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Relationships between surface-detected EMG signals and motor unit activation

2002; Lippincott Williams & Wilkins; Volume: 34; Issue: 9 Linguagem: Inglês

10.1097/00005768-200209000-00018

1530-0315

Hiromasa Suzuki, Robin Conwit, Dan Stashuk, LYNN SANTARSIERO, E. Jeffrey Metter,

Advanced Sensor and Energy Harvesting Materials

SUZUKI, H., R. A. CONWIT, D. STASHUK, L. SANTARSIERO, and E. J. METTER. Relationships between surface-detected EMG signals and motor unit activation. Med. Sci. Sports Exerc., Vol. 34, No. 9, pp. 1509–1517, 2002. Introduction Surface-detected electromyographic (S-EMG) signals are used in exercise science to assess the extent of muscle activation, muscle fatigue, and neural activity during muscle contraction. However, the relationship has not been studied between S-EMG signal amplitude and motor unit activation at different muscle force levels. Methods S-EMG signals were measured from 76 healthy subjects during target force levels of 5, 10, 20, 30, and 50% of maximal voluntary contraction (MVC) of the knee extensors over 20-30 s. Mean absolute S-EMG amplitude, surface-detected motor unit action potential amplitude (S-MUAP), motor unit mean firing rate (mFR), and motor unit mean voltage, which is the product of S-MUAP amplitude and mFR, were assessed in the vastus medialis by using EMG signal-decomposition and spike-triggered averaging techniques. Results Motor unit mean voltage increased to the same degree as mean absolute S-EMG amplitude with increasing force, implying that motor unit size and firing rate explain the increase in mean absolute S-EMG amplitude with increasing force generation. In addition, mean absolute S-EMG amplitude increased linearly during the course of each 20-30 s contraction, with the slope being greater at higher force levels. A small change was observed in the shape of needle-detected motor unit action potentials during the contraction, but this change was not sufficient to explain the large change in mean absolute S-EMG amplitude during the contraction. Conclusion Mean absolute S-EMG amplitude at different force levels and its changes during the course of a submaximal contraction are dependent on the number of motor units active, their size, and firing rates.