title: Influence of timing variability between motor unit potentials on M-wave characteristics.
abstract: The transient enlargement of the compound muscle action potential (M wave) after a conditioning contraction is referred to as potentiation. It has been recently shown that the potentiation of the first and second phases of a monopolar M wave differed drastically; namely, the first phase remained largely unchanged, whereas the second phase underwent a marked enlargement and shortening. This dissimilar potentiation of the first and second phases has been suggested to be attributed to a transient increase in conduction velocity after the contraction. Here, we present a series of simulations to test if changes in the timing variability between motor unit potentials (MUPs) can be responsible for the unequal potentiation (and shortening) of the first and the second M-wave phases. We found that an increase in the mean motor unit conduction velocity resulted in a marked enlargement and narrowing of both the first and second M-wave phases. The enlargement of the first phase caused by a global increase in motor unit conduction velocities was apparent even for the electrode located over the innervation zone and became more pronounced with increasing distance to the innervation zone, whereas the potentiation of the second phase was largely independent of electrode position. Our simulations indicate that it is unlikely that an increase in motor unit conduction velocities (accompanied or not by changes in their distribution) could account for the experimental observation that only the second phase of a monopolar M wave, but not the first, is enlarged after a brief contraction. However, the combination of an increase in the motor unit conduction velocities and a spreading of the motor unit activation times could potentially explain the asymmetric potentiation of the M-wave phases.
authors: Department of Electrical and Electronical Engineering, Public University of Navarra, Pamplona, Spain. Electronic address: javier.rodriguez.falces@gmail.com.
authors: Javier
authors: J
authors: Rodriguez-Falces
authors: Department of Electrical and Electronical Engineering, Public University of Navarra, Pamplona, Spain.
authors: Armando
authors: A
authors: Malanda
authors: Department of Electrical and Electronical Engineering, Public University of Navarra, Pamplona, Spain.
authors: Iban
authors: I
authors: Latasa
authors: Pediatric Neurology Unit, Virgen del Camino Hospital, Pamplona, Spain.
authors: Ana
authors: A
authors: Lavilla-Oiz
authors: Department of Electrical and Electronical Engineering, Public University of Navarra, Pamplona, Spain.
authors: Javier
authors: J
authors: Navallas
mesh_terms: D004576:Electromyography / Q000379:methods*; D019054:Evoked Potentials, Motor; D006801:Humans; D009046:Motor Neurons / Q000502:physiology; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000502:physiology; D009431:Neural Conduction; D011930:Reaction Time
keywords: Compound muscle action potential; Conduction velocity; Electromyogram; M-wave potentiation; Motor unit potentials