Original Article DOI: http://dx.doi.org/10.1590/2446-4740.0736 *e-mail: rani@univap.br Received: 23 February 2015 / Accepted: 01 September 2015 Effect of low-level laser therapy (GaAlAs - λ660 nm) on muscle function Vania Maria de Araújo Giaretta, Luiz Prudêncio Santos, Ana Maria Barbosa, Stephen Hyslop, Alexandre Pinto Corrado, Renata Amadei Nicolau*, José Carlos Cogo Abstract Introduction: Low-level laser therapy (LLLT) is effective in preventing fatigue and in stimulating the microcirculation and cellular activity. In this study, we examined the effect of LLLT on injured tibial muscle in vivo by assessing muscle function during fatigue. Methods: Twenty-four male mice were used. Each mouse received an injection of sterile 0.9% saline solution (50 µL) in the right tibialis anterior muscle, after which the tendon of the muscle was exposed, connected to an isometric transducer and subjected to a resting tension of 1 g. A bipolar electrode was attached to the tibial nerve for electrical stimulation. The mice were randomly allocated to one of two groups: G1 (control: 3 h – n=8 and 9 h – n=5) and G2 (treated with GaAlAs laser, λ660 nm, 35 mW, 0.6 J, 17 s: 3 h – n=6 and 9 h – n=5). Results: In G1 mice, the amplitude of the tetanic contracture in response to induced fatigue remained unchanged during six consecutive tetani. The amplitude of the tetanic contractions in response to electrical stimulation (4-8 mV) was also unchanged. These results indicated muscle intactness in response to the load imposed by tetanus. In G2 mice, there was an increase in the amplitude of contraction after 3 h and 9 h when compared to G1 at 83% tetanus. Conclusion: These results indicate that exposure of muscle to LLLT enhanced the contractile force and increased the resistance to muscle fatigue without causing morphological damage to cellular structures. Keywords: Low-level laser therapy, Muscle activity, Muscle fatigue, Tetanus. Introduction Most studies of the effects of low-level laser therapy (LLLT) in laboratory animals have been done in rats and have shown that LLLT can reduce the weakness of local fatigue on muscle force and possibly reduce muscle damage after strenuous exercises (Albuquerque- Pontes et al., 2015; Caetano and Zanuto et al., 2013; Lopes-Martins et al., 2006; Santos et al., 2014). LLLT significantly enhances skeletal muscle performance and protects skeletal muscle tissue against damage when applied immediately (Santos et al., 2014). LLLT also causes a dose- and wavelength-dependent increase in cytochrome c oxidase expression in intact skeletal muscle tissue, indicating that muscular metabolism can be enhanced by phototherapy (Albuquerque- Pontes et al., 2015). Compared to rats, little is known of the responses to LLLT in other species, although various reports have demonstrated the benefits of LLLT in improving muscle function in dystrophic (mdx) mice (Leal-Junior et al., 2014; Oron et al., 2014; Silva et al., 2015). Mice are extensively used in toxinology to study the local effects (pain, edema, hemorrhage and necrosis) of a variety of venoms. Myonecrosis caused by Bothrops snake venoms is a well-known phenomenon experimentally (Gutiérrez and Ownby, 2003) and clinically (França and Málaque, 2009; Warrell, 2004) and, in the latter case, can result in extensive tissue loss and permanent functional damage to the bitten limb. Experimental studies have shown that laser therapy can attenuate the local damage caused by snake venoms, as assessed histologically and based on the quantification of marker proteins or enzymes (Barbosa et al., 2008; 2009; Doin-Silva et al., 2009; Dourado et al., 2003; Nadur-Andrade et al., 2012; 2014). However, none of these investigations has provided a functional assessment of the potential improvement in the contractile activity of muscle injected with venom and treated with LLLT. In view of the lack of detailed studies on the beneficial effects of LLLT on muscle function in normal mice, and the potential usefulness of such a model for studying venom-induced and other lesions, in this work we examined the effect of LLLT on normal (non-inflamed and non-lesioned) tibial muscle by assessing muscle function in response to tetanic stimulation. The standardization of this model should provide a useful basis for assessing the effects of LLLT on muscle function in a variety of situations in mice. Volume 31, Number 3, p. 241-248, 2015