Stimulation Sites and Fatigue Induced by Neuromuscular Electrical Stimulation in Healthy Individuals

Neuromuscular electrical stimulation (NMES) is a tool used in training protocols and in clinical practice to prevent or attenuate atrophy and improve the ability to produce muscle strength in different populations. Although widely used, the effects of NMES can be limited by discomfort and early fatigue induced by electrical current. Previous studies have investigated alternatives to minimize muscle fatigue, reduce muscle discomfort and increase muscle performance. A measure adopted to reduce the NMES's deleterious effects is the choice of stimulation site. More specifically, muscle contractions can be evoked by applying electrical pulses to the trunk of peripheral nerves (nNMES) or terminal branches of the nerve at the muscle belly level (mNMES). There is evidence that the mNMES stimulates the more superficial motor units (MUs), while the deeper MUs of the muscle remain inactivated, or, to recruit them, an additional increase in current intensity and stimulation frequency may be required. On the other hand, in direct nerve stimulation (nNMES) both superficial and deep MUs are recruited regardless of NMES intensity. Based on these observations, a new application modality of NMES emerged, the intercalated nerve and muscle stimulation (iNMES). In this strategy, electrical pulses are intercalated or alternated between the mNMES and nNMES sites, intending to reduce the high frequencies at which the MUs are activated during NMES, recruiting both superficial and deep MUs, and reducing muscle fatigue during evoked contractions. Although iEENM is a promising strategy to potentiate the NMES effects, few studies have investigated the iNMES effects on neuromuscular fatigue, and the existing literature is solely focused on the analysis of the tibialis anterior muscle, limiting the findings' inferences for other muscles important for lower limb functionality (e.g., quadriceps femoris). Therefore, the objective of this study is to compare the effects of nNMES applied to the femoral nerve (FN-NMES), of mNMES applied to the rectus femoris' motor point (MP-NMES), and iNMES applied simultaneously to both sites (FNMP-NMES) on knee extensors' functional (muscle fatigue) and clinical (discomfort) parameters in healthy individuals, through a randomized clinical trial. Our study has three hypotheses. In our first hypothesis, muscle fatigue during an electrical stimulation protocol will be lower with the FNMP-NMES modality, followed by FN-NMES, and will be higher with MP-NMES. Thus, FNMP-NMES will present a smaller reduction in maximal voluntary isometric contractions (MVICs) immediately after the fatigue protocol, a smaller relative reduction between the final compared to the initial evoked torque, a greater number of contractions for the evoked torque to reduce 50% with respect to the initial torque during the NMES fatigue protocol, and a greater total work compared the FN-NMES and MP-NMES modalities. In our second hypothesis, low frequency (20 Hz) NMES will produce greater total work and less fatigability of the knee extensors (smaller reduction from pre to post MVIC, smaller percentage reduction at the final compared to the initial evoked torque, a greater number of contractions for the evoked torque to reduce 50% compared to the initial evoked torque, and greater total work) compared to a high stimulation frequency (100 Hz). Furthermore, the total work will be higher and the fatigability lower with FNMP-NMES, followed by FN-NMES, and finally MP-NMES, regardless of stimulation frequency. Finally, the third hypothesis is that discomfort will be less with FNMP-NMES, followed by FN-NMES, and finally MP-NMES, regardless of stimulation frequency.

Inclusion Criteria:Healthy individuals:young people of both sexesage between 18 and 40 yearsbody mass index between 20 and 25 kg/m2and normal knee range of motionwith no complaints of pain or presence of pathology in the dominant lower limb. -Exclusion Criteria:Individuals with:any contraindication to maximal exercisehaving been treated with NMES in the last 3 months in the lower limbnot tolerate electrical stimulation