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 (EMG(Th)) protocol, supported the hypothesis, showing children's EMG(Th) intensities to be higher than adults'. Conclusions, however, were hampered by children's low EMG(Th) 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 EMG(Th) testing of adults. Purpose Determine whether an IICT protocol would enhance EMG(Th) detection in children, and consequently increase the magnitude of the previously observed child-adult EMG(Th) 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 similar to 3%1RM to exhaustion. Vastus lateralis surface EMG was recorded and EMG(Th), expressed as %1RM, was defined as the onset of the EMG-response's steeper segment. Results EMG(Th) 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 EMG(Th) 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.