhancing sports performance, Physiology, Randomised controlled trial
WHAT IS ALREADY KNOWN ON THIS TOPIC
Photobiomodulation therapy combined with a static magnetic field (PBMT-sMF) has been shown to enhance performance, accelerate recovery and support muscle adaptation when applied before or after different types of exercise.
Resistance strength training is widely recognised for enhancing muscle strength, mass and functional capacity.
Detraining, defined as the partial or complete loss of training-induced adaptations, typically results in declines in muscle strength, volume and functional capacity.
WHAT THIS STUDY ADDS
This study demonstrates that PBMT-sMF, when applied during or prior to a detraining period, helps preserve muscle strength and structural properties of the quadriceps in healthy young men.
The benefits of PBMT-sMF were evident even when applied exclusively during the training phase, suggesting lasting effects.
HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY
Supports PBMT-sMF as a promising non-invasive strategy to attenuate detraining-induced declines in muscle strength and structure.
Highlights potential applications in both clinical rehabilitation and athletic settings, particularly when training must be temporarily suspended.
Supports the need for further studies to confirm these results and assess their applicability across diverse populations and longer follow-up durations.
Introduction
Resistance training is well established as a key intervention for improving muscle strength, physical function and overall quality of life.1 However, periods of interrupted training, due to illness, injury or seasonal breaks, can lead to a reduction or cessation of mechanical and metabolic stimuli, resulting in a phenomenon known as detraining.2 Detraining refers to the partial or complete reversal of training-induced adaptations, affecting anatomical, physiological and performance-related outcomes.2 This process is characterised by a range of physiological alterations. Cardiorespiratory effects include increased resting heart rate, reduced maximal oxygen uptake and decreased blood volume.3 4 Metabolic changes may involve depleted muscle glycogen stores and lower blood lactate levels.4 At the muscular level, reductions in capillary density and mitochondrial ATP production capacity have been reported, accompanied by a decline in strength and power performance.25,7 These regressions are particularly concerning in clinical and athletic contexts where maintaining physical capacity is critical.2
To mitigate the effects of detraining, several strategies have been proposed, including reduced-frequency training, combined strength and balance exercises, retraining and recovery interventions such as optimised protein intake, nutritional support and neuromuscular electrical stimulation.8,12 Among these, photobiomodulation therapy (PBMT) has emerged as a promising non-thermal intervention that employs visible to infrared light to trigger photochemical processes in biological tissues, promoting reduction of pain, modulation of inflammation and tissue repair.13 When applied before exercise, PBMT—either alone or in combination with a static magnetic field (PBMT-sMF)—has demonstrated ergogenic and protective effects, including improved strength, delayed fatigue, enhanced recovery and reduced muscle damage.14 Furthermore, PBMT-sMF has been shown to enhance muscular adaptations during strength training and improve endurance capacity in aerobic exercise.15,18 Preclinical evidence suggests that PBMT may also help maintain muscle integrity during periods of reduced activity by stimulating satellite cell activation and angiogenesis—mechanisms similarly impaired during disuse atrophy and detraining.19 These findings support the potential use of PBMT and PBMT-sMF as countermeasures to prevent neuromuscular decline during training interruptions.
Despite encouraging evidence supporting PBMT and PBMT-sMF in enhancing performance and recovery when used before or after physical exercise, no studies to date have assessed whether these interventions can preserve muscular adaptations in the absence of training. Investigating their potential to prevent strength and morphological loss during detraining could represent a significant advance in muscle physiology and rehabilitation. Therefore, the present study aimed to evaluate the effects of PBMT-sMF on the maintenance of muscle strength and structural properties of the quadriceps during a 4-week detraining period following a 12-week resistance training programme. We hypothesised that PBMT-sMF applied during the detraining phase would attenuate neuromuscular regression, preserving both functional performance and muscle morphology achieved during the training phase.
Methods
Study design
A randomised, triple-blind, placebo-controlled trial—blinding participants, therapists and outcome assessors—was conducted at the Laboratory of Phototherapy and Innovative Technologies in Health. The study protocol was prospectively registered at ClinicalTrials.gov (NCT03858179), and its methodological details have been previously described in full (online supplemental file 1).20 No deviations from the original protocol occurred throughout the trial. All participants provided written informed consent prior to enrolment.
Participants and recruitment
Healthy male participants aged 18–35 years were included in the study if they were not engaged in a regular exercise programme (defined as performing physical activity more than once per week), had no history of musculoskeletal injury involving the trunk, hips, knees or ankles in the 2 months preceding enrolment and did not regularly use pharmacological agents or nutritional supplements. Participants were excluded if they had any musculoskeletal injury that limited or prevented participation in the strength training protocol within the 2 months prior to the study, or if they sustained such an injury during the intervention period (as self-reported or confirmed by medical diagnosis). Additional exclusion criteria included regular use of nutritional supplements or pharmacological agents, as well as the presence of signs or symptoms of neurological, metabolic, inflammatory, pulmonary, oncological or cardiovascular disorders that could interfere with the performance of high-intensity exercise.
Randomisation and blinding
The randomisation sequence was generated using the website random.org by a researcher who was not involved in participant assessment or treatment. Another researcher was responsible for programming the device to deliver either the active or placebo intervention and labelling the treatments according to the randomisation schedule. Participants were assigned to one of four experimental groups following a simple randomisation procedure with an allocation ratio of 1:1:1:1. Allocation concealment was ensured through the use of sequentially numbered, sealed and opaque envelopes.
Study outcomes were assessed by an evaluator who was blinded to group allocation. The PBMT-sMF device employed in the study did not produce thermal effects and emitted identical sounds and lights and displayed information regardless of whether it was set to active or placebo mode. These features ensured the blinding of both the therapist and the participants throughout the intervention.
Experimental groups
Participants were randomly assigned to one of four experimental groups, with 12 individuals in each group. All participants received PBMT-sMF or placebo irradiations before and after the exercise sessions, in accordance with their group assignment. The experimental groups were defined as follows:
Group A (PBMT-sMF+PBMT-sMF): Received PBMT-sMF before the strength training sessions (12 weeks, twice weekly) and PBMT-sMF during the detraining phase (4 weeks, twice weekly).
Group B (PBMT-sMF+placebo): Received PBMT-sMF before the strength training sessions and placebo during the detraining phase.
Group C (placebo+PBMT-sMF): Received a placebo before the strength training sessions and PBMT-sMF during the detraining phase.
Group D (placebo+placebo): Received a placebo both before the strength training sessions and during the detraining phase.
Interventions
Active PBMT-sMF and placebo
PBMT-sMF or its placebo equivalent was administered in two phases of the study: before each training session and throughout the detraining period, according to the participants’ group allocation. All applications were performed by a single trained researcher who was blinded to group assignments and randomisation, ensuring consistency and reducing bias. To maintain blinding, both the visual display and acoustic signals of the device were identical in active and placebo treatments. The intervention targeted six specific sites on the anterior region of each thigh—two medial, two central and two lateral points—using a direct contact technique with gentle pressure applied to the skin. The treatment was delivered bilaterally. A cluster probe comprising 12 diodes, manufactured by Multi Radiance Medical (Solon, Ohio, USA), was used to accommodate the large treatment area. The device incorporated four super-pulsed laser diodes (905 nm and 12.5 W peak power each), four infrared light-emitting diodes (LEDs) (875 nm and 17.5 mW average power), four red LEDs (640 nm and 15 mW average power) and a static magnetic field of 35 mT. Each irradiation site received a dose of 30 J, resulting in a total of 180 J per thigh. For the placebo treatment, the 905 nm lasers, 875 nm infrared LEDs and the magnetic field were deactivated. The red LEDs (640 nm) remained active but operated at a reduced output of 1 mW (mean power for each diode), preserving the visual appearance of red light without delivering a physiologically meaningful dose (5.47 J per thigh). For detailed information, please refer to the previously published study protocol.20 A comprehensive summary of all PBMT-sMF parameters is presented in table 1.
Table 1. Irradiation parameters for active photobiomodulation therapy with static magnetic field application.
| Parameter | Super-pulsed lasers | Red LEDs | Infrared LEDs |
|---|---|---|---|
| Number of diodes | 4 | 4 | 4 |
| Wavelength (nm) | 905 (±1) | 640 (±10) | 875 (±10) |
| Frequency (Hz) | 250 | 2 | 16 |
| Peak power (W) | 12.5 | – | – |
| Average output power (mW) | 0.3125 | 15 | 17.5 |
| Power density (mW/cm²) | 0.71 | 16.66 | 19.44 |
| Energy density (J/cm²) | 0.162 | 3.80 | 4.43 |
| Dose (J) | 0.07125 | 3.42 | 3.99 |
| Spot size (cm²) | 0.44 | 0.9 | 0.9 |
| General application parameters | |||
| Magnetic field (mT) | 35 | ||
| Irradiation time per site (s) | 228 | ||
| Total dose per site (J) | 30 | ||
| Total dose applied in muscular group (J) | 180 | ||
| Aperture of device (cm2) | 20 | ||
| Application mode | The cluster probe was applied perpendicularly to the skin, maintaining full contact and light pressure | ||
LED, light emitting diode.
Training and detraining protocol
For detailed information on the training and detraining procedures, please refer to the previously published protocol.20
Training protocol (training phase)
Following a 2-day period dedicated to baseline assessments, participants initiated the strength training protocol. The programme consisted of unilateral exercises, with each leg (right and left) trained separately, performed twice weekly on non-consecutive days, with at least 72 hours between sessions, over a period of 12 weeks (3 months), totalling 24 sessions. Each training session included five sets of 10 repetitions for each leg, performed separately, using a workload equivalent to 80% of one-repetition maximum (1RM). The exercises performed were the leg press and leg extension, using leg press and knee extension machines. A rest period of 2 min was provided between sets. In cases where participants could not complete a set, they were instructed to continue the exercise until the point of concentric failure (ie, until no further concentric contractions were possible). To ensure progressive overload, resistance loads were recalibrated every 4 weeks (after every eight sessions). All training sessions were conducted in a temperature-controlled environment, maintained between 22°C and 24°C. For detailed information, please refer to the previously published study protocol.
Detraining protocol (detraining phase)
Following the training phase, a 4-week detraining period was implemented, also scheduled twice weekly, during which participants abstained from strength training. They were instructed and closely monitored to avoid any structured training or recreational physical activity performed more than once per week.
Throughout both the training and detraining phases, participants received PBMT-sMF or placebo in accordance with their assigned group and the randomisation schedule.
Outcomes
Participants were evaluated at baseline, during the training period after 4, 8 and 12 weeks, and once again 4 weeks after cessation of training (16 weeks) to evaluate the effects of detraining. These assessments were deliberately scheduled to occur at least 24 hours apart from any intervention session (whether PBMT-sMF, placebo or strength training) to prevent acute effects from influencing the results. Importantly, evaluation days were never aligned with training days. To ensure consistency and control for circadian influences, all measurements were taken at the same time of day for each participant across all timepoints. Morning sessions included ultrasonographic imaging of the quadriceps femoris muscle and the measurement of maximal voluntary contraction (MVC). On those same days, the 1RM test was conducted in the afternoon. Assessments were carried out on both legs (unilaterally). Participants were asked to maintain regular physical activity and nutritional habits and were advised to avoid alcohol consumption and ensure adequate sleep prior to each evaluation.
The primary outcome was MVC.
Maximal voluntary contraction
To evaluate the maximal isometric strength of the knee extensors, participants were tested using an isokinetic dynamometer (Biodex System 4, Biodex Medical Systems, Shirley, New York, USA). Volunteers were positioned in the equipment’s seat with the trunk inclined at 100° relative to the hip. Both lower limbs were stabilised: the tested leg was fixed at 60° of knee flexion (with 0° indicating full extension), and the non-tested leg was also positioned at 100° hip flexion and secured to prevent movement. The trunk was stabilised using two crossing straps, and participants were instructed to fold their arms across their chest. Proper alignment of the dynamometer’s mechanical axis with the anatomical axis of the knee joint was ensured before initiating the test. Each assessment consisted of three isometric contractions of 5 s each, targeting the knee extensor muscles of one limb at a time. Prior to the test, standardised instructions were given, and participants received verbal encouragement during each contraction. The peak torque value obtained from the three attempts was used for analysis, as it reflects the muscle’s maximum capacity to produce force.21
The secondary outcomes were 1RM and morphological characteristics of the quadriceps muscle.
1RM test
To evaluate maximal dynamic muscular strength. Before testing, participants engaged in a brief standardised warm-up consisting of 5 min of unloaded cycling on a stationary bike (Inbramed, Brazil) at a cadence of 100 revolutions per minute. This light activity aimed to prepare the musculature for subsequent maximal effort testing. For both exercises employed in the study (leg press and leg extension), the range of motion was standardised between 0° (full extension) and 90° of knee flexion. Anatomical landmarks, including the greater trochanter, lateral epicondyle of the femur and lateral malleolus, were used to define and monitor the movement angle. Participants were first familiarised with the movement pattern by completing a warm-up set of five repetitions using a submaximal load (less than 60% of the estimated 1RM). Perceived effort during this familiarisation was evaluated using the OMNI scale for resistance exercise, which ranges from 0 (‘extremely easy’) to 10 (‘extremely hard’).22 The 1RM was identified using a progressive loading approach: resistance was gradually increased until the participant could no longer complete one full repetition with proper form and within the defined range of motion.23 A maximum of five attempts was allowed to determine the 1RM value, with 5-min rest intervals between trials to minimise fatigue and ensure test reliability. During each attempt, participants were strongly encouraged to exert maximal effort. The protocol was performed unilaterally using both the leg-extension and leg-press machines. The resistance load prescribed during training sessions was adjusted based on the 1RM results obtained at weeks 4 and 8.
Morphological characteristics of the quadriceps muscle
To evaluate training-induced structural changes in the quadriceps muscles, ultrasound imaging was employed.24 All images were acquired using a 60 mm linear transducer operating at 7.5 MHz. A single examiner, highly trained and experienced in musculoskeletal ultrasonography, conducted all acquisitions. During image capture, participants remained at rest, and a water-based conductive gel was applied to the probe to ensure optimal acoustic coupling and avoid direct pressure that could deform the underlying tissues.25 For the assessment of the rectus femoris and vastus lateralis, the transducer was aligned longitudinally with the direction of the muscle fibres. Participants were positioned supine, with their hips in a neutral position and knees fully extended. To ensure consistency across the five evaluation time points, anatomical landmarks were identified—specifically, the midpoint between the greater trochanter and the lateral condyle of the femur—and marked using transparencies placed over the skin.26 The following architectural parameters were extracted from these longitudinal images: (1) muscle thickness, measured perpendicularly between the superficial and deep aponeuroses; (2) fascicle length, corresponding to the estimated length of the muscle fibres and indicative of the number of sarcomeres arranged in series and (3) pennation angle, calculated from the angle formed between the muscle fascicles and the deep aponeurosis, reflecting the degree of fibre orientation and parallel sarcomere arrangement. Additionally, cross-sectional views of the rectus femoris and vastus medialis muscles were obtained by placing the probe transversely to the muscle fibres. These images were captured at the midpoint between the greater trochanter and the intercondylar fossa of the femur (positioned between the femur and tibia) and used to calculate the anatomical cross-sectional area (ACSA) of the rectus femoris and to estimate the overall quadriceps volume. Muscle volume was estimated using the equation:
V=113.7X+11.6Y–443.7.
where V=muscle vol, X=combined thickness of the rectus femoris and vastus medialis and Y=length of the thigh segment.27
Sample size
To date, no published studies have investigated the effects of PBMT-sMF during a detraining period following a structured strength training programme. Due to the absence of prior data, the sample size for the present trial was determined based on findings from a preliminary pilot study conducted by our research team, which included five participants per group.
The sample size estimation was based on a statistical power (β) of 80% and a significance level (α) of 5%. In the pilot phase, individuals who received PBMT-sMF during the detraining period demonstrated a peak torque of 257.25 Nm (SD=33.73) during the MVC assessment. In contrast, those in the placebo group showed a lower peak torque of 222.05 Nm (SD=29.87). Using these data and the Sample Size Calculator from DSS Research (https://www.dssresearch.com/KnowledgeCenter/toolkitcalculators/samplesizecalculators.aspx), the required number of participants was estimated at 10 per group, resulting in a total of 40 individuals. To account for potential attrition of up to 20%, a total of 48 volunteers (12 per group) were recruited.
Statistical analysis
All analyses were conducted according to the intention-to-treat principle.28 The distribution of the data was assessed using the Shapiro-Wilk test. Variables that followed a normal distribution were reported as mean±SD, while non-normally distributed data were presented as median values with corresponding IQRs. Parametric data were analysed using a two-way repeated measures analysis of variance with time and group as factors. When applicable, post hoc comparisons were carried out using the Bonferroni correction. Non-parametric outcomes were evaluated using the Friedman test, followed by Wilcoxon signed-rank tests for pairwise comparisons. Both absolute values and percentage changes from baseline were considered in the analysis. Statistical significance was set at p<0.05. Effect sizes were calculated using Cohen’s d and interpreted according to established thresholds: small (0.2), moderate (0.5) and large (0.8).29 All statistical analyses were carried out by one of the authors, who remained blinded to group assignments throughout the process and conducted the analyses based solely on the randomisation codes.
Results
All 48 participants completed the 16-week study protocol, with no dropouts reported. The study was conducted between March 2019 and May 2022. The baseline characteristics of the volunteers are summarised in table 2. Statistical analysis revealed no significant differences (p>0.05) among the four experimental groups regarding anthropometric variables or baseline measures. No adverse events or important harms related to the intervention were observed in any of the study groups.
Table 2. Anthropometric characteristics of participants at baseline (n=48).
| PBMT-sMF+PBMT-sMF | PBMT-sMF+placebo | Placebo+PBMT-sMF | Placebo+placebo | |
|---|---|---|---|---|
| Age (years) | 25.30±6.63 | 25.09±5.11 | 24.42±4.08 | 26.80±6.91 |
| Weight (kg) | 80.58±13.52 | 81.62±13.61 | 73.88±14.43 | 75.21±18.44 |
| Height (cm) | 173.40±4.95 | 176.45±7.85 | 170.92±5.71 | 174.80±7.32 |
Continuous variables are expressed as mean (SD).
PBMT-sMF, photobiomodulation therapy combined with static magnetic field.
Maximal voluntary contraction
The percentage change in MVC across all assessment points is illustrated in figure 1. During the training period, the PBMT-sMF+PBMT-sMF group demonstrated a statistically significant improvement of approximately 15% at weeks 4, 8 and 12 compared with the placebo+placebo group. In the detraining phase (week 16), the group that received PBMT-sMF only during the training phase (PBMT-sMF+placebo) exhibited a statistically significant change compared with the placebo+placebo group, with a difference of around 20%. Notably, when PBMT-sMF was applied throughout the entire study period (PBMT-sMF+PBMT-sMF), the difference at week 16 was even greater, reaching approximately 30% compared with the placebo+placebo group. Additionally, the group treated with PBMT-sMF exclusively during the detraining period (placebo+PBMT/sMF) also showed a statistically significant change in percentage compared with the placebo+placebo group at week 16.
Figure 1. Percentage change in MVC test. The data are presented in mean and SEM. ‘a’ versus placebo+placebo, ‘b’ versus placebo+PBMT-sMF and ‘c’ versus PBMT-sMF+placebo. *p<0.05; ***p<0.001 and ****p<0.0001. MVC, maximal voluntary contraction; PBMT-sMF, photobiomodulation therapy combined with static magnetic field.
1RM test
The percentage change in the 1RM leg extension test among study participants is graphically represented in figure 2. The PBMT-sMF+PBMT-sMF group exhibited a statistically significant improvement compared with the placebo+placebo group at all experimental time points, with a difference of approximately 20% at the end of the training phase and 30% after 4 weeks without training (detraining period). Furthermore, at week 16, the PBMT-sMF+PBMT-sMF group also showed a greater percentage change when compared with the PBMT-sMF+placebo group. It is also noteworthy that the group receiving PBMT-sMF exclusively during the detraining period (placebo+PBMT-sMF) demonstrated a statistically significant improvement relative to the placebo+placebo group, with a difference of approximately 8% at week 16.
Figure 2. Percentage change in 1RM—leg extension—test. The data are presented in mean and SEM. ‘a’ versus placebo+placebo, ‘b’ versus placebo+PBMT-sMF and ‘c’ versus PBMT-sMF+placebo. *p<0.05, **p<0.01, ***p<0.001 and ****p<0.0001. 1RM, one-repetition maximum; PBMT-sMF, photobiomodulation therapy with static magnetic field.
The percentage change in the 1RM leg press test is illustrated in figure 3. The group that received PBMT-sMF throughout the entire study period (PBMT-sMF+PBMT-sMF) showed a significantly greater percentage change compared with the placebo+placebo group—approximately 35% at the end of the training phase (week 12) and around 47% after 4 weeks without training (week 16). The PBMT-sMF+PBMT-sMF group also demonstrated a statistically significant difference at week 16 compared with the PBMT-sMF+placebo group, with an increase of approximately 12%. Additionally, PBMT-sMF applied exclusively during the detraining period (placebo+PBMT-sMF) resulted in a statistically significant percentage change of about 11% in comparison to the placebo+placebo group at the same time point.
Figure 3. Percentage change in 1RM—leg press—test. The data are presented in mean and SEM. ‘a’ versus placebo+placebo, ‘b’ versus placebo+PBMT-sMF and ‘c’ versus PBMT-sMF+placebo. *p<0.05; ****p<0.0001. 1RM, one-repetition maximum; PBMT-sMF, photobiomodulation therapy with static magnetic field.
Morphological characteristics of the quadriceps muscle
Both the PBMT-sMF+PBMT-sMF group and the PBMT-sMF+placebo group, shown in figure 4, demonstrated a statistically significant percentage change in ACSA compared with the placebo+placebo group from week 8 onwards. By the end of the study, the PBMT-sMF+PBMT-sMF group exhibited a markedly greater change—approximately 35% versus 11% in the placebo+placebo group. It is also noteworthy that PBMT-sMF applied exclusively during the detraining phase (placebo+PBMT-sMF group) resulted in a higher percentage change compared with the placebo+placebo group, with values around 21% and 11%, respectively.
Figure 4. Percentage change in ACSA. The data are presented in mean and SEM. ‘a’ versus placebo+placebo, ‘b’ versus placebo+PBMT-sMF and ‘c’ versus PBMT-sMF+placebo. *p<0.05, **p<0.01, ***p<0.001 and ****p<0.0001. ACSA, anatomical cross-sectional area; PBMT-sMF, photobiomodulation therapy with static magnetic field.
The percentage change in muscle volume throughout the study is presented in figure 5. Both the PBMT-sMF+PBMT-sMF and PBMT-sMF+placebo groups showed statistically significant increases in muscle volume after 12 weeks of training when compared with the placebo+placebo and placebo+PBMT/-sMF groups. At the end of the study (week 16), the PBMT-sMF+PBMT-sMF group exhibited the greatest percentage change, particularly in comparison to the placebo+placebo group—approximately 29% versus 9%, respectively. Once again, PBMT-sMF applied exclusively during the detraining period (placebo+PBMT-sMF group) led to a higher percentage change relative to the placebo+placebo group, with values around 18% and 9%, respectively.
Figure 5. Percentage change in muscle volume. The data are presented in mean and SEM. ‘a‘ versus placebo+placebo, ‘b’ versus placebo+PBMT-sMF and ‘c’ versus PBMT-sMF+placebo. *p<0.05, **p<0.01, ***p<0.001 and ****p<0.0001. PBMT-sMF, photobiomodulation therapy with static magnetic field.
The absolute values of functional outcomes (MVC and 1RM) and structural properties of the quadriceps (ACSA and muscle volume) for both lower limbs are summarised in table 3. Regarding MVC, both the PBMT-sMF+PBMT-sMF and PBMT-sMF+placebo groups outperformed the placebo+placebo group during the training (weeks 4, 8 and 12) and detraining (week 16) phases. Notably, PBMT-sMF+placebo also showed superior results compared with placebo+PBMT-sMF at all time points. Additionally, PBMT-sMF+PBMT-sMF yielded greater improvements than placebo+PBMT-sMF from week 8 in the right leg and from week 4 in the left leg.
Table 3. Outcome measures in absolute values (n=48).
| Outcomes | Groups (n=12/group) | Baseline (n=12/group) | 4 weeks (n=12/group) | 8 weeks (n=12/group) | 12 weeks (n=12/group) | 16 weeks (n=12/group) |
|---|---|---|---|---|---|---|
| MVC—right leg (Nm) | PBMT-sMF+PBMT-sMF | 238.07±44.12 | 289.28±41.53a* | 309.95±44.00a* b* | 333.09±42.00a* b** | 332.08±42.56a**** b* |
| PBMT-sMF+placebo | 244.03±41.74 | 296.06±39.71a** b* |
317.54±41.13a**
b** |
337.75±42.45a** b** | 308.72±40.18a** | |
| Placebo+PBMT-sMF | 227.40±52.09 | 242.94±50.16 | 260.30±50.47 | 276.83±48.51 | 277.38±49.64 | |
| Placebo+placebo | 222.82±36.98 | 239.02±35.37 | 258.85±33.86 | 279.65±36.73 | 246.95±32.85 | |
| MVC—left leg (Nm) | PBMT-sMF+PBMT-sMF | 238.20±26.97 | 296.10±37.31a*** b** | 317.33±37.20a*** b** | 340.38±35.10a**** b** | 335.58±32.45a**** b** |
| PBMT-sMF+placebo | 241.55±33.95 | 296.75±32.37a*** b** | 317.42±31.53a*** b** | 338.67±32.10a*** b** | 318.62±36.08a**** | |
| Placebo+PBMT-sMF | 220.12±35.99 | 245.23±36.61 | 265.77±36.66 | 285.77±38.05 | 283.03±35.70a* | |
| Placebo+placebo | 217.94±31,28 | 238.04±37.91 | 257.04±40.12 | 276.82±38.99 | 238.24±34.02 | |
| Leg extension—1RM—right leg (kg) | PBMT-sMF+PBMT-sMF | 73.50±12.70 | 89.80±14.97a* | 101.40±16.74a** | 109.50±16.46a*** b* | 109.90±16.91a**** b* |
| PBMT-sMF+placebo | 71.82±11.89 | 87.55±14.91 | 98.73±17.27a** | 105.64±18.06a** | 98.36±16.55a** | |
| Placebo+PBMT-sMF | 73.33±15.86 | 80.58±17.22 | 86.92±17.76 | 91.75±18.55 | 91.33±17.53 | |
| Placebo+placebo | 63.50±9.14 | 72.60±9.36 | 78.20±12.87 | 82.40±12.06 | 75.00±9.13 | |
| Leg extension—1RM—left leg (kg) | PBMT-sMF+PBMT-sMF | 72.50±12.08 | 87.90±14.51 | 99.40±16.97a** | 105.70±20.68a** | 105.10±21.40a**** |
| PBMT-sMF+placebo | 65.91±13.00 | 81.18±15.34 | 89.73±16.13 | 95.09±15.18 | 89.91±15.06 | |
| Placebo+PBMT-sMF | 72.50±15.15 | 80.67±15.44 | 87.17±15.54 | 92.83±18.99 | 93.50±20.10 | |
| Placebo+placebo | 64.00±10.22 | 71.60±9.98 | 77.80±12.56 | 83.30±14.18 | 76.70±13.22 | |
| Leg press—1RM—right leg (kg) | PBMT-sMF+PBMT-sMF | 58.00±21.50 | 80.70±25.82 | 91.10±28.25a* | 100.80±32.92a** | 100.00±33.20a*** |
| PBMT-sMF+placebo | 50.91±15.78 | 72.45±20.50 | 81.27±22.58 | 90.18±24.83 | 81.82±22.66 | |
| Placebo+PBMT-sMF | 60.83±18.32 | 70.08±20.15 | 76.58±21.63 | 83.42±22.36 | 82.33±21.50 | |
| Placebo+placebo | 51.00±9.94 | 57.50±10.14 | 64.20±10.44 | 71.30±12.00 | 63.90±11.29 | |
| Leg press—1RM—left leg (kg) | PBMT-sMF+PBMT-sMF | 54.00±18.38 | 73.70±21.19 | 83.30±23.56 | 92.90±25.68a* | 91.30±25.65a** |
| PBMT-sMF+placebo | 51.82±16.01 | 71.45±19.50 | 80.27±20.12 | 89.09±25.81 | 82.45±24.83 | |
| Placebo+PBMT-sMF | 56.67±16.14 | 66.58±17.34 | 72.25±17.96 | 78.92±19.23 | 76.92±18.04 | |
| Placebo+placebo | 51.00±11.01 | 58.30±11.91 | 64.40±14.22 | 71.60±16.89 | 65.00±15.08 | |
| ACSA—right leg (cm³) | PBMT-sMF+PBMT-sMF | 6.87±0.66 | 8.05±0.74 | 8.91±0.78 | 9.63±0.85 | 9.41±1.11 |
| PBMT-sMF+placebo | 7.72±1.59 | 9.09±1.99 | 9.88±1.59a* | 10.44±1.87a* | 9.80±1.75a** | |
| Placebo+PBMT-sMF | 7.65±1.24 | 8.50±1.20 | 8.85±1.29 | 9.53±1.02 | 9.33±1.11 | |
| Placebo+placebo | 7.08±1.11 | 8.07±1.64 | 8.34±1.50 | 8.90±1.61 | 7.96±1.54 | |
| ACSA—left leg (cm³) | PBMT-sMF+PBMT-sMF | 6.14±0.78 | 7.30±0.72 | 8.08±0.75 | 8.44±0.97c* | 8.20±0.94 |
| PBMT-sMF+placebo | 7.05±1.31 | 8.44±1.68 | 9.25±1.61b** | 9.68±1.73b* | 8.73±1.33 | |
| Placebo+PBMT-sMF | 6.74±1.00 | 7.58±1.10 | 7.76±0.92 | 8.45±0.86 | 8.07±0.77 | |
| Placebo+placebo | 7.13±0.91 | 7.89±1.07 | 8.26±1.02 | 8.90±1.05 | 7.84±0.93 | |
| Muscle volume—right leg (cm³) | PBMT-sMF+PBMT-sMF | 578.71±89.35 | 636.33±80.68 | 692.19±76.22 | 753.64±100.87a** b* | 749.48±92.44a*** |
| PBMT-sMF+placebo | 564.57±71.20 | 635.17±54.25 | 677.49±51.56 | 735.59±85.12a* | 672.94±85.71 | |
| Placebo+PBMT-sMF | 570.44±83.65 | 628.11±74.95 | 640.23±82.18 | 665.65±81.98 | 680.99±93.85 | |
| Placebo+placebo | 548.92±53.48 | 610.04±67.60 | 630.65±77.97 | 649.07±81.46 | 609.30±73.78 | |
| Muscle volume—left leg (cm³) | PBMT-sMF+PBMT-sMF | 552.66±93.52 | 627.82±118.43 | 661.42±123.67 | 729.38±125.93 | 711.67±127.85a* |
| PBMT-sMF+placebo | 560.75±59.26 | 637.40±57.19 | 672.21±81.13 | 734.32±105.90 | 657.37±70.39 | |
| Placebo+PBMT-sMF | 566.01±81.03 | 625.15±83.87 | 647.89±105.02 | 671.48±84.28 | 663.57±102.16 | |
| Placebo+placebo | 557.61±43.45 | 612.53±52.92 | 641.57±71.74 | 662.64±60.04 | 604.57±58.65 |
Continuous variables are expressed as mean (SD). Statistically significant differences are highlighted in bold. ‘a‘ versus placebo+placebo, ‘b’ versus placebo+PBMT-sMF and ‘c’ versus PBMT-sMF+placebo. *p<0.05, **p<0.01, ***p<0.001 and ****p<0.0001.
ACSA, anatomical cross-sectional area; PBMT-sMF, photobiomodulation therapy with static magnetic field.
For the 1RM test (leg extension), the PBMT-sMF+PBMT-sMF group showed consistently better results for the right leg across all phases, while PBMT-sMF+Placebo was significantly superior to placebo+placebo from week 8. In the left leg, only PBMT-sMF+PBMT-sMF demonstrated superiority, beginning at week 8. Similar patterns were observed in the leg press test: the right leg showed consistent improvements with PBMT-sMF+PBMT-sMF, whereas in the left leg, significant differences were observed only at weeks 12 and 16.
Concerning quadriceps structure, the PBMT-sMF+placebo group exhibited greater ACSA values than placebo+placebo from week 8 in both legs and also outperformed placebo+PBMT-sMF at weeks 8 and 12 in the left leg. Furthermore, PBMT-sMF+PBMT-sMF showed superior ACSA compared with all other groups at week 12. As for muscle volume, significant increases were found in the right leg for the PBMT-sMF+PBMT-sMF group compared with placebo+placebo (weeks 12 and 16) and with placebo+PBMT-sMF (week 12). In the left leg, this difference was observed only during the detraining phase.
Discussion
This is the first randomised, triple-blind, placebo-controlled trial to investigate the effects of PBMT-sMF on the preservation of muscular adaptations during a detraining period following a 12-week resistance training programme. Our results demonstrate that PBMT-sMF significantly preserved muscle strength, measured by MVC and 1RM tests, regardless of the phase in which it was applied, when compared with placebo. Furthermore, PBMT-sMF effectively attenuated losses in quadriceps muscle volume and ACSA. Overall, PBMT-sMF provided superior benefits compared with placebo, regardless of whether it was applied during the training phase, the detraining phase or both.
Our results during the training period align with existing evidence demonstrating that PBMT-sMF, when applied prior to resistance training, enhances muscle strength in both athletic and non-athletic populations.14 Moreover, they are consistent with previous findings showing that PBMT alone, applied before eccentric training sessions twice a week over 8 weeks, can improve muscle thickness and strength gains.15 Despite this consistency with the current literature, our study uniquely investigates its use during a detraining period following a resistance training programme, which limits direct comparisons and highlights the novelty of our findings.
To date, no prior evidence has evaluated the use of PBMT-sMF during a detraining period following a prolonged 12-week resistance training programme. However, a previous study investigated the effects of PBMT-sMF applied during both a 12-week aerobic training period and subsequent detraining, reporting improvements in time to exhaustion and maximal oxygen uptake.18 Although the type of exercise (endurance vs resistance) and the primary outcomes (cardiorespiratory vs muscular) differ, their findings align with ours in demonstrating that PBMT-sMF can enhance training adaptations and attenuate losses during detraining. Notably, both studies shared key methodological similarities, including comparable participant profiles, identical durations for training (12 weeks) and detraining (4 weeks), and the use of the same PBMT device, dose and irradiation time per point, reinforcing the consistency and relevance of these results.
Some studies have shown that strength gains may be partially preserved for several weeks after training cessation. However, the findings remain inconsistent, and no clear consensus has been established.230,32 While earlier reports have suggested that strength adaptations can be retained following training interruption,2 our results demonstrated that participants who received only a placebo during both training and detraining phases experienced a significant reduction in muscle strength and quadriceps morphology. These findings suggest that the maintenance of muscle adaptations observed in our study may be attributed to the use of PBMT-sMF.
Our findings are further supported by a previous investigation that compared the effects of PBMT, neuromuscular electrical stimulation and placebo when applied prior to a combined plyometric and strength training programme performed three times per week over 6 weeks.16 In that study, strength in the non-dominant limb increased after PBMT combined with training and was sustained even after a 2-week period without further training or intervention. This reinforces our hypothesis that the preservation of strength adaptations achieved through resistance training may be attributed to the adjunct use of PBMT.
The findings of this study offer novel insights into the potential of PBMT-sMF as a non-invasive strategy to preserve muscular adaptations during periods of physical inactivity. The beneficial effects observed may be explained by mechanisms such as improved mitochondrial efficiency, reduced oxidative damage, activation of satellite cells and increased capillary formation. These processes have been previously associated with photobiomodulation therapy in both preclinical and clinical settings19 33 34 and may play a key role in mitigating the typical loss of muscle strength and morphology associated with detraining.25,7
For clinicians, PBMT-sMF represents a promising adjunct to rehabilitation strategies, particularly for individuals temporarily unable to maintain regular physical training due to injury, surgery or chronic health conditions. From a practical standpoint, the use of PBMT-sMF during periods of detraining may also play a role in injury prevention when athletes resume training at full capacity. By helping to preserve muscle strength and structural integrity of the quadriceps, PBMT-sMF could mitigate the risk associated with abrupt increases in training load following either scheduled breaks (eg, transition or recovery phases) or unscheduled interruptions (eg, illness or personal reasons). In this context, PBMT-sMF may represent a supportive strategy not only for maintaining performance but also for facilitating a safer reintegration into training routines. On a broader level, incorporating PBMT-sMF into rehabilitation guidelines or public health strategies may help reduce the functional decline associated with physical inactivity, especially among older adults, accelerate a safer return to sport in athletes and improve outcomes in clinical populations. Further studies in diverse populations are essential to confirm these findings, and it will be important to translate them into practical recommendations and inform evidence-based policy development.
This study presents several strengths. The experimental design was methodologically rigorous, incorporating triple-blinding of participants, therapists and outcome assessors, which strengthens the internal validity of the findings. Functional and structural muscle outcomes were assessed using validated tools, ensuring reliable and clinically meaningful results. Importantly, the study protocol was prospectively registered and adhered to throughout the trial, with no deviations from the planned methodology. Randomisation procedures were appropriately conducted with allocation concealment, and data were analysed following the intention-to-treat principle. The inclusion of a placebo group was critical for minimising potential sources of bias, including placebo effects, regression to the mean and therapist-related influences. Additionally, the placebo device emitted a minimal amount of light energy (5.47 J per thigh), which was several orders of magnitude below the therapeutic threshold reported in the literature and therefore considered biologically insignificant.35 This negligible emission was intended solely to maintain the visual appearance of active treatment and did not produce measurable photobiomodulation effects.
Nonetheless, some limitations must be acknowledged. The results are specific to healthy, physically inactive young men and may not be generalisable to other populations such as women, older adults or individuals with clinical conditions. Additionally, the study did not directly investigate the cellular or molecular mechanisms responsible for the observed effects. Another limitation is the inability to fully separate the specific contributions of PBMT and sMF to the observed outcomes. However, previous evidence suggests that the combination of PBMT and sMF can produce synergistic biological effects, potentially enhancing cellular metabolism and redox balance compared with photobiomodulation alone.36 Finally, the follow-up period after training cessation was limited to 4 weeks, and the long-term sustainability of the benefits remains unknown.
Future studies involving more diverse populations, including women, older adults, athletes and individuals with various clinical conditions, are needed to confirm our findings and enhance generalisability. Additionally, mechanistic investigations using muscle biopsies or molecular analyses would be valuable for elucidating the biological pathways underlying the observed effects of PBMT-sMF and for better understanding the potential synergy between PBMT and sMF. Finally, it is essential to explore the efficacy of PBMT-sMF across a range of training and detraining protocols to determine its applicability in different exercise contexts and timeframes.
Conclusions
PBMT-sMF, particularly when applied both during training and detraining phases, or exclusively during the training phase, significantly attenuated strength loss and structural muscle changes during the detraining period. These findings support the use of this intervention as a promising strategy to preserve neuromuscular adaptations when continued physical training is not feasible.
Supplementary material
Footnotes
Funding: This study was supported by the São Paulo Research Foundation (FAPESP) under the following grants: doctoral scholarship to PRVdP (grant number 16/11878-5), research funding to ECPL-J (grant number 18/21982-0), and visiting researcher grant to JMB (grant number 19/15719-7). ECPL-J also receives a research productivity scholarship from the National Council for Scientific and Technological Development (CNPq, grant number 310468/2021-3).
Provenance and peer review: Not commissioned; externally peer reviewed.
Patient consent for publication: Not applicable.
Ethics approval: This study involves human participants. The study adhered to ethical standards and received approval from the Research Ethics Committee of Universidade Nove de Julho (protocol number 1781602). Participants gave informed consent to participate in the study before taking part.
Data availability free text: The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.
Patient and public involvement: Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Data availability statement
Data are available upon reasonable request.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
Data are available upon reasonable request.