IDENTIFYING DOSAGE EFFECT OF LIGHT-EMITTING DIODE THERAPY ON MUSCULAR FATIGUE IN QUADRICEPS THOMAS J. HEMMINGS,KRISTINA L. KENDALL, AND JOHN L. DOBSON Department of Health and Kinesiology, Georgia Southern University, Statesboro, Georgia ABSTRACT Hemmings, TJ, Kendall, KL, and Dobson, JL. Identifying dosage effect of light-emitting diode therapy on muscular fatigue in quadriceps. J Strength Cond Res 31(2): 395–402, 2017—The purpose of this study was to compare the effects of various dosages of light-emitting diode therapy (LEDT) on muscle fatigue while performing a single-leg extension to exhaustion. A total of 34 recreationally resistance-trained athletes between the ages of 18 and 26 participated in 4 trials. Each trial included pre-exercise/postexercise blood lactate measure- ments and 2 sets of 3 maximal voluntary isometric contractions (MVICs), followed by LEDT on 6 points across the superficial quadriceps. Each randomized trial consisted of a placebo, 30, 60, or 120 seconds on each point on the quadriceps. Three minutes after LEDT, the participants performed an eccentric leg extension with 120% of MVIC until fatigue. There was significant increase in the number of repetitions performed between the placebo treatment and 60 seconds (p = 0.023), as well as placebo and 120 seconds (p = 0.004) of irradiation on each point. There were no significant differences in blood lactate levels between any of the 4 trials. In conclusion, LEDT had a positive effect on performance when irradiating 6 points on the superficial quadriceps for 60 and 120 seconds before an eccentric leg extension. KEY WORDS light emitting diode therapy, performance, MVIC, eccentric INTRODUCTION A decrease in muscle strength, motor control, and muscle coordination are characteristics that can be used to identify muscle fatigue (14). During intense or maximal exercise, muscles reach a point where muscle function is inhibited, which leads to a decrease in performance. There are numerous determinants of muscle fatigue, including the type of contractions being performed, the lactate concentration, and the production of reactive oxy- gen species. For example, Newham et al. (26) suggest that eccentric contractions cause significant muscle fatigue when compared to concentric and isometric contractions. Elevated blood lactate concentrations during intense exercise are strongly associated with muscle fatigue (25), and reactive oxi- dative species may contribute to fatigue by inhibiting the pro- duction of adenosine triphosphate (ATP) (1). Recently, low-level laser therapy (LLLT) or photobiomo- dulation has been used as a modality to enhance performance by decreasing muscle fatigue. Low-level laser therapy is an umbrella term used to describe laser light therapy (LLT) and light-emitting diode therapy (LEDT) (6). The difference between LLT and LEDT is the power output and depth of penetration because of various patterns in wavelengths (6). A study by Byrne et al. (5) indicates that fatigued muscle cells are less capable of metabolizing substrates for energy, which could be a related to the reactive oxidative species that are present during exercise. Research suggests that LLLT can pro- long the binding of nitric oxide to the cytochrome C oxidase enzyme, which permits the muscle to produce more ATP in the preferred oxidative pathway (17,19,20,22). Leal-Junior et al. (19) completed one of the first studies to identify the effects of LLT on muscular performance in human participants. Twelve male professional volleyball players received either a placebo or an active LLT treatment before performing voluntary bicep contractions until exhaustion. During the experimental trial, when LLT was applied, partic- ipants performed significantly more repetitions (means = 20 6 3 to 28 6 3 repetitions) when compared with the placebo trial. The placebo group improved their performance by 2.7 repetitions (pre and post means = 16 6 2 to 19 6 3 repeti- tions), whereas the LLT group improved by 8.5 6 1.9 repeti- tions when LLT was applied for 100 seconds. Another study performed by Baroni et al. (3) used LEDT before knee exten- sor eccentric contractions. When compared to Leal-Junior et al. (18), Baroni et al. (3) irradiated a group of muscles instead of just one muscle. This protocol focused on eccentric motion, which is known for causing elevated levels of muscle damage, when compared with concentric contractions. The Address correspondence to Thomas J. Hemmings, Th03873@ georgiasouthern.edu. 31(2)/395–402 Journal of Strength and Conditioning Research Ó 2016 National Strength and Conditioning Association VOLUME 31 | NUMBER 2 | FEBRUARY 2017 | 395 Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.