Differences in the firing rate versus recruitment threshold relationships of the vastus lateralis in children ages 7-10 years and adults.

Motor unit (MU) firing rates of the vastus lateralis in children and adults were examined. Seven healthy adult males (mean ± SD, age = 21 ± 2.6 yrs) and six healthy male children (mean ± SD age = 8.8 ± 1.7) volunteered. Surface electromyography (EMG) signals were recorded from 20% and 60% maximal voluntary contractions (MVC). Surface EMG signals were decomposed into firing events of individual MUs and slopes and y-intercepts were calculated for the mean firing rate (MFR, pps) at steady torque vs. recruitment thresholds (RT) relationships for each subject. Muscle cross-sectional area (mCSA) was measured, via ultrasonography, with specific torque calculated (MVC/mCSA). Adults possessed greater mCSA (p = .002; children = 11.5 ± 2.1 cm2; adults = 31.80 ± 12.15 cm2) and greater specific torque (p = .018; children = 4.63 ± 1.4 Nm/cm2; adults = 7.1 ± 1.8 Nm/cm2) compared to children. The y-intercepts were significantly (p < .001) greater during the 60% (28.91 ± 4.56 pps) than the 20% MVC (23.5 ± 4.9 pps) collapsed across groups while the children had significantly (p = .036) lower y-intercepts (23.9 ± 5.4 pps) than the adults (28.2 ± 4.8 pps) when collapsed across intensities. Slopes of the MFR vs RT relationships were greater for the 60% (-0.342 ± 0.127 pps/%MVC) contraction than the 20% (-0.50 ± 0.159 pps/%MVC) MVC when collapsed across groups. Adults had greater firing rates regardless of recruitment threshold than children. This may be due to lower recruitment potential and overall excitation to the motoneuron pool of children as indicated with differences in specific torque and/or differences in antagonist co-activation.