Isometric-based EMG threshold in girls and women.

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.

Isometric-based test improves EMG-threshold determination in boys vs. men.

BACKGROUND: Children have been hypothesized to utilize higher-threshold (type-II) motor units (MUs) to a lesser extent than adults. Two recent studies, using a cycling-based EMG-threshold (EMGTh) protocol, supported the hypothesis, showing children's EMGTh intensities to be higher than adults'. Conclusions, however, were hampered by children's low EMGTh detection rates. Insufficiently high contractile forces at exhaustion were postulated as the reason for non-detection, predominantly in children. An intermittent isometric contraction test (IICT) protocol facilitates higher contractile forces prior to exhaustion and was shown effective in EMGTh testing of adults. PURPOSE: Determine whether an IICT protocol would enhance EMGTh detection in children, and consequently increase the magnitude of the previously observed child-adult EMGTh differences. METHODS: 18 boys and 21 men completed one-repetition-maximum (1RM) isometric knee-extension test. The IICT protocol followed, commencing at 25%1RM and comprising five isometric contractions per load, incremented by ~ 3%1RM to exhaustion. Vastus lateralis surface EMG was recorded and EMGTh, expressed as %1RM, was defined as the onset of the EMG-response's steeper segment. RESULTS: EMGTh was detected in 88.9% of boys and 95.2% of men, and occurred at higher relative intensities in boys (56.4 ± 9.2%1RM) than in men (46.0 ± 6.8%1RM). This 10.4% difference was 57% greater than the corresponding, previously reported cycling-based age-related difference. CONCLUSIONS: With the boys' detection rate nearly on par with the men's, the IICT protocol appears to overcome much of the intensity limitation of cycling-based protocols and provide a more sensitive EMGTh detection tool, thus extending the previously observed boys‒men difference. This difference adds supports to the notion of children's more limited type-II MU recruitment capacity.