Abstract
This study examined motor unit (MU) amplitudes (APAMPS) and firing rates during moderate-intensity contractions and muscle cross-sectional area (mCSA) and echo intensity (mEI) of the vastus lateralis (VL) in chronically endurance-trained and sedentary females. Eight endurance-trained (ET) and nine sedentary controls (SED) volunteered for this study. Surface electromyographic (EMG) signals from a five-pin electrode array were recorded from the VL during isometric trapezoid muscle actions at 40% of maximal voluntary contraction (MVC). Decomposition methods were applied to the EMG signals to extract the firing events and amplitudes of single MUs. The mean firing rate (MFR) during steady force and MUAPAMP for each MU was regressed against recruitment threshold (RT, expressed as %MVC). The y-intercepts and slopes from the MFR and MUAPAMP vs. RT relationships were calculated. EMG amplitude during steady force was normalized (N-EMGRMS) to peak EMG amplitude recorded during the MVC. Ultrasonography was used to measure mCSA and mEI. Significant differences existed between the ET and SED for the slopes (P = 0.005, P = 0.001) from the MFR and MUAPAMP vs. RT relationships with no differences for the y-intercepts (P > 0.05). N-EMGRMS was significantly (P = 0.033) lower for the ET than SED. There were no differences between groups for mCSA; however, the SED possessed significantly (P = 0.001) greater mEI. Subsequently, the ET likely possessed hypertrophied and stronger MUs that allowed for lower necessary muscle activation to maintain the same relative task as the SED. The larger MUs for the ET is supported via the MFR vs. RT relationships and ultrasound data.
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Abbreviations
- CNS:
-
Central nervous system
- EMG:
-
Electromyography
- ET:
-
Endurance-trained group
- mCSA:
-
Muscle cross-sectional area
- mEI:
-
Muscle echo intensity
- MFR:
-
Mean firing rate
- MU:
-
Motor unit
- MUAP:
-
Motor unit action potential
- MUAPAMP :
-
Motor unit action potential amplitude
- MUAPDUR :
-
Motor unit action potential duration
- MVC:
-
Maximum voluntary contraction
- N-EMGRMS :
-
EMG amplitude normalized to peak EMG amplitude
- RMS:
-
Root mean square
- RT:
-
Recruitment threshold
- SED:
-
Sedentary group
- sEMG:
-
Surface electromyography
- sFAT:
-
Subcutaneous fat
- VL:
-
Vastus lateralis
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Acknowledgements
The authors thank the undergraduate and graduate research assistants in the Neuromechanics Laboratory for their assistance in performing the experiments.
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HD and TH conceived and designed research. HD, JM, AS, and MT conducted experiments. HD and TH analyzed data and wrote the manuscript. All authors read and approved the manuscript.
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Communicated by Guido Ferretti.
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Dimmick, H.L., Miller, J.D., Sterczala, A.J. et al. Vastus lateralis muscle tissue composition and motor unit properties in chronically endurance-trained vs. sedentary women. Eur J Appl Physiol 118, 1789–1800 (2018). https://doi.org/10.1007/s00421-018-3909-9
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DOI: https://doi.org/10.1007/s00421-018-3909-9