Effects of muscle shortening on single-fiber, motor unit, and compound muscle action potentials

Abstract

Even under isometric conditions, muscle contractions are associated with some degree of fiber shortening. The effects of muscle shortening on extracellular electromyographic potentials have not been characterized in detail. Moreover, the anatomical, biophysical, and detection factors influencing the muscle-shortening effects have been neither identified nor understood completely. Herein, we investigated the effects of muscle shortening on the amplitude and duration characteristics of single-fiber, motor unit, and compound muscle action potentials. We found that, at the single-fiber level, two main factors influenced the muscle-shortening effects: (1) the electrode position and distance relative to the myotendinous zone and (2) the electrode distance to the maxima of the dipole field arising from the stationary dipole created at the fiber-tendon junction. Besides, at the motor unit and muscle level, two additional factors were involved: (3) the overlapping between the propagating component of some fibers with the non-propagating component of other fibers and (4) the spatial spreading of the fiber-tendon junctions. The muscle-shortening effects depend critically on the electrode longitudinal distance to the myotendinous zone. When the electrode was placed far from the myotendinous zone, muscle shortening resulted in an enlargement and narrowing of the final (negative) phase of the potential, and this enlargement became less pronounced as the electrode approached the fiber endings. For electrode locations close to the myotendinous zone, muscle shortening caused a depression of both the main (positive) and final (negative) phases of the potential. Beyond the myotendinous zone, muscle shortening led to a decrease of the final (positive) phase. The present results provide reference information that will help to identify changes in MUPs and M waves due to muscle shortening, and thus to differentiate these changes from those caused by muscle fatigue. Graphical abstract

M-wave changes caused by brief voluntary and stimulated isometric contractions

“…This model revealed two main effects: (1) as the muscle shortens and bulges, the fibers increase their distance from the detection electrode, which results in a decrease of the amplitude of the propagating component; (2) as the muscle shortens, the tendon endings approach the electrodes, thus causing an enlargement of the non-propagating component. Our group proposes a second analytical model, which confirmed the findings of Mesin and colleagues (Rodriguez-Falces and Place 2014 ; Rodriguez-Falces et al 2022 ). In addition to the adjustments in amplitude, our model showed that muscle shortening also provokes a decrease in the duration of the M wave (Rodriguez-Falces et al 2022 ).…”

“…Our group proposes a second analytical model, which confirmed the findings of Mesin and colleagues (Rodriguez-Falces and Place 2014 ; Rodriguez-Falces et al 2022 ). In addition to the adjustments in amplitude, our model showed that muscle shortening also provokes a decrease in the duration of the M wave (Rodriguez-Falces et al 2022 ). In the present study, the above simulation results were verified using real M waves evoked at different muscle lengths.…”

“…The compound muscle action potential (M wave) is one of the most utilized tools to study the changes in sarcolemmal membrane excitability. The M wave comprises two phases with a different electrical origin: the first “propagating” phase is generated by the propagation of the intracellular action potentials along the fibers, the second “non-propagating” phase reflects the extinction of these action potentials at the tendon (Rodriguez-Falces et al 2022 ). Despite the importance of the M wave, no experimental study has been conducted to characterize the effects of muscle shortening on the M-wave profile.…”

The contribution of the tendon electrode to M‐wave characteristics in the biceps brachii, vastus lateralis and tibialis anterior

“…In view of the present findings the assumption that 'the first phase of the belly-tendon M wave results from the propagation of the action potentials along muscle fibres, whereas the second reflects the extinction of these action potentials at the tendon' proposed by Rodriguez-Falces et al (2022) should be revisited. Indeed, here we showed that the belly potential of the vastus lateralis (and tibialis anterior) exhibited a distinct shoulder (or peak) in the first phase, which had a non-propagating character.…”

Doença degenerativa dos discos intervertebrais de indivíduos adultos interfere na eficiência dos ciclos mastigatórios e amplitude dos movimentos mandibulares?

“…Existe uma interação entre os sistemas que compõe a mandíbula e a região cervical devido às comunicações neurológicas e biomecânicas existentes (PACKER et al, 2014). Os potenciais de ação das fibras musculares promovem o deslizamento de filamentos de actina e miosina, que produz padrões funcionais como a contração muscular isométrica e/ou isotônica que é interceptado pela eletromiografia de superfície, sendo um importante método de estudo clínico que produz resultados com a função de demonstrar alterações que o organismo humano pode estar sofrendo decorrentes das patologias degenerativas sistêmicas (MATSUI et al, 2017;MOUSA;PATIL;LYNCH, 2017;RODRIGUEZ-FALCES;MALANDA;NAVALLAS, 2021).…”