Isometric-based EMG threshold in girls and women

Abstract

Background: The electromyographic threshold (EMGTh) has been suggested to indicate the onset of accelerated higher-threshold (type-II) MU recruitment. Previous research has demonstrated that boys’ EMGTh occurs at higher relative exercise intensities than men’s in both cycling- and isometric-based testing. Girls‒women EMGTh differences were demonstrated only in cycling, but findings were clouded by low EMGTh-detection rates in women (68%) and particularly in girls (45%). Purpose: To examine the EMGTh, in girls and women, using the same males-employed isometric-based test protocol, and compare the females’ findings with those previously obtained in the males. Methods: Seventeen girls and 17 women had their EMGTh determined as well as their one repetition-maximum isometric knee-extension strength (1RM). Vastus-lateralis sEMG root mean square was recorded and the EMGTh was defined as the exercise intensity (%1RM) at the bi-segmental point of the least sum of squares. Results: EMGTh was detected in 88.2% of girls and 94.1% of women, and occurred at higher relative intensities in the girls than in women (56.0±11.1 vs. 47.7±8.0 %1RM). The girls’ 1RM (normalized to lean body mass) was only 69.1% that of the women. Conclusions: Girls’ EMGTh values are higher compared with women’s, possibly reflecting lower ability to activate higher-threshold (type-II) motor units. The females’ EMGTh and detection rate values were similar to the corresponding values previously observed in males. The females’ age-related difference in the recruitment of higher-threshold motor units, as reflected by the EMGTh, appears to be on par with the males.