The Association between Accuracy of Motor Control and Motor Unit Activity in Developing Children

Abstract

The accuracy of force production in children is recognized to be less than that of adults, yet the motor unit （MU） activation pattern, the smallest unit of the neural system and muscle fiber, remains incompletely understood. This research aimed to clarify the accuracy of force production and MU activity in children. Eighteen children aged 6 to 12 and 18 healthy young adults performed the maximal voluntary isometric contraction （MViC） and ramp-up contraction to 50 %MViC in knee extension. High-density surface electromyography was recorded from the vastus lateralis and decomposed into individual MU activity. Children exhibited significantly lower accuracy of force production during the ramp-up phase compared to adults （p<0.001）． Regarding MU activity, children demonstrated a significantly higher MU firing rate （FR） compared to adults （p<0.05）． The increase in FR was significantly greater for the lower-threshold MUs in children during the initial recruitment phase （p<0.05）， whereas adults displayed a greater increase in FR for higher-threshold MUs during the later phase of ramp-up contraction （p<0.05）． These findings suggest that children not only regulate MUs at a high FR but also show a tendency to rapidly increase FR during the initial recruitment phase. Differences in force production accuracy between children and adults could be linked to the characteristics of MU activity.