Heart failure (HF) is an important public health problem being considered an emerging epidemic, therefore, strategies are essential to improve symptoms and to decrease the money expenditure on healthcare. Based on the knowledge about the syndrome of heart failure, new approaches are needed to decrease the symptoms, to enhance the quality of life of these patients and highlight the non-pharmacological therapies such as the use of phototherapy. The objective of this study is to study the effect of low level laser therapy in muscle function and tissue oxygenation of the lower limbs in patients with heart failure. It is a randomized, double-blind, including 20 individuals with heart failure, with stable, class II or III according to the New York Heart Association (NYHA). It will be evaluated the acute effects of a LASER intervention applied before a fatigue protocol throught muscle function and tissue oxygenation in the participants. Evaluations will be performed by an isokinetic dynamometer, near infrared spectroscopy (NIRS) and electromyography . It will be analyzed the concentration of oxyhemoglobin and deoxyhemoglobin, total hemoglobin concentration, the difference between these concentrations, muscular torque, fatigue index and muscle activation. The data will be subjected to distribution normality test and if they have normal distribution, the differences between the interventions will be checked by analysis of variance, considering significant ap <0.05. If the data is non-parametric distribution will be using the Mann-Whitney or Kruskal-Wallis. The SPSS 19.0 software will be used as a tool for data analysis. The investigators expected to find as outcomes of this study an increase in peripheral muscle function and tissue oxygenation and also a delay in the development of muscle fatigue in patients with HF submitted to acute intervention with LASER.

Day 1: Informed Consent signature and Clear (IC); Application of the Minnesota Living with Heart Failure Questionnaire (MLHFQ); Measurement of the muscle thickness with use of ultrasound; Anatomical points definition for electrode placement of Electromyography (EMG); Warming up on stationary bike; Subject's position in the Isokinetic, placement of EMG electrodes and NIRS´ optodes; Baseline measurement (peak torque and muscle fatigue protocol); Stretching exercises for the lower limbs muscles; 48-hour interval.

Day 2: Warming up on stationary bike; LASER / Placebo application; Subject's position in the Isokinetic, placement of EMG electrodes and NIRS´ optodes; Peak torque and muscle fatigue protocol in the Isokinetic equipment; Stretching exercises for the lower limbs muscles; 48-hour interval.

Day 3: Warming up on stationary bike; LASER / Placebo application; Subject's position in the Isokinetic, placement of EMG electrodes and NIRS´ optodes; Peak torque and muscle fatigue protocol in the Isokinetic equipment; Stretching exercises for the lower limbs muscles.

Inclusion Criteria:

• Participants who consent to cooperate with this study and meet the following inclusion criteria: a) to have a clinical diagnosis of heart failure (ejection fraction of the left ventricle less than 40% secondary to ischemia or idiopathic dilated cardiomyopathy) functional class II or III, according to the NYHA classification ; b) be medically stable in the 30 days prior to enrollment in the study and, c) not have participated or are participating in cardiac rehabilitation program.

Exclusion Criteria:

• The sample shall be excluded: a) participants who did not consent to participate in the study, b) participants with present insufficient level of understanding for the tests, c) participants that are making use of anti-inflammatory drugs steroids and nonsteroidal, d) the ones that have limitations to perform physical activity (exacerbation of fatigue, extreme breathlessness, arthritis, claudication, angina), e) participants with musculoskeletal disease, brain vascular disease and psychiatric illness and f) the ones who not adhere to the schedule of activities proposed in the study.