This study aims To investigate the effects of Quadriceps femoris muscle length on neuromuscular fatigue induced by NMES, muscle performance, discomfort and peripheral oxygen extraction.

This is a crossover, experimental, randomized and double-blind trial (participant and statistician). The procedures will be performed at the Neuromuscular Performance Laboratory and the Musculotendineal Plasticity Laboratory (LaPlasT) of the Faculty of Ceilândia/University of Brasília to evaluate the neuromuscular fatigue generated during NMES of the QF muscle in different muscle lengths, according to the articular angles of the hip and knee. Participants aged between 18 and 45 years of age, of both sexes, healthy, with body mass index (BMI) between 18.5 and 24.9 kg/ m² (ie eutrophic), who did not perform systematic training to strengthen the lower limbs in the last six months, practitioners or not of recreational sports activities, physically active according to the International Physical Activity Questionnaire (IPAQ), and with minimum torque reach of 20% of MVC during NMES without excessive discomfort. Those who have: edema, dermal injury, limitation of the range of joint motion, deformity or amputation in any part of the lower limbs, as well as a history of patellar dislocation or trauma to the lower limbs or trunk that compromises the results. Also excluded are those with conditions that affect musculotendineal morphology or neuromuscular excitability such as diabetes mellitus type II, familial hypercholesterolemia, neuromuscular disease and severe cardiopathy, or conditions that procedures, such as cognitive impairment, psychiatric disease, chemical dependence or behavioral problems (Dudley-Javoroski et al., 2010). Participants will be initially familiar with assessments and training with EENM. In familiarization, will be performed: Antropometry (height and body mass); location of the motor points; randomization of the order of articular positioning that will be evaluated; muscle enhancement; and three CVIM. The fatigue protocol with NMES will occur at 20% of MVC in positions: bench press with knee at 60º of flexion (Sup60), sitting with knee at 60º (Sen60), bench press at 20º (Sup20) and Sitting with knee at 20º (Sen20). These positions were chosen considering that: (1) SJ60 is the position in which the knee angle provides the optimal QF length for maximum torque production (Scott et al., 2019), and the hip angle provides a neutral length for the RF (Bampouras et al., 2017); (2) QF is commonly stimulated with knees fully extended (Fitzgerald et al., 2003). However, with the knees extended, it is not possible to measure the extensor torque of the knee properly in the isokinetic dynamometer (Babault et al., 2003). Therefore, we chose 20º knee flexion as an approximate position; (3) the hip angle affects the length of RF and myotendinous stiffness of QF (Bampouras et al., 2017), so the knee angles (60º and 20º) will be evaluated with the participants both seated (hip in flexion; 85º) when lying down (hip in extension; 0º). During familiarization, for anthropometric assessment (height, body mass and BMI), participants will be barefoot and wearing light clothes. The BMI will be obtained by the ratio of the weight of the participants in kilos with the square of the height (kg/m2). To determine the level of engagement in physical activities, the IPAQ will be applied, which contains 7 items that assess the frequency (days) and duration (minutes and/ or hours) spent on physical activities in the last week (Scholes et al., 2016). After familiarization, there will be four sessions of fatigue induced by NMES, composed of 20 electrically induced contractions at 20% of CVM (CEI20%). Each session will be as confirmed by the volunteer regarding the absence of any residual muscle discomfort arising from the previous session. The sessions will be initiated with heating and muscle enhancement. The following outcomes will be observed before, during and after the fatigue protocol: CVIM; reflex H, Wave M and voluntary activation level; electromyographic activity; muscular architecture; tendinous properties and tissue oxygen extraction. During the fatigue protocol (five first and five last evoked contractions, will be evaluated: (1) fatigability by the torque decay curve; (2) integral force-time; (3) tissue extraction of oxygen.

Inclusion Criteria:

• Participants between 18 and 45 years of age;
• Both sexes;
• Healthy;
• Body mass index (BMI) between 18.5 and 24.9 kg/ m² (ie eutrophic);
• Who did not perform systematic training strengthening of the lower limbs in the last six months;
• Practitioners or not of recreational sports activities;
• Physically active according to the International Physical Activity Questionnaire (IPAQ);
• Who with minimum torque reach of 20% of MVC during NMES without excessive discomfort.

Exclusion Criteria:

• Edema;
• Dermal injury;
• Limitation of the range of joint motion;
• Deformity or amputation in any part of the lower limbs, as well as a history of patellar dislocation or trauma to the lower limbs or trunk that compromises the results;
• Those with conditions that affect musculotendineal morphology or neuromuscular excitability such as diabetes mellitus type II, familial hypercholesterolemia, neuromuscular disease and severe cardiopathy;
• Conditions that make it impossible to cooperate with procedures, such as cognitive impairment, psychiatric illness, chemical dependence or behavioral problems.

The authors consider sharing the data depending on the situation