End-of-Fiber Signals Strongly Influence the First and Second Phases of the M Wave in the Vastus Lateralis: Implications for the Study of Muscle Excitability

Abstract

It has been recurrently observed that, for compound muscle action potentials (M wave) recorded over the innervation zone of the vastus lateralis, the descending portion of the first phase generally shows an “inflection” or “shoulder.” We sought to clarify the electrical origin of this shoulder-like feature and examine its implications. M waves evoked by maximal single shocks to the femoral nerve were recorded in monopolar and bipolar configurations from 126 individuals using classical (10-mm recording diameter, 20-mm inter-electrode distance) electrodes and from eight individuals using small electrodes arranged in a linear array. The changes of the M-wave waveform at different positions along the muscle fibers' direction were examined. The shoulder was identified more frequently in monopolar (97%) than in bipolar (46%) M waves. The shoulder of M waves recorded at different distances from the innervation zone had the same latency. Furthermore, the shoulder of the M wave recorded over the innervation zone coincided in latency with the positive peak of that recorded beyond the muscle. The positive phase of the M wave detected 20 mm away from the innervation zone was essentially composed of non-propagating components. The shoulder-like feature in monopolar and bipolar M waves results from the termination of action potentials at the superficial aponeurosis of the vastus lateralis. We conclude that, only the amplitude of the first phase, and not the second, of M waves recorded monopolarly and/or bipolarly in close proximity to the innervation zone can be used reliably to monitor possible changes in muscle membrane excitability.

Neuromuscular Factors Contributing to Reductions in Muscle Force After Repeated, High-Intensity Muscular Efforts

“…To obtain clearer evoked responses (i.e., MEP, cSP, and M max data), a pseudo-monopolar EMG configuration was also used over SOL (SOL PSU ). This pseudo-monopolar configuration provides a better representation of the electrical characteristics of the action potentials, compared with the bipolar (single-differential) where part of the electrical signal is lost due to phase cancelation (Rodriguez-Falces and Place, 2018). The active electrode was positioned just medial to the SOL bipolar EMG electrodes and the dispersive electrode ∼3 cm superior to the medial malleolus and over the Achilles tendon–soleus muscle–tendon junction (Blazevich et al, 2012).…”

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

“…The presence of this 'shoulder' was first reported by Kincaid et al (1993) in the hypothenar muscles' M waves, and attracted the attention of subsequent authors (Nandedkar & Barkhaus, 2007;Van Dijk et al, 1999). More recently, our group has demonstrated that the shoulder is also present in most belly-tendon M waves recorded in the vastus lateralis (Rodriguez-Falces & Place, 2018) and biceps brachhi (Rodriguez-Falces et al, 2021). Interestingly, Kincaid and coworkers argued that the shoulder was present only in those…”

“…( 1993 ) in the hypothenar muscles’ M waves, and attracted the attention of subsequent authors (Nandedkar & Barkhaus, 2007 ; Van Dijk et al., 1999 ). More recently, our group has demonstrated that the shoulder is also present in most belly–tendon M waves recorded in the vastus lateralis (Rodriguez‐Falces & Place, 2018 ) and biceps brachhi (Rodriguez‐Falces et al., 2021 ). Interestingly, Kincaid and coworkers argued that the shoulder was present only in those muscles in which the tendon potential makes a significant contribution to the belly–tendon M wave and, what is more, they theorized that the shoulder was indeed generated by the tendon potential.…”

The M waves of the biceps brachii have a stationary (shoulder-like) component in the first phase: implications and recommendations for M-wave analysis

“…Previous research has detected the presence of a 'shoulder-like' feature in the M waves recorded in pennate muscles (Thomas et al 1989, Rodriguez-Falces andPlace 2018b). However, interpretation of the M-wave shape in pennate muscles is complex: therefore, here we selected a simpler 'model' (i.e.…”