EMG Threshold Detection and Reliability of Boys and Men Using an Isometric Contraction Protocol

Abstract

Recently it has been suggested that, as children grow, they utilize more of their higher-threshold (type II) motor units (MU). Such developmental changes in MU utilization can potentially underlie many observed physiological differences between children and adults. The EMG threshold (EMGTh¬) is a proposed non-invasive tool to investigate the recruitment of type II MU activation. In progressive cycling, children’s EMGTh was demonstrated to occur at higher relative workloads compared with adults. However, many of the participants did not exhibit a threshold, presumably due to exhaustion at relatively low muscle force. By using an isometric contraction protocol to determine EMGth, participants will reach higher relative muscle forces at exhaustion, thus facilitating the determination of EMGTh. Twenty-one boys and 21 men participated in the study. Participants completed a one repetition maximum (1RM) of knee extension, and the progressive isometric contraction protocol, consisting of repetitive isometric contractions (5 seconds on, 3 seconds off) at increasing loads: Starting at 25% of 1RM, the load increased by 3% 1RM after every five contractions. Using this isometric contraction protocol, detection rate of EMGTh was 88.2 % in boys and 90.0 % in men. EMGTh occurred at higher relative intensity in boys (56.09% ± 9.42%) compared with men (46.16% ± 7.71%). Intra-class correlation coefficients showed that EMGTh is moderately reliable from visit 1 to visit 2 in both boys (ICC = 0.70) and men (ICC = 0. 56). EMG¬Th occurs at significantly higher exercise intensities in visit 2 (boys 61.85% ± 8.74%; men 54.45% ± 8.88%) than compared with visit 1 (boys 56.09% ± 9.42%; men 46.16% ± 7.71%) in both boys and men. Overall the results provide evidence to support the use of an isometric contraction protocol to study the EMGTh in children.