Two-dimensional spatial distribution of surface mechanomyography response to single motor unit activity

Cescon, Corrado and Madeleine, Pascal and Graven-Nielsen, Thomas and Merletti, Roberto and Farina, Dario (2006) Two-dimensional spatial distribution of surface mechanomyography response to single motor unit activity. In: Proc. of the XVI Congress of the International Society of Electrophysiology and Kinesiology, 29 Jun - 1 Jul 2006, Torino, Italy.

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In order to better understand the mechanisms of generation of the mechanomyography (MMG) signal, The the two-dimensional distribution of surface mechanomyography (MMG) produced by the activity of single motor units was analyzed by a novel two-dimensional MMG recording method. Motor unit action potentials were identified from intramuscular electromyographic (EMG) signals and used to trigger the averaging of MMG signals detected over the tibialis anterior muscle of 11 volunteers with a grid of 53 accelerometers, with (20-mm inter-accelerometer distance). The intramuscular wires were inserted between the first and second accelerometer in the middle column of the grid, proximal to the innervation zone. The subjects performed three contractions with visual feedback onf the intramuscular EMG signals. In each contraction, a new motor unit was recruited at the minimum stable discharge rate (mean±SD, N = 11 subjects, 7.3±2.3 pulses per second), resulting in torque of 2.4±2.8 % of the maximal voluntary contraction (MVC), 4.6±2.7 % MVC, and 6.3±3.1 %MVC (all different, P<0.01). For 23 out of 33 detected motor units, it was possible to extract the motor unit surface acceleration map (MUAM). A negative MUAM peak (-2.7±2.2 mm/s2) was detected laterally and a positive MUAM peak (4.1±2.4 mm/s2) medially (P<0.001). The time-to-peak was shorter in the medial part of the muscle (2.9±0.4 ms) than in the other locations (3.4±0.5 ms, P<0.001). The double integrated signals (muscle displacement) indicated negative deflection in the lateral part and inflation close to the tibia bone. The maps of acceleration showed spatial dependency in single motor unit MMG activities. The technique provides a new insight into motor unit contractile properties.

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