Mechanomyographic and electromyographic responses to repeated concentric muscle actions of the quadriceps femoris.

In comparison to isometric muscle action models, little is known about the electromyographic (EMG) and mechanomyographic (MMG) amplitude and mean power frequency (MPF) responses to fatiguing dynamic muscle actions. Simultaneous examination of the EMG and MMG amplitude and MPF may provide additional insight with regard to the motor control strategies utilized by the superficial muscles of the quadriceps femoris during a concentric fatiguing task. Thus, the purpose of this study was to examine the EMG and MMG amplitude and MPF responses of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) during repeated, concentric muscle actions of the dominant leg. Seventeen adults (21.8+/-1.7 yr) performed 50 consecutive, maximal concentric muscle actions of the dominant leg extensors on a Biodex System 3 Dynamometer at velocities of 60 degrees s(-1) and 300 degrees s(-1). Bipolar surface electrode arrangements were placed over the mid portion of the VL, RF, and VM muscles with a MMG contact sensor placed adjacent to the superior EMG electrode on each muscle. Torque, MMG and EMG amplitude and MPF values were calculated for each of the 50 repetitions. All values were normalized to the value recorded during the first repetition and then averaged across all subjects. The cubic decreases in torque at 60 degrees s(-1) (R2 = 0.972) and 300 degrees s(-1) (R2 = 0.931) was associated with a decline in torque of 59+/-24% and 53+/-11%, respectively. The muscle and velocity specific responses for the MMG amplitude and MPF demonstrated that each of the superficial muscles of the quadriceps femoris uniquely contributed to the control of force output across the 50 repetitions. These results suggested that the MMG responses for the VL, RF, VM during a fatiguing task may be influenced by a number of factors such as fiber type differences, alterations in activation strategy including motor unit recruitment and firing rate and possibly muscle wisdom.     

Mechanomyographic and electromyographic responses of the vastus medialis muscle during isometric and concentric muscle actions

The purpose of this study was to examine the patterns for the mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) vs. torque relationships during submaximal to maximal isometric and isokinetic muscle actions. Seven men (mean +/- SD age, 22.4 +/- 1.3 years) volunteered to perform isometric and concentric isokinetic leg extension muscle actions at 20, 40, 60, 80, and 100% of maximal voluntary contraction (MVC) and peak torque (PT) on a Cybex II dynamometer. A piezoelectric MMG recording sensor was placed between bipolar surface EMG electrodes on the vastus medialis. Polynomial regression and separate 1-way repeated-measures analysis of variance were used to analyze the EMG amplitude, MMG amplitude, EMG MPF, and MMG MPF data for the isometric and isokinetic muscle actions. For the isometric muscle actions, EMG amplitude (R(2) = 0.999) and MMG MPF (R(2) = 0.946) increased to MVC, mean MMG amplitude increased to 60% MVC and then plateaued, and mean EMG MPF did not change (p > 0.05) across torque levels. For the isokinetic muscle actions, EMG amplitude (R(2) = 0.988) and MMG amplitude (R(2) = 0.933) increased to PT, but there were no significant mean changes with torque for EMG MPF or MMG MPF. The different torque-related responses for EMG and MMG amplitude and MPF may reflect differences in the motor control strategies that modulate torque production for isometric vs. dynamic muscle actions. These results support the findings of others and suggest that isometric torque production was modulated by a combination of recruitment and firing rate, whereas dynamic torque production was modulated primarily through recruitment.     

Time and frequency domain responses of the mechanomyogram and electromyogram during isometric ramp contractions: a comparison of the short-time Fourier and continuous wavelet transforms.

The purposes of this study were to examine the mechanomyographic (MMG) and electromyographic (EMG) time and frequency domain responses of the vastus lateralis (VL) and rectus femoris (RF) muscles during isometric ramp contractions and compare the time-frequency of the MMG and EMG signals generated by the short-time Fourier transform (STFT) and continuous wavelet transform (CWT). Nineteen healthy subjects (mean+/-SD age=24+/-4 years) performed two isometric maximal voluntary contractions (MVCs) before and after completing 2-3, 6-s isometric ramp contractions from 5% to 100% MVC with the right leg extensors. MMG and surface EMG signals were recorded from the VL and RF muscles. Time domains were represented as root mean squared amplitude values, and time-frequency representations were generated using the STFT and CWT. Polynomial regression analyses indicated cubic increases in MMG amplitude, MMG frequency, and EMG frequency, whereas EMG amplitude increased quadratically. From 5% to 24-28% MVC, MMG amplitude remained stable while MMG frequency increased. From 24-28% to 76-78% MVC, MMG amplitude increased rapidly while MMG frequency plateaued. From 76-78% to 100% MVC, MMG amplitude plateaued (VL) or decreased (RF) while MMG frequency increased. EMG amplitude increased while EMG frequency changed only marginally across the force spectrum with no clear deflection points. Overall, these findings suggested that MMG may offer more unique information regarding the interactions between motor unit recruitment and firing rate that control muscle force production during ramp contractions than traditional surface EMG. In addition, although the STFT frequency patterns were more pronounced than the CWT, both algorithms produced similar time-frequency representations for tracking changes in MMG or EMG frequency.     

Non-uniform mechanical activity of quadriceps muscle during fatigue by repeated maximal voluntary contraction in humans

To determine the non-uniform surface mechanical activity of human quadriceps muscle during fatiguing activity, surface mechanomyogram (MMG), or muscle sound, and surface electromyogram (EMG) were recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles of seven subjects during unilateral isometric knee extension exercise. Time- and frequency-domain analyses of MMG and of EMG fatigued by 50 repeated maximal voluntary contractions (MVC) for 3 s, with 3-s relaxation in between, were compared among the muscles. The mean MVC force fell to 49.5 (SEM 2.0)% at the end of the repeated MVC. Integrated EMG decreased in a similar manner in each muscle head, but a marked non-uniformity was found for the decline in integrated MMG (iMMG). The fall in iMMG was most prominent for RF, followed by VM and VL. Moreover, the median frequency of MMG and the relative decrease in that of EMG in RF were significantly greater (P < 0.05) than those recorded for VL and VM. These results would suggest a divergence of mechanical activity within the quadriceps muscle during fatiguing activity by repeated MVC. Accepted: 19 January 1999     

The impact of seatback angle on electromyographical activity of the lower back and quadriceps muscles during bilateral knee extension

This study investigated variations in electromyographic (EMG) responses of the erector spinae (ES), vastus medialis (VM), rectus femoris (RF), and vastus lateralis (VL) to different seatback angles during leg extension. Twenty men and women (10 men, 10 women; age 27.49 +/- 6.16 years) performed 8 repetitions at 70% of 8 repetition maximum at seatback angles of 1.57, 1.75, and 1.92 radius (rad). Analyses using repeated-measures analysis of variance indicated the greatest root square mean of the EMG (rmsEMG) and integrated EMG (intEMG) for the ES were at 1.92 rad, and the greatest for the VM (concentric) and VL (eccentric) were at 1.57 rad. No differences were observed among seat angles for the RF except for a higher normalized intEMG at 1.92 than 1.75 rad (concentric). Throughout all sets for all conditions and muscles, rmsEMG and intEMG significantly increased and median power frequency significantly decreased. These data indicate that a seatback angle of 1.57 rad is best for a leg extension machine, because this angle maximizes quadriceps activity while minimizing stress on the lower back muscles.     

Contraction mode shift in quadriceps femoris muscle activation during dynamic knee extensor exercise with increasing loads

The objective of the present study was to examine the superficial quadriceps femoris (QF) muscle electromyogram (EMG) during dynamic sub-maximal knee extension exercise between young adult men and women. Thirty subjects completed, in a random order, 2 sub-maximal repetitions of single-leg knee extensions at 20-90% of their one-repetition maximum (1RM). Vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) muscle integrated EMG (IEMG) during each sub-maximal lift was normalized to the respective 1RM for concentric, isometric and eccentric modes. The EMG median frequency (f(med)) was determined over the isometric mode. Men attained a significantly (p<0.05) greater knee angular velocity than the women during the concentric mode (83.6+/-19.1 degrees /s and 67.4+/-19.8 degrees /s, respectively). RF IEMG was significantly lesser than the VM (p=0.014) and VL (p<0.001) muscles, when collapsed across all contraction modes, loads, and sex. Overall IEMG was significantly greater during the concentric (p<0.001) and isometric (p<0.001) modes, than the eccentric mode. Men generated significantly (p=0.03) greater VL muscle IEMG than the women, while the opposite pattern emerged for the RF muscle. VM f(med) (105.1+/-11.1Hz) was significantly lesser than the VL (180.3+/-19.5Hz) and RF (127.7+/-13.9Hz) muscles across all lifting intensities, while the men (137.7+/-10.7Hz) generated greater values than the women (129.0+/-11.4Hz). The findings demonstrate a reduction in QF muscle activation across the concentric to eccentric transition, which may be related to the mode-specific velocity pattern.     

Angle- and gender-specific quadriceps femoris muscle recruitment and knee extensor torque

The objectives were to examine knee angle-, and gender-specific knee extensor torque output and quadriceps femoris (QF) muscle recruitment during maximal effort, voluntary contractions. Fourteen young adult men and 15 young adult women performed three isometric maximal voluntary contractions (MVC), in a random order, with the knee at 0 degrees (terminal extension), 10 degrees, 30 degrees, 50 degrees, 70 degrees, and 90 degrees flexion. Knee extensor peak torque (PT), and average torque (AT) were expressed in absolute (N m), relative (N m kg(-1)) and allometric-modeled (N m kg(-n)) units. Vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscle EMG signals were full-wave rectified and integrated over the middle 3 s of each contraction, averaged over the three trials at each knee angle, and normalized to the activity recorded at 0 degrees. Muscle recruitment efficiency was calculated as the ratio of the normalized EMG of each muscle to the allometric-modeled average torque (normalized to the values at 0 degrees flexion), and expressed as a percent. Men generated significantly greater knee extensor PT and AT than women in absolute, relative and allometric-modeled units. Absolute and relative PT and AT were significantly highest at 70 degrees, while allometric-modeled values were observed to increase significantly across knee joint angles 10-90 degrees. VM EMG was significantly greater than the VL and RF muscles across all angles, and followed a similar pattern to absolute knee extensor torque. Recruitment efficiency improved across knee joint angles 10-90 degrees and was highest for the VL muscle. VM recruitment efficiency improved more than the VL and RF muscles across 70-90 degrees flexion. The findings demonstrate angle-, and gender-specific responses of knee extensor torque to maximal-effort contractions, while superficial QF muscle recruitment was most efficient at 90 degrees, and less dependent on gender.     

Torque-related changes in mechanomyographic intensity patterns for the superficial quadriceps femoris muscles

The purpose of this study was to investigate changes in mechanomyographic (MMG) intensity patterns for the vastus lateralis (VL), rectus femoris (RF) and vastus medialis (VM) during submaximal to maximal concentric isokinetic, eccentric isokinetic and isometric muscle actions of the leg extensors. Eleven men (mean ± SD age = 20.1 ± 1.1 years) performed concentric, eccentric and isometric muscle actions of the dominant leg extensors on 3 separate days. Surface MMG signals were detected from the VL, RF and VM, processed with a wavelet analysis and examined with a trend plot. The results indicated that the trend plot was capable of tracking systematic changes in MMG amplitude and frequency with an increase in torque. However, these changes were statistically significant in only 26% of the cases. There were also no consistent differences between muscles or contraction types for the significance of the trend plots.     

Muscle-specific atrophy of the quadriceps femoris with aging.

We examined the size of the four muscles of the quadriceps femoris in young and old men and women to assess whether the vastus lateralis is an appropriate surrogate for the quadriceps femoris in human studies of aging skeletal muscle. Ten young (24 +/- 2 yr) and ten old (79 +/- 7 yr) sedentary individuals underwent magnetic resonance imaging of the quadriceps femoris after 60 min of supine rest. Volume (cm3) and average cross-sectional area (CSA, cm2) of the rectus femoris (RF), vastus lateralis (VL), vastus intermedius (VI), vastus medialis (VM), and the total quadriceps femoris were decreased (P < 0.05) in older compared with younger women and men. However, percentage of the total quadriceps femoris taken up by each muscle was similar (P > 0.05) between young and old (RF: 10 +/- 0.3 vs. 11 +/- 0.4; VL: 33 +/- 1 vs. 33 +/- 1; VI: 31 +/- 1 vs. 31 +/- 0.4; VM: 26 +/- 1 vs. 25 +/- 1%). These results suggest that each of the four muscles of the quadriceps femoris atrophy similarly in aging men and women. Our data support the use of vastus lateralis tissue to represent the quadriceps femoris muscle in aging research.     

Quadriceps femoris electromyogram during concentric, isometric and eccentric phases of fatiguing dynamic knee extensions

The objective of this study was to examine the superficial quadriceps femoris (QF) muscle electromyogram (EMG) during fatiguing knee extensions. Thirty young adults were evaluated for their one-repetition maximum (1RM) during a seated, right-leg, inertial knee extension. All subjects then completed a single set of repeated knee extensions at 50% 1RM, to failure. Subjects performed a knee extension (concentric phase), held the weight with the knee extended for 2s (isometric phase), and lowered the weight in a controlled manner (eccentric phase). Raw EMG of the vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) muscles were full-wave rectified, integrated and normalized to the 1RM EMG, for each respective phase and repetition. The EMG median frequency (f(med)) was computed during the isometric phase. An increase in QF muscle EMG was observed during the concentric phase across the exercise duration. VL EMG was greater than the VM and RF muscles during the isometric phase, in which no significant changes occurred in any of the muscles across the exercise duration. A significant decrease in EMG across the exercise duration was observed during the eccentric phase, with the VL EMG greater than the VM and RF muscles. A greater decrease in VL and RF muscle f(med) during the isometric phase, than the VM muscle, was observed with no gender differences. The findings demonstrated differential recruitment of the superficial QF muscle, depending on the contraction mode during dynamic knee extension exercise, where VL muscle dominance appears to manifest across the concentric-isometric-eccentric transition.     

The effects of electrode placement and innervation zone location on the electromyographic amplitude and mean power frequency versus isometric torque relationships for the vastus lateralis muscle.

The purposes of this investigation were to examine the effects of electrode placement and innervation zone (IZ) location on: (a) the torque-related patterns of responses for absolute and normalized electromyographic (EMG) amplitude and mean power frequency (MPF) and (b) the mean absolute and normalized EMG amplitude and MPF values. In addition, the present study examined the variability between subjects for the location of the IZ for the vastus lateralis (VL). Eight men (mean+/-SD age=23.0+/-4.3yr) performed submaximal to maximal isometric muscle actions of the dominant leg extensors. During each muscle action, fifteen channels of bipolar surface EMG signals were detected from the vastus lateralis using a linear electrode array aligned with the long axis of the muscle fibers. The results indicated that there were differences among channels 1-15 for the patterns of responses and mean values for absolute and normalized EMG amplitude and MPF versus isometric torque. Thus, normalized EMG amplitude and MPF values from different individuals cannot be compared if the EMG signals were detected from different locations over the muscle. In addition, absolute and relative (to femur length) estimates of IZ location for the VL resulted in similar inter-subject variability.     

Knee extensor torque and quadriceps femoris EMG during perceptually-guided isometric contractions.

The aim of this study was to examine superficial quadriceps femoris (QF) EMG and torque at perceived voluntary contraction efforts. Thirty subjects (15 males, 15 females) performed 9, 5 s, sub-maximal contractions at prescribed levels of perceived voluntary effort at points 1-9 on an 11-point scale (0-10), in a random order. Surface electromyograms (EMG) of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles, as well as QF peak torque (PT), average torque (AT), and torque coefficient of variation (C.V.), were sampled. The raw EMG signals were full-wave rectified and integrated over the middle three s of each contraction. The sampled EMG signals, and PT and AT at each perceived exertion level were normalized to the average of three maximal voluntary contractions. The normalized EMG and torque values at each perceived exertion level were then compared to equivalent percent values (i.e., 10% at a perceived level of 1). The results demonstrated that at all perceived exertion levels, with the exception of the RF at a level of 2 which was equivalent to 20%, and the VL and RF muscles at a level 1 in which activation was greater than 10%, activation was significantly less than the equivalent percent value at each point on the scale. VM EMG was found to be less than the VL and RF from contraction levels 3-9. PT was shown to be less than the equivalent percent values at contraction levels 6-9. The AT was found to be lower than the expected percent value at perceived effort levels 2-9. Torque C.V. was not found to be different across the range of perceived effort. The major findings of this study suggested that humans over-estimate voluntary QF muscle torque when guided by perceptual sensations. It is also suggested that the produced EMG signals revealed a reliance on the VL muscle for knee extensor torque generation at sub-maximal levels.     

Mechanomyographic and electromyographic responses during submaximal to maximal eccentric isokinetic muscle actions of the biceps brachii

The purpose of this investigation was to determine the mechanomyography (MMG) and electromyography (EMG) amplitude and mean power frequency (MPF) vs. eccentric isokinetic torque relationships for the biceps brachii muscle. Nine adults (mean +/- SD age = 23.1 +/- 2.9 years) performed submaximal to maximal eccentric isokinetic muscle actions of the dominant forearm flexors. After determination of isokinetic peak torque (PT), the subjects randomly performed submaximal step muscle actions in 10% increments from 10 to 90% PT. Polynomial regression analyses indicated that the MMG amplitude vs. eccentric isokinetic torque relationship was best fit with a quadratic model (R(2) = 0.951), where MMG amplitude increased from 10 to 60% PT and then plateaued from 60 to 100% PT. There were linear increases in MMG MPF (r(2) = 0.751) and EMG amplitude (r(2) = 0.988) with increases in eccentric isokinetic torque, but there was no significant change in EMG MPF from 10 to 100% PT. The results suggested that for the biceps brachii, eccentric isokinetic torque was increased to approximately 60% PT through concurrent modulation of the number of active motor units and their firing rates, whereas additional torque above 60% PT was produced only by increases in firing rates. These findings contribute to current knowledge of motor-control strategies during eccentric isokinetic muscle actions and could be useful in the design of training programs.     

Force fluctuations are modulated by alternate muscle activity of knee extensor synergists during low-level sustained contraction.

The study examined the hypothesis that altered synergistic activation of the knee extensors leads to cyclic modulation of the force fluctuations. To test this hypothesis, the force fluctuations were investigated during sustained knee extension at 2.5% of maximal voluntary contraction force for 60 min in 11 men. Surface electromyograms (EMG) were recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles. The SD of force and average EMG (AEMG) of each muscle were calculated for 30-s periods during alternate muscle activity. Power spectrum of force was calculated for the low- (< or =3 Hz), middle- (4-6 Hz), and high-frequency (8-12 Hz) components. Alternate muscle activity was observed between RF and the set of VL and VM muscles. The SD of force was not constant but variable due to the alternate muscle activity. The SD was significantly greater during high RF activity compared with high VL and VM activity (P < 0.05), and the correlation coefficient between the SD and AEMG was significantly greater in RF [0.736 (SD 0.095), P < 0.05] compared with VL and VM. Large changes were found in the high-frequency component. During high RF activity, the correlation coefficient between the SD and high-frequency component [0.832 (SD 0.087)] was significantly (P < 0.05) greater compared with other frequency components. It is suggested that modulations in knee extension force fluctuations are caused by the unique muscle activity in RF during the alternate muscle activity, which augments the high-frequency component of the fluctuations.     

Mean power frequency and amplitude of the mechanomyographic and electromyographic signals during incremental cycle ergometry.

The purpose of this investigation was to determine the relationships for mechanomyographic (MMG) amplitude, MMG mean power frequency (MPF), electromyographic (EMG) amplitude, and EMG MPF versus power output during incremental cycle ergometry. Seventeen adults volunteered to perform an incremental test to exhaustion on a cycle ergometer. The test began at 50 W and the power output was increased by 30 W every 2 min until the subject could no longer maintain 70 rev min(-1). The MMG and EMG signals were recorded simultaneously from the vastus lateralis during the final 10 s of each power output and analyzed. MMG amplitude, MMG MPF, EMG amplitude, EMG MPF, and power output were normalized as a percentage of the maximal value from the cycle ergometer test. Polynomial regression analyses indicated that MMG amplitude increased (P<0.05) linearly across power output, but there was no change (P>0.05) in MMG MPF. EMG amplitude and MPF were fit best (P<0.05) with quadratic models. These results demonstrated dissociations among the time and frequency domains of MMG and EMG signals, which may provide information about motor control strategies during incremental cycle ergometry. The patterns for amplitude and frequency of the MMG signal may be useful for examining the relationship between motor-unit recruitment and firing rate during dynamic tasks.     

EMG frequency content changes with increasing force and during fatigue in the quadriceps femoris muscle of men and women.

The purpose of this study was to determine the effect of gender on changes in electromyographic (EMG) signal characteristics of the quadriceps muscles with increasing force and with fatigue. A total of fourteen healthy adults (seven men, seven women) participated in the study. Subjects had to perform isometric ramp contractions in knee extension with the force gradually increasing from 0 to 100% of the maximal voluntary contraction (MVC) in a 6-s period. Subjects then performed a fatigue task, consisting of a sustained maximum isometric knee extension contraction held until force decreased below 50% of the pre-fatigue MVC. Subjects also performed a single ramp contraction immediately after the fatigue task. The Root Mean Square (RMS) amplitude, mean power frequency (MPF) and median frequency (MF) of EMG signals obtained from the vastus lateralis, vastus medialis and rectus femoris were calculated at nine different force levels from the ramp contractions (10, 20, 30, 40, 50, 60, 70, 80 and 90% MVC), as well as every 5 s during the fatigue task. The main results were a more pronounced increase in EMG RMS amplitude for the three muscles and in MPF for the VL muscle with force in men compared with women. No significant effect of gender was found with regards to fatigue. These observations most likely reflect a moderately greater type II fiber content and/or area in the VL muscle of men compared to that of women.     

Influence of contraction intensity, muscle, and gender on median frequency of the quadriceps femoris.

The purpose of this study was to assess the effects of contraction intensity, gender, and muscle on median frequency of the three superficial portions of the quadriceps femoris muscle. Thirty healthy volunteers were assessed for isometric electromyogram activity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles with the knee at 60 degrees flexion. Subjects performed 5-s isometric contractions at 10, 20, 30, 40, 50, 60, 70, 80, and 90% of the average of three maximal voluntary contractions. Median frequency (f(med)) of the three muscles was assessed through a power spectral analysis performed over 11 consecutive 512-ms epochs overlapping each other by one-half their length. The f(med) for each of the 11 epochs was then determined, followed by calculation of the mean and SD. The major findings of this study demonstrated that overall f(med) was significantly highest for the VL and lowest for the VM, whereas RF f(med) was between that of the other two muscles. Similar findings were observed for f(med) variability as the VL was significantly higher than the VM and RF, with no gender differences or differences between the latter two muscles. The results demonstrate that the largest change in f(med) as a function of contraction intensity occurred for the VL in men (18.6%) and women (7.6%). These findings suggest that muscle fiber-type homogeneity may exist in the VM and RF, which displayed negligible changes in f(med), whereas the VL may possess greater morphological variability.     

Agonist-antagonist common drive during fatiguing knee extension efforts using surface electromyography.

AIM: This study examined the electromyographic (EMG) activity of knee extensor agonists and a knee extensor antagonist muscle during fatiguing isometric extensions across a range of force levels. METHODS: Five female subjects performed isometric knee extensions at 25%, 50%, 75% and 100% of their maximal voluntary contraction (MVC) with the knee flexed to 75 degrees. Surface EMG (SEMG) was recorded with bipolar electrodes from the vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF) and biceps femoris (BF) and the root-mean-squared (RMS) amplitude and the percentage frequency compression of these recordings were calculated. Commonality and cross talk between recordings were also examined. RESULTS: Cross talk between recordings was deemed negligible despite significant levels of commonality between the agonist and antagonist SEMG, which was attributed to common drive. SEMG RMS amplitude increased significantly for all muscles during the 25%, 50%, 75% MVC knee extensions until task failure, and decreased significantly for 100% MVC. The frequency spectrum of the SEMG compressed significantly for all muscles and % MVC levels. The VM, VL and BF SEMG recordings responded similarly to fatigue. The RF's frequency spectrum compressed to a significantly higher degree. CONCLUSIONS: The VM, VL, RF, and BF fatigue in parallel, with high similarity between VM, VL and BF, giving support to the concept of a shared agonist-antagonist motoneuron pool.     

Gender and muscle differences in EMG amplitude and median frequency, and variability during maximal voluntary contractions of the quadriceps femoris.

The purpose of this study was to evaluate gender and muscle differences in electromyographic (EMG) amplitude and median frequency mean and standard deviation during maximal voluntary contractions of the quadriceps femoris. Thirty recreationally active volunteers were assessed for isometric EMG activity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles during three 5-s maximal isometric voluntary contractions (MVCs). Median frequency of the three muscles was assessed through a power spectral analysis (fast Fourier transformation, Hanning window processing, 512 points). The power spectral analysis was performed during the middle 3 s of each contraction over 11 consecutive, 512 ms epochs overlapping each other by half their length (256 ms). The median frequency (F(med)) for each of the 11 windows was determined for each muscle. The mean and standard deviation of the F(med) across the 11 overlapping windows were then calculated for each contraction and muscle. EMG amplitude was determined by calculating the root mean square (RMS-50 ms time constant) over the same contraction period for each muscle. The mean amplitude and standard deviation about the mean value were then determined. A three-factor ANOVA with repeated measures was performed on the calculated F(med) mean and standard deviation values, and RMS standard deviations, to assess any gender, muscle, or trial differences, or interactions. A two-factor (gender by muscle) ANOVA with repeated measures was performed on the RMS mean amplitude for each muscle. Intraclass correlation coefficients (ICCs-2,1), standard errors of measurement (SEMs), and associated 95% confidence intervals were then calculated for maximal quadriceps torque and F(med) for each muscle. The results from this study demonstrated that the VL muscle displayed significantly higher F(med) values than the RF and VM muscles. The RF muscle showed significantly higher F(med) values (mean of 11 overlapping windows) than the VM muscle. Intrasession reliability was found to be high for the calculated mean values (ICC=0.85-0.96), but was shown to be low for variability (ICC=0.13-0.45). The major findings of this study support the notion that the EMG signal is "quasi-random" in nature, as demonstrated by the reproducible F(med) means and unreliable variability.     

The impact of foot position on electromyographical activity of the superficial quadriceps muscles during leg extension

This study investigated variations in electromyographic (EMG) responses of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) due to foot position during leg extension. Twenty-four men and women (23.67 +/= 4.02 years) performed 8 repetitions at 70% of 8 repetition maximum with their leg medially rotated, laterally rotated, and neutral. Repeated-measures analyses of variance indicated that the highest normalized root mean square (NrmsEMG) for the VM and VL occurred with medial rotation, and the highest NrmsEMG for the RF occurred with lateral rotation. Significant NrmsEMG increases and median power frequency decreases occurred across repetitions regardless of foot position. Therefore, medial rotation produced the greatest muscle activation for the VL and VM, whereas lateral rotation produced the greatest activation in the RF. These findings are applicable to athletes or bodybuilders who are seeking to selectively increase either the size or performance of a specific muscle of the quadriceps group.