EMG-Torque Relationship and Reliability of the Medial and Lateral Hamstring Muscles

The objectives of the present investigation were to examine the EMG-joint torque relation and day-to-day reliability of the medial (MH) and lateral hamstring (LH) muscles during isometric contractions. Twenty young adults performed five maximal voluntary isometric hamstring contractions (MVC) followed by contractions at 10%-90% MVC, in a random order. Full-wave-rectified and -integrated EMG during the middle 3 s of each contraction was expressed as a percent of the EMG during the three highest averaged MVC. A three-factor ANOVA (muscle x intensity x gender) with repeated measures was performed on normalized EMG activity. Reliability coefficients were determined for 10 randomly selected subjects that repeated the testing procedures 1 wk after the first session. The MH and LH EMG were observed to be significantly greater than equivalent percent MVC values at all intensity levels. A significant linear increase in MH and LH EMG was observed across contraction intensity levels, with no muscle or gender differences. Reliability was high for MH EMG across contraction intensities of 10%-60% MVC (intraclass correlation (ICC) = 0.70-0.82), moderate for the LH across 10%-30% MVC (ICC = 0.57-0.68) and the MH at 70% MVC (ICC = 0.52), and unreliable across all other contraction intensities for both muscles. The linear increase in MH and LH EMG, as a function of contraction intensity, was similar between both muscles but was highly reliable only for the MH during low to moderate intensities.