Neuromuscular Electrical Stimulation and Isokinetic Training

Subjects Twenty active and healthy undergraduate students (men: 21 ± 1.9 years; 1.77 ± 0.1 m; 73 ± 8.4 kg; 23.3 ± 1.4 kg/m²; women: 20.9 ± 0.7 years; 1.64 ± 0.1 m; 56.3 ± 5.5 kg; 21 ± 1.0 kg/m²) were selected for this study. All of them underwent physical examination before the study and had already participated in strength training programs before this study. All 10 men selected concluded the study. Although 15 women were selected and began the training program, 5 were excluded: 3 reported episodes of pain in the knee joint during training sessions, and 2 of them could not conclude the training program for personal reasons. Therefore, the final data refers to 10 women.

All procedures for evaluation and training were explained before and a consent term was signed. The project was approved by the Ethics Committee for Medical Research of Federal University of São Carlos and is in agreement with the Declaration of Helsinki.

Evaluation protocol Before all evaluations, subjects were familiarized with testing and training procedures. All the evaluation and training procedures were applied for both right and left limbs of subjects before and after training program. To avoid possible dominance effects, volunteers were randomly separated to determine in which limb (right or left) training would be associated with NMES. The investigator who made the data analysis was blind to this randomization.

After 5 min period of warm-up in a stationary bicycle at a speed of 20 km/h and load of 20 W, the subjects stretched the quadriceps femoris and the hamstring muscles of both limbs. Each muscle group was stretched 3 times of 30 s alternately. Afterwards, the subject was positioned in an isokinetic dynamometer (Biodex® Multi-joint System 3, Biodex, Shirley, NY), with the angulation of 100º on hips. Trunk, pelvis, and thigh of subjects were stabilized by strips, following equipment description36. Rotation axis of dynamometer was aligned with axis of the knee, at the level of lateral epicondyle of the femur, while the attachment was fixed at the distal part of leg of subjects, about 5 cm above medial malleolus. The gravity effect correction was calculated with the limb at 60° of flexion. The isokinetic device was calibrated before each evaluation and training session as requested by the manufacturer.

After that, volunteers had their knees positioned at 60° and were familiarized to the action they would do. After that, they were asked to perform 3 maximum voluntary isometric knee extension contractions for 5 s, with 1 min of rest interval between the contractions. Three min after the last contraction, subjects were familiarized with the isokinetic contraction and then performed 5 maximum isokinetic concentric knee extension contractions at 30º/s speed, from 90° to 15°, 0° being full extension, totaling up 75° of Range of Motion (ROM). Verbal encouragement37 as well as visual feedback from the equipment38 were given in an attempt to achieve a maximal voluntary effort level during all contractions each subject was asked to perform. The same procedures were repeated with the contralateral limb.

Inclusion criteria

• The inclusion criteria were to be young (age range between 18 and 25 years), body mass index considered normal (between 20 and 25 kg/m²), practice of physical activity other than resistance training during the training period, absence of any neuromuscular disorders in lower limbs, no knee pain history.

Exclusion criteria

• The exclusion criteria were pregnancy, epilepsy, cancer, and suspicion on heart disease, recent surgery, respiratory disorders, non-compensated diabetes, and history of trauma, fracture or surgery in lower limbs, reports of pain at knee joint.