Mean Muscle Fiber Conduction Velocity Research Articles

Does vibration superimposed on low-level isometric contraction alter motor unit recruitment strategy?

Objective. Beneficial effects, including improved muscle strength and power performance, have been observed during vibration exercise (VE) and partially ascribed to a specific reflex mechanism referred to as Tonic vibration reflex (TVR). TVR involves motor unit (MU) activation synchronized and un-synchronized with the vibration cycle; this suggests VE to alter the temporal MU recruitment strategy. However, the effects of VE on MU recruitment remain poorly understood. This study aims to elucidate the influence of VE on MU recruitment indirectly, by investigating the effects of low-intensity VE on muscle activation. Approach. Twenty volunteers performed isometric contractions on the biceps brachii of the right arm at a baseline (low) force equal to 30% of the maximum voluntary contraction without vibration (control) and with vibration at 20, 30, 40, and 55 Hz. Three vibration amplitudes were employed at 12.5%, 25%, and 50% of the baseline. Mean muscle-fiber conduction velocity (mCV), mean frequency (MF), and root mean square (RMS) value were estimated from surface electromyography as indicators of the alteration in MU recruitment strategies. Main results. The mCV estimates during VE were significantly (p < 0.05) higher compared to the control condition. Furthermore, six VE conditions produced significantly larger RMS values compared to control condition. The estimated MF did not show any consistent trend. Significance. These results suggest that vibration superimposed on low-level isometric contraction alters the MU recruitment strategy, activating larger and faster MUs.

Intramuscular fiber conduction velocity, isometric force and explosive performance.

Conduction of electrical signals along the surface of muscle fibers is acknowledged as an essential neuromuscular component which is linked with muscle force production. However, it remains unclear whether muscle fiber conduction velocity (MFCV) is also linked with explosive performance. The aim of the present study was to investigate the relationship between vastus lateralis MFCV and countermovement jumping performance, the rate of force development and maximum isometric force. Fifteen moderately-trained young females performed countermovement jumps as well as an isometric leg press test in order to determine the rate of force development and maximum isometric force. Vastus lateralis MFCV was measured with intramuscular microelectrodes at rest on a different occasion. Maximum MFCV was significantly correlated with maximum isometric force (r = 0.66, p < 0.01), nevertheless even closer with the leg press rate of force development at 100 ms, 150 ms, 200 ms, and 250 ms (r = 0.85, r = 0.89, r = 0.91, r = 0.92, respectively, p < 0.01). Similarly, mean MFCV and type II MFCV were better correlated with the rate of force development than with maximum isometric leg press force. Lower, but significant correlations were found between mean MFCV and countermovement jump power (r = 0.65, p < 0.01). These data suggest that muscle fiber conduction velocity is better linked with the rate of force development than with isometric force, perhaps because conduction velocity is higher in the larger and fastest muscle fibers which are recognized to contribute to explosive actions.

Multi-channel electromyography during maximal isometric and dynamic contractions

Motor unit behavior differs between contraction types at submaximal contraction levels, however is challenging to study during maximal voluntary contractions (MVCs). With multi-channel surface electromyography (sEMG), mean physiological characteristics of the active motor units can be extracted. Two 8-electrode sEMG arrays were attached on biceps brachii muscle (one on each head) to examine behavior of sEMG variables during isometric, eccentric and concentric MVCs of elbow flexors in 36volunteers.On average, isometric (364±88N) and eccentric (353±74N) MVCs were higher than concentric (290±73N) MVC (p<0.001). Mean muscle fiber conduction velocity (CV) was highest during eccentric MVC (4.42±0.49m/s) than concentric (4.25±0.49m/s, p<0.01) and isometric (4.14±0.45m/s, p<0.001) MVCs. Furthermore, eccentric MVC showed lower sEMG amplitude at the largest elbow joint angles (120–170°) and higher CV at the smallest (70–150°) elbow joint angles (p<0.05–0.001) than concentric MVC.The differences in CV and sEMG amplitude between the MVCs suggest that the control strategy of motor units differs between the contraction types during MVCs, and is dependent on the muscle length between the dynamic MVCs.

Changes in motor unit characteristics after eccentric elbow flexor exercise

Morphological evidence suggests that fast-twitch fibers are prone to disruption of their membrane structures by eccentric exercise. However, it is unclear how this is reflected in the discharge rate and action potential propagation of individual motor units, especially at high contraction levels. High-density surface electromyograms were recorded from biceps brachii muscle and decomposed to individual motor unit action potentials at isometric contraction levels between 10% and 75% of maximal voluntary contraction (MVC) before intermittent maximal elbow flexor eccentric exercise, and two hours (2H), two days (2D) and four days (4D) post-exercise. Maximal voluntary force decreased by 21.3±5.6% 2H and by 12.6±11.1% 2D post-exercise. Motor unit discharge rate increased and mean muscle fiber conduction velocity decreased, at the highest isometric contraction levels only (50% and 75% of MVC) at 2H post-exercise. These results indicate that eccentric exercise can disturb the function of motor units active at high contraction levels in the early stages after exercise, which seems to be compensated by the central nervous system with an increase in neural drive during submaximal isometric contractions.

Muscle fiber conduction velocity is more affected after eccentric than concentric exercise

It has been shown that mean muscle fiber conduction velocity (CV) can be acutely impaired after eccentric exercise. However, it is not known whether this applies to other exercise modes. Therefore, the purpose of this experiment was to compare the effects of eccentric and concentric exercises on CV, and amplitude and frequency content of surface electromyography (sEMG) signals up to 24 h post-exercise. Multichannel sEMG signals were recorded from biceps brachii muscle of the exercised arm during isometric maximal voluntary contraction (MVC) and electrically evoked contractions induced by motor-point stimulation before, immediately after and 2 h after maximal eccentric (ECC group, N = 12) and concentric (CON group, N = 12) elbow flexor exercises. Isometric MVC decreased in CON by 21.7 ± 12.0% (± SD, p < 0.01) and by 30.0 ± 17.7% (p < 0.001) in ECC immediately post-exercise when compared to baseline. At 2 h post-exercise, ECC showed a reduction in isometric MVC by 24.7 ± 13.7% (p < 0.01) when compared to baseline, while no significant reduction (by 8.0 ± 17.0%, ns) was observed in CON. Similarly, reduction in CV was observed only in ECC both during the isometric MVC (from baseline of 4.16 ± 0.3 to 3.43 ± 0.4 m/s, p < 0.001) and the electrically evoked contractions (from baseline of 4.33 ± 0.4 to 3.82 ± 0.3 m/s, p < 0.001). In conclusion, eccentric exercise can induce a greater and more prolonged reduction in muscle force production capability and CV than concentric exercise.

Changes in motor unit characteristics after eccentric elbow flexor exercise

Morphological evidence suggests that fast-twitch fibers are prone to disruption of their membrane structures by eccentric exercise. However, it is unclear how this is reflected in the discharge rate and action potential propagation of individual motor units, especially at high contraction levels. High-density surface electromyograms were recorded from biceps brachii muscle and decomposed to individual motor unit action potentials at isometric contraction levels between 10% and 75% of maximal voluntary contraction (MVC) before intermittent maximal elbow flexor eccentric exercise, and two hours (2H), two days (2D) and four days (4D) post-exercise. Maximal voluntary force decreased by 21.3±5.6% 2H and by 12.6±11.1% 2D post-exercise. Motor unit discharge rate increased and mean muscle fiber conduction velocity decreased, at the highest isometric contraction levels only (50% and 75% of MVC) at 2H post-exercise. These results indicate that eccentric exercise can disturb the function of motor units active at high contraction levels in the early stages after exercise, which seems to be compensated by the central nervous system with an increase in neural drive during submaximal isometric contractions.

Impaired action potential conduction at high force levels after eccentric exercise

High-density surface electromyography was used to examine whether gross sarcolemmal function is impaired in m. biceps brachii after intensive eccentric elbow flexor exercise, when measured at wide range of isometric contraction levels. Root mean square (RMS), mean power frequency (MNF) and mean muscle fibre conduction velocity (CV) were calculated before and up to four days post-exercise. Maximal isometric voluntary (MVC) force decreased by 21.3 ± 5.6% two hours after exercise, and by 12.6 ± 11.1% two days post-exercise. CV and MNF decreased both during MVC (CV from 4.1 ± 0.3 m/s to 3.8 ± 0.4 m/s and MNF from 92.6 ± 10 Hz to 85.2 ± 11 Hz) and during electrically evoked maximal M-wave (CV from 4.1 ± 0.3 m/s to 3.0 ± 0.5 m/s and MNF from 97.1 ± 27.2 Hz to 78.0 ± 24.4 Hz) two hours post-exercise. Furthermore, at submaximal isometric force levels, CV and MNF decreased only at higher contraction levels (40%, 50% and 75% of MVC) two hour post-exercise. It can be concluded that intensive exercise can temporarily impair gross sarcolemmal function. In addition, since this only occurred at high force levels, based on Henneman’s size principle, it seems that higher threshold motor units were predominantly affected.

Effect of Short-Term Creatine Supplementation on Neuromuscular Function

The purpose of the present investigation was to determine whether short-term creatine (Cr) supplementation would affect 1) muscle contractile properties assessed by evoked and voluntary contractions, 2) force-velocity relationship, and 3) mean muscle fiber conduction velocity (CV). Using a double-blind random design, 16 moderately trained men (25.2 ± 5.1 yr) were assigned to a Cr (CRE) or a placebo (PLA) group. Subjects supplemented their diet four times a day for 5 d with 5 g of Cr + 15 g maltodextrin (CRE) or 20 g maltodextrin (PLA). Isometric maximal voluntary contraction, maximal twitch, force-velocity relationship, and dynamic fatiguing contractions were assessed in the elbow flexors. Mechanical and EMG signals were recorded and analyzed. CV was estimated from the EMG and used as a parameter of interest. After supplementation, peak torque (PT) of maximal twitch was 33.4% higher, and the time to reach the PT was 54.7% lower in CRE than in PLA (P < 0.05). Torque-angular velocity curve was enhanced after Cr supplementation, especially at the higher velocities. Mean fiber CV was, on average, 8.9% higher in CRE at all angular velocities after supplementation (P < 0.05). EMG and mechanical parameters during the fatiguing exercise protocol did not show significant differences in muscle fatigue between the two groups after supplementation. The present study shows that oral Cr supplementation improves neuromuscular function of the elbow flexor muscle during both voluntary and electrically induced contractions.

Effect of electrode location on surface electromyography changes due to eccentric elbow flexor exercise

Experiments were carried out to determine whether the location of electrodes has an effect on eccentric exercise-induced changes in surface electromyography (sEMG) variables in the biceps brachii muscle. sEMG signals were recorded with a grid of 64 electrodes before and up to 4 days post-exercise. Root mean square (RMS) and mean power frequency (MNF) were calculated for: (1) each channel; (2) as an average of all channels; and (3) as an average of individual channel rows and columns. Mean muscle-fiber conduction velocity (CV) was estimated similarly but was based on double-differential channels. Maximal isometric voluntary torque decreased 21.3 +/- 5.6% post-exercise. The average sEMG variables decreased after the exercise and recovered 2 days (RMS and CV) or 4 days (MNF) post-exercise. Site-dependent changes were observed in sEMG variables. We conclude that site-dependent changes in sEMG variables after eccentric exercise can be detected and are influenced in part by anatomical factors.

Distribution of motor unit potential velocities in short static and prolonged dynamic contractions at low forces: use of the within-subject’s skewness and standard deviation variables

Behaviour of motor unit potential (MUP) velocities in relation to (low) force and duration was investigated in biceps brachii muscle using a surface electrode array. Short static tests of 3.8 s (41 subjects) and prolonged dynamic tests (prolonged tests) of 4 min (30 subjects) were performed as position tasks, applying forces up to 20% of maximal voluntary contraction (MVC). Four variables, derived from the inter-peak latency technique, were used to describe changes in the surface electromyography signal: the mean muscle fibre conduction velocity (CV), the proportion between slow and fast MUPs expressed as the within-subject skewness of MUP velocities, the within-subject standard deviation of MUP velocities [SD-peak velocity (PV)], and the amount of MUPs per second (peak frequency = PF). In short static tests and the initial phase of prolonged tests, larger forces induced an increase of the CV and PF, accompanied with the shift of MUP velocities towards higher values, whereas the SD-PV did not change. During the first 1.5–2 min of the prolonged lower force levels tests (unloaded, and loaded 5 and 10% MVC) the CV and SD-PV slightly decreased and the MUP velocities shifted towards lower values; then the three variables stabilized. The PF values did not change in these tests. However, during the prolonged higher force (20% MVC) test, the CV decreased and MUP velocities shifted towards lower values without stabilization, while the SD-PV broadened and the PF decreased progressively. It is argued that these combined results reflect changes in both neural regulatory strategies and muscle membrane state.

Prediction of endurance capacity of quadriceps muscles in humans using surface electromyogram spectrum analysis during submaximal voluntary isometric contractions.

The measurement of endurance time ( t(lim)) is the procedure commonly used to quantify the ability of a muscle to maintain force. The relationship between surface electromyographic (sEMG) manifestations of localised muscle fatigue and t(lim) during an effort at 50% of maximal voluntary isometric torque of the knee extensors (vastus lateralis and vastus medialis) until exhaustion was studied in 14 healthy volunteers. It was carried out to test whether changes in sEMG computed over shorter periods than expected t(lim) could be used to predict t(lim). Changes in mean muscle fibre conduction velocity, mean power frequency, median frequency, root mean square ), in the relative power in the 6-30 Hz and 30-60 Hz frequency bands were monitored using linear slope and area ratio index as statistical indicators. These indicators were computed over fixed periods shorter than t(lim). The subjects were able to maintain the required force level for [mean (SD)] 78.8 (9.5) s. During the fatigue trial, it was the greatest of the increases in the 6-30 Hz frequency band, recorded for either of the two muscles investigated, that was the only variable which correlated with t(lim). Significant relationships between t(lim) and changes in this low frequency band were observed as early as the first 15-30 s of the contraction. These results suggest that sEMG frequency banding may predict mechanical endurance without the need to maintain the contraction until exhaustion. From a clinical perspective, this could be an advantage for patients who might not be able to tolerate contractions to exhaustion.

Effect of muscle length on distribution of muscle fiber conduction velocity for M. biceps brachii.

The influence of the muscle length on the distribution of muscle fiber conduction velocity (MFCV) was estimated at 3 different elbow angles (90 degrees, 120 degrees and 150 degrees) during the contraction of 30% of isometric maximum voluntary contraction (MVC). Surface electromyogram was recorded in the distal region of m. biceps brachii using a surface array electrode. MFCVs at the different locations from the end-plate to the distal tendon of the muscle were measured directly using the averaging method. MFCVs near the end-plate and the distal tendon of the muscle showed the high values, while MFCVs in the middle region between the end-plate and the distal tendon of the muscle had low values at the same elbow angles. The mean MFCVs in the middle region decreased when the muscle length increased during the contraction of 30% MVC. The middle region length changed when the elbow joint was extended from the elbow angle of 90 degrees to that of 150 degrees. Although previous studies showed the influence of muscle length on MFCV for special regions (like end-plate, tendon, and middle region), MFCV for whole regions, that is, the distribution of MFCV has not been studied for various muscle length. The MFCV for the same region in the some muscle length could not be estimated. In the study, the distribution of MFCV in various muscle length for the contraction of 30% MVC was obtained, and the model with the function including muscle length for the estimation of MFCV was presented.

Muscle fiber conduction velocity in the diagnosis of familial hypokalemic periodic paralysis--invasive versus surface determination.

Muscle fiber conduction velocity (MFCV) in the brachial biceps muscle was determined in a large family of patients with hypokalemic periodic paralysis (HOPP) by both a surface and an invasive method. Other surface EMG parameters and the muscle force were also determined. Both the surface and the invasive method showed a significantly lower mean MFCV in the proven gene carriers but only the invasive method showed a lower MFCV in all proven carriers. It can be concluded that MFCV determination is a reliable method to detect the membrane defect in HOPP carriers and that the invasive method is not only easy to perform, but also more sensitive. The muscle force and the integrated EMG at maximal voluntary contraction were lower in the carrier group. A positive correlation between the surface MFCV and the neuromuscular efficiency (the quotient of force and integrated EMG) was found in the controls but not in the HOPP carriers. Since type II fibers have a higher neuromuscular efficiency, this suggests a preferential involvement of type II fibers in HOPP.

Long‐lasting supernormal conduction velocity after sustained maximal isometric contraction in human muscle

Local muscle fatigue (1 min maximal voluntary contraction) and recovery were studied by means of surface and invasive EMG on elbow flexors to record the changes in muscle fiber conduction velocity (MFCV), median power frequency (MPF), integrated EMG (IEMG), and force. The main finding was a long-lasting "supernormal" MFCV during recovery, for at least 1 hour. After a normalization phase, the MFCV and MPF continued to increase reaching a steady state at supernormal values after 10-12 min. Mean MFCV increase at 20% MVC after 15-min recovery was 0.58 m.s-1 (12%). Postfatigue IEMG values were increased at all contraction levels. In combination with near normal force levels, this resulted in a decrease in "neuromuscular efficiency" (force/IEMG). We suggest that this IEMG increase is mainly a result of the MFCV increase. The MFCV changes in fastest and slowest fibers found with the invasive method indicate a relatively equal effect on type I and II fiber types. A possible explanation of the supernormal MFCV is muscle fiber swelling, in combination with altered membrane properties.

Muscle fiber action potential changes and surface EMG: A simulation study

The muscle fiber membrane electrophysiology may change under fatiguing circumstances. In a simulation study, the influence of muscle fiber membrane electrophysiological changes on two major characteristics of the surface electromyogram interference pattern (SEMG) is quantified [median frequency ( F med) and SEMG amplitude ( RMS)]. Frequency and amplitude parameters are calculated from a model of the motor unit action potential in which the mean muscle fiber conduction velocity ( U) and the intracellular action potential duration ( T) characterize the state of the sarcolemma. F med appears to be proportionally related to U, while T does not play a significant role in F med. The simulations further reveal that the RMS amplitude is proportional to T and the square root of U. The simple dependencies of SEMG parameters on U and T can be disturbed by a different sensitivity of thick and thin muscle fibers to factors that decrease the membrane excitability. It is stressed that the results of this study concern bipolarly recorded interference EMGs. It is concluded that volume conductor parameters and interelectrode distances, although important with respect to the absolute values of F med and RMS, do not interfere with the influences of T and U when relative SEMG changes are considered.

The clinical use of multichannel surface electromyography.

Surface EMG motor unit action potentials (MUAPs), recorded with "array" or "matrix" types of electrodes, can be seen to propagate from the motor end plates in both directions along the muscle fibers. We attempted to obtain diagnostic information in muscle disease by using the multichannel surface EMG. The subjects examined were 28 normal controls (5-40 years old), seven patients with myopathy and three patients with neuropathy. The tibialis anterior and the biceps brachii muscles were investigated at weak and moderate contraction levels. In normal muscles, the mean muscle fiber conduction velocity increased with muscle force. In four of seven cases with myopathy, the conduction velocities were reduced compared to normal subjects (p less than 0.01), and there was little or no correlation between the conduction velocity and muscle force. In Duchenne muscular dystrophy, abnormal propagation patterns of MUAPs were obtained. The conduction velocities were within the normal range in the muscles of patients with neuropathy. However, abnormal MUAPs, giant and fasciculation potentials, and diminished discharge patterns of motor units (MUs) were detected during voluntary contraction and at rest in neuropathy.

Relationship between average muscle fibre conduction velocity and EMG power spectra during isometric contraction, recovery and applied ischemia.

The relationship between muscle fibre conduction velocity (MFCV) and the power spectrum of surface EMGs in 3 human volunteers was studied during isometric contractions at 40% maximum voluntary contraction. In addition, the recovery of these two parameters was measured during short lasting contractions at the same force level every 30 s. The recovery phase was also studied during ischaemia, thereby preventing the recovery of MFCV. The mean MFCV was calculated by the cross-correlation method. The measurements were facilitated by a real-time estimation of the cross-correlation and the MFCV and by a graphic display of the digitised signal. During contraction a nearly linear relation was found between MFCV and the median frequency of the power spectrum (MPF). During recovery this relationship was lost in one subject: MPF restored much faster then MFCV. During recovery under ischemia MFCV did not recover, but MPF recovered partially in all subjects. It is concluded that the shift of the power spectrum to lower frequencies during fatigue cannot be explained by changes in MFCV alone. Central mechanisms also influence the power spectrum and studying the recovery of local muscle fatigue during ischemia may separate these influences from that of MFCV on the power spectrum during fatigue.

Normal and myopathic propagation of surface motor unit action potentials

With the aid of a computer-assisted multichannel EMG system, the propagation of motor unit action potentials is analysed during isometric voluntary muscular contraction under normal and myopathic conditions. A linear array of 30 surface electrodes is fixed above the biceps brachii muscle, parallel to the longitudinal muscle axis and centred over the end-plate zone. The EMG is simultaneously recorded on all channels and displays the propagation of surface potentials on both sides of the innervation band. The mean muscle fibre conduction velocity is computed by a cross-correlation technique. Five adult patients in a late stage of progressive muscular dystrophy show a highly altered propagation behaviour of motor unit action potentials without a time shift between potentials of adjacent electrodes. A mathematical model is described which predicts such a behaviour and suggests that it must be due to a pathological longitudinal spread of end-plates in this patient group. In 5 boys with Duchenne muscular dystrophy and only moderate impairment of the biceps muscle the mean muscle fibre velocity was reduced (2.81 ± 0.34 m/sec) compared to 17 healthy subjects (4.42 ± 0.37 m/sec). Each myopathic patient examined could be separated clearly from the group of healthy subjects on the basis of the surface EMG analysis.