The Effect of Low Level Laser Irradiation (Ga-AI-As-655nm) On Skeletal Muscle Fatigue induced by Electrical Stimulation in Rats

We investigated if low level laser irradiation (LLLI) can reduce muscular fatigue during tetanic contractions in rats. 32 male Wistar rats were divided in 4 groups receiving LLLT doses of 0 (control group), 0.5, 1.0 and 2.5 J/cm2. Irradiation lasted 32, 80 and 160 seconds respectively with a fixed power density of 31.25 mW/ cm2. The total energy doses were 0.08, 0.2 and 0.4 Joules respectively. Electrical stimulation induced 6 tetanic muscle contractions in the tibial anterior muscle. Contractions were stopped when the muscle force fell to 50% of the initial value for each contraction (T50%). There was no significant difference between the 2.5 J/cm2 laser-irradiated group and the control group in mean T50%-values. Laser-irradiated groups 0.5 J/cm2 and 1.0 J/cm2 had significantly longer T50% values than the control group. The relative peak force for the 6th contraction in the laser irradiated groups were significantly higher at 92.2 % (SD +/12.6%) for 0.5 J/cm2, 83.2 %(SD +/20.5%) for 1.0 J/cm2 and 82.9 %(SD +/18.3%) for 2.5 J/cm2 respectively, than for the control group, 50% (SD +/15%). Laser groups receiving 0.5 J/cm2 and 1.0 J/cm2, showed significant increases in mean performed work compared both to the control group and their 1st contraction values. Groups receiving laser irradiation with doses of 1 and 2.5 J/cm2 also showed significantly lower levels of Creatine Kinase (CK) in plasma than the non-irradiated control group. We conclude that these doses of LLLT inhibit development of muscular fatigue in rats. INTRODUCTION Page 2 of 30 by 10.0. 33.1 on A uust 8, 2017 http://jaysiology.org/ D ow nladed fom Fatigue is a common experience in daily life, although the cellular and physiological mechanisms involved are not fully understood. The main features of muscular fatigue are decreased muscle strength and impaired motor control, and subsequently muscular pain. Several studies have investigated how fatigue develops during different types of exercise in order to elucidate the process and mechanisms involved. Decreased muscle strength has been reported after fatigue in different parts of the human body (7, 24, 40). Physical fitness is a major factor in preventing fatigue, but within common physiological limits anyone may experience a state of neuromuscular fatigue if the time period is long enough (16). Whether age-dependent changes in physical performance result from alterations in muscle fibers, neuromuscular junction, peripheral nerve and/or central motor neurons have been difficult to ascertain in vivo (19). In patients with chronic neck pain of muscular origin, both early onset of increased electromyographic activity and a lack of post exercise vasodilatation have been observed compared to healthy controls (28). Exercise-induced muscle injury in humans may occur after strenuous exercises, particularly if the exercises involve a large amount of eccentric contractions (9). There are several definitions for muscular fatigue and one of them is the incapacity to maintain force by muscular contraction for a period of time. From an athlete`s perspective, Vollestad (52) definition of fatigue consist as any exerciseinduced reduction in the maximal capacity to generate muscle force might be appropriate. Verburg et al. (51) also states that fatigue develops during maximal or high-intensity exercise and during prolonged sub-maximal exercises, as evidenced by a decline in the force-generating capacity during maximal (test) contractions. It has been shown that a considerable component of fatigue is due Page 3 of 30 by 10.0. 33.1 on A uust 8, 2017 http://jaysiology.org/ D ow nladed fom to processes on the surroundings of the muscular junction (4, 31, 42) i.e., inhibition of neural excitation-muscular contraction coupling (1, 18, 44). Most hypotheses for the mechanism of this type of fatigue are based on changes in muscle function and local biochemical changes which occur during high-intensity exercise or tetanic stimulation (51). It is also well established that altering oxygen (O2) delivery to the contracting skeletal muscle affects human performance. In this respect, reducing O2 supply (hypoxia) will increase the rate of muscle fatigue, whereas increasing O2 supply (hyperoxia) will reduce the rate of fatigue (23). Numerous studies have used experimental models involving maximal voluntary contractions, tetanic stimulation and ischemic conditions (37, 53, 5) to study skeletal muscle fatigue. In experimental models, it has been demonstrated that a reduced force generation is following intense, sustained voluntary contractions and tetanic electrical stimulation of animal muscles in vitro (4, 13, 15). Delayed onset muscle soreness (DOMS) is often observed after eccentric exercises. DOMS is manifested clinically through transient muscle pain and impaired maximal muscle strength. Physiological manifestation may, for example, be increased levels of creatine kinase (CK), which is considered to be an indirect sign of widespread microtraumas inside the muscle. It has been shown that the anti-inflammatory drug diclofenac reduces minor muscle damage and CK-levels in DOMS. Several tools has been used in order to prevent muscle fatigue in healthy athletic subjects as well as in pathological conditions such as nutritional supplements (14, 47, 48) fluid reposition (10, 27), Creatine (33) and Arginine (32, 38). Low level laser irradiation (LLLI) is a therapy form which has been used to treat muscle damage and muscular pain, but the biological mechanisms behind Page 4 of 30 by 10.0. 33.1 on A uust 8, 2017 http://jaysiology.org/ D ow nladed fom observed beneficial results in clinical trials, remain unclear (8) LLLI may be administered with different wavelengths of the visible and near-infrared spectra including HeNe (632.8nm), Ga-Al-As (805 or 650 nm) and Ga-As (904 nm) (34). LLLI has been shown to reduce the duration of acute inflammation (36), and accelerate tissue repair in tendon and muscle injuries (3). Positive effects from LLLI may be related to modulation of the oxidative chain, and a recent review concluded that LLLI increased cellular redox activity (29). Stimulatory effects from LLLI have been observed for mitochondrial activity in cell cultures (53), and in hypoxic animal tissue (54) of diabetic mice. Marked increases in arteriolar and collateral blood flow have also been observed in injured tissue after LLLI (25,30). In clinical settings, painful fibromyalgia is associated with impaired local circulation and can be attenuated by an increase in muscle blood flow (26), and this disorder also seems to benefit from LLLI (21). In this perspective it is a clear need to investigate if LLLI can prevent hypoxia and muscular fatigue. We decided to use the experimental model of electrical nerve stimulation to induce fatigue by tetanic tibial muscle contraction in rats, and test if LLLI could prevent the anticipated reduction of muscle force or the anticipated increase in CK levels. This model with electrically stimulated tetanic contraction has limitations, but we considered it suitable for initial experimental studies with laser irradiation in muscular fatigue. Page 5 of 30 by 10.0. 33.1 on A uust 8, 2017 http://jaysiology.org/ D ow nladed fom