Twitch and M-wave potentiation induced by intermittent maximal voluntary quadriceps contractions: differences between direct quadriceps and femoral nerve stimulation

Neural Adjustments during Repeated Braking and Throttle Actions on a Motorcycle Setup

The aim of the study was to assess neuromuscular changes during an intermittent fatiguing task designed to replicate fundamental actions and ergonomics of road race motorcycling. Twenty-eight participants repeated a sequence of submaximal brake-pulling and gas throttle actions, interspaced by one maximal brake-pulling, until failure. During the submaximal brake-pulling actions performed at 30% MVC, force fluctuations, surface EMG, maximal M-wave (Mmax) and H-reflex were measured in the flexor digitorum superficialis. At the end of the task, the MVC force and associated EMG activity decreased (P<0.001) by 46% and 26%, respectively. During the task, force fluctuation and EMG activity increased gradually (106% and 61%, respectively) with respect to the pre-fatigue state (P≤0.029). The Mmax first phase did not change (P≥0.524), whereas the H-reflex amplitude, normalized to Mmax, increased (149%; P≤0.039). Noteworthy, the relative increase in H-reflex amplitude was correlated with the increase in EMG activity during the task (r=0.63; P<0.001). During the 10-min recovery, MVC force and EMG activity remained depressed (P≤0.05) whereas H-reflex amplitude and force fluctuation returned to pre-fatigue values. In conclusion, contrarily to other studies, our results bring forward that when mimicking motorcycling brake-pulling and gas throttle actions, supraspinal neural mechanisms primarily limit the duration of the performance.

Quadriceps Neuromuscular Impairments after Arthroscopic Knee Surgery: Comparison between Procedures

Quadriceps neuromuscular function remains impaired in the short- and long-term following knee arthroscopy for meniscal surgery and/or anterior cruciate ligament (ACL) reconstruction. The aim of this study was to compare quadriceps neuromuscular impairments in patients following meniscal surgery with and without ACL reconstruction. Thirty patients were tested six months after meniscal surgery with (n = 15) and without (n = 15) ACL reconstruction. We bilaterally assessed knee extension maximal voluntary contraction (MVC) torque using dynamometry, vastus lateralis thickness using ultrasound, quadriceps voluntary activation and evoked knee extension torque with transcutaneous electrical stimulation. Patient-reported outcomes were evaluated with the Knee Injury and Osteoarthritis Outcome Score (KOOS). Compared with meniscus patients, ACL patients demonstrated larger asymmetries in MVC torque (15% vs. 5%, p = 0.049) and vastus lateralis thickness (6% vs. 0%, p = 0.021). In ACL patients, asymmetries in MVC torque correlated with asymmetries in evoked torque (r = 0.622, p = 0.013). In meniscus patients, asymmetries in muscle activation correlated with KOOS quality of life (r = 0.619, p = 0.018). Patients demonstrated persistent quadriceps muscle weakness six months after ACL reconstruction, but not after isolated meniscal surgery. Quantitative and/or qualitative muscular changes likely underlie quadriceps muscle weakness in ACL patients, whereas activation failure is associated with poor quality of life in some meniscus patients.

Potentiation of the first and second phases of the M wave after maximal voluntary contractions in the biceps brachii muscle

The study was undertaken to examine separately the potentiation of the first and second phases of the M wave in biceps brachii after conditioning maximal voluntary contractions (MVCs) of different durations. M waves were evoked in the biceps brachii muscle before and after isometric MVCs of 1, 3, 6, 10, 30, and 60 s. The amplitude, duration, and area of the first and second phases of monopolar M waves were measured during the 10-min period following each contraction. Our results indicated that the amplitude and area of the M-wave first phase increased after MVCs of long (≥ 30 s) duration (P < 0.05), while it decreased after MVCs of short (≤ 10 s) duration (P < 0.05). The enlargement after the long MVCs persisted for 5 min, whereas the depression after the short contractions lasted only for 15 s. The amplitude of the second phase increased immediately (1 s) after all MVCs tested (P < 0.05), regardless of their duration, and then returned rapidly (10 s) to control levels. Unexpectedly, the amplitude of the second phase decreased below control values between 15 s and 1 min after the MVCs lasting ≥ 6 s (P < 0.05). Our results reinforce the idea that the presence of fatigue is a necessary condition to induce an enlargement of the M-wave first phase and that this enlargement would be greater (and occur sooner) in muscles with a predominance of type II fibers (quadriceps and biceps brachii) compared to type-I predominant muscles (tibialis anterior). The unique findings observed for the M-wave second phase indicate that changes in this phase are highly muscle dependent. Graphical abstract Left panel-Representative examples of M waves recorded in one participant before (control) and at various times after conditioning maximal voluntary contractions (MVCs) of short (a1) and long (a2) duration. Left panel-Time course of recovery of the amplitude of the first (b1) and second (b2) phases of the M wave after conditioning MVCs of different durations.

Sarcolemmal membrane excitability during repeated intermittent maximal voluntary contractions

NEW FINDINGS

What is the central question of this study? Is impaired membrane excitability reflected by an increase or by a decrease in M-wave amplitude? What is the main finding and its importance? The magnitude of the M-wave first and second phases changed in completely different ways during intermittent maximal voluntary contractions, leading to the counterintuitive conclusion that it is an increase (and not a decrease) of the M-wave first phase that reflects impaired membrane excitability.

ABSTRACT

The study was undertaken to investigate separately the changes in the first and second phases of the muscle compound action potential (M-wave) during 4 min of intermittent maximal voluntary contractions (MVCs) of the quadriceps. M-waves were evoked by supramaximal single electrical stimulation to the femoral nerve delivered in the resting periods between 48 successive MVCs of 3 s. The amplitude, duration and area of the M-wave first and second phases were measured separately, together with muscle conduction velocity and MVC force. During the intermittent MVCs, the amplitude of the M-wave first phase increased uninterruptedly for the first 3 min (12-16%, P < 0.05) and stabilized thereafter, whereas the second phase initially increased for 55-75 s (11-22%, P < 0.05), but decreased subsequently. The enlargement of the first phase occurred in parallel with an increase in its duration, and concomitantly with a decline in conduction velocity (maximal cross-correlations, 0.89-0.97; time lag, 0 s). Also, a significant temporal association was found between the amplitude of the first phase and MVC force (time lag, 0 s; maximal cross-correlations, 0.85-0.97). Conversely, there was no temporal association between the second phase amplitude and conduction velocity or MVC force (time lag, 73-117 s; maximal cross-correlations, 0.65-0.77). It is concluded that the enlargement of the M-wave first phase is the electrical manifestation of impaired muscle membrane excitability. The results highlight the importance of independently analysing the first and second phases, as only the first phase can be used reliably to detect changes in membrane excitability, while the second might be affected by muscle architecture.

Knee joint angle and vasti muscle electromyograms during fatiguing contractions

We compared vasti muscle electromyograms for two knee joint angles during fatiguing tetanic contractions. Tetanic contraction of the knee extensors was evoked for 70 s by electrical stimulation of the femoral nerve at knee joint angles of 60° (extended, with 0° indicating full extension) and 110° (flexed) in eight healthy men. Surface electromyography was recorded from the vastus intermedius (VI), vastus lateralis (VL) and vastus medialis (VM) muscles. Knee extension force and M-wave amplitudes and durations were calculated every 7 s, which were normalized by the initial value. Normalized knee extension force was decreased at the flexed knee joint angle compared with that of the extended knee joint angle (P<0·05). Decreased normalized M-wave amplitude and increased normalized M-wave duration of the VI were greater at the flexed knee joint angle than the extended knee joint angle (P<0·05), whereas those for the VL and VM were similar (P>0·05). These results suggest that peripheral fatigue profiles of the VI might be greater at the flexed than the extended knee joint angles, but that of VL and VM might be similar in the tested range of knee joint angles (i.e. 60°-110°) during continuous tetanic contraction induced by electrical stimulation. Therefore, greater reduction of knee extension force at the flexed knee joint angle than the extended knee joint angle may reflect fatigue development of the VI more than other quadriceps femoris components.

Task Failure during Exercise to Exhaustion in Normoxia and Hypoxia Is Due to Reduced Muscle Activation Caused by Central Mechanisms While Muscle Metaboreflex Does Not Limit Performance

To determine whether task failure during incremental exercise to exhaustion (IE) is principally due to reduced neural drive and increased metaboreflex activation eleven men (22 ± 2 years) performed a 10 s control isokinetic sprint (IS; 80 rpm) after a short warm-up. This was immediately followed by an IE in normoxia (Nx, P_IO₂:143 mmHg) and hypoxia (Hyp, P_IO₂:73 mmHg) in random order, separated by a 120 min resting period. At exhaustion, the circulation of both legs was occluded instantaneously (300 mmHg) during 10 or 60 s to impede recovery and increase metaboreflex activation. This was immediately followed by an IS with open circulation. Electromyographic recordings were obtained from the vastus medialis and lateralis. Muscle biopsies and blood gases were obtained in separate experiments. During the last 10 s of the IE, pulmonary ventilation, VO₂, power output and muscle activation were lower in hypoxia than in normoxia, while pedaling rate was similar. Compared to the control sprint, performance (IS-Wpeak) was reduced to a greater extent after the IE-Nx (11% lower P < 0.05) than IE-Hyp. The root mean square (EMG_RMS) was reduced by 38 and 27% during IS performed after IE-Nx and IE-Hyp, respectively (Nx vs. Hyp: P < 0.05). Post-ischemia IS-EMG_RMS values were higher than during the last 10 s of IE. Sprint exercise mean (IS-MPF) and median (IS-MdPF) power frequencies, and burst duration, were more reduced after IE-Nx than IE-Hyp (P < 0.05). Despite increased muscle lactate accumulation, acidification, and metaboreflex activation from 10 to 60 s of ischemia, IS-Wmean (+23%) and burst duration (+10%) increased, while IS-EMG_RMS decreased (−24%, P < 0.05), with IS-MPF and IS-MdPF remaining unchanged. In conclusion, close to task failure, muscle activation is lower in hypoxia than in normoxia. Task failure is predominantly caused by central mechanisms, which recover to great extent within 1 min even when the legs remain ischemic. There is dissociation between the recovery of EMG_RMS and performance. The reduction of surface electromyogram MPF, MdPF and burst duration due to fatigue is associated but not caused by muscle acidification and lactate accumulation. Despite metaboreflex stimulation, muscle activation and power output recovers partly in ischemia indicating that metaboreflex activation has a minor impact on sprint performance.

M-wave potentiation after voluntary contractions of different durations and intensities in the tibialis anterior

The study was undertaken to provide insight into the mechanisms underlying the potentiation of the muscle compound action potential (M wave) after conditioning contractions. M waves were evoked in the tibialis anterior before and after isometric maximal voluntary contractions (MVC) of 1, 3, 6, 10, 30, and 60 s, and after 3-s contractions at 10, 30, 50, 70, 90, and 100% MVC. The amplitude, duration, and area of the first and second phases of the M wave, together with the median frequency (Fmedian) and muscle fiber conduction velocity (MFCV) were recorded. Furthermore, twitch force, muscle fascicle length, and pennation angle were measured at rest, before, and 1 s after the conditioning contractions. The results indicate that only the amplitude of the second phase of the M wave was significantly increased after conditioning contractions. The extent of this potentiation was similar for MVC durations ranging from 1 to 10 s and augmented progressively with contraction intensity from 30 to 70% MVC. After these conditioning contractions, the duration and area of the two M-wave phases decreased ( P < 0.05), whereas MFCV and Fmedian increased ( P < 0.05). For all of these parameters, the greatest changes occurred 1 s after the conditioning contraction. Changes in MFCV after the contractions were correlated with those in M-wave second-phase amplitude ( r2 = 0.42; P < 0.05) and Fmedian ( r2 = 0.53; P < 0.05). In contrast, fascicle length and pennation angle did not change after the conditioning contractions. It is concluded that the potentiation of the second phase of the M wave is mainly due to an increased MFCV.

Comparison of the power spectral changes of the voluntary surface electromyogram and M wave during intermittent maximal voluntary contractions

INTRODUCTION

To compare the power spectral changes of the voluntary surface electromyogram (sEMG) and of the compound action potential (M wave) in the vastus medialis and vastus lateralis muscles during fatiguing contractions.

METHODS

Interference sEMG and force were recorded during 48 intermittent 3-s isometric maximal voluntary contractions (MVC) from 13 young, healthy subjects. M waves and twitches were evoked using supramaximal femoral nerve stimulation between the successive MVCs. Mean frequency (F mean), and median frequency were calculated from the sEMG and M waves. Muscle fiber conduction velocity (MFCV) was computed by cross-correlation.

RESULTS

The power spectral shift to lower frequencies was significantly greater for the voluntary sEMG than for the M waves (P < 0.05). Over the fatiguing protocol, the overall average decrease in MFCV (~25%) was comparable to that of sEMG F mean (~22%), but significantly greater than that of M-wave F mean (~9%) (P < 0.001). The mean decline in MFCV was highly correlated with the mean decreases in both sEMG and M-wave F mean.

CONCLUSIONS

The present findings indicated that, as fatigue progressed, central mechanisms could enhance the relative weight of the low-frequency components of the voluntary sEMG power spectrum, and/or the end-of-fiber (non-propagating) components could reduce the sensitivity of the M-wave spectrum to changes in conduction velocity.

The increase in surface EMG could be a misleading measure of neural adaptation during the early gains in strength

PURPOSE

To test the validity of using the increase in surface EMG as a measure of neural adaptation during the early gains in strength.

METHODS

Simulation of EMG signals detected by surface bipolar electrode with 20-mm inter-pole distance at different radial distances from the muscle and longitudinal distances from the end-plate area. The increases in the root mean square (RMS) of the EMG signal due to possible alteration in the neural drive or elevation of the intracellular negative after-potentials, detected in fast fatigable muscle fibres during post-tetanic potentiation and assumed to accompany post-activation potentiation, were compared.

RESULTS

Lengthening of the intracellular action potential (IAP) profile due to elevation of the negative after-potentials could affect amplitude characteristics of surface EMG detected at any axial distance stronger than alteration in the neural drive. This was irrespective of the fact that the elevation of IAP negative after-potential was applied to fast fatigable motor units (MUs) only, while changes in frequency of activation (simulating neural drive changes) were applied to all MUs. In deeper muscles, where the fibre-electrode distance was larger, the peripheral effect was more pronounced. The normalization of EMG amplitude characteristics to an M-wave one could result only in partial elimination of peripheral factor influence

CONCLUSIONS

The increase in RMS of surface EMG during the early gains in strength should not be directly related to the changes in the neural drive. The relatively small but long-lasting elevated free resting calcium after high-resistance strength training could result in force potentiation and EMG increase.

Comparison of the duration and power spectral changes of monopolar and bipolar M waves caused by alterations in muscle fibre conduction velocity

The muscle compound action potential (M wave) recorded under monopolar configuration reflects both the propagation of the action potentials along the muscle fibres and their extinction at the tendon. M waves recorded under a bipolar configuration contain less cross talk and noise than monopolar M waves, but they do not contain the entire informative content of the propagating potential. The objective of this study was to compare the effect of changes in muscle fibre conduction velocity (MFCV) on monopolar and bipolar M waves and how this effect depends on the distance between the recording electrodes and tendon. The study was based on a simulation approach and on an experimental investigation of the characteristics of surface M waves evoked in the vastus lateralis during 4-s step-wise isometric contractions in knee extension at 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% MVC. The peak-to-peak duration (Durpp) and median frequency (Fmedian) of the M waves were calculated. For monopolar M waves, changes in Durpp and Fmedian produced by MFCV depended on the distance from the electrode to the tendon, whereas, for bipolar M waves, changes in Durpp and Fmedian were largely independent of the electrode-to-tendon distance. When the distance between the detection point and tendon lay between approximately 15 and 40mm, changes in Durpp of bipolar M waves were more pronounced than those of distal monopolar M waves but less marked than those of proximal monopolar M waves, and the opposite occurred for Fmedian. Since, for bipolar M waves, changes in duration and power spectral features produced by alterations in MFCV are not influenced by the electrode-to-tendon distance, the bipolar electrode configuration is a preferable choice over monopolar arrangements to estimate changes in conduction velocity.

The effect of muscle fatigue on stimulus intensity requirements for central and peripheral fatigue quantification

PURPOSE

The present study was designed to determine the stimulation intensity necessary for an adequate assessment of central and peripheral components of neuromuscular fatigue of the knee extensors.

METHODS

Three different stimulation intensities (100, 120 and 150% of the lowest intensity evoking a plateau in M-waves and twitch amplitudes, optimal stimulation intensity, OSI) were used to assess voluntary activation level (VAL) as well as M-wave, twitch and doublet amplitudes before, during and after an incremental isometric exercise performed by 14 (8 men) healthy and physically active volunteers. A visual analog scale was used to evaluate the associated discomfort.

RESULTS

There was no difference (p > 0.05) in VAL between the three intensities before and after exercise. However, we found that stimulating at 100% OSI may overestimate the extent of peripheral fatigue during exercise, whereas 150% OSI stimulations led to greater discomfort associated with doublet stimulations as well as to an increased antagonist co-activation compared to 100% OSI.

CONCLUSION

We recommend using 120% OSI, as it constitutes a good trade-off between discomfort and reliable measurements.