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

Objective. We recently documented that compound muscle action potentials (M waves) recorded over the 'pennate' vastus lateralis showed a sharp deflection (named as a shoulder) in the first phase. Here, we investigated whether such a shoulder was also present in M waves evoked in a muscle with different architecture, such as the biceps brachii, with the purpose of elucidating the electrical origin of such afeature. Approach. M waves evoked by maximal single shocks to the brachial plexus were recorded in monopolar and bipolar configurations from 72 individuals using large (10 mm diameter) electrodes and from eight individuals using small (1 mm diameter) electrodes arranged in a linear array. The changes in M-wave features at different locations along the muscle fiber direction were examined. Main results. The shoulder was recognizable in most (87%) monopolar M waves, whereas it was rarely observed (6%) in bipolar derivations. Recordings made along the fiber direction showed that the shoulder was a stationary (non-propagating) feature, with short duration (spiky), which had positive polarity at all locations along the fibers. The latency of the shoulder (9.5 ± 0.5 ms) was significantly shorter than the estimated time taken for the action potentials to reach the biceps tendon (12.8 ms). Significance. The shoulder must be generated by a dipole source, i.e. a source created at a fixed anatomical position, although the exact origin of this dipole is uncertain. Our results suggest that the shoulder may not be due to the end-of-fiber signals formed at the biceps brachii tendon. The shoulder is not related to any specific arrangement of muscle fibers, as it has been observed in both pennate and fusiform muscles. Being a stationary (non-propagating) component, the shoulder is not reliable for studying changes in sarcolemmal excitability, and thus should be excluded from the M-wave analysis

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

It has been recurrently observed that, for compound muscle action potentials (M wave) recorded over the innervation zone of the &lt;i&gt;vastus lateralis&lt;/i&gt; , 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 &lt;i&gt;vastus lateralis&lt;/i&gt; . 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

Electromyographic, cerebral, and muscle hemodynamic responses during intermittent, isometric contractions of the biceps brachii at three submaximal intensities.

This study examined the electromyographic, cerebral and muscle hemodynamic responses during intermittent isometric contractions of biceps brachii at 20, 40, and 60% of maximal voluntary contraction (MVC). Eleven volunteers completed 2 min of intermittent isometric contractions (12/min) at an elbow angle of 90° interspersed with 3 min rest between intensities in systematic order. Surface electromyography (EMG) was recorded from the right biceps brachii and near infrared spectroscopy (NIRS) was used to simultaneously measure left prefrontal and right biceps brachii oxyhemoglobin (HbO2), deoxyhemoglobin (HHb), and total hemoglobin (Hbtot). Transcranial Doppler ultrasound was used to measure middle cerebral artery velocity (MCAv) bilaterally. Finger photoplethysmography was used to record beat-to-beat blood pressure and heart rate. EMG increased with force output from 20 to 60% MVC (P &lt; 0.05). Cerebral HbO2 and Hbtot increased while HHb decreased during contractions with differences observed between 60% vs. 40% and 20% MVC (P &lt; 0.05). Muscle HbO2 decreased while HHb increased during contractions with differences being observed among intensities (P &lt; 0.05). Muscle Hbtot increased from rest at 20% MVC (P &lt; 0.05), while no further change was observed at 40 and 60% MVC (P &gt; 0.05). MCAv increased from rest to exercise but was not different among intensities (P &gt; 0.05). Force output correlated with the root mean square EMG and changes in muscle HbO2 (P &lt; 0.05), but not changes in cerebral HbO2 (P &gt; 0.05) at all three intensities. Force output declined by 8% from the 1st to the 24th contraction only at 60% MVC and was accompanied by systematic increases in RMS, cerebral HbO2 and Hbtot with a leveling off in muscle HbO2 and Hbtot. These changes were independent of alterations in mean arterial pressure. Since cerebral blood flow and oxygenation were elevated at 60% MVC, we attribute the development of fatigue to reduced muscle oxygen availability rather than impaired central neuronal activation

Determinants, analysis and interpretation of the muscle compound action potential (M wave) in humans: implications for the study of muscle fatigue.

The compound muscle action potential (M wave) has been commonly used to assess the peripheral properties of the neuromuscular system. More specifically, changes in the M-wave features are used to examine alterations in neuromuscular propagation that can occur during fatiguing contractions. The utility of the M wave is based on the assumption that impaired neuromuscular propagation results in a decrease in M-wave size. However, there remains controversy on whether the size of the M wave is increased or decreased during and/or after high-intensity exercise. The controversy partly arises from the fact that previous authors have considered the M wave as a whole, i.e., without analyzing separately its first and second phases. However, in a series of studies we have demonstrated that the first and second phases of the M wave behave in a different manner during and after fatiguing contractions. The present review is aimed at five main objectives: (1) to describe the mechanistic factors that determine the M-wave shape; (2) to analyze the various factors influencing M-wave properties; (3) to emphasize the need to analyze separately the first and second M-wave phases to adequately identify and interpret changes in muscle fiber membrane properties; (4) to advance the hypothesis that it is an increase (and not a decrease) of the M-wave first phase which reflects impaired sarcolemmal membrane excitability; and (5) to revisit the involvement of impaired sarcolemmal membrane excitability in the reduction of the force generating capacity

New insights into the potentiation of the first and second phases of the M-wave after voluntary contractions in the quadriceps muscle

We investigated the mechanisms underlying the potentiation of the first and second phases of the compound action potential (M-wave) after conditioning contractions. M-waves were evoked in the knee extensors before and after isometric maximal voluntary contractions (MVCs) of 1 s, 3 s, 6 s, 10 s, 30 s, and 60 s. The amplitude, duration, and area of the M-wave first and second phases were measured during the 10-min period after each contraction. The magnitude of the M-wave first phase was enlarged only after MVCs of 30 s and 60 s, whereas the second phase increased after all MVCs, regardless of their duration. The enlargement of the first phase remained for longer than 2 min, whereas the potentiation of the second phase lasted only 20 s. Potentiation of the first phase is the result of fatigue-induced membrane changes, whereas enlargement of the second phase is probably related to shortening of muscle fascicles. Muscle Nerve 55: 35-45, 2017

Different recoveries of the first and second phases of the M-wave after intermittent maximal voluntary contractions.

We investigated the recovery of muscle electrical properties after intermittent intense exercise by examining separately the first and second phases of the muscle compound action potential (M-wave). M-waves and mechanical twitches were obtained using femoral nerve stimulation throughout the 30-min recovery period following 48 successive intermittent 3-s MVCs. The amplitude, duration, and area of the M-wave first and second phases, and the peak twitch force were measured from the knee extensors. The amplitudes of both the first and second M-wave phases were increased immediately after exercise (P &lt; 0.05), but, whereas the first phase remained enlarged for 5 min after exercise, the increase of the second phase only lasted for 10 s. After 30 min of recovery, the amplitude, area, and duration of both the first and second phases were decreased compared to control values (10-20%, P &lt; 0.05). A significant temporal association was found between the changes in the amplitude and duration of the M-wave first phase (maximal cross correlations, 0.9-0.93; time lag, 0 s). A significant, negative temporal relation was found between the amplitude of the M-wave first phase and the peak twitch force during recovery (P &lt; 0.05). The prolonged enlargement of the M-wave first phase during recovery seems primarily related to fatigue-induced changes in membrane properties, whereas the extremely short recovery of the second phase might be related to changes in muscle architectural features. It is concluded that muscle excitability is impaired even after intermittent fatiguing contractions which allow partial clearance of extracellular K(+)

Differences in the recruitment curves obtained with monopolar and bipolar electrode configurations in the quadriceps femoris.

We sought to verify whether the stimulation intensity at which M-wave amplitude reaches a plateau actually corresponds to full motor unit activation in monopolar and bipolar configurations. M-waves and twitches were evoked using femoral nerve stimulation of gradually increasing intensity in 21 subjects. Recruitment curves corresponding to the amplitude of the first phase (AmpliFIRST ) and peak-to-peak amplitude (AmpliPP ) of the M-wave were obtained in the vastus lateralis, vastus medialis, and rectus femoris in monopolar and bipolar configurations. In all muscles, bipolar M-waves and twitches reached plateau at a significantly lower stimulus intensity compared with monopolar M-waves (P &lt; 0.05). The different behavior of monopolar and bipolar M-waves with stimulus intensity was found for both AmpliFIRST and AmpliPP . In a bipolar configuration, the stimulus intensity at which M-waves plateau should be increased by at least 10%-20% to achieve complete motor unit recruitment. Muscle Nerve 54: 118-131, 2016

Effects of muscle fibre shortening on the characteristics of surface motor unit potentials.

Traditionally, studies dealing with muscle shortening have concentrated on assessing its impact on conduction velocity, and to this end, electrodes have been located between the end-plate and tendon regions. Possible morphologic changes in surface motor unit potentials (MUPs) as a result of muscle shortening have not, as yet, been evaluated or characterized. Using a convolutional MUP model, we investigated the effects of muscle shortening on the shape, amplitude, and duration characteristics of MUPs for different electrode positions relative to the fibre-tendon junction and for different depths of the MU in the muscle (MU-to-electrode distance). It was found that the effects of muscle shortening on MUP morphology depended not only on whether the electrodes were between the end-plate and the tendon junction or beyond the tendon junction, but also on the specific distance to this junction. When the electrodes lie between the end-plate and tendon junction, it was found that (1) the muscle shortening effect is not important for superficial MUs, (2) the sensitivity of MUP amplitude to muscle shortening increases with MU-to-electrode distance, and (3) the amplitude of the MUP negative phase is not affected by muscle shortening. This study provides a basis for the interpretation of the changes in MUP characteristics in experiments where both physiological and geometrical aspects of the muscle are varied

Muscle fibre conduction velocity varies in opposite directions after short- vs. long-duration muscle contractions.

The effects of muscle contractions on muscle fibre conduction velocity have normally been investigated for contractions of a given duration and intensity, with most studies being focused on the decline on conduction velocity during/after prolonged contractions. Herein, we perform a systematic analysis of the changes in conduction velocity after voluntary contractions of different durations and intensities. Conduction velocity was estimated in the vastus lateralis before and after knee extensor isometric maximal voluntary contractions (MVCs) of 1, 3, 6, 10, 30 and 60 s, and after brief (3 s) contractions at 10, 30, 50, 70, and 90% of MVC force. Measurements were made during the 10-min period following each contraction. (1) Conduction velocity was increased immediately after (1 s) the MVCs of brief (≤ 10 s) duration (12 ± 2%, P &lt; 0.05), and then returned rapidly (within 15 s) to control levels; (2) the extent of the increase in conduction velocity was similar after the 3-s, 6-s, and 10-s MVCs (P &gt; 0.05); (3) the magnitude of the increase in conduction velocity after a brief contraction augmented with the intensity of the contraction (increases of 4.6, 7.7, 11.4, 14.8, and 15.2% for contractions at 10, 30, 50, 70, and 90% of MVC force, respectively); (4) conduction velocity was not decreased immediately after the 30-s MVC (P &gt; 0.05); and (5) conduction velocity did not reach its minimum 1 s after the long (≥ 30 s) MVCs. Brief (≤ 10 s) muscle contractions induce a short-term increase in conduction velocity, lasting 15 s, while long (≥ 30 s) contractions produce a long-term decrease in conduction velocity, lasting more than 2 min