Difference in the Motor Unit Firing Behavior During Submaximal Isometric Ramp Contraction Between Healthy Children Aged 6-12 Years and Young Adults.

INTRODUCTION/AIMS: Although nervous system maturation is a key factor underlying the difference in muscle strength between children and adults, specific neural strategies for modulating motor unit (MU) firing during graded force production remain largely unexplored. This study aimed to clarify the differences in MU firing behavior between children and young adults during submaximal isometric ramp contractions. METHODS: Eighteen healthy children (aged 6-12 years) and 18 healthy young adults performed maximal voluntary contractions (MVCs) and submaximal ramp contractions to 50% of MVC for isometric knee extension. High-density surface electromyography data were collected from the vastus lateralis muscle and were decomposed to identify individual MU firing. MUs were analyzed according to their recruitment thresholds (RTs) to compare their firing rates (FRs) and the change in FR (ΔMU FR) at various force intervals. RESULTS: Children consistently exhibited significantly higher MU FRs than adults across almost all RT groups and force levels. ΔMU FR was higher in children only during the initial 10%-20% MVC interval for the lowest-threshold MUs but was significantly lower for higher threshold MUs at higher force levels. DISCUSSION: Higher MU FRs in children likely represent a functional adaptation to compensate for immature muscle contractile properties, thereby ensuring effective force generation. This distinct neural control strategy, which combines high initial rates with subsequent firing saturation, may reflect the ongoing maturation of spinal motor control. These findings provide a valuable reference for assessing pediatric neuromuscular disorders and can inform the design of more effective exercise and rehabilitation programs.