Red light therapy and resistance exercise

Research and practical recommendations

Takeaways/Highlights:

-Red light therapy has strong evidence showing performance enhancement when it comes to multiple-repetition strength.

-Red light therapy likely increases 1RM (1 rep max) strength, though the research isn't as high quality as the research showing benefit to multiple-repetition strength

-Red light therapy appears to reduce muscle damage, which likely doesn't negatively impact muscle hypertrophy and may speed recovery from training.

-Red light therapy is likely more effective than cryotherapy when it comes to improving post-exercise recovery.

-Red light therapy has evidence showing concrete enhancement of muscle hypertrophy.

-Red light therapy is probably not effective for tendon injuries.

-Red light therapy is plausibly effective for muscle injuries but human data is lacking, there are quite a few animal studies with good results, however.

Glossary:

Photobiomodulation(PBM)/LLLT/LEDT: Some of the terms often used in research for red light therapy. LLLT stands for low level laser therapy, or low level light therapy. LEDT stands for Light-Emitting Diode Therapy.

Ergogenic: Performance-enhancing

Creatine Kinase: For the purposes of this article, a marker of muscle damage

Electromyography: A technique for tracking the electrical activity of skeletal muscles

Strength training

STRENGTH TRAINING

In this article we'll take an evidence-based look at how to best go about implementing red light therapy for performance and recovery enhancement when it comes to resistance exercise. 

An excellent systematic review of human studies was published in 2013 by Borsa et al. [1], it's a little bit old considering the amount of literature on the topic that has come out since, but well worth a look at. Their criteria for inclusion were: “Eligible studies had to be original research published in English as full papers, involve human participants, and receive a minimum score of 7 out of 10 on the Physiotherapy Evidence Database (PEDro) scale.”, not a low bar. They tracked fatigue, number of repetitions to fatigue, total work performed, strength, electromyographic activity, and a number of postexercise biomarkers. They concluded “Phototherapy administered before resistance exercise consistently has been found to provide ergogenic and prophylactic benefits to skeletal muscle.” In layman's terms, this means the studies examined found a benefit both to performance, and to the prevention of adverse events, like injuries. It states “In human studies, skeletal muscle exposed to selected doses of laser or LED therapy demonstrated enhanced performance by maintaining contractile force output and delaying the onset of fatigue when challenged with resistance exercise. Skeletal muscle exposed to laser or LED therapy also had less cell damage after exercise, indicating that phototherapy provided protection from exercise-induced damage.” This reduction in muscle damage particularly is an interesting finding, that is replicated in many other studies. Is there any reason to believe this might hinder muscle growth? 

Muscle damage

Muscle damage has long been thought to be a contributing factor to muscle hypertrophy. However, as Brad Schoenfeld (2012) [2] points out, this has yet to be established, and there are researchers who believe any of four things:

-Muscle damage is the major factor when it comes to hypertrophy
-Muscle damage contributes to hypertrophy to a small degree
-Muscle damage is neutral when it comes to hypertrophy
-Muscle damage negatively impacts hypertrophy

What is muscle damage?  

Muscle damage involves the internal structures or our wrapping layers of muscle fibers being damaged. When muscle fibers are only mildly damaged, their existing structures are retained, and the damaged parts are replaced with new proteins. When they are too severely damaged, they die off and are replaced with entirely new fibers.

Damas et al. (2018) [3] concluded "Increases in muscle protein synthesis post-resistance training session only contribute to muscle hypertrophy after a progressive attenuation of muscle damage, and even more significantly when damage is minimal." citing one of their earlier papers (2016) [4]. It goes on, "Furthermore, resistance training protocols that do not promote significant muscle damage still induce similar muscle hypertrophy and strength gains compared to conditions that do promote initial muscle damage. Thus, we conclude that muscle damage is not the process that mediates or potentiates resistance training-induced muscle hypertrophy.". Let's look at some more evidence for this: Wilson et al. (2013) [5] showed that blood flow restriction training, despite causing similar hypertrophy, didn't score nearly as high on the indirect indices used to measure muscle damage. Loenneke et al. (2014) [6], found the same, while also including some blood measurements that correlate with muscle damage. Another hit to the idea of muscle damage being a core driver of muscle hypertrophy are the effects of non-steroidal anti-inflammatories, which are mixed. Some studies find detriment, for instance Lilja et al. (2017) [7]. Others find the opposite, like Trappe et al. (2011) [8]. Interestingly, the first is on younger subjects, and the second one on older ones, the dose used was the same. It should be noted that the inflammatory signal is not the only mechanism by which muscle damage is hypothesized by some to promote muscle hypertrophy. So, there you have it, the evidence on muscle damage and hypertrophy is, at best, mixed and inconclusive. 

Muscle hypertrophy

Now that we've covered muscle damage, it would be a good idea to take a look at research on the direct effects, or lack thereof, that red and near-infrared light therapy has on muscle hypertrophy. There is an interesting twin study by Ferraresi et al. (2016) [9] that takes a look at this. The findings look promising, it found an up-regulation of protein synthesis as well as increased muscle hypertrophy as shown by MRI: “There was an increase in the volume in thigh muscles measured by MRI when LEDT was combined with the training program (+20%, from 2937 to 3523 cm3), whereas placebo therapy gave a smaller increase in muscle volume (+5%, from 3152 to 3316 cm3)”. The total energy used in this study was quite small, only 75 joules. This is smaller than the amounts that found the biggest benefits in some other studies. The wavelength used was exclusively 850 nm. An obvious problem here is the small sample size, just two people, it would be hard to draw any conclusions. Another study looking at hypertrophy is the one by Baroni et al. (2014) [10], this one has a more reasonable sample size of 30 male subjects, it finds over 150% the muscle hypertrophy when using red light therapy + training compared to training alone (15.4% vs 9.4%).

How about strength gains over time? This study states that “Subjects from the Training+Light therapy Group reached significantly higher percent changes compared to subjects from the Training-only Group for sum of muscles’ thicknesses (15.4 vs. 9.4 %), isometric peak torque (20.5 vs. 13.7 %), and eccentric peak torque (32.2 vs. 20.0 %).”, indicating the strength increase difference is just about as big as the hypertrophy difference between the Training-only Group and the Train+Light therapy Group. In another paper worth a look at, Barbosa et al. (2017) [11] state: “The aim of this study was to evaluate LLLT effects on grip strength. The protocol has shown to be efficient in improving the grip strength. This condition was more evident in 904 nm, in which there was a difference between the final and baseline. Thereby, it can be suggested that, for 904 nm group, irradiation was effective to improve the grip strength.”

Performance enhancement

Next, let's consider the ergogenic effects of red light therapy, the 2013 review [1] states: “The decrease in muscle function associated with fatigue is believed to be a result of metabolic alterations, such as substrate depletion (lack of ATP and glycogen), oxidative stress, tissue hypoxia, and blood acidification. Researchers also have indicated that specific doses of phototherapy reduce blood lactate and inflammatory biomarker levels after strenuous upper and lower extremity exercise.” There are a number of studies directly measuring performance enhancement, such as Hemmings et al. (2017) [12]. The experimental groups in this study were able to perform 13.3 and 13.2 more reps on average than placebo, with 60 seconds and 120 seconds exposure to near-infrared radiation. 30 seconds of exposure differed from placebo only by 2.3 reps on average. The irradiance was unfortunately not reported, so we're not sure what those exposure times correspond to. It is interesting however, to see near identical benefit from 60 and 120 second exposure here, suggesting a clear cutoff point. The total amounts of reps were 48.6 (placebo) to 61.9 (60 second light exposure). This study used LEDs, so it's unfortunate the irradiance wasn't reported. Another interesting study by Pinto et al. (2008) [13] found the light-treated group to perform 5.8 reps above placebo on average. The light was administered immediately before performing the exercise. Only 655 nm was used, which is red light, this may not be ideal for penetration purposes, as red light tends to penetrate less deeply than near-infrared light. The exact number of reps performed isn't mentioned, only provided as an image, suggesting roughly 16 (day 1, placebo) to 28 reps (day 8, light therapy) were performed on average. The increase of 6 reps is therefore quite significant, as it's a pretty high percentage of total repetitions performed. The participants weren't entirely untrained, as they were professional volleyball players. It would be interesting to see how these results hold up with intermediate or advanced strength trainees.

For a more comprehensive overview, we can look at the meta-analysis by Vanin et al. (2017) [14], 39 clinical trials were included, with a sample size of 5 to 60 participants. Meta-analysis was performed on four variables: time to exhaustion, number of repetitions, blood lactate, and isometric peak torque, which is strongly correlated with 1RM (1 rep max) strength. The evidence for decreased blood lactate was found to be the strongest, followed by time to exhaustion and number of repetitions performed. The evidence for increased isometric peak torque was found to be of the lowest quality. Of the 39 studies included, 32 found positive results in at least one of the variables outlined above, a few found benefits to other variables. It states: “The main reasons for the lack of positive results at any variable found in five studies are the small area covered by the photobiomodulation therapy irradiation or parameters used, showing the importance of the establishment of an optimal therapeutic window to reach the effects of photobiostimulation. The scanning mode of application used by Gorgey et al. (2008) [19] did not show positive results, which can be explained by the high refraction of the light and energy loss provided by this kind of application.” This is something our article on research doses versus at-home LED panel doses goes into in a lot more depth. Also noteworthy, and slightly surprising: “Interestingly, positive results were found in most studies that combined different wavelengths and sources of light, and it must be explored because few studies used this kind of device.” [14] Our devices feature a mix of 660 and 850 nm, both wavelengths that have been studied quite extensively.

Injuries

What about muscle and tendon injuries? 

When it comes to red light therapy and tendon injuries, the research doesn't seem impressive, a 2018 meta-analysis on lateral elbow tendinopathy by Mamais et al. [15] states: “This umbrella review found poor results for the effectiveness of LLLT in the management of lateral elbow tendinopathy. However, LLLT cannot be ruled out, as it is a dose-response modality, and the optimal treatment dose needs yet to be discovered. The current review recommends that practitioners do not use LLLT as sole treatment for lateral elbow tendinopathy but can be used in combination with other suggested treatments. In addition, the included studies had methodological shortcomings. Therefore, further research with well-designed RCTs is required to provide meaningful evidence on the effectiveness (absolute and relative) of LLLT for the management of lateral elbow tendinopathy.” When looking at the individual human trials, there is indeed very little research we could find. This also applies to the meta-analysis on achilles tendinopathy by Martimbianco et al. (2020) [16], stating: “Concluding, the evidence is of low to very low certainty, and there were insufficient data to support clinical effects of low-level laser therapy for Achilles tendinopathy.”. However, it should be noted the doses used were fairly low at 2-18 joules, perhaps that influenced the results.

When it comes to research on red light therapy and muscle injuries, unfortunately nearly all of it is animal research, but the results are more positive generally. A 2017 review by dos Santos et al. [17] states: “Although the small number of studies limits the systematic review on photobiomodulation, evidence was found to suggest that photobiomodulation is an effective short-term approach for reducing oxidative stress in muscle injury.”

Recovery

Lastly, on to recovery. A 2017 review article by Fisher et al. [18] states: “There is moderate evidence to support the use of photobiomodulation over cryotherapy when using this modality postexercise for muscle recovery in trained and untrained athletes. Shorter recovery times, identified by a fast return to baseline muscle torque and subjective muscle soreness values, can be seen 24 to 96 hours following photobiomodulation application. Lower markers of muscle damage, creatine kinase (CK), which lead to less inflammation markers, were found 24 to 96 hours after photobiomodulation treatments; however, CK levels after cryotherapy treatments followed similar patterns to placebo treatments.” There is not a lot of research truly assessing effects on longer-term recovery in practice unfortunately.

Dosage recommendations

The dosages in the studies often correspond to something along the lines of using one of our lights for 1-3 minutes, at our usually recommended 60cm distance. However, we suspect that exceeding this dose is appropriate. Our lights come equipped with a timer that by default shuts off after a 10 minute session. This seems to be anecdotally where the majority of users feel “satiated” by the light. Why this discrepancy? There are a number of factors that may play a role, such as the effect of coherence of the lasers in some studies on penetration, as well as the fact that the LEDs are often pressed up against the skin, causing less light to get lost from reflection, as well as the mixed wavelengths. The original review we cited [1] states “This finding makes sense because the light emanating from LEDs has a wider bandwidth, is not coherent, and is more divergent than the light emanating from laser diodes, resulting in more reflection and less transmission of LED-generated light through the skin. Therefore, a higher dose when using LED therapy may compensate for beam reflection and divergence.” For a much more in-depth look into this topic, we recommend a look at our research-doses versus at-home LED panel doses article here. Another takeaway from the research is that for acute performance-enhancement, light should be administered pre-exercise, the research suggesting greater gradual improvement from training over time also administered the light pre-training. Whether there is any additional benefit to using light post-exercise is less clear, but anecdotally many report finding it very pleasant to do so. Overall, there is a good amount of evidence that red light therapy can be of benefit to performance when it comes to resistance exercise. Have you been using a CytoLED or other red light therapy device and noticed benefit or lack thereof when it comes to your training? Please let us know your experiences by email at info@CytoLED.com, we're always highly interested to hear how people personally perceive their response to red light therapy.

References

1. Borsa et al. “Does phototherapy enhance skeletal muscle contractile function and postexercise recovery? A systematic review”. J Athl Train. 2013 Jan-Feb; 48(1): 57–67. doi: 10.4085/1062-6050-48.1.12
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3554033/

2. Schoenfeld, Brad J. "Does Exercise-Induced Muscle Damage Play a Role in Skeletal Muscle Hypertrophy?". Journal of Strength and Conditioning Research: May 2012 - Volume 26 - Issue 5 - p 1441-1453. doi: 10.1519/JSC.0b013e31824f207e.
https://journals.lww.com/nscajscr/Fulltext/2012/05000/Does_Exercise_Induced_Muscle_Damage_Play_a_Role_in.37.aspx

3. Damas et al. "The development of skeletal muscle hypertrophy through resistance training: the role of muscle damage and muscle protein synthesis". Eur J Appl Physiol. 2018 Mar;118(3):485-500. doi: 10.1007/s00421-017-3792-9.
https://pubmed.ncbi.nlm.nih.gov/29282529/

4. Damas et al. "Resistance training-induced changes in integrated myofibrillar protein synthesis are related to hypertrophy only after attenuation of muscle damage". J Physiol. 2016 Sep 15;594(18):5209-22. doi: 10.1113/JP272472.
https://pubmed.ncbi.nlm.nih.gov/27219125/

5. Wilson et al. "Practical Blood Flow Restriction Training Increases Acute Determinants of Hypertrophy Without Increasing Indices of Muscle Damage". Journal of Strength and Conditioning Research: November 2013 - Volume 27 - Issue 11 - p 3068-3075. doi: 10.1519/JSC.0b013e31828a1ffa.
https://journals.lww.com/nsca-jscr/Fulltext/2013/11000/Practical_Blood_Flow_Restriction_Training.20

6. Loenneke et al. "Does blood flow restriction result in skeletal muscle damage? A critical review of available evidence" Scandinavian Journal Of Medicine & Science In Sports Volume 24, Issue 6, December 2014 Pages e415-422. doi: 10.1111/sms.12210.
https://onlinelibrary.wiley.com/doi/abs/10.1111/sms.12210

7. Lilja et al. "High doses of anti-inflammatory drugs compromise muscle strength and hypertrophic adaptations to resistance training in young adults." Acta Physiologica (Oxford, England), 16 Sep 2017, 222(2). doi: 10.1111/apha.12948.
https://europepmc.org/article/med/28834248

8. Trappe et al. "Influence of acetaminophen and ibuprofen on skeletal muscle adaptations to resistance exercise in older adults". Am J Physiol Regul Integr Comp Physiol. 2011 Mar; 300(3): R655–R662. doi: 10.1152/ajpregu.00611.2010.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3064281/

9. Ferraresi et al. "Effects of Light-Emitting Diode Therapy on Muscle Hypertrophy, Gene Expression, Performance, Damage, and Delayed-Onset Muscle Soreness". Am J Phys Med Rehabil. 2016 Oct; 95(10): 746–757. doi: 10.1097/PHM.0000000000000490.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5026559/

10. Baroni et al. "Effect of low-level laser therapy on muscle adaptation to knee extensor eccentric training". Eur J Appl Physiol 2014, volume 115, 639–647. doi:10.1007/s00421-014-3055-y.
https://link.springer.com/article/10.1007/s00421-014-3055-y

11. Barbosa et al. "Effect of Low-Level Laser Therapy and Strength Training Protocol on Hand Grip by Dynamometry" J Lasers Med Sci. 2017 Summer; 8(3): 112–117. doi: 10.15171/jlms.2017.20.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5662498/

12. Hemmings et al. "Identifying Dosage Effect of Light-Emitting Diode Therapy on Muscular Fatigue in Quadriceps". Journal of Strength and Conditioning Research: February 2017 - Volume 31 - Issue 2 - p 395-402. doi: 10.1519/JSC.0000000000001523.
https://journals.lww.com/nsca-jscr/Fulltext/2017/02000/Identifying_Dosage_Effect_of_Light_Emitting_Diode.15.aspx

13. Pinto et al. "Effect of 655-nm Low-Level Laser Therapy on Exercise-Induced Skeletal Muscle Fatigue in Humans". October 2008 Photomedicine and laser surgery 26(5):419-24 doi: 10.1089/pho.2007.2160.
https://www.researchgate.net/publication/23282176_Effect_of_655-nm_Low-Level_Laser_Therapy_on_Exercise-Induced_Skeletal_Muscle_Fatigue_in_Humans

14.Vanin et al. "Photobiomodulation therapy for the improvement of muscular performance and reduction of muscular fatigue associated with exercise in healthy people: a systematic review and meta-analysis". Lasers Med Sci 33, 181–214 (2018). doi: 10.1007/s10103-017-2368-6.
https://link.springer.com/article/10.1007%2Fs10103-017-2368-6

15. Mamais et al. "Effectiveness of Low Level Laser Therapy (LLLT) in the treatment of Lateral elbow tendinopathy (LET): an umbrella review". Laser Therapy 27(3):174-186.
https://www.jstage.jst.go.jp/article/islsm/27/3/27_27_18-OR-16/_pdf

16. Martimbianco et al. "Photobiomodulation with low-level laser therapy for treating Achilles tendinopathy: a systematic review and meta-analysis". Clin Rehabil. 2020 Jun;34(6):713-722. doi: 10.1177/0269215520912820.
https://pubmed.ncbi.nlm.nih.gov/32204620/

17. Santos et al. "Effects of Photobiomodulation Therapy on Oxidative Stress in Muscle Injury Animal Models: A Systematic Review". Oxid Med Cell Longev. 2017; 2017: 5273403. doi: 10.1155/2017/5273403.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5623775/

18. Fisher et al. "The Effectiveness of Photobiomodulation Therapy Versus Cryotherapy for Skeletal Muscle Recovery: A Critically Appraised Topic". Journal of Sport Rehabilitation 2017. Volume 28: Issue 5 Pages: 526–531. doi: 10.1123/jsr.2017-0359
https://journals.humankinetics.com/view/journals/jsr/28/5/article-p526.xml

19. Gorgey et al. "The Effect of Low-Level Laser Therapy on Electrically Induced Muscle Fatigue: A Pilot Study". Photomedicine and Laser Surgery. Oct 2008. Volume: 26 Issue 5. 501-506. doi: 10.1089/pho.2007.2161.
https://www.liebertpub.com/doi/10.1089/pho.2007.2161