Effects of Transcranial Direct Current Stimulation (tDCS) on Rotator Cuff Fatigue.

To assess endurance performance, participants will perform a submaximal isometric time to exhaustion (TTE) task of the shoulder external rotator muscles at 50% of their maximal voluntary contraction (MVC), which will be performed during each visit. During the TTE each participant will receive visual feedback on a computer monitor showing the target force. The task will finish when their force is below the required target value for more than 3 s. None of the participants will be aware of the time elapsed during the test and results of all the sessions will be provided only after the completion of all visits. Participants' perception of effort will be measured using the RPE scale (Borg, 1998) every 20 s of the TTE task. Leg muscle pain will be assessed every 20 s by using the Numerical Pain Rating Scale (NPRS).

After a brief standardized warm-up with submaximal isometric contractions, all participants will perform a 6 seconds MVC of infraspinatus muscle. For the MVC, subjects will be standing, with the shoulder in adduction position and the elbow flexed at 90°. The record will be evaluated by a load cell S200 (Miotec™, Porto Alegre, Brazil). The MVC produced during this test will be used to calculate the participants' 50% MVC used in the subsequent TTE task of that visit.

Borg rating of perceived exertion (RPE) is an outcome measure scale used in knowing exercise intensity prescription. The RPE will be evaluated using the BORG CR10 scale, its ranges are from "0" (no exertion) to "10" (maximum effort). The RPE will be evaluated using the BORG CR10 scale, its ranges are from "0" (no effort) to "10" (maximum effort). The perception of the effort of the participants will be measured every 20 s during the TTE task.

For the Global Rating of Change Scale, in the second and third visit, subjects will be asked about their perception of change in relation to the endurance task relative to their status at the first visit.

Surface electromyography (sEMG) assessment will be performed on the infraspinatus and deltoid muscles, using an acquisition module with four analog channels (Miotec™, Biomedical Equipments, Porto Alegre, Brazil). The conversion from analog to digital signals will be performed by an A/D board with 16-bit resolution input range, sampling frequency of 2 kHz, common rejection module greater than 100 dB, signal-noise ratio less than 03 μV Root Mean Square and impedance of 109 Ω. The sEMG signals will be recorded through surface Ag/AgCl electrodes (Meditrace™, Canada) with a centre-to-centre distance of 2 centimeters, in a parallel orientation to the underlying muscle fibers. A ground electrode will be placed in the lateral epicondyle of the contralateral arm The skin will be shaved and cleaned using alcohol swabs, following standardized guidelines. A fatigue study will be carried out using a frequency analysis.