The aim of the study was to verify the usefulness of the measurement of muscle activation level (AL) by comparing the maximal isometric and eccentric torque (torques ratio method - TRM), measured using a simple home-made hydraulic system built into the leg extension machine. The results obtained with TRM were compared with those measured with the already established double-interpolated twitch method (DTM). At this purpose, we wanted to determine the reliability and sensitivity of measurements and their validity under conditions of acute (fatigue) and chronic (strength training) effects on the AL. 28 recreational trained male subject (average age and standard deviation 30,3 ± 10,6 years, height 181,6 ± 8,0 cm, body mass 83,6 ± 9,7 kg) voluntarily participated in the study. The research was conducted in two parts. In the first part, 9 subjects within the measurement day performed 2 measurement of AL on both method (2 repetitions for each method, order was randomly selected), followed by a fatigue protocol containing 2-min maximal voluntary contraction. Immediately afterwards, subjects repeated 2 measurements of NA for each method (in the same order). The second part included 6 subject of experimental group (EG) and 6 of control group (CG). They performed within the measurement day in random order 2 repetition of AL measurement with TRM and 1 with DTM. Initial (IM) and final (FM) measurements were performed. Between the measurement days, EG performed a 4-week exercise program for the development of AL, while CG performed daily activities. For single measurement of the TRM, the maximal voluntary isometric torque in the knee angle 60° was measured first, followed, in the same movement action, by the measurement of maximal voluntary eccentric torque in the range of motion from 55° to 70° of knee extension, made possible by the hydraulic sistem while pushing the trainer lever at the constant angle velocity 35°/s to 65°/s. AL, measured with TRM (AL_TRM) was then calculated as a ratio between maximum voluntary isometric and eccentric torque. Measurement with DTW were performed on an isometric dynamometer for leg extension. When measured with DTW, the muscle quadriceps femoris was stimulated across the surface of the skin on the belly of the muscle by a double-superimposed, supramaximal electrical stimulus (two-phase pulse duration 0,3 ms, interval between pulses 10 ms). AL, measured with DTW (AL_DTW) was obtained with comparing the twitch amplitude superimposed on maximal voluntary isometric contraction and the twitch amplitude of the relaxed muscle immediately after it at the knee angle 60° of knee extension. The results of the MRN measurements at baseline (restfulness of subjects, measurements taken before both interventions) showed excellent repeatibility for all variables (ICC > 0,90; p < 0,001). Comparison within the measurement day showed no differences between the methods in the mean values of the maximum voluntary isometric torques, measured at baseline and after fatigue protocol, with medium to high correlation coefficients (0,60 > r > 0,94; p > 0,05). Strength training did not produce the expected results, as we did not find differences between the IM and FM in measured values of maximum voluntary isometric torques for the two groups, as well as between the groups (p > 0,05). Despite the high correlations between the methods in the maximum torque isometric torques, the comparison of the average NA values measured in all three conditions (i - restful, ii - after fatigue protocol and iii - after exercise) did not show significant match between the methods, since the values differed (p < 0,05) and the correlations were low, negative and nonsignificant (-0,10 > ICC < 0,20). We found that the use of a hydraulic system for measuring AL with TRM does not provide comparable results with the DTW.
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