Isometric-based EMG threshold in girls and women

The electromyographic threshold (EMG_Th) has been suggested to indicate the onset of accelerated higher-threshold (type-II) MU recruitment. Previous research has demonstrated that boys' EMG_Th occurs at higher relative exercise intensities than men's in both cycling- and isometric-based testing. Girls‒women EMG_Th differences were demonstrated only in cycling, but findings were clouded by low EMG_Th-detection rates in women (68%) and particularly in girls (45%) PURPOSE: To examine the EMG_Th, 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.

Seventeen girls and 17 women had their EMG_Th determined as well as their one-repetition-maximum isometric knee-extension strength (1RM). Vastus-lateralis sEMG root mean square was recorded and the EMG_Th was defined as the exercise intensity (%1RM) at the bi-segmental point of the least sum of squares.

EMG_Th 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.

Girls' EMG_Th values are higher compared with women's, possibly reflecting lower ability to activate higher-threshold (type-II) motor units. The females' EMG_Th 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 EMG_Th, appears to be on par with the males.