Abstract
Common drive to a motor unit (MU) pool manifests as low-frequency oscillations in MU discharge rate, producing fluctuations in muscle force. The aim of the study was to examine the temporal correlation between instantaneous MU discharge rate and rectified EMG in low frequencies. Additionally, we attempted to examine whether there is a temporal correlation between the low-frequency oscillations in MU discharge rate and the first derivative of force (dF/dt). Healthy young subjects produced steady submaximal force with their right finger as a single task or while maintaining a pinch-grip force with the left hand as a dual task. Surface EMG and fine-wire MU potentials were recorded from the first dorsal interosseous muscle in the right hand. Surface EMG was band-pass filtered (5–1,000 Hz) and full-wave rectified. Rectified surface EMG and the instantaneous discharge rate of MUs were smoothed by a Hann-window of 400 ms duration (equivalent to 2 Hz low-pass filtering). In each of the identified MUs, the smoothed MU discharge rate was positively correlated with the rectified-and-smoothed EMG as confirmed by the distinct peak in cross-correlation function with greater values in the dual task compared with the single task. Additionally, the smoothed MU discharge rate was temporally correlated with dF/dt more than with force and with rectified-and-smoothed EMG. The results indicated that the low-frequency component of rectified surface EMG and the first derivative of force provide temporal information on the low-frequency oscillations in the MU discharge rate.
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Acknowledgments
The authors would like to thank Drs. Hiroaki Kanehisa, Naokazu Miyamoto, Yohei Takai, and Tetsuo Fukunaga (National Institute of Fitness and Sports in Kanoya) for helpful comments on the manuscript. The authors acknowledge the advice from Dr. Francesco Negro (University Medical Center Goettingen) with motor unit analysis. We thank Mr. Sumiaki Maeo, Mr. Hiroki Honma, and Miss. Miyuki Nakatani (National Institute of Fitness and Sports in Kanoya) for assistance with the experiment. We thank Dr. Vasiliy Buharin and Ms. Ellenor Brown (Georgia Institute of Technology) for proofreading the manuscript. This study was funded, in part, by the Grant-in-Aid for Scientific Research (B) in Japan (24300207) and by The Nakatomi Foundation to YY.
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Yoshitake, Y., Shinohara, M. Oscillations in motor unit discharge are reflected in the low-frequency component of rectified surface EMG and the rate of change in force. Exp Brain Res 231, 267–276 (2013). https://doi.org/10.1007/s00221-013-3689-8
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DOI: https://doi.org/10.1007/s00221-013-3689-8