Reliability of antagonist EMG activation of the biceps femoris during maximal and submaximal voluntary isometric efforts in adolescent males

The purpose of the study was to examine the test -retest reliability of the hamstring antagonist EMG activity during maximum isometric efforts of the knee extensors. Ten male subjects (age: 15 years, mass: 52.7 ± 5.0 kg and stature: 169.1 ± 0.5 cm, mean ± SD) performed 5 maximal isometric efforts (MVC) of the knee extensors and flexors with the knee flexed at 90°. These were followed by five knee extension efforts at 50% (50MVC) and 30% (30MVC) of the maximum value. A digital myometer was used for the measurement of isometric moment of force. The EMG signal of vastus medialis and biceps femoris was also recorded using a remote EMG system. The antagonist raw average EMG was normalised using the approach of Kellis 1996. The tests were repeated one-week later. A one-way repeated measures analysis of variance (ANOVA) was used to examine the differences between test and retest raw EMG values of the hamstrings. Reliability was established using the reliability coefficients and the 95% limits of agreement. The antagonist activity was 4.4 ± 2.6%, 5.8 ± 3.4% and 19.8 ± 8.8 %, for 30MVC, 50MVC and MVC efforts, respectively. For the retest, the values were 4.0 ± 1.9 %, 6.1 ± 2.3 % and 17.9 ± 9.1%. The ANOVA results indicated no significant differences between test and retest raw EMG values at 30MVC, 50 MVC and MVC. The reliability coefficients were 0.82, 0.91 and 0.70, respectively. 95% of the differences between test and retest moments were less than 1.9 %, 2.0 % and 13.6%, for 30MVC, 50MVC and MVC. Reliability coefficients indicate that antagonist activity of the biceps femoris demonstrate moderate to high test-retest reliability.