Knee extensor torque,work, and EMG during subjectively graded dynamic contractions

We examined knee extensor peak torque, work, and electromyogram (EMG) during dynamic contractions to perceived exertion levels in men and women. Thirty subjects performed three maximal effort isokinetic knee extensions (60 deg x s(-1)), followed by three contractions to each of nine separate levels of perceived exertion. Surface EMG of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF), and knee extensor peak torque and work were normalized to a percent of each respective value obtained during the maximal effort contractions. The results demonstrated a significant linear increase in voluntary knee extensor peak torque and work across perceived exertion levels. Knee extensor peak torque and work were less than 70%, 80%, and 90% maximal voluntary contraction (MVC) at perceived exertion levels 7, 8, and 9, respectively. A significant increase in VM, VL, and RF muscle EMG was observed across perceived exertion levels 1 through 9, with EMG increase highest for the VL. The findings demonstrate that dynamic contractions guided by perceived exertion are underproduced at relatively high perceptual intensities, and that reliance on VL activation occurs across submaximal torque levels. The overestimation of knee extensor peak torque and work at relatively high perceptual intensities may suggest the presence of a subconscious mechanism aiming to reduce high muscle and joint forces.     

A wavelet‐based analysis of surface mechanomyographic signals from the quadriceps femoris

The purpose of this study was to use a wavelet analysis designed specifically for surface mechanomyographic (MMG) signals to examine the MMG responses of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) muscles. Fifteen healthy men [age (mean ± SD): 26.4 ± 6.1 years] volunteered to perform isometric muscle actions of the dominant leg extensors at 20%, 40%, 60%, 80%, and 100% of the maximum voluntary contraction (MVC). During each muscle action, surface MMG signals were detected from the VL, RF, and VM and processed with the MMG wavelet analysis. The results show that, for the VL and VM muscles, there was compression of the total MMG intensity spectra toward low frequencies for most force levels above 20% MVC. For the RF, however, the peak of the total MMG intensity spectrum occurred at approximately 30–40 HZ for all force levels. Because the VL, RF, and VM are all innervated by the femoral nerve, the discrepancies among the three muscles for total MMG intensity in each wavelet band may have been due to differences in architecture, muscle stiffness, and/or intramuscular pressure. Muscle Nerve 39: 355–363, 2009     

MMG and EMG responses during fatiguing isokinetic muscle contractions at different velocities

The purpose of this study was to determine mechanomyographic (MMG) and electromyographic (EMG) responses of the superficial quadriceps muscles during repeated isokinetic contractions in order to provide information about motor control strategies during such activity, and to assess uniformity in mechanical activity (MMG) between the investigated muscles. Ten adults performed 50 maximal concentric muscle contractions at three randomly selected contraction velocities (60, 180, and 300 degrees.s(-1)) on different days. Surface electrodes and an MMG sensor were placed on the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM). EMG and MMG amplitude and peak torque (PT) were calculated for each contraction, normalized, and averaged across all subjects. The results demonstrated that MMG amplitude more closely tracked the fatigue-induced decline in torque production at each velocity than did EMG amplitude. This indicates that MMG amplitude may be useful for estimating force production during fatiguing dynamic contractions when a direct measure is not available, such as during certain rehabilitative exercises. MMG amplitude responses of the VL, RF, and VM were not uniform for each velocity or across velocities, indicating that it may be possible to detect the individual contribution of each muscle to force production during repeated dynamic contractions. Therefore, MMG amplitude may be clinically useful for detecting abnormal force contributions of individual muscles during dynamic contractions, and determining whether various treatments are successful at correcting such abnormalities.     

Comparison of electromyographic responses for the superficial quadriceps muscles: Cycle versus knee‐extensor ergometry

The purpose of this study was to compare the electromyographic (EMG) amplitude and mean power frequency (MPF) versus power output relationships for the three superficial quadriceps muscles during incremental cycle (CE) and knee‐extensor (KE) ergometry in the same subjects. Eight men performed incremental CE and KE tests to exhaustion. Surface EMG signals were recorded simultaneously from the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM). Polynomial regression analyses on a subject‐by‐subject basis indicated that the relationship between EMG amplitude versus power output was best‐fit with either a linear, quadratic, or cubic model for CE, whereas the relationship was best‐fit with a linear model for all subjects for the KE test. No consistent relationship was found for EMG MPF within subjects and between muscle groups for CE or KE. Compared with CE, however, the EMG amplitude during KE exercise was, on average, approximately 87% and 30% higher for the RF and VM muscles, respectively. These results suggest that KE exercise may be a better mode of examining EMG amplitude in the quadriceps muscle during incremental exercise than traditional CE. Muscle Nerve, 2009     

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

Introduction: The aim of this study was to investigate differences in twitch and M‐wave potentiation in the quadriceps femoris when electrical stimulation is applied over the quadriceps muscle belly versus the femoral nerve trunk. Methods: M‐waves and mechanical twitches were evoked using direct quadriceps muscle and femoral nerve stimulation between 48 successive isometric maximal voluntary contractions (MVC) from 10 young, healthy subjects. Potentiation was investigated by analyzing the changes in M‐wave amplitude recorded from the vastus medialis (VM) and vastus lateralis (VL) muscles and in quadriceps peak twitch force. Results: Potentiation of twitch, VM M‐wave, and VL M‐wave were greater for femoral nerve than for direct quadriceps stimulation (P < 0.05). Despite a 50% decrease in MVC force, the amplitude of the M‐waves increased significantly during exercise. Conclusions: In addition to enhanced electrogenic Na⁺‐K⁺ pumping, other factors (such as synchronization in activation of muscle fibers and muscle architectural properties) may significantly influence the magnitude of M‐wave enlargement. Muscle Nerve 48 : 920–929, 2013     

EMG spectral differences among the quadriceps femoris during the stretch reflex

Introduction: The purpose of this study was to examine the electromyographic (EMG) spectral characteristics of the quadriceps femoris muscles during tendon tap stretch reflexes. Methods: Sixteen healthy subjects (mean ± SD age = 21.2 ± 2.8 years) performed tendon tap reflexes of the leg extensors as surface EMG signals were detected from the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) muscles of the dominant thigh. All EMG signals were processed with a wavelet analysis, and the resulting spectra were decomposed with nonparametric spectral decomposition. Results: The results showed that the spectra for the VL had significantly more high‐frequency power than those for the RF and VM, with similar spectral shapes for the RF and VM. Conclusions: These findings could be due to differences in the width of the innervation zone, or the fiber type composition of the muscles, although the latter seems to be more likely. Muscle Nerve 52 : 826–831, 2015     

Linearity and reliability of the mechanomyographic amplitude versus dynamic torque relationships for the superficial quadriceps femoris muscles

The purpose of this investigation was to examine the linearity and reliability of the mechanomyographic (MMG) amplitude versus dynamic torque relationships for the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) muscles. Nine healthy men and 11 healthy women performed submaximal to maximal, concentric, isokinetic muscle actions of the leg extensors at 30° s^?1 on two occasions. Surface MMG signals were detected from the VL, RF, and VM of the dominant thigh during both trials. The ranges of the coefficients of determination for the MMG amplitude versus dynamic torque relationships were 0.01–0.94 for the VL, 0.01–0.84 for the RF, and 0.19–0.96 for the VM. The intraclass correlation coefficients for the linear MMG amplitude versus torque slope coefficients were 0.823 (VL), 0.792 (RF), and 0.927 (VM). These results indicate that, when analyzed for individual subjects, the MMG amplitude versus dynamic torque relationships demonstrated inconsistent linearity. When using MMG in the clinical setting, dynamic muscle actions of the superficial quadriceps femoris muscles do not appear to be appropriate for assessing changes in muscle function during strength training. Muscle Nerve, 2009     

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

The purpose of this study was to determine the velocity-related patterns for mechanomyographic (MMG) amplitude, electromyographic (EMG) amplitude, mean power output (MP), and peak torque (PT) of the superficial muscles of the quadriceps femoris (vastus lateralis [VL], rectus femoris [RF], and vastus medialis [VM]) during maximal, concentric, isokinetic leg extensions. Twelve adult women (mean +/- SD: 22 +/- 3 years of age) performed such leg extensions at velocities of 60 degrees, 120 degrees, 180 degrees, 240 degrees, and 300 degrees /s on a Cybex 6000 dynamometer. PT decreased (P < 0.05) across velocity to 240 degrees /s. MP and MMG amplitude for each muscle (VL, RF, and VM) increased (P < 0.05) with velocity to 240 degrees /s and then plateaued. EMG amplitude increased (P < 0.05) to 240°/s for the VL, remained unchanged across velocity (P > 0.05) for the RF, and increased (P < 0.05) to 300 degrees /s for the VM. The results indicated close similarities between the velocity-related patterns for MMG amplitude and MP, but dissociations among EMG amplitude, MMG amplitude, and PT. These findings support the recent hypothesis that MMG amplitude is more closely related to MP than PT during maximal, concentric, isokinetic muscle actions and, therefore, may be useful for monitoring training-induced changes in muscle power.     

Assessment of muscular fatigue using vibromyography

The purpose of this study was to investigate the behavior of electromyographical (EMG) and vibromyographical (VMG) signals in the time and frequency domains during a fatigue protocol. EMG and VMG records were obtained from the rectus femoris (RF) and vastus lateralis (VL) muscles of 11 adult male subjects during sustained, isometric knee extensor contractions performed at 70% of maximal voluntary contraction (MVC). The average median frequencies of the power density spectra decreased during the fatigue protocol for the EMG (from 73 to 54 Hz for RF, and from 75 to 57 Hz for VL) and the VMG signals (from 40 to 19 Hz for RF, and from 25 to 12 Hz for VL). Raw EMG signals remained the same qualitatively throughout the fatigue protocol, whereas corresponding VMG records appeared to become “smoother.” The results of this study indicate that the pronounced decrease in the high-frequency content of the VMG signal may be observed in the time domain as a “smoothing” of the signal, and thus, that the raw VMG records (which may be displayed readily online) can be used to assess qualitatively the onset and progression of muscular fatigue. © 1994 John Wiley & Sons, Inc.     

Acoustic myography of the human quadriceps muscle during intermittent fatiguing activity.

Integrated acoustic myography (IAMG) and electromyography (IEMG) were recorded over rectus femoris (RF) in six healthy subjects during a series of intermittent isometric contractions of quadriceps. Contractions were held for 10 sec with 10 sec rest between each, commencing at 75% maximum voluntary contraction (MVC) force and continuing to 40% MVC. The IAMG activity initially decreased (75%-60% MVC) in a linear relationship (r = 0.9) with fatigue (i.e. force loss) but then plateaued and increased once force fell below 52% MVC. The AMG/force relationship for the whole fatiguing protocol (i.e. 75%-40% MVC) was quadratic (r = 0.87). The IEMG also showed a quadratic relationship with force (r = 0.85) but activity initially increased before decreasing. The results of the present study quantify the relationship between AMG and force in quadriceps during fatigue from intermittent contractions commencing at 75% MVC. The findings confirm previous observations that AMG decreases with fatigue during strong contractions but the quadratic relationship found in the present study differs to that for other muscles during sustained contractions. The results also suggest that simultaneous recordings of AMG and EMG may help distinguish central and peripheral fatigue. Acoustic myography may therefore be a useful non-invasive monitor of force during early fatiguing activity using the present protocol but the need to study AMG during fatigue of different muscles and force levels is stressed.     

Reliability of surface electromyographic measurements.

OBJECTIVES: The aim of the study was to investigate short-term, intermediate-term and long-term reliability of surface electromyographic (EMG) measurements. METHODS: Eighteen healthy subjects performed 810 isometric knee extension tests. Reliability for maximum voluntary contraction (MVC) and 50% MVC was assessed with retest intervals of 3 min, 90 min and 6 weeks. Reliability for sustained contractions was assessed with retest intervals of 90 min and 6 weeks. EMG was recorded from the rectus femoris, vastus lateralis and vastus medialis muscles. The root mean square (RMS) and the median frequency (MF) parameters were extracted. At sustained contraction tasks, estimated linear regression values of both parameters were analyzed. Bland-Altman-plots, coefficient of repeatability, Pearson's coefficient of correlation and intra class correlation (ICC) procedures were applied to assess test-retest reliability. RESULTS: EMG recordings taken at short-term intervals were generally better reproducible than those of the longer-term intervals. Moreover, 50% MVC EMG recordings demonstrated better reproducibility than 100% MVC measurements, and EMG recorded from the rectus femoris were more constant than that from the vastus lateralis or vastus medialis. The MF parameter recorded from the rectus femoris was the only reliable parameter of EMG fatigue change. CONCLUSION: In our set up, EMG measurement is best suited for clinical applications if submaximal MVC measurements are performed and signal is taken from rectus femoris muscle.     

Quadriceps activation and perceived exertion during a high intensity, steady state contraction to failure

The ability to sustain a high-intensity, steady-state muscle contraction may have differential effects on neuromuscular activation and perceived exertion. The purpose of this study was to examine changes in neuromuscular activation and perceived exertion at a near-maximal steady-state contraction of the quadriceps in healthy men. Seventeen healthy, college-aged male volunteers were studied during isometric contractions equivalent to 80% of the maximum voluntary contraction (MVC). Perceived exertion was measured with a modified category-ratio scale (CR-10). The CR-10 scale was anchored with one high anchor at 100% MVC and one low anchor at 10% MVC. Subjects then performed an 80% MVC for as long as they could sustain it. Subjects were asked to rate the feelings in their quadriceps every 5 s during the contraction. The results demonstrated significant increases in neuromuscular activation of the vastus medialis and vastus lateralis muscles (P < 0.05) during the 80% MVC, but there were no significant muscle by time interactions. The results also demonstrated a significant increase (P < 0.05) in perceived exertion during the 80% MVC. Neuromuscular activation of both muscles, and perceived exertion, were found to increase in linear (P < 0.05) and quadratic (P < 0.05) trends. Alterations in motor unit discharge properties or impairments in muscle fiber membrane excitability may account for nonlinear increases in vastii muscle activation and perceived exertion.     

Wavelet analysis of quadriceps power spectra and amplitude under varying levels of contraction intensity and velocity

Introduction: We investigated the effect of contraction intensity [100%, 75%, 50%, and 25% maximum voluntary contraction (MVC)] and movement velocity (50°, 100°, 200°, and 400°/s) on surface electromyography root mean square amplitude (SEMG_RMS) and median frequency (SEMG_MDF) of rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM). Methods: SEMGs during knee extension were resolved into their respective frequencies using wavelet transformations. Results: RF, VL, and VM muscles displayed increased SEMG_MDF as contraction intensity increased from 25% to 50% MVC and from 75% to 100% MVC, and each muscle displayed its own unique frequency shifting patterns. The SEMG_MDF was not influenced by movement velocity. SEMG_RMS increased in all 3 muscles as contraction intensity increased and was influenced by movement velocity, with the highest values observed at 400° and 200°/s. Conclusions: We infer that increasing contraction intensity facilitates greater recruitment of fast‐twitch muscle fibers, but there are differing responses in RF, VL, and VM muscles. Muscle Nerve 50 : 844–853, 2014     

Assessing contractile ability of the quadriceps muscle using ultrasound imaging

Muscle dimension changes on ultrasound imaging (USI) indicate contractile activity. Quadriceps force and rectus femoris (RF) dimensions were examined to assess USI for estimating contraction level. In 15 healthy males, mean age 24.8 years, isometric quadriceps force was measured in 90° knee flexion during submaximal and maximal voluntary contractions (MVCs) and at MVC in extension. Mid‐thigh cross‐sectional area (CSA), depth and width, and surface electromyography (EMG) of RF were recorded. Muscle width decreased curvilinearly as both force and EMG increased. At MVC, width was 3.2 ± 0.5 cm, 25% smaller than at rest (4.4 ± 0.7 cm), and similar to MVC in extension (?23%). CSA decreased linearly to ?18% at MVC. RF thickness increased by only 10% (at 30% to MVC). Similar width changes at MVC in flexion and extension indicate the clinical potential of USI for assessing quadriceps contractility. Sensitivity is limited to assessing strength to within 25% of MVC in young males. Muscle Nerve, 2010     

Differential response of the dog quadriceps muscle to external skeletal fixation of the knee

The change in muscle fiber size and fiber percentage was studied in three heads of the dog quadriceps following 10 weeks of immobilization using an external skeletal fixator. Muscle biopsy morphometric analysis revealed that muscle fiber atrophy was greatest for the vastus medialis and least for the rectus femoris. The atrophic response for type 1 fibers was, in order from most to least atrophied: VM greater than VL greater than RF, whereas for type 2 fibers the corresponding order was VM = VL greater than RF. An increase in connective tissue was also observed for all muscles. These results are discussed in terms of skeletal muscle architecture, initial fiber-type distribution, and level of use. As a result, predictions are made as to the muscles that are most vulnerable to disuse atrophy, namely the postural muscles that contain a relatively large proportion of slow muscle fibers and cross a single joint. Conversely, those that are least susceptible to atrophy are those that are not used as postural muscles, that cross multiple joints, and that are predominantly composed of fast muscle fibers.     

Mechanomyographic amplitude and mean power frequency responses during isometric ramp vs. step muscle actions

The purpose of the present study was to compare the mechanomyographic amplitude (MMG(RMS)) and mean power frequency (MMG(MPF)) vs. torque relationships during isometric ramp and step muscle actions for the vastus lateralis (VL) and rectus femoris (RF) muscles. Nineteen subjects (mean+/-S.D. age=24+/-4 years) performed 2 isometric maximal voluntary contractions (MVCs) before and after 2 or 3 isometric ramp muscle actions from (5-95% MVC) to 9 submaximal step muscle actions (15, 25, 35, 45, 55, 65, 75, 85, and 95% MVC). MMG signals were recorded from the VL and RF muscles, and MMG(RMS) and MMG(MPF) values were computed for each corresponding percentage of the MVC. Absolute and normalized MMG(RMS) and MMG(MPF) vs. torque relationships were analyzed and interpreted on a subject-by-subject and composite pattern basis using polynomial regression and repeated measures ANOVAs. For MMG(RMS) and MMG(MPF), only 16-53% and 11-26% of the individual responses were consistent with the composite polynomial models, respectively. In addition, the normalized composite MMG(RMS) values were greater for the RF than the VL from 35 to 85% MVC. Only 47% of the MMG(RMS) and 5% of the MMG(MPF) individual patterns of responses were the same for the ramp and step muscle actions, and differences were also observed for the composite MMG(RMS) and MMG(MPF) patterns between the ramp and step muscle actions. Overall, these findings indicated that the torque-related patterns of responses for MMG(RMS) and MMG(MPF) were different among subjects (i.e., inter-individual variability) and were muscle- (VL vs. RF) and mode-specific (ramp vs. step).     

Effects of transcranial magnetic stimulation on results of the twitch interpolation technique

To investigate whether transcranial magnetic stimulation (TMS) has an effect on isometric muscle force elicited by maximal voluntary contractions (MVC) and the ability to activate a muscle voluntarily (VA, as a percentage of full muscle activation), a twitch-interpolation technique was applied on the quadriceps femoris muscles of six volunteers before and within 1 min after TMS. VA improved by 5% (P = 0.019) and MVC by 17% (P = 0.002), whereas these parameters were unchanged in a control experiment. The results suggest that TMS has an effect on the central motor drive, at least within 1 min after stimulation.     

Acoustic myographic activity increases linearly up to maximal voluntary isometric force in the human quadriceps muscle

Sounds produced during voluntary isometric contractions of the quadriceps muscle were recorded by acoustic myography (AMG) in seven healthy adults. With the subject seated, surface AMG and electromyography (EMG) were recorded over rectus femoris (RF) during isometric contractions, at different levels of maximum voluntary force. The AMG and EMG signals were amplified and integrated (IAMG and IEMG). The relationships between force and IAMG (r = 0.98 +/- 0.01, mean +/- 1 SD) and force and IEMG (r = 0.99 +/- 0) were linear in all subjects. The results for EMG confirm previous reports but those for AMG differ from the relationships reported for other muscles. Physiological and technical explanations are proposed for these differences and the necessity for further validation of the AMG technique is stressed.     

Muscle‐related differences in mechanomyography–force relationships are model‐dependent

Introduction: In this study we examined the mechanomyographic amplitude (MMG_RMS)–force relationships with log‐transform and polynomial regression models for the vastus lateralis (VL), rectus femoris (RF), and first dorsal interosseous (FDI) muscles. Methods: Twelve healthy (age 23 ± 3 years) men performed isometric ramp contractions of the leg extensors and index finger from 10% to 80% of their maximal voluntary contraction (MVC) with MMG sensors positioned on the VL, RF, and FDI. Log‐transform and polynomial regression models were fitted to the relationships. Results: There were differences for the a terms (intercepts) and b terms (slopes) from the log‐transform model between the FDI, VL, and RF; however, there were no consistent differences identified with the polynomial regression models. Conclusions: The log‐transform model quantified differences in the patterns of responses between the FDI and the leg extensors, but polynomial regression could not distinguish such differences. Muscle Nerve 49 : 202–208, 2014     

The relationship of motor unit size, firing rate and force.

OBJECTIVE: Using a clinical electromyographic (EMG) protocol, motor units were sampled from the quadriceps femoris during isometric contractions at fixed force levels to examine how average motor unit size and firing rate relate to force generation. METHODS: Mean firing rates (mFRs) and sizes (mean surface-detected motor unit action potential (mS-MUAP) area) of samples of active motor units were assessed at various force levels in 79 subjects. RESULTS: MS-MUAP size increased linearly with increased force generation, while mFR remained relatively constant up to 30% of a maximal force and increased appreciably only at higher force levels. A relationship was found between muscle force and mS-MUAP area (r2 = 0.67), mFR (r2 = 0.38), and the product of mS-MUAP area and mFR (mS-MUAP x mFR) (r2 = 0.70). CONCLUSIONS: The results support the hypothesis that motor units are recruited in an orderly manner during forceful contractions, and that in large muscles only at higher levels of contraction ( > 30% MVC) do mFRs increase appreciably. MS-MUAP and mFR can be assessed using clinical EMG techniques and they may provide a physiological basis for analyzing the role of motor units during muscle force generation.