Introduction: Neuromuscular electrical stimulation (NMES) has the purpose of generating muscle contractions to minimize muscular atrophy and to improve neuromuscular performance. NMES has been performed using monophasic or biphasic currents, applied over a nerve trunk or muscle belly, and both can generate contractions by the peripheral and central nervous system. Pulse width (wide or narrow) is an essential parameter for NMES. Although NMES studies using wide pulses have been performed with monophasic currents, it is known that this current induces discomfort during NMES. Therefore, it is necessary to analyze if biphasic currents have the same effect as monophasic currents using the same parameters.

Objectives:

To compare the effects of NMES with narrow and wide pulse widths associated with monophasic and biphasic currents, applied over a tibial nerve and triceps surae muscles in healthy individuals in terms of muscle fatigue, central and peripheral contribution, voluntary and evoked force and sensory discomfort.

Methods:

A crossover, experimental controlled and randomized study will be developed with healthy male and female (age: 18-45 years). The following dependent variables will be: amplitude of H-reflex and M-wave (single and double pulses), voluntary and evoked triceps surae muscles torque, fatigability (force time integral), perceived discomfort and neuromuscular adaptations. The independent variables will be related to current phase, pulse width and location of electrical stimulation electrodes. There will be a familiarization session followed by 9 sessions with 7 rest days between them (10 weeks). Data will be reported as mean and standard deviation (± SD). Parametric tests will be used for the normally distributed data (Shapiro-Wilk test) that show homogeneous variations (Levene test). A repeated measure mixed-model ANOVA will be performed and, in the case of major effects or significant interactions, the Tukey post-hoc test will be applied. In addition, the power and size of the effect (reported as partial eta square, partial η2) will be calculated. The significance threshold will be set at p <0.05 for all procedures.

Expected results: Biphasic currents will be more comfortable and will generate less muscle fatigue when compared to monophasic currents. There will be less fatigue and greater central contribution when wider pulse currents will be applied over a nerve trunk concerning the application with a wide pulse over a muscle belly.

This is a randomized controlled trial, a crossover study, with healthy male and female participants. The procedures will take place in 10 distinct visits for a 7-day interval.

The following visit in the lab will be:

• Familiarization session.
• Monophasic Current (100 Hz) with 1ms pulse width applied to the Tibial Nerve;
• Biphasic current (100 Hz) with pulse width 0.5ms applied to the Tibial Nerve;
• Biphasic current (100 Hz) with 1ms pulse width applied to the Tibial Nerve;
• Biphasic current (100 Hz) with 2ms pulse width applied to the Tibial Nerve;
• Monophasic (100 Hz) with 1ms pulse width applied to the Triceps Surae Muscle Belly;
• Biphasic current (100 Hz) with pulse width 0.5ms applied to the Triceps Surae muscle Belly;
• Biphasic current (100 Hz) with 1ms pulse width applied to the Triceps Surae Muscle Belly;
• Biphasic current (100 Hz) with 2ms pulse width applied to the Triceps Surae Muscle Belly;
• Biphasic current (25 Hz) with 0.5ms pulse width applied to the Triceps Surae Muscle Belly.

The dependent variables will be:

• Muscle fatigability;
• Central and peripheral contribution (with H-reflex and M-wave amplitude analysis);
• Maximum voluntary isometric contraction;
• Force production for plantar flexion evoked by NMES;
• Perceived discomfort.

Inclusion Criteria:

• Classified as physically active according to the INTERNATIONAL QUESTIONNAIRE OF PHYSICAL ACTIVITY;
• To practice only recreational physical activity;
• Be at least 3 months without practicing strength training.

Exclusion Criteria:

• Present some type of skeletal muscle dysfunction that may interfere with the tests;
• NMES intolerance in the triceps surae muscle belly or tibial nerve;
• Use of analgesics, antidepressants, tranquilizers or other centrally acting agents;
• Cardiovascular or peripheral vascular problems, chronic diseases, neurological or muscular disorders that may hinder the complete execution of the study design by the volunteer.