Effects of Kilohertz-frequency and Low-frequency Current on Triceps Surae

Neuromuscular electrical Stimulation (NMES) can minimize muscle atrophy and complications related to muscle disuse and help improve neuromuscular performance. Medium and low-frequency currents have been assessed regarding the generation of evoked torque, sensory discomfort, muscle fatigue, and peripheral oxygen extraction. In addition, metabolic stress is also linked to muscle strength gain, an important aspect to be evaluated in addition to NMES physical parameters. Thus, the aim of this study is to compare the effects of different NMES protocols applied to the triceps surae muscle for evoked torque, muscle fatigue, sensory discomfort, and peripheral oxygen extraction in healthy individuals. This is a crossover, experimental, randomized, double-blind trial composed of apparently healthy participants. All NMES protocols will be tested on the same individual with randomization of the sequence of intervention protocols. There will be a total of 5 encounters with seven days between them. Session 1 will evaluate the anthropometric measures, the maximum intensity for each intervention protocol, and the sequence of intervention protocols for each individual will be randomized. Sessions 2, 3, 4, and 5 will be composed equally with the assessment of the maximum voluntary and evoked joint torque of the triceps surae muscle through the isokinetic dynamometer, evaluation of muscle fatigue through the H-reflex, M-wave, fatigue index, time-torque-integral, and recruitment curve, evaluation of peripheral oxygen extraction through NIRS (Near Infrared Spectroscopy), electromyographic signals to assessed the RMS (root mean square) and the median frequency, evaluation of the level of sensory discomfort through the Visual Analog Pain Scale and finally by the NMES protocol. The EENM protocols will be as follows: CR10% (Russian Current at 2500 Hz, modulated in 50 Hz bursts, 200 µs and 10% duty cycle - 2 ms of 18 ms bursts and interbusrts), CA10% (Aussie Current with 1000 Hz, modulated in 50 Hz bursts, 500 µs and 10% duty cycle - 2 ms of 18 ms bursts and interbusrts), CP500 (pulsed current with 50 Hz, 500 µs phase) and CP200 (Pulsed current with 50 Hz, 200 µs phase). all protocols will be performed on the triceps surae muscle.

This is a crossover, experimental, randomized, double-blind trial composed of apparently healthy participants. The objective is to compare the effects of different NMES protocols applied to the triceps surae muscle for evoked torque, muscle fatigue, sensory discomfort, and peripheral oxygen extraction. The effects of the types of neuromuscular electrical stimulation (NMES) protocols on the aforementioned outcomes will be evaluated in the same participant by randomizing the sequences of interventions for each visit in the laboratory. The study is considered double-blind, as individuals will not know the sequence of the protocols applied. The evaluator will also not know which protocol will be used at the time of the intervention. It will consist of a total of 5 sessions with seven days between them. In the first session, anthropometry, the maximum intensity level for each electrical stimulation protocol as well as the protocol sequence for each individual will be evaluated. From the second to the fifth session, the following will be considered: voluntary and evoked maximum joint torque of the triceps surae muscle, muscle fatigue through the evaluation of the H-reflex, M-wave, fatigue index, torque-time-integral and recruitment curve, peripheral oxygen extraction, electromyographic signals through RMS (root mean square) and median frequency, and level of sensory discomfort with the Visual Analog Scale (VAS). From the second to the fifth session will be composed by the following evaluation sequence: warm-up with six submaximal contractions with 6 seconds of duration and 10 seconds of rest between them; then the assessment of muscle fatigue; then two maximal isometric contractions, then two maximal evoked contractions; fatigue protocol at 20% of the maximum isometric contraction (this fatigue protocol will use the NMES sequence randomized in the first session; after the fatigue protocol, two maximum evoked contractions will be performed again; then two maximal isometric contractions and at the end, the muscle fatigue evaluation will be performed again. The NMES protocols will be CR10% (Russian Current at 2500 Hz, modulated in 50 Hz bursts, 200 µs and 10% duty cycle - 2 ms of 18 ms bursts and interbusrts), CA10% (Aussie Current with 1000 Hz, modulated in 50 Hz bursts, 500 µs and 10% duty cycle - 2 ms of 18 ms bursts and interbusrts), CP500 (pulsed current with 50 Hz, 500 µs phase) and CP200 (Pulsed current with 50 Hz, 200 µs phase). All protocols will be performed on the triceps surae muscle.

Eligibility Criteria:
Inclusion Criteria:

* Female and male, aged between 18-40 years;
* Be classified as physically active according to the International Physical Activity Questionnaire (IPAQ);
* The practice of just recreational physical activity;
* Achieve a minimum torque of 20% of the MVIC during the NMES;
* Be at least three months without strength training;

Exclusion Criteria:

* Present musculoskeletal dysfunction that may interfere with the tests, present intolerance to NMES in the triceps surae muscle;
* Use analgesics, antidepressants, tranquilizers, or other centrally acting agents;
* Present cardiovascular or peripheral vascular problems, chronic diseases, neurological or muscle disorders that may impair the complete execution of the study design by the volunteer;