Motor unit action potential amplitudes and firing rates during repetitive muscle actions of the first dorsal interosseous in children and adults.

PURPOSE: Previous research has indicated greater muscle activation is needed for children (CH) to match relative intensity submaximal contractions in comparison with adults (AD). However, no study has compared motor unit (MU) firing and recruitment patterns between children and adults. Therefore, MU action potential amplitudes (MUAPAMP ) and firing rates were examined during two repetitive submaximal contractions of the first dorsal interosseous in children and adults.

METHODS: Twenty-two children (age 9.0 ± 0.8 years) and 13 adults (age 22.9 ± 4.8 years) completed three maximum voluntary contractions (MVC) and two repetitive isometric contractions at 30% MVC for 40 s. Surface electromyography (EMG) was recorded and decomposed into action potential trains. MUAPAMPS , recruitment thresholds (RTs), and mean firing rates (MFRs) were calculated, and EMG amplitude was normalized (N-EMG) to MVC. For each subject and repetition, linear MFR vs. RT and exponential MUAPAMP vs. RT and MFR vs. MUAPAMP relationships were calculated.

RESULTS: N-EMG (P = 0.001, CH = 56.5 ± 31.7%, AD = 30.3 ± 9.1%), MFRs regardless of RT, according to greater y-intercepts of the MFR vs. RT relationships [P = 0.013, CH = 31.1 ± 5.1 pulses per second (pps), AD = 25.9 ± 4.3 pps] and MFRs of MUs with smaller action potential amplitudes (P = 0.017, CH = 29.4 ± 6.8 pps, AD = 23.5 ± 3.5 pps), were greater for children. MUAPAMPS in relation with RT were similar between groups except the highest threshold MUs (RT = 28% MVC) were greater for the adults (1.02 ± 0.43 mV) than children (0.67 ± 0.24 mV) (P = 0.010).

CONCLUSIONS: Muscle activation and MU firing rates were greater for children, which likely indicated a greater operating point of MU control in comparison with adults during an isometric contraction performed at a relative submaximal intensity.