Motor unit action potential amplitudes and firing rates during repetitive muscle actions of the first dorsal interosseous in children and adults
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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.
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.
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).
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.
KeywordsChildren Muscle activation Motor units Electromyography Ultrasound
First dorsal interosseous
Mean firing rate
Motor unit action potential amplitude
Motor unit action potential duration
Maximum voluntary contraction
Normalized electromyographic amplitude
Peak electromyographic amplitude
Spike trigger average
We would like to thank the undergraduate students who assisted in the collection of data, as well as each of the subjects for their selfless participation.
TJH and JDM conceived and designed the study. JDM, AJS, HLD, MEW, and MAT collected and analyzed the data. All authors interpreted the results. JDM prepared the first draft of the manuscript and figures. All authors edited the figures and manuscript. All authors have approved the final version of the manuscript and agree to be accountable for all aspects of the work. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. All experiments were performed in the University of Kansas Neuromechanics Laboratory.
This study was supported financially by the National Strength and Conditioning Association Foundation’s Graduate Research Grant (#0000010446) and by the University of Kansas via the General Research Fund (GRF 2301166-RSC).
Compliance with ethical standards
Conflict of interest
The authors declare no conflicts of interest, financial or otherwise.
All procedures performed in this study involving human participants were in accordance with the ethical standards of the University Institutional Review Boards for Human Subjects and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
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