In 48 previously inactive young men (12 per arm) doing 12 weeks of leg training, a contact device combining red/near-infrared light with a static magnetic field (PBMT-sMF), held on the front of the thighs before each session, gave larger strength gains than placebo (right-leg knee-extension MVC at week 12: 333 and 338 vs 280 Nm with placebo throughout), and after 4 weeks off those two groups were still stronger than placebo throughout (332 and 309 vs 247 Nm); the benefit of using it during the break rests mainly on percentage-change analyses; one small trial whose senior author receives research support from the device maker
Main caveat: Funding and ties: funded by FAPESP and CNPq (Brazilian public agencies).
De Paiva and colleagues asked whether photobiomodulation therapy combined with a static magnetic field (PBMT-sMF) helps hold strength and muscle size when training stops.
Forty-eight healthy men aged 18-35 who were not exercising regularly were randomised to four groups of 12: active device in both the 12-week training phase and the 4-week detraining phase, active then placebo, placebo then active, or placebo throughout.
Each leg was trained separately on leg press and leg extension, 5 × 10 at 80% 1RM, twice a week for 24 sessions; then all training stopped for 4 weeks. The device, a cluster probe of lasers, infrared and red LEDs and a magnet, was pressed on six spots of each front thigh (30 J per spot, 180 J per thigh) before every session and twice a week during the break.
Placebo used the same device with the lasers, infrared LEDs and magnet off and the red LEDs dimmed. All 48 finished. Knee-extension maximal voluntary contraction (MVC, primary), 1RM on both machines, quadriceps cross-sectional area and estimated volume were measured every 4 weeks.
Both groups that had the active device during training gained more MVC than placebo-throughout; for the both-phases group the authors give about 15% more at weeks 4, 8 and 12.
Over the 4 weeks off, the placebo-throughout group lost much of its gain, while the groups with the active device during the break held their week-12 values; at week 16 the both-phases group was about 30% above placebo-throughout on MVC. The authors conclude that the device attenuated strength and muscle loss in detraining.
Participants
48 healthy men, 18-35 years, not in a regular exercise programme (activity no more than once a week), no recent lower-limb or trunk injury, no regular supplements or drugs.
Mean age 24.4-26.8 years, weight 73.9-81.6 kg by group; no significant baseline differences reported (p > 0.05).
12 per group; all 48 completed the 16 weeks; no adverse events. Conducted March 2019 to May 2022 in São Paulo, Brazil.
Protocol
Training (all groups): each leg trained on its own, leg press and leg extension, 5 sets × 10 at 80% 1RM, 2 min between sets, sets taken to concentric failure when 10 could not be completed; loads reset from new 1RM tests every 4 weeks; twice a week, at least 72 h apart, 24 sessions. Detraining: 4 weeks with no strength training and no structured or recreational activity more than once a week.
What was applied: a Multi Radiance Medical (Solon, Ohio, USA) cluster probe of 12 diodes plus a static magnet: 4 super-pulsed lasers at 905 nm (12.5 W peak, 0.3125 mW average, 250 Hz), 4 infrared LEDs at 875 nm (17.5 mW, 16 Hz), 4 red LEDs at 640 nm (15 mW, 2 Hz), magnetic field 35 mT, aperture 20 cm². 228 s and 30 J per spot, six spots per thigh (two medial, two central, two lateral), 180 J per thigh, both thighs.In full skin contact with light pressure, applied by one therapist. Light was not tested alone: the magnet was always on with it.
When: before each training session (one sentence of the methods says before and after; the time between application and lifting is not given) and twice a week during the 4 weeks off. Tests were kept at least 24 h away from any device or training session.
Placebo: the same device with lasers, infrared LEDs and magnet switched off and the red LEDs at 1 mW (5.47 J per thigh), so it glowed red; display and sounds identical; no heat from either mode. Whether participants could tell which they had was not asked or reported.
Blinding and allocation: random.org sequence, 1:1:1:1, sealed opaque envelopes; a separate researcher programmed the device; therapist, participants, assessor and statistician blinded.
Outcomes: MVC (primary), peak isometric knee-extension torque at 60° on a Biodex 4 dynamometer, each leg; 1RM on leg extension and leg press, each leg; ultrasound anatomical cross-sectional area (ACSA) of rectus femoris and quadriceps volume estimated from muscle thickness and thigh length by an equation.
Analysis: two-way repeated-measures ANOVA with Bonferroni post hoc tests, on absolute values and on percentage change from baseline; legs analysed separately; Cohen's d said to be calculated but not reported; no confidence intervals. Sample size from a 5-per-group pilot: 10 per group needed, 12 recruited for attrition.
Registration: NCT03858179, prospectively registered, protocol published; the authors report no deviations.
Key Results
Absolute means (SD omitted), right leg, Table 3. Groups: device+device / device+placebo / placebo+device / placebo+placebo.
| Outcome, right leg | Baseline | Week 12 (end of training) | Week 16 (after 4 weeks off) |
|---|---|---|---|
| MVC, Nm | 238.1 / 244.0 / 227.4 / 222.8 | 333.1 / 337.8 / 276.8 / 279.7 | 332.1 / 308.7 / 277.4 / 247.0 |
| Leg-extension 1RM, kg | 73.5 / 71.8 / 73.3 / 63.5 | 109.5 / 105.6 / 91.8 / 82.4 | 109.9 / 98.4 / 91.3 / 75.0 |
| Leg-press 1RM, kg | 58.0 / 50.9 / 60.8 / 51.0 | 100.8 / 90.2 / 83.4 / 71.3 | 100.0 / 81.8 / 82.3 / 63.9 |
| Rectus femoris ACSA, cm² (printed cm³) | 6.87 / 7.72 / 7.65 / 7.08 | 9.63 / 10.44 / 9.53 / 8.90 | 9.41 / 9.80 / 9.33 / 7.96 |
| Quadriceps volume, cm³ | 578.7 / 564.6 / 570.4 / 548.9 | 753.6 / 735.6 / 665.7 / 649.1 | 749.5 / 672.9 / 681.0 / 609.3 |
Left-leg MVC, same order: week 12 340.4 / 338.7 / 285.8 / 276.8; week 16 335.6 / 318.6 / 283.0 / 238.2 Nm.
Training phase (weeks 0-12), authors' between-group differences in percentage change:
MVC: device+device about 15% above placebo+placebo at weeks 4, 8 and 12; in absolute values both device-in-training groups were above placebo+placebo at every training time point in both legs (p < 0.05 to < 0.0001).
Leg-extension 1RM: device+device about 20% above placebo+placebo at week 12. Leg-press 1RM: about 35% at week 12.
Muscle volume: both device-in-training groups above both placebo-in-training groups at week 12 (percentage change).
Detraining phase (week 16), authors' between-group differences in percentage change versus placebo+placebo:
| Outcome | Device+device | Device+placebo | Placebo+device |
|---|---|---|---|
| MVC | about 30% | about 20% | significant, size not given |
| Leg-extension 1RM | about 30% (also above device+placebo) | not stated | about 8% |
| Leg-press 1RM | about 47% (about 12% above device+placebo) | not stated | about 11% |
| ACSA (change from baseline) | about 35% vs 11% | significant from week 8 | about 21% vs 11% |
| Muscle volume (change from baseline) | about 29% vs 9% | not stated | about 18% vs 9% |
No confidence intervals or effect sizes are reported for any comparison; significance is shown as p-value bands only.
The absolute-value table marks fewer differences than the percentage-change figures: for ACSA it shows no significant difference between device+device and placebo+placebo at any time in either leg, although the text and figure report one from week 8; for leg-extension 1RM the figure text reports device+device above placebo+placebo at every time point, while the table marks the left leg only from week 8.At week 16 the table marks no significant difference between device+device and device+placebo on any outcome, and between placebo+device and placebo+placebo only for left-leg MVC (283.0 vs 238.2 Nm, p < 0.05), so the contrasts that isolate use during the break rest on the percentage-change analyses.
Practical Application
One triple-blind trial in previously inactive men found that a contact probe of red/near-infrared light plus a static magnet, pressed on the thighs, added to strength gains over 12 weeks of leg training, and those groups stayed stronger than placebo throughout after 4 weeks off; that using it during the break itself helps rests mainly on percentage-change analyses, the senior author receives research support from the device maker, and light was never tested without the magnet. It is not evidence for a red-light panel, a light bed or light in the room.
To "will red light stop me losing strength on a break?": say one trial of a specific combined device (180 J per thigh, in contact, twice a week) found less loss over 4 weeks in men who had just learned to lift, mostly in percentage-change analyses (used only during the break, it beat placebo in absolute values for left-leg MVC alone); no trial in trained athletes is in the base.The base's own answer for a break is a reduced dose rather than none: in small trials one strength session a week held strength for about 12 weeks (SYNTH-106), and a few days off is not an emergency (SYNTH-36).
SYNTH-83 has a row on infrared-emitting clothing (markers, no outcome) and a separate row on red and near-infrared light, which reports this trial under the light-plus-magnet probe: larger strength gains than placebo in untrained men, the senior author's research support from the maker, light never tested alone.
Inference, not tested here: the size of the effect in beginners, whose early gains are large and partly neural, may not carry to trained lifters; whether the light, the magnet or both did the work cannot be told from this design.
Limitations
Funding and ties: funded by FAPESP and CNPq (Brazilian public agencies). The senior author (Leal-Junior) receives research support from Multi Radiance Medical, which makes the device tested; the corresponding author (Tomazoni) has a personal relationship with him, and both list ELJ Consultancy, Scientific Consultants as an affiliation.
12 per arm; the sample size came from a 5-per-group pilot; no confidence intervals or effect sizes are reported, and the text, figures and absolute table do not agree on every significant difference (ACSA most clearly).
Light and a 35 mT magnet were always given together; there is no light-only arm.
Placebo kept a dim red glow and identical sounds; whether blinding held was not checked.
Participants were inactive young men starting to lift; the placebo-throughout group also started with the lowest leg-extension 1RM (63.5 kg against 71.8-73.5 kg) and the device+device group with the smallest rectus femoris ACSA (6.87 against 7.08-7.72 cm²), which favours both in percentage change, and results were analysed partly as percentage change from baseline.
Detraining lasted 4 weeks; activity during it was limited by instruction and monitoring, not measured.
The text says the device was given before each session in one place and before and after in another.
The published protocol and supplement were not read.
How this entry was checked and changed
Why this level
a parallel trial with 12 per arm (10-19 band), read in full; not lowered for markers, since the outcomes are strength and muscle size