Motor Unit Firing Properties During Force Control Task and Associations With Neurological Tests in Children

Abstract

The present study aimed to clarify the development of motor unit (MU) firing properties and the association between those neural properties and force steadiness (FS)/neurological tests in 6- to 12-year-old children. Fifty-eight school-aged children performed maximal voluntary knee extension contraction, a submaximal FS test at 10% of maximal voluntary knee extension contraction, knee extension reaction time to light stimulus test, and single-leg standing test, and data from 38 children who passed the criteria were subject to analysis. During the FS test, high-density surface electromyography was recorded from the vastus lateralis muscle to identify individual MU firing activity. FS was improved with an increase in age (r = −.540, P < .001). The MU firing rate (MUFR) was significantly decreased with an increase in age (r = −.343, P = .035). MUFR variability was not associated with age. Although there was no significant correlation between FS and MUFR, FS was significantly correlated with MUFR variability even after adjustment for the effect of age (r = .551, P = .002). Neither the reaction time nor the single-leg standing test was correlated with any MU firing properties. These findings suggest that MUFR variability makes an important contribution to precise force control in children but does not naturally develop with age.

Intrinsic motoneuron properties in typical human development

“…Previous reports have suggested that in prepubescent children, firing rates in first dorsal interosseous motor units are higher compared to adults at matched levels of MVC force to compensate for the inability to activate motor units with high recruitment thresholds (Dotan et al., 2012; Miller et al., 2019). In agreement with this and other studies (Okudaira et al., 2023; Piotrkiewicz et al., 2007), we also show that the maximum and end firing rates of TA motor units decrease with age from 7 to 28 years, with the young development group having higher firing rates compared to the young adult and/or adult groups. Although superimposition of motor unit action potentials at peak contractions may reduce the accuracy of decomposition and measures of maximum firing rates, the finding that the end firing rates were also higher, where there is less superimposition, supports the conclusion of higher maximum firing rates in the young development group.…”