Large base of clinical research shows that red light therapy promotes muscle recovery
The healing and regenerative effects of red light therapy on muscle recovery have been demonstrated in countless studies and it is backed by many world-class pro athletes and trainers who swear by red light therapy for daily muscle recovery at the highest level of competition.
In 2015, researchers conducted a systematic review and meta-analysis of numerous randomized, placebo-controlled studies on red light therapy and muscle recovery and performance. Based on the study, the authors concluded that red light therapy "improves muscle performance and speeds up recovery." 
Less pain and damage after intense workouts: In 2014, red light therapy was tested for muscle recovery in healthy young men after "damaging eccentric exercise." The light therapy group showed significantly reduced loss of muscle strength, less muscle soreness, and less restriction of movement, and this was demonstrated up to 4 days after harmful exercise. 
Recovery from demanding workouts: Researchers in the 2014 study cited above found that light therapy reduces muscle soreness after strenuous exercise, and also that a single red light therapy treatment immediately following damaging exercise was effective in reducing muscle soreness and loss of strength. 
Less delayed muscle soreness: Delayed onset muscle soreness (DOMS) is pain and stiffness that you feel 24 to 72 hours after a workout or strenuous activity. A double-blind, randomized trial assessed light therapy, exercise, and DOMS and concluded:
- "The light therapy group showed a significant decrease in pain associated with DOMS."
- “The McGill Pain Questionnaire showed a significant difference in pain scores at the 48 hour period.”
- Conclusion: Light therapy "provides a beneficial effect for patients who may experience DOMS after an exercise session."
A 2014 trial assessed skeletal muscle performance and recovery after exercise for healthy men. The researchers found that pre-exercise light therapy "significantly increases performance, decreases delayed onset muscle soreness and improves biochemical markers associated with skeletal muscle damage."  This ties in with previous research showing that pre-exercise red light therapy treatments delay the onset of muscle fatigue. 
Fight inflammation: Research shows that red light therapy also helps reduce inflammation that can lead to cell damage. [6-7-8]
Less running damage: In 2012, it was concluded that runners in the light therapy group experienced reduced "exercise-related oxidative stress and muscle damage." Researchers suggested that the modulation of the redox system (reduction of oxidative stress) by light therapy likely led to a delay and reduction of skeletal muscle fatigue after running. 
Less knee fatigue: A recent 2018 study found that pre- and post-exercise red light therapy reduced knee muscle fatigue.  Similar results from a 2010 study of knee muscle fatigue found that pre-exercise light therapy was an effective way to prevent the increase in muscle protein in the blood serum. "
Lower creatine kinase levels indicate less muscle damage with light therapy: Creatine kinase (CK) is an essential enzyme in your body that you make in greater numbers in response to muscle and tissue damage. As a result, it is often used in blood tests to assess muscle damage. 
Many studies have shown a strong decrease in CK levels for light therapy groups versus placebo and control groups.
A recent 2018 systematic review and meta-analysis was conducted in theClinical Journal of Sports Medicine to assess the muscle effects of light therapy, especially on post-exercise creatine kinase levels. After reviewing 14 studies, the authors concluded that light therapy has a positive effect on the control of creatine kinase activity. 
Red Light therapy beats cryotherapy for muscle recovery in clinical tests: A 2016 study evaluated both cryotherapy and red light therapy for muscle repair. Researchers found that only light therapy was "optimal for post-exercise recovery," with reduced DOMS and creatine kinase activity versus placebo or cryotherapy. They concluded that light therapy alone led to "full recovery to baseline values from 24 hours after high-intensity eccentric contractions." 
These positive results build on previous research. For example, a 1990 study assessed numerous soft tissue injuries and concluded that light therapy was an effective recovery method, with significantly longer healing times. 
Research shows that pro football and rugby players recover faster with red light therapy: Recent clinical studies on professional rugby and soccer players assessed men's stamina, speed and ability to recover from demanding matches and training. Researchers found that the red light therapy accelerated players' recovery times, leading the authors of both studies to recommend red light therapy for athletic recovery.
The results on the rugby field were consistent with the results in the laboratory: rugby players who received phototherapy treatments saw a "significantly reduced percentage change in blood lactate levels (p ≤ 0.05) and perceived fatigue (p ≤ 0.05). " [15-16]
a double blind,placebo controlled trial tested pro volleyball players and found this about the light therapy group:
"Post-exercise levels of biochemical markers decreased significantly: blood lactate, creatine kinase and C-reactive protein levels." 
Women feel less pain from exercise in postmenopause:
Light therapy has been shown to be clinically effective in reducing muscle pain in a wide range of populations, with particularly encouraging results for postmenopausal women who combine red light therapy with exercise.
In addition to improving quadricep performance, a 2014 study also found that red light therapy "reduces peripheral fatigue in postmenopausal women."  Infrared LED therapy associated with treadmill exercise has also been shown to improve muscle strength and delay leg fatigue in postmenopausal women. 
Faster recovery for postmenopausal women: Similar to the study on preventing muscle fatigue, postmenopausal women showed impressive results of muscle recovery with red light therapy. In 2013, a team of researchers analyzed the exercise tolerance and muscle recovery of 30 postmenopausal women over 6 months. One group received shortwave infrared light therapy and did the treadmill workout, the other only the treadmill.
Researchers found that "recovery time showed a significant decrease only in the LED group." They concluded that light therapy and exercise "may improve maximal performance and post-exercise recovery in postmenopausal women." 
Your strength increases with this therapy
Here are even more reasons why so many professional strength trainers are incorporating red light therapy into their routines.
Harder strength training: ina 2016 test assessed the effects of light therapy on men aged 18 to 35 who did strength training. Men in the light therapy group "showed significant changes" in maximum torque for both leg extension and leg muscle exercises.
Researchers concluded: "The application of phototherapy yields improved strength gains if applied before exercise. The application can have an extra favorable value in rehabilitation after an injury, where strength improvements are required. "
In 2011, researchers who tested light therapy and strength training on healthy young men concluded, “Strength training associated with light therapy may improve muscle performance compared to strength training alone.”
Stronger legs: That increase mentioned above amounted to a 55% increase in leg-press tests, much higher than the non-therapy group. The men who received light therapy were "the only group to experience an increase in muscle performance in the isokinetic dynamometer test compared to baseline." 
Stronger grip: A 2014 placebo-controlled study found that red light treatments increased maximum repetitions of hand and grip exercises by 52%, as measured by an isokinetic dynamometer.  Numerous other studies have also shown significant increases in grip strength during exercise following light therapy treatments. 
Scientific sources and medical references: