Mechanomyographic amplitude and mean power output during maximal, concentric, isokinetic muscle actions

Torque and power of knee extensor muscles at individualized isokinetic angular velocities

OBJECTIVE

Existing isokinetic contractions are characterized using standardized angular velocities, which can induce differing adaptations. Here, we characterized the variation in the isokinetic parameters of knee extensors according to individualized angular velocity (IAV).

METHODS

We performed a cross-sectional study of 19 young, healthy men. We measured the maximum angular velocity (MAV) of concentric knee extension using the isotonic mode of an isokinetic dynamometer. Isometric and isokinetic (at angular velocities corresponding to 100%, 70%, 40%, and 10% of each individual's MAV) knee extensor contractions were performed, and the peak torque and mean power were recorded.

RESULTS

Peak torque significantly decreased with increasing IAV (129.42 ± 25.04, 84.37 ± 20.97, and 56.42 ± 16.18 Nm at 40%, 70%, and 100%, respectively), except for isometric contraction (233.36 ± 47.85) and at 10% of MAV (208 ± 48.55). At the mean power, 10% of MAV (74.52 ± 20.84 W) was significantly lower than the faster IAV (176.32 ± 49.64, 161.53 ± 56.55, and 145.95 ± 50.64 W at 40%, 70%, and 100%, respectively), and 100% was significantly lower than 40%.

CONCLUSION

The optimized IAV for isokinetic contraction to improve power output while maintaining torque is 10% to 40% of MAV. IAV may reflect both the velocity and force components of power because individuals do not have the same angular velocity.

Influence of Pennation Angle and Muscle Thickness on Mechanomyographic Amplitude-Torque Relationships and Sex-Related Differences in the Vastus Lateralis

This study examined potential sex-related differences and correlations among the pennation angle (PA), muscle thickness (MT), and mechanomyographic amplitude (MMG_RMS)-torque relationships of the vastus lateralis (VL) in 11 healthy males and 12 healthy females. The PA and MT of the VL were quantified with ultrasound. Participants performed an isometric muscle action of the knee extensors that linearly increased to 70% of maximal strength followed by a 12 s plateau. MMG was recorded from the VL. Linear regression models were fit to the log-transformed MMG_RMS-torque relationships to calculate b terms (slopes) for the linearly increasing segment. MMG_RMS was averaged during the plateau. Males exhibited greater PA (p < 0.001), MT (p = 0.027), b terms (p = 0.005), and MMG_RMS (p = 0.016). The b terms were strongly (p < 0.001, r = 0.772) and moderately correlated (p = 0.004, r = 0.571) with PA and MT, respectively, while MMG_RMS was moderately correlated with PA (p = 0.018, r = 0.500) and MT (p = 0.014, r = 0.515). The greater mechanical behavior of individuals possessing a larger PA and MT of the VL may reflect increased cross-bridge activity within the muscle fibers. Additionally, PA may help explain sex-related differences in MMG_RMS between sexes.

A non-volitional skeletal muscle endurance test measures functional changes associated with impaired blood flow

Introduction: An electrically stimulated intermittent fatigue test using mechanomyography was recently proposed as a possible tool for detecting clinically relevant changes in muscle function. This study was designed to determine whether the proposed test can detect additional fatigue when it should be present. Methods: Subjects (n = 10) underwent two trials each (occluded and normal blood flow) with a standardized fatigue protocol on the Ankle Dorsiflexors (AD) and Wrist Extensors (WE) using a clinical electrical stimulator. Results: Mean normalized twitch acceleration was strongly predictive of mean normalized torque (R ² = 0.828). The WE experienced lower twitch magnitudes throughout the tourniquet trial (10.81 ± 1.25 m/s²) compared to normal blood flow (18.05 ± 1.06 m/s²). The AD twitches were overall reduced in the tourniquet trial (3.87 ± 0.48 m/s²) compared with the control trial (8.57 ± 0.91 m/s²). Conclusion: Occluding blood flow to a muscle should cause greater muscle fatigue. The ability to detect reduced contraction magnitudes during an electrically stimulated fatigue protocol resulting from low blood flow suggests the proposed test may be capable of detecting clinically relevant muscle deficits.

Sex differences in kinematics and quadriceps activity for fast isokinetic knee extension

BACKGROUND: The quantitative recruitment of motor units depends on specific demands, including movement velocity. DNA microarrays differ according to sex, and these sex-related differences affect the fiber type composition. OBJECTIVE: The aim of this study is to demonstrate inter-sex differences in the isokinetic parameters, isokinetic phases, and muscle activities and to investigate the correlation between muscle activity and isokinetic data. METHODS: A total of 41 healthy adults (20 male, 21 female) performed concentric knee extension at angular velocities of 60∘/s, 180∘/s, and 240∘/s. The outcome measures consisted of the isokinetic peak moment (PM), normalized PM (PM/BW), total work, and power, alongside the acceleration, iso-speed and deceleration, sub-phases. Muscle activity was recorded from the rectus femoris, vastus lateralis, and vastus medialis using surface EMG. RESULTS: There were significant two-factor main effect and interaction between sex and angular velocity on the power of knee extension and isokinetic phase (p< 0.05). As the velocity increased, the increase in power of males was greater than that of females. In contrast, with the increase in velocity, PM, PM/BW, and total work decreased, but no significant interaction was observed between velocity and sex. At high velocity, males showed higher acceleration ability than females. CONCLUSION: The sex-dependent responses to velocity were more affected by differences in total movement time than force production. Fast isokinetic exercise should consider the acceleration ability rather than the ability to produce force.

Comparing the torque- and power-velocity relationships between children and adolescents during isokinetic leg extension muscle actions

The purpose of this study was to use polynomial regression analyses to examine the torque- and power-velocity relationships and calculate and compare the vertices of these nonlinear models, and how they relate to measurements of muscle size and maximal strength, between male and female children and adolescents during maximal isokinetic leg extension muscle actions. Sixteen children (n = 8 males, n = 8 females) and 22 adolescents (n = 11 males, n = 11 females) participated in this study. Measurements of growth included age, maturity offset, height, body mass, fat-free mass, and quadriceps femoris muscle cross-sectional area (CSA). Participants completed maximal voluntary isometric contractions (MVICs) of the leg extensors and maximal voluntary isokinetic leg extensions at 60, 120, 180, 240, and 300°·s^-1. Variables calculated during all leg extension muscle actions included peak torque (PT, Nm) and mean power (MP, W). Polynomial regression analyses determined the model of best fit for the PT- and MP-velocity relationships. For each participant, the vertex from the PT- and MP-velocity quadratic models were quantified as the predicted maximum velocity of last measurable torque (V_PT) and the predicted velocity of maximum mean power (V_MP), respectively. Measurements of growth, PT and MP at all velocities, V_PT, and V_MP were greater in the adolescents than children. When normalized to CSA, V_PT and V_MP remained greater for adolescents than children, and exhibited low to very high relationships with measurements of growth. When normalized to MVIC strength, V_PT and V_MP were no longer different between children and adolescents and exhibited negligible to low relationships with measurements of growth. The results of the present study suggest that the ability to produce torque and power at high velocities may be more dependent on muscle strength than muscle size, which suggests that mechanisms other than muscular hypertrophy affect torque and power production at high velocities in young males and females.

Inspiratory muscle activation increases with COPD severity as confirmed by non-invasive mechanomyographic analysis

There is a lack of instruments for assessing respiratory muscle activation during the breathing cycle in clinical conditions. The aim of the present study was to evaluate the usefulness of the respiratory muscle mechanomyogram (MMG) for non-invasively assessing the mechanical activation of the inspiratory muscles of the lower chest wall in both patients with chronic obstructive pulmonary disease (COPD) and healthy subjects, and to investigate the relationship between inspiratory muscle activation and pulmonary function parameters. Both inspiratory mouth pressure and respiratory muscle MMG were simultaneously recorded under two different respiratory conditions, quiet breathing and incremental ventilatory effort, in 13 COPD patients and 7 healthy subjects. The mechanical activation of the inspiratory muscles was characterised by the non-linear multistate Lempel–Ziv index (MLZ) calculated over the inspiratory time of the MMG signal. Subsequently, the efficiency of the inspiratory muscle mechanical activation was expressed as the ratio between the peak inspiratory mouth pressure to the amplitude of the mechanical activation. This activation estimated using the MLZ index correlated strongly with peak inspiratory mouth pressure throughout the respiratory protocol in both COPD patients (r = 0.80, p<0.001) and healthy (r = 0.82, p<0.001). Moreover, the greater the COPD severity in patients, the greater the level of muscle activation (r = -0.68, p = 0.001, between muscle activation at incremental ventilator effort and FEV₁). Furthermore, the efficiency of the mechanical activation of inspiratory muscle was lower in COPD patients than healthy subjects (7.61±2.06 vs 20.42±10.81, respectively, p = 0.0002), and decreased with increasing COPD severity (r = 0.78, p<0.001, between efficiency of the mechanical activation at incremental ventilatory effort and FEV₁). These results suggest that the respiratory muscle mechanomyogram is a good reflection of inspiratory effort and can be used to estimate the efficiency of the mechanical activation of the inspiratory muscles. Both, inspiratory muscle activation and inspiratory muscle mechanical activation efficiency are strongly correlated with the pulmonary function. Therefore, the use of the respiratory muscle mechanomyogram can improve the assessment of inspiratory muscle activation in clinical conditions, contributing to a better understanding of breathing in COPD patients.

Mechanomyographic responses during recruitment curves in the soleus muscle

INTRODUCTION

In this study we examined relationships among mechanomyographic (MMG), electromyographic (EMG), and peak twitch torque (PTT) responses as well as test-retest reliability when recorded during recruitment curves in the soleus muscle.

METHODS

PTT, EMG (M-wave, H-reflex), and MMG responses were recorded during recruitment curves in 16 subjects (age 24 ± 2 years) on 2 separate days. The sum of the M-wave and H-reflex (M+H) was calculated. Correlations among variables and test-retest reliability were determined.

RESULTS

MMG was correlated with PTT (mean r = 0.93, range r = 0.59-0.99), the M-wave (0.95, 0.04-0.98), and M+H (0.91, 0.42-0.97), but was unrelated to the H-reflex (-0.06, -0.56 to 0.47). Reliability was consistently high among most variables, but normalizing to the maximum value improved MMG reliability and the minimum detectable change.

CONCLUSION

MMG responses predicted 86%-90% of the variability in PTT, M-wave, and M+H; thus, MMG may be a useful alternative for estimating twitch torque and maximal activation. Muscle Nerve 56: 107-116, 2017.

Evidence towards improved estimation of respiratory muscle effort from diaphragm mechanomyographic signals with cardiac vibration interference using sample entropy with fixed tolerance values

The analysis of amplitude parameters of the diaphragm mechanomyographic (MMGdi) signal is a non-invasive technique to assess respiratory muscle effort and to detect and quantify the severity of respiratory muscle weakness. The amplitude of the MMGdi signal is usually evaluated using the average rectified value or the root mean square of the signal. However, these estimations are greatly affected by the presence of cardiac vibration or mechanocardiographic (MCG) noise. In this study, we present a method for improving the estimation of the respiratory muscle effort from MMGdi signals that is robust to the presence of MCG. This method is based on the calculation of the sample entropy using fixed tolerance values (fSampEn), that is, with tolerance values that are not normalized by the local standard deviation of the window analyzed. The behavior of the fSampEn parameter was tested in synthesized mechanomyographic signals, with different ratios between the amplitude of the MCG and clean mechanomyographic components. As an example of application of this technique, the use of fSampEn was explored also in recorded MMGdi signals, with different inspiratory loads. The results with both synthetic and recorded signals indicate that the entropy parameter is less affected by the MCG noise, especially at low signal-to-noise ratios. Therefore, we believe that the proposed fSampEn parameter could improve estimates of respiratory muscle effort from MMGdi signals with the presence of MCG interference.

Effects of diverting activity on strength, electromyographic, and mechanomyographic signals

The purpose of this study was to investigate the effects of different recovery interventions on peak torque, electromyographic (EMG), and mechanomyographic (MMG) measures. Ten (23.40 ± 1.00 years; 178.40 ± 5.03 cm; 84.80 ± 15.85 kg) recreationally trained college men performed 4 experimental visits consisting of 2 bouts of 50 maximal isokinetic leg extensions at 180°·s(-1). Between each bout of maximal exercise, 2 minutes of recovery involving one of the 4 interventions (passive, active, passive diverting, and active diverting) was completed. Electromyographic and MMG measures were collected during the preintervention and postintervention maximal isokinetic strength tests. Peak torque declined to a significantly greater degree during the postintervention test in the passive condition than the other interventions. Electromyographic amplitude decreased on the postintervention test but did not differ between conditions. There was a significant 2-way interaction for EMG mean power frequency (MPF) between time and repetitions. There was a significant decrease in EMG MPF from the first 3 to the last 3 repetitions for both the preintervention and postintervention tests. The decrease in EMG MPF from the initial to final repetitions was greater for the preintervention test than the postintervention test. Mechanomyographic amplitude significantly decreased from the initial to final repetitions regardless of time or condition. These findings suggest that active, passive diverting, and active diverting strategies provide the same extent of recovery between maximal, fatiguing isokinetic leg extension tasks, and that these strategies can be used by coaches and exercise professionals to select the most effective interset recovery strategy.

Electromyographic normalization procedures for determining exercise intensity of closed chain exercises for strengthening the quadriceps femoris muscles

The purpose of this study was to compare the electromyographic (EMG) amplitudes of the quadriceps femoris (QF) muscles during a maximum voluntary isometric contraction (MVIC) to submaximal and maximal dynamic concentric contractions during active exercises. A secondary purpose was to provide information about the type of contraction that may be most appropriate for normalization of EMG data if one wants to determine if a lower extremity closed chain exercise is of sufficient intensity to produce a strengthening response for the QF muscles. Sixty-eight young healthy volunteers (39 female, 29 male) with no lower extremity pain or injury participated in the study. Surface electrodes recorded EMG amplitudes from the vastus medialis obliquus (VMO), rectus femoris (RF), and vastus lateralis (VL) muscles during 5 different isometric and dynamic concentric exercises. The last 27 subjects performed an additional 4 exercises from which a second data set could be analyzed. Maximum isokinetic knee extension and moderate to maximum closed chain exercises activated the QF significantly more than a MVIC. A 40-cm. lateral step-up exercise produced EMG amplitudes of the QF muscles of similar magnitude as the maximum isokinetic knee extension exercises and would be an exercise that could be considered for strengthening the QF muscles. Most published EMG studies of exercises for the QF have been performed by comparing EMG amplitudes during dynamic exercises to a MVIC. This procedure can lead one to overestimate the value of a dynamic exercise for strengthening the QF muscles. We suggest that when studying the efficacy of a dynamic closed chain exercise for strengthening the QF muscles, the exercise be normalized to a dynamic maximum muscle contraction such as that obtained with knee extension during isokinetic testing.

Time course of stretching-induced changes in mechanomyogram and force characteristics

To evaluate the time-course of stretching-induced changes in mechanical properties of the muscle-tendon unit (MTU), 11 participants (age 22±1 yr; body mass 77±5 kg; stature 1.78±0.05 m; mean±SD) underwent tetanic electrical stimulations of the medial gastrocnemius muscle before and after (up to 2h) stretching administration. During contractions, surface electromyogram (EMG), mechanomyogram (MMG) and force were recorded simultaneously. From MMG, peak-to-peak (p-p) and root mean square (RMS) were calculated during the on-phase and plateau phase of tetanic contraction, respectively. After stretching: (i) no differences were found in EMG parameters; (ii) MMG p-p and slope decreased (-16% and -10%, respectively; P<0.05) and remained depressed for the entire recovery period; (iii) MMG RMS increased (+20%; P<0.05), returning to pre-stretching values within 15 min; and (iv) peak force (pF), with its first (dF/dt) and second (d(2)F/dt(2)) derivative, decreased significantly by 32%, 35% and 54%, respectively, and remained depressed for the entire recovery period. The lack of MMG p-p and pF recovery could be ascribable to a reduced muscle force generating capacity due to persisting changes in viscoelastic characteristics of series elastic components. The early return of MMG RMS to pre-stretching values suggests that changes in viscoelastic parallel components recovered after few minutes.

Relationships among peak power output, peak bar velocity, and mechanomyographic amplitude during the free-weight bench press exercise

The purpose of this study was to examine the relationships among mechanomyographic (MMG) amplitude, power output, and bar velocity during the free-weight bench press exercise. Twenty-one resistance-trained men [one-repetition maximum (1-RM) bench press = 125.4+18.4 kg] performed bench press muscle actions as explosively as possible from 10% to 90% of the 1-RM while peak power output and peak bar velocity were assessed with a TENDO Weightlifting Analyzer. During each muscle action, surface MMG signals were detected from the right and left pectoralis major and triceps brachii, and the concentric portion of the range of motion was selected for analysis. Results indicated that power output increased from 10% to 50% 1-RM, followed by decreases from 50% to 90% 1-RM, but MMG amplitude for each of the muscles increased from 10 to 80% 1-RM. The results of this study indicate that during the free-weight bench press exercise, MMG amplitude was not related to power output, but was inversely related to bar velocity and directly related to the external load being lifted. In future research, coaches and sport scientists may be able to estimate force/torque production from individual muscles during multi-joint, dynamic constant external resistance muscle actions.

Acute effects of static stretching on characteristics of the isokinetic angle - torque relationship, surface electromyography, and mechanomyography

The aims of this study were to examine the acute effects of static stretching on peak torque, work, the joint angle at peak torque, acceleration time, isokinetic range of motion, mechanomyographic amplitude, and electromyographic amplitude of the rectus femoris during maximal concentric isokinetic leg extensions at 1.04 and 5.23 rad x s(-1) in men and women. Ten women (mean +/- s: age 23.0 +/- 2.9 years, stature 1.61 +/- 0.12 m, mass 63.3 +/- 9.9 kg) and eight men (age 21.4 +/- 3.0 years, stature 1.83 +/- 0.11 m, mass 83.1 +/- 15.2 kg) performed maximal voluntary concentric isokinetic leg extensions at 1.04 and 5.23 rad x s(-1). Following the initial isokinetic tests, the dominant leg extensors were stretched using four static stretching exercises. After the stretching, the isokinetic tests were repeated. Peak torque, acceleration time, and electromyographic amplitude decreased (P< or = 0.05) from pre- to post-stretching at 1.04 and 5.23 rad . s(-1); there were no changes (P > 0.05) in work, joint angle at peak torque, isokinetic range of motion, or mechanomyographic amplitude. These findings indicate no stretching-related changes in the area under the angle - torque curve (work), but a significant decrease in peak torque, which suggests that static stretching may cause a "flattening" of the angle - torque curve that reduces peak strength but allows for greater force production at other joint angles. These findings, in conjunction with the increased limb acceleration rates (decreased acceleration time) observed in the present study, provide tentative support for the hypothesis that static stretching alters the angle - torque relationship and/or sarcomere shortening velocity.

Comparison of a piezoelectric contact sensor and an accelerometer for examining mechanomyographic amplitude and mean power frequency versus torque relationships during isokinetic and isometric muscle actions of the biceps brachii

The purpose of this study was to compare a piezoelectric contact sensor with an accelerometer for measuring the mechanomyographic (MMG) signal from the biceps brachii during submaximal to maximal isokinetic and isometric forearm flexion muscle actions. Following determination of isokinetic peak torque (PT) and the isometric maximum voluntary contraction (MVC), 10 adults (mean+/-SD age=22.8+/-2.7yrs) performed randomly ordered, submaximal step muscle actions of the dominant forearm flexors in 20% increments from 20% to 80% PT and MVC. Surface MMG signals were recorded simultaneously from a contact sensor and an accelerometer placed over the belly of the biceps brachii muscle. During the isokinetic and isometric muscle actions, the contact sensor and accelerometer resulted in linear increases in normalized MMG amplitude with torque (r(2) range=0.84-0.97) but the linear slope of the normalized MMG amplitude versus isokinetic torque relationship for the accelerometer was less (p<0.10) than that of the contact sensor. There was no significant (p>0.05) relationship for normalized MMG mean power frequency (MPF, %max) versus isokinetic and isometric torque for the contact sensor, but the accelerometer demonstrated a quadratic (R(2)=0.94) or linear (r(2)=0.83) relationship for the isokinetic and isometric muscle actions, respectively. There were also a number of significant (p<0.05) mean differences between the contact sensor and accelerometer for normalized MMG amplitude or MPF values. These findings indicated that in some cases involving dynamic and isometric muscle actions, the contact sensor and accelerometer resulted in different torque-related responses that may affect the interpretation of the motor control strategies involved.

Is fatigue all in your head? A critical review of the central governor model

The central governor model has recently been proposed as a general model to explain the phenomenon of fatigue. It proposes that the subconscious brain regulates power output (pacing strategy) by modulating motor unit recruitment to preserve whole body homoeostasis and prevent catastrophic physiological failure such as rigor. In this model, the word fatigue is redefined from a term that describes an exercise decline in the ability to produce force and power to one of sensation or emotion. The underpinnings of the central governor model are the refutation of what is described variously as peripheral fatigue, limitations models, and the cardiovascular/anaerobic/catastrophe model. This argument centres on the inability of lactic acid models of fatigue to adequately explain fatigue. In this review, it is argued that a variety of peripheral factors other than lactic acid are known to compromise muscle force and power and that these effects may protect against "catastrophe". Further, it is shown that a variety of studies indicate that fatigue induced decreases in performance cannot be adequately explained by the central governor model. Instead, it is suggested that the concept of task dependency, in which the mechanisms of fatigue vary depending on the specific exercise stressor, is a more comprehensive and defensible model of fatigue. This model includes aspects of both central and peripheral contributions to fatigue, and the relative importance of each probably varies with the type of exercise.

Mechanomyographic amplitude and frequency responses during dynamic muscle actions: a comprehensive review

The purpose of this review is to examine the literature that has investigated mechanomyographic (MMG) amplitude and frequency responses during dynamic muscle actions. To date, the majority of MMG research has focused on isometric muscle actions. Recent studies, however, have examined the MMG time and/or frequency domain responses during various types of dynamic activities, including dynamic constant external resistance (DCER) and isokinetic muscle actions, as well as cycle ergometry. Despite the potential influences of factors such as changes in muscle length and the thickness of the tissue between the muscle and the MMG sensor, there is convincing evidence that during dynamic muscle actions, the MMG signal provides valid information regarding muscle function. This argument is supported by consistencies in the MMG literature, such as the close relationship between MMG amplitude and power output and a linear increase in MMG amplitude with concentric torque production. There are still many issues, however, that have yet to be resolved, and the literature base for MMG during both dynamic and isometric muscle actions is far from complete. Thus, it is important to investigate the unique applications of MMG amplitude and frequency responses with different experimental designs/methodologies to continually reassess the uses/limitations of MMG.

Sonomyography: monitoring morphological changes of forearm muscles in actions with the feasibility for the control of powered prosthesis

Electromyography (EMG) has been widely used for the assessment of musculoskeletal functions and the control of electrical prostheses, which make use of the EMG signal generated by the contraction of the residual muscles. In spite of the successful applications of EMG in different fields, it has some inherent limitations, such as the difficulty to differentiate the actions of neighboring muscles and to collect signals from deep muscles using the surface EMG. The majority of current EMG controlled prostheses can only provide sequential on-off controls using signals from two groups of muscles, so the users are required to put many conscious efforts in monitoring the speed and range of motion of the terminal devices being controlled. Recently, many alternative signals based on the detection of dimensional changes of muscles or tendons during actions have been reported. The objective of this study was to investigate the potential of the dimensional change of muscles detected using sonography for musculoskeletal assessment and control. A portable B-mode ultrasound scanner was used to collect the dynamic ultrasound images of the forearm muscles of six normally limbed young adults and three amputee subjects. A motion analysis system was used to collect the movement of the wrist angle during the experiments for the normal subjects. It was demonstrated that the morphological changes of forearm muscles during actions can be successfully detected by ultrasound and linearly correlated (R(2)=0.876+/-0.042, mean+/-S.D.) with the wrist angle. We named these sonographically detected signals about the architectural change of the muscle as sonomyography (SMG). The mean ratio between the wrist angle and the percentage deformation of the forearm muscle was 7.2+/-3.7 degrees /% for the normal subjects. The intraclass correlation coefficient (ICC) of this ratio among the three repeated tests was 0.868. The SMG signals from the residual forearms were also successfully detected when the three amputee subjects contracted their residual muscles. The results demonstrated that SMG had potentials for the musculoskeletal control and assessment.

Comparison of Fourier and wavelet transform procedures for examining the mechanomyographic and electromyographic frequency domain responses during fatiguing isokinetic muscle actions of the biceps brachii

The primary purpose of the present study was to compare the fast Fourier transform (FFT) with the discrete wavelet transform (DWT) for determining the mechanomyographic (MMG) and electromyographic (EMG) center frequency [mean power frequency (mpf), median frequency (mdf), or wavelet center frequency (cf)] patterns during fatiguing isokinetic muscle actions of the biceps brachii. Seven men (mean+/-SD age=23+/-3 years) volunteered to perform 50 consecutive maximal, concentric isokinetic muscle actions of the dominant forearm flexors at a velocity of 180 degrees s(-1). Non-parametric "run" tests indicated significant (p<0.05) trends in the MMG and EMG signals for the 5th, 25th, and 45th muscle actions for all subjects, thereby confirming non-stationarity of the MMG and EMG signals. There were significant (p<0.05) correlations among the average normalized mpf, mdf, and cf values for contractions 1-50 for both MMG (r=0.671-0.935) and EMG (r=0.956-0.987). Polynomial regression analyses demonstrated quadratic decreases in normalized MMG mpf (R2=0.439), MMG mdf (R2=0.258), MMG cf (R2=0.359), EMG mpf (R2=0.952), EMG mdf (R2=0.914) and EMG cf (R2=0.888) across repetitions. The primary finding of this study was the similarity in the mpf, mdf, and cf patterns for both MMG and EMG, which suggested that, despite the concerns over non-stationarity, Fourier based methods are acceptable for determining the patterns for normalized MMG and EMG center frequency during fatiguing dynamic muscle actions at moderate velocities.

The acute effects of static stretching on peak torque, mean power output, electromyography, and mechanomyography

The purpose of this study was to examine the acute effects of static stretching on peak torque (PT), the joint angle at PT, mean power output (MP), electromyographic (EMG) amplitude, and mechanomyographic (MMG) amplitude of the vastus lateralis (VL) and rectus femoris (RF) muscles during maximal, voluntary concentric isokinetic leg extensions at 60 and 240 degrees x s(-1) of the stretched and unstretched limbs. Twenty-one volunteers [mean age (SD) 21.5 (1.3) years] performed maximal, voluntary concentric isokinetic leg extensions for the dominant and non-dominant limbs at 60 and 240 degrees x s(-1). Surface EMG (muVrms) and MMG (mVrms) signals were recorded from the VL and RF muscles during the isokinetic tests. PT (Nm), the joint angle at PT, and MP (W) were calculated by a dynamometer. Following the initial isokinetic tests, the dominant leg extensors were stretched using four static stretching exercises. After the stretching, the isokinetic tests were repeated. PT decreased (P< or =0.05) from pre- to post-stretching for the stretched limb at 60 and 240 degrees x s(-1) and for the unstretched limb at 60 degrees x s(-1). EMG amplitude of the VL and RF also decreased (P< or =0.05) from pre- to post-stretching for the stretched and unstretched limbs. There were no stretching-induced changes (P>0.05) for the joint angle at PT, MP, or MMG amplitude. These findings indicated stretching-induced decreases in force production and muscle activation. The decreases in PT and EMG amplitude for the unstretched limb suggested that the stretching-induced decreases may be due to a central nervous system inhibitory mechanism.

Gender, muscle, and velocity comparisons of mechanomyographic and electromyographic responses during isokinetic muscle actions

The purpose of this study was to examine the responses of peak torque (PT), mean power output (MP), mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) in males and females during maximal, concentric isokinetic muscle actions. Subjects performed maximal leg extensions at 60 degrees s(-1), 120 degrees s(-1), 180 degrees s(-1), 240 degrees s(-1), 300 degrees s(-1), 360 degrees s(-1), 420 degrees s(-1), and 480 degrees s(-1). No gender differences were observed, but there were muscle-specific differences for the patterns of MMG MPF, EMG amplitude, and EMG MPF. The MP and MMG amplitude increased to 180-240 degrees s(-1), plateaued, and then decreased to 480 degrees s(-1). MMG MPF for the VL and VM remained unchanged to 300 degrees s(-1), but then increased to 480 degrees s(-1). The EMG amplitude for the RF and EMG MPF for the VL decreased across velocity. Overall, these findings indicated that there were muscle-specific, velocity-related differences in the associations among motor control strategies (EMG amplitude and MPF) and the mechanical aspects of isokinetic muscular activity (MMG amplitude and MPF).