Cancellation of surface action potential amplitude in motor units of the vastus medialis muscle

Cescon, Corrado and Negro, Francesco and Enoka, Roger Maro and Farina, Dario (2007) Cancellation of surface action potential amplitude in motor units of the vastus medialis muscle. In: Proceedings of the 37th annual meeting of the Society for Neuroscience, 3-7 Nov 2007, San Diego, CA, USA.

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Abstract

The surface EMG underestimates the amount of motor unit activity due to the loss of information that occurs when positive and negative phases of action potentials cancel one another and reduce the amplitude of the signal (cancellation) It has been shown in simulation [1] that cancellation of action potentials of individual motor units is linearly correlated to the decrease in size of the spike-triggered average of the rectified or squared EMG Thus, spike-triggered averaging [2,3] of the surface EMG can be in principle used to estimate amplitude cancellation but this has not been supported theoretically or verified experimentally Theoretical analysis of cancellation of single motor unit action potentials in relation to spike-triggered averaging to explain the results reported previously in simulation [1] Validation of the theoretical predictions with experimental recordings of motor units in the vastus medialis muscle The amount of cancellation of action potentials of individual motor units has been theoretically proven to depend only on the ratio between the RMS of the motor unit action potential and the RMS of the interference signal. According to this theoretical derivation, small surface action potentials are cancelled more than large action potentials. The ratio between the spike-triggered average of the interference and the squared surface EMG is theoretically equivalent to the degree of cancellation of the action potential, which explains the results reported in [1]. The theoretical predictions have been experimentally validated for the vastus medialis muscle. Cancellation of action potential amplitude of single motor units can thus be theoretically predicted and experimentally quantified.

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