Abstract
Background
Lower activation of higher threshold (type-II) motor units (MUs) has been suggested in children compared with adults. We examined child–adult differences in discrete MU activation of the flexor carpi radialis (FCR).
Methods
Fifteen boys (10.2 ± 1.4 years), and 17 men (25.0 ± 2.7 years) completed 2 laboratory sessions. Following a habituation session, maximal voluntary isometric wrist flexion torque (MVIC) was determined before completing trapezoidal isometric contractions at 70%MVIC. Surface electromyography was captured by Delsys Trigno Galileo sensors and decomposed into individual MU action potential trains. Recruitment threshold (RT), and MU firing rates (MUFR) were calculated.
Results
MVIC was significantly greater in men (10.19 ± 1.92 Nm) than in boys (4.33 ± 1.47 Nm) (p < 0.05), but not statistically different after accounting for differences in body size. Mean MUFR was not different between boys (17.41 ± 7.83 pps) and men (17.47 ± 7.64 pps). However, the MUFR–RT slope was significantly (p < 0.05) steeper (more negative) in boys, reflecting a progressively greater decrease in MUFR with increasing RT. Additionally, boys recruited more of their MUs early in the ramped contraction.
Conclusion
Compared with men, boys tended to recruit their MUs earlier and at a lower percentage of MVIC. This difference in MU recruitment may explain the greater decrease in MUFR with increasing RT in boys compared with men. Overall, these findings suggest an age-related difference in the neural strategy used to develop moderate–high torque in wrist flexors, where boys recruit more of their MUs earlier in the force gradation process, possibly resulting in a narrower recruitment range.
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Data availability
Data are available from the corresponding author upon reasonable request.
Abbreviations
- CV:
-
Coefficient of variation
- EMGRMS :
-
Root mean square of the electromyography signal
- ECR:
-
Extensor Carpi Radialis
- FCR:
-
Flexor Carpi Radialis
- FDI:
-
First dorsal interossei
- LBM:
-
Lean body mass
- MU:
-
Motor unit
- MUAP:
-
Motor-unit action potential
- MUFR:
-
Motor-unit firing rates
- MVIC:
-
Maximal voluntary isometric contraction
- P–P:
-
Peak to peak
- PHV:
-
Peak height velocity
- RT:
-
Recruitment threshold
- sEMG:
-
Electromyography
- STA:
-
Spike trigger average
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Acknowledgements
We would like to thank all the participants for their time, effort and hard work. Additionally, we would like to thank the parents/guardians for making it all happen, even when faced with COVID-related limitations and hardships.
Funding
This work was supported by the Natural Science and Engineering Research Council of Canada (NSERC grants to B. Falk and to M. Holmes). S Woods and A McKiel were supported by Ontario Graduate Scholarship.
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SW, MH, DG and BF conceived and designed research. SW, AM conducted experiments. SW, TH, DG and BF analyzed data and wrote the manuscript. TH and PK commented on previous versions. All authors read and approved the manuscript.
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Communicated by Toshio Moritani.
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Woods, S., McKiel, A., Herda, T. et al. Different discrete motor-unit activation patterns in the flexor carpi radialis in boys and men. Eur J Appl Physiol (2024). https://doi.org/10.1007/s00421-024-05417-2
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DOI: https://doi.org/10.1007/s00421-024-05417-2