Comparison of the electromyographic variables at different muscle lengths and contraction intensities

Thirty-three (33) healthy volunteers (2.29 +/- 2.4 years) participated. Two active surface electrodes were placed on the rectus femoris muscle: one on the motor point (M) and the other on the thickest distal portion of the muscle (V). The volunteer was positioned on a Bonet table, with his/her trunk fixed, thigh at 90 degrees and leg with 105 degrees and 45 degrees flexion. The signal was collected simultaneously in the 2 electrodes and in the load cell, during the extension of the leg at 50% or 100% of the maximum voluntary isometric contraction (MVIC) for 5 seconds, repeated 3 times, with an interval of 1 minute. The EMG signal was obtained through a data acquisition module (EMG1000 - Lynx) of 16 bits (LYNX), with band- pass filter of 20-1.000 Hz and sampling frequency of 2.000 Hz. RMS and mean frequency (MF) were assessed with Matlab software. Statistical analysis consisted of the Wilcoxon test and Student t test (p < 0.05). At 50% of the MVIC, the RMS was greater in the shortened muscle than the lengthened one. Whereas at 100% there was an inversion, with the lengthened muscle presenting greater amplitude. The FM both at 50% and 100% MVIC was greater for the muscle in the shortened position. It was possible to conclude that changes in muscular length and in contraction intensity alter the electromyographic variables.