The extraction of neural strategies from the surface EMG

Factors responsible for force steadiness impairment with fatigue

In this study we investigated the contribution of muscle activation to the impairment of fine force control with fatigue. In three experiments, we manipulated muscle activation and measured force variability before and after a fatigue protocol. When muscle activation was left free (subjects had to match the same absolute force pre- and post-fatigue), fatigue increased muscle activation at moderate force levels only, and force variability increased regardless of the level of force. When muscle activation was controlled (subjects had to match the same electromyographic activity), fatigue no longer increased force variability, except at low force levels. When voluntary muscle activation was suppressed (muscles were electrically stimulated), force variability was unaffected by fatigue. We conclude that the impairment of force steadiness with fatigue is mainly due to the increase in muscle activation at moderate forces, but there are other central sources of force fluctuation present at low force levels.

Assessment of hip abductor muscle strength. A validity and reliability study

BACKGROUND

Hip abductors are the most important muscles around the hip joint. It is therefore essential to assess their function in a valid and reliable way. Since the optimal body posture for the assessment of hip abductor strength is unknown, we tested the validity and reliability of unilateral hip abductor strength assessment in three different body positions. We hypothesized that the validity would be better in the side-lying position because of the consistent stabilization of the contralateral (untested) hip.

METHODS

Sixteen healthy subjects participated in two identical testing sessions. Unilateral isometric hip abductor muscle strength was measured, with use of a stabilized commercial dynamometer, with the subject in the side-lying, supine, and standing positions. Construct validity was based on concomitant recordings of gluteus medius electromyographic activity from the tested and contralateral hips. The body position permitting greater muscle activation and abductor strength on the tested hip, while minimizing muscle activation in the contralateral hip (that is, lower contralateral-to-tested electromyographic ratio), was considered the most valid. Coefficients of variation, the Bland and Altman limits of agreement, and intraclass correlation coefficients were calculated to determine test-retest reliability of hip abductor strength.

RESULTS

Maximal hip abductor strength was significantly higher in the side-lying position compared with the standing and supine positions (p < 0.05). The contralateral-to-tested electromyographic ratio for the side-lying position was significantly lower than that for the supine and the standing position (p < 0.01). Test-retest reliability of strength measurements in terms of coefficients of variation (3.7% for side-lying, 6.1% for supine, and 4.2% for standing) and limits of agreement (+/-6.9% for side-lying, +/-8.4% for supine, and +/-7.5% for standing) was better in the side-lying position. All intraclass correlation coefficients were high to moderate (0.90 for side-lying, 0.83 for supine, and 0.88 for standing).

CONCLUSIONS

The side-lying body position offers the most valid and reliable assessment of unilateral hip abductor strength.

Repeated maximal volitional effort contractions in human spinal cord injury: initial torque increases and reduced fatigue

BACKGROUND

Substantial data indicate greater muscle fatigue in individuals with spinal cord injury (SCI) compared with healthy control subjects when tested by using electrical stimulation protocols. Few studies have investigated the extent of volitional fatigue in motor incomplete SCI.

METHODS

Repeated, maximal volitional effort (MVE) isometric contractions of the knee extensors (KE) were performed in 14 subjects with a motor incomplete SCI and in 10 intact subjects. Subjects performed 20 repeated, intermittent MVEs (5 seconds contraction/5 seconds rest) with KE torques and thigh electromyographic (EMG) activity recorded.

RESULTS

Peak KE torques declined to 64% of baseline MVEs with repeated efforts in control subjects. Conversely, subjects with SCI increased peak torques during the first 5 contractions by 15%, with little evidence of fatigue after 20 repeated efforts. Increases in peak KE torques and the rate of torque increase during the first 5 contractions were attributed primarily to increases in quadriceps EMG activity, but not to decreased knee flexor co-activation. The observed initial increases in peak torque were dependent on the subject's volitional activation and were consistent on the same or different days, indicating little contribution of learning or accommodation to the testing conditions. Sustained MVEs did not elicit substantial increases in peak KE torques as compared to repeated intermittent efforts.

CONCLUSIONS

These data revealed a marked divergence from expected results of increased fatigability in subjects with SCI, and may be a result of complex interactions between mechanisms underlying spastic motor activity and changes in intrinsic motoneuron properties.

On the feasibility of obtaining multiple muscular maximal voluntary excitation levels from test exertions: a shoulder example

Currently, contrasting views exist regarding which body and arm postures are most effective for eliciting maximal voluntary exertions in the shoulder muscles. Informed exertion standardization may improve comparisons between subjects and muscle groups for normalized electromyography values. Additionally, identifying exertions that can produce equivalent maximal electrical activity values can reduce experimental setup time and reduce the likelihood of fatigue development. This research study examined twelve posture and force direction defined test exertions to identify those that elicited maximal electrical activity from the deltoid (anterior and middle fibres) and pectoralis major (clavicular and sternal heads). Further, the question of whether a single test exertion could obtain maximal electrical activity from multiple muscle fascicles was explored. Maximal activation was demonstrated for the deltoid during several exertions that incorporated an upward force exertion and the pectoralis major for multiple exertions that included an inward force direction. Finally, two test exertions produced maximal electrical activity from both muscles of interest. This research supports the notion that a range of exertions can elicit maximal electrical activity from a muscle, rather than one specific exertion. This suggests that researchers may be able to leverage a smaller set of test exertions to evaluate multiple muscles simultaneously without loss of data quality, and thereby decrease overall experimental data collection time while maintaining high fidelity data.

Sex alters impact of repeated bouts of sprint exercise on neuromuscular activity in trained athletes

This study characterized the effect of sex on neuromuscular activity during repeated bouts of sprint exercise. Thirty-three healthy male and female athletes performed twenty 5-s cycle sprints separated by 25 s of rest. Mechanical work and integrated electromyograhs (iEMG) of 4 muscles of the dominant lower limb were calculated in every sprint. The iEMG signals from individual muscles were summed to represent overall electrical activity of these muscles (sum-iEMG). Neuromuscular efficiency (NME) was calculated as the ratio of mechanical work and sum-iEMG for every sprint. Arterial oxygen saturation was estimated (SpO2) with pulse oximetry throughout the protocol. The sprint-induced work decrement (18.9% vs. 29.6%; p < 0.05) and sum-iEMG reduction (11.4% vs. 19.4%; p < 0.05) were less for the women than for the men. However, the sprints decreased NME (10.1%; p < 0.05) and SpO2 (3.4%; p < 0.05) without showing sex dimorphism. Changes in SpO2 and sum-iEMG were strongly correlated in both sexes (men, R2 = 0.87; women, R2 = 0.91; all p < 0.05), although the slope of this relationship differed (6.3 +/- 2.9 vs. 3.8 +/- 1.6, respectively; p < 0.05). It is suggested that the sex difference in fatigue during repeated bouts of sprint exercise is not likely to be explained by a difference in muscle contractility impairment in men and women, but may be due to a sex difference in muscle recruitment strategy. We speculate that women would be less sensitive to arterial O2 desaturation than men, which may trigger lower neuromuscular adjustments to exhaustive exercise.

Comparison between the degree of motor unit short-term synchronization and recurrence quantification analysis of the surface EMG in two human muscles

OBJECTIVE

To verify if non-linear recurrence analysis of the surface EMG is a suitable tool for assessing motor unit short-term synchronization.

METHODS

Surface and intramuscular EMG signals were recorded from the abductor digiti minimi and vastus medialis muscles of 12 and 10 healthy men, respectively, during isometric contractions. In the abductor digiti minimi, EMG signals were additionally recorded after a contraction sustained for 1min at 50% of the maximal force. In both muscles, percent of determinism (%DET) was estimated from the surface EMG and common input strength (CIS) index was computed from motor unit recordings.

RESULTS

For both muscles, CIS did not correlate with %DET (abductor digiti minimi: R(2)=0.11, P=0.12; vastus medialis: R(2)=0.04, P=0.56). Although the values of CIS for the vastus medialis were lower than those of the abductor digiti minimi (P<0.001), the %DET values did not differ between the two muscles (71.6+/-5.5% vs 66.9+/-8.7%; P=0.12).

CONCLUSION

The variable %DET extracted from the surface EMG is a poor indicator of the degree of motor unit short-term synchronization.

SIGNIFICANCE

The study provides a systematic evaluation of a technique previously proposed for the estimation of a clinically relevant characteristic of motor unit behavior.

Influence of motor unit synchronization on amplitude characteristics of surface and intramuscularly recorded EMG signals

The increase in muscle strength without noticeable hypertrophic adaptations is very important in some sports. Motor unit (MU) synchronisation and higher rate of MU activation are proposed as possible mechanisms for such a strength and electromyogram (EMG) increase in the early phase of a training regimen. Root mean square and/or integrated EMG are amplitude measures commonly used to estimate the adaptive changes in efferent neural drive. EMG amplitude characteristics could change also because of alteration in intracellular action potential (IAP) spatial profile. We simulated MUs synchronization under different length of the IAP profile. Different synchronization was simulated by variation of the percent of discharges in a referent MU, to which a variable percent of remaining MUs was synchronized. Population synchrony index estimated the degree of MU synchronization in EMG signals. We demonstrate that the increase in amplitude characteristics due to MU synchronization is stronger in surface than in intramuscularly detected EMG signals. However, the effect of IAP profile lengthening on surface detected EMG signals could be much stronger than that of MU synchronization. Thus, changes in amplitude characteristics of surface detected EMG signals with progressive strength training could hardly be used as an indicator of changes in neural drive without testing possible changes in IAPs.

No effect of prior caffeine ingestion on neuromuscular recovery after maximal fatiguing contractions

The purpose of this study was to test the hypothesis that prior caffeine ingestion would enhance neural recovery after isometric fatiguing maximal intermittent plantar flexions, and thus would enhance the recovery of voluntary muscle strength. After a familiarisation session, 13 males randomly participated in two experimental trials where they ingested either caffeine (approximately 6 mg/kg) or identical placebo pills 1 h prior to testing. Subjects were tested for electromyogram (EMG) activity and evoked V-waves in the soleus and gastrocnemius medialis muscles. These measurements were obtained during brief plantar flexion maximum voluntary isometric contractions (MVICs), and normalised by the superimposed maximal M-wave (EMG/M(SUP) and V/M(SUP), respectively), before and after (20 s, 10 min and 20 min) a fatigue protocol (seven 25 s MVICs, 5 s rest). There were no effects (P > 0.05) of caffeine ingestion on EMG/M(SUP), V/M(SUP), MVIC or M(SUP). The central neural modulation (EMG/M(SUP) and V/M(SUP)) and voluntary strength changes followed a similar time-course with a substantial reduction 20 s post-fatigue and a gradual return towards baseline values.

Methodological aspects of SEMG recordings for force estimation--a tutorial and review

Insight into the magnitude of muscle forces is important in biomechanics research, for example because muscle forces are the main determinants of joint loading. Unfortunately muscle forces cannot be calculated directly and can only be measured using invasive procedures. Therefore, estimates of muscle force based on surface EMG measurements are frequently used. This review discusses the problems associated with surface EMG in muscle force estimation and the solutions that novel methodological developments provide to this problem. First, some basic aspects of muscle activity and EMG are reviewed and related to EMG amplitude estimation. The main methodological issues in EMG amplitude estimation are precision and representativeness. Lack of precision arises directly from the stochastic nature of the EMG signal as the summation of a series of randomly occurring polyphasic motor unit potentials and the resulting random constructive and destructive (phase cancellation) superimpositions. Representativeness is an issue due the structural and functional heterogeneity of muscles. Novel methods, i.e. multi-channel monopolar EMG and high-pass filtering or whitening of conventional bipolar EMG allow substantially less variable estimates of the EMG amplitude and yield better estimates of muscle force by (1) reducing effects of phase cancellation, and (2) adequate representation of the heterogeneous activity of motor units within a muscle. With such methods, highly accurate predictions of force, even of the minute force fluctuations that occur during an isometric and isotonic contraction have been achieved. For dynamic contractions, EMG-based force estimates are confounded by the effects of muscle length and contraction velocity on force producing capacity. These contractions require EMG amplitude estimates to be combined with modeling of muscle contraction dynamics to achieve valid force predictions.

Sex differences in response to cognitive stress during a fatiguing contraction

This study compared the time to task failure for a submaximal fatiguing contraction in the presence and absence of a cognitive stressor in men and women. In study 1, 10 men and 10 women (22 +/- 3 yr of age) performed an isometric fatiguing contraction at 20% maximal voluntary contraction force until task failure with the elbow flexor muscles during two separate sessions. Subjects performed a mental-math task during one of the fatiguing contractions that aimed to increase anxiety and stress (stressor session). Salivary cortisol and reported levels of arousal (visual analog scale for anxiety, and State-Trait Anxiety Inventory scores) were elevated during the stressor session compared with a control session for both sexes (P < 0.05). Time to task failure, however, was briefer during the stressor session compared with control (P = 0.005) but more so for the women (27.3 +/- 20.1%) than the men (8.6 +/- 23.1%) (P = 0.03). The briefer time to task failure was associated with target force (r(2) = 0.21) and accompanied by a higher mean arterial pressure, heart rate, and rate-pressure product during the fatiguing contraction in the stressor session compared with control in women. In study 2 (11 men and 8 women, 20 +/- 3 yr of age), time to task failure was similar for a fatiguing contraction with simple mental-math that did not increase stress (mental-attentiveness session) and control for both men and women. The greater change in fatigability of women than men with performance of a cognitive stressor involved initial strength and increases in indexes of sympathetic neural activity and cardiac work compared with control conditions.

Relationship between oxygen uptake slow component and surface EMG during heavy exercise in humans: influence of pedal rate

The aim of this study was to test the hypothesis that extreme pedal rates contributed to the slow component of oxygen uptake (VO(2) SC) in association with changes in surface electromyographic (sEMG) during heavy-cycle exercise. Eight male trained cyclists performed two square-wave transitions at 50 and 110 rpm at a work rate that would elicit a VO(2) corresponding to 50% of the difference between peak VO(2) and the ventilatory threshold. Pulmonary gas exchange was measured breath-by-breath and sEMG was obtained from the vastus lateralis and medialis muscles. Integrated EMG flow (QiEMG) and mean power frequency (MPF) were computed. The relative amplitude of the VO(2) SC was significantly higher during the 110-rpm bout (556+/-186 ml min(-1), P<0.05) with compared to the 50-rpm bout (372+/-227 ml min(-1)). QiEMG values increased throughout exercise only during the 110-rpm bout and were associated with the greater amplitude of the VO(2) SC observed for this condition (P<0.05). MPF values remained relatively constant whatever the cycle bout. These findings indicated a VO(2) SC at the two pedal rates but the association with sEMG responses was observed only at high pedal rate. Possible changes in motor units recruitment pattern, muscle energy turnover and muscle temperature have been suggested to explain the different VO(2) SC to heavy pedal rate bouts.

Implantable myoelectric sensors (IMESs) for intramuscular electromyogram recording

We have developed a multichannel electrogmyography sensor system capable of receiving and processing signals from up to 32 implanted myoelectric sensors (IMES). The appeal of implanted sensors for myoelectric control is that electromyography (EMG) signals can be measured at their source providing relatively cross-talk-free signals that can be treated as independent control sites. An external telemetry controller receives telemetry sent over a transcutaneous magnetic link by the implanted electrodes. The same link provides power and commands to the implanted electrodes. Wireless telemetry of EMG signals from sensors implanted in the residual musculature eliminates the problems associated with percutaneous wires, such as infection, breakage, and marsupialization. Each implantable sensor consists of a custom-designed application-specified integrated circuit that is packaged into a biocompatible RF BION capsule from the Alfred E. Mann Foundation. Implants are designed for permanent long-term implantation with no servicing requirements. We have a fully operational system. The system has been tested in animals. Implants have been chronically implanted in the legs of three cats and are still completely operational four months after implantation.

Muscle functional magnetic resonance imaging and acute low back pain: a pilot study to characterize lumbar muscle activity asymmetries and examine the effects of osteopathic manipulative treatment

Background

Muscle functional magnetic resonance imaging (mfMRI) measures transverse relaxation time (T2), and allows for determination of the spatial pattern of muscle activation. The purposes of this pilot study were to examine whether MRI-derived T2 or side-to-side differences in T2 (asymmetries) differ in low back muscles between subjects with acute low back pain (LBP) compared to asymptomatic controls, and to determine if a single osteopathic manipulative treatment (OMT) session alters these T2 properties immediately and 48-hours after treatment.

Methods

Subjects with non-specific acute LBP (mean score on 1-10 visual analog score = 3.02 ± 2.81) and asymptomatic controls (n = 9/group) underwent an MRI, and subsequently the LBP subjects received OMT and then underwent another MRI. The LBP subjects reported back for an additional MRI 48-hours following their initial visit. T2 and T2 asymmetry were calculated from regions of interest for the psoas, quadratus lumborum (QL), multifidus, and iliocostalis lumborum/longissimus thoracis (IL/LT) muscles.

Results

No differences were observed between the groups when T2 was averaged for the left and right side muscles. However, the QL displayed a significantly greater T2 asymmetry in LBP subjects when compared to controls (29.1 ± 4.3 vs. 15.9 ± 4.1%; p = 0.05). The psoas muscle also displayed a relatively large, albeit non-significant, mean difference (22.7 ± 6.9 vs. 9.5 ± 2.8%; p = 0.11). In the subjects with LBP, psoas T2 asymmetry was significantly reduced immediately following OMT (25.3 ± 6.9 to 6.1 ± 1.8%, p = 0.05), and the change in LBP immediately following OMT was correlated with the change in psoas T2 asymmetry (r = 0.75, p = 0.02).

Conclusion

Collectively, this pilot work demonstrates the feasibility of mfMRI for quantification and localization of muscle abnormalities in patients with acute low back pain. Additionally, this pilot work provides insight into the mechanistic actions of OMT during acute LBP, as it suggests that it may attenuate muscle activity asymmetries of some of the intrinsic low back muscles.

Differentiation of power and endurance athletes based on their muscle fatigability assessed by new spectral electromyographic indices

The aim of this study was to differentiate between endurance and power athletes based on electromyographic (EMG) data analysed using new spectral indices. Nine endurance and six strength athletes were recruited to complete sets of knee extension repetitions (15 per set) until exhaustion, with each set followed by a maximal voluntary isometric knee extensor contraction. Peripheral muscle fatigue of the vastus lateralis, vastus medialis, and rectus femoris (bilaterally) was quantified by the changes in median frequency of the EMG power spectrum and a new spectral EMG fatigue index. Cluster analysis of the fatigue indices differentiated athletes into two groups: endurance (fatigue resistant) and strength (faster fatigue), whereas cluster analysis of the median EMG power spectrum frequency produced six indistinct groups. The average fatigue index for the quadriceps group increased across repetitions by 40 +/- 24% in the endurance group and by 184 +/- 12% in the strength group. The decrease in peak force and power across repetitions, and the rate of force decrease during maximal voluntary contraction per set, were significantly smaller for the endurance than for the strength group. The new spectral EMG indices effectively discriminated between strength and endurance athletes, thus providing a useful functional index that could be applied to track training adaptations as well as potentially talent identification.

EMG-based and gaze-tracking-based man-machine interfaces

A great demand for brain-machine and, more generally, man-machine interfaces is arising nowadays, pushed by several promising scientific and technological results, which are encouraging the concentration of efforts in this field. The possibility of measuring, processing and decoding brain activity, so as to interpret neural signals, is often looked at as a possibility to bypass lost or damaged neural and/or motor structures. Beyond that, such interfaces currently show a potential for applications in other fields, space science being certainly one of them. At present, the concept of "reading" the brain to detect intended actions and use these to control external devices is being studied with several technical and methodological approaches; among these, interfaces based on electroencephalographic signals play today a prominent role. Within such a context, the aim of this section is to present a brief survey on two types of noninvasive man-machine interfaces based on a different approach. In particular, they rely on the extraction of control signals from the user with techniques that adopt electromyography and gaze tracking. Working principles, implementations, typical features, and applications of these two types of interfaces are reported.

Comparison of critical force to EMG fatigue thresholds during isometric leg extension

Theoretically, the critical force (CF) and the EMG fatigue threshold (EMGFT) tests demarcate fatiguing from nonfatiguing isometric torque levels.

PURPOSE

The purpose of this study was twofold: 1) to determine whether the mathematical model for estimating the EMGFT during cycle ergometry was applicable to isometric leg extension muscle actions and 2) to compare the mean torque level from the CF test to those of EMGFT tests for the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) muscles during isometric muscle actions.

METHODS

The slope coefficient of the linear relationship between total "isometric work" (Wlim in newton-meters per second) and time to exhaustion (Tlim in seconds) was defined as the CF. The EMGFT was defined as the y-intercept of the isometric torque versus EMG fatigue curve slope coefficient relationship.

RESULTS

There was a significant (P < 0.05) mean difference between the CF (25.9 +/- 12.1 N.m) and the EMGFT value for the RF (41.1 +/- 20.7 N.m) muscle. There were no significant differences, however, in EMGFT values among the three superficial muscles of the quadriceps femoris. In addition, the mean CF (17.6% maximum voluntary isometric contraction [MVIC]) occurred at a percentage of MVIC that is typically not affected by circulatory occlusion (20% MVIC), whereas the mean EMGFT values for the VL (25.9% MVIC), VM (22.9% MVIC), and RF (27.8% MVIC) exceeded this threshold.

Central motor control failure in fibromyalgia: a surface electromyography study

Background

Fibromyalgia (FM) is characterised by diffuse musculoskeletal pain and stiffness at multiple sites, tender points in characteristic locations, and the frequent presence of symptoms such as fatigue. The aim of this study was to assess whether the myoelectrical manifestations of fatigue in patients affected by FM are central or peripheral in origin.

Methods

Eight female patients aged 55.6 ± 13.6 years (FM group) and eight healthy female volunteers aged 50.3 ± 9.3 years (MCG) were studied by means of non-invasive surface electromyography (s-EMG) involving a linear array of 16 electrodes placed on the skin overlying the biceps brachii muscle, with muscle fatigue being evoked by means of voluntary and involuntary (electrically elicited) contractions. Maximal voluntary contractions (MVCs), motor unit action potential conduction velocity distributions (mean ± SD and skewness), and the mean power frequency of the spectrum (MNF) were estimated in order to assess whether there were any significant differences between the two groups and contraction types.

Results

The motor pattern of recruitment during voluntary contractions was altered in the FM patients, who also showed fewer myoelectrical manifestations of fatigue (normalised conduction velocity rate of changes: -0.074 ± 0.052%/s in FM vs -0.196 ± 0.133%/s in MCG; normalised MNF rate of changes: -0.29 ± 0.16%/s in FM vs -0.66 ± 0.34%/s in MCG). Mean conduction velocity distribution and skewnesses values were higher (p < 0.01) in the FM group. There were no between-group differences in the results obtained from the electrically elicited contractions.

Conclusion

The apparent paradox of fewer myoelectrical manifestations of fatigue in FM is the electrophysiological expression of muscle remodelling in terms of the prevalence of slow conducting fatigue-resistant type I fibres. As the only between-group differences concerned voluntary contractions, they are probably more related to central motor control failure than muscle membrane alterations, which suggests pathological muscle fibre remodelling related to altered suprasegmental control.

Can the electromyographic fatigue threshold be determined from superficial elbow flexor muscles during an isometric single-joint task?

The purpose of this study was to compare the electromyographic fatigue threshold (EMG(FT)) values determined simultaneously from superficial elbow flexor muscles during an isometric single-joint task. Eight subjects performed isometric elbow flexions at randomly ordered percentages of maximal voluntary contraction (20, 30, 40, 50 and 60%). During these bouts, electromyographic (EMG) activity was measured in the anterior head of Deltoïd, lateral head of Triceps brachii, Brachioradialis and both short and long head of Biceps brachii. For each subject and each muscle, the EMG amplitude data were plotted as function of time for the five submaximal bouts. The slope coefficient of the EMG amplitude versus time linear relationships were plotted against force level. EMG(FT) was determined as the y-intercept of this relationship and considered as valid only if the following criteria were met: (1) significant positive linear regression (P < 0.05) between force and slope coefficient, (2) an adjusted coefficient of determination for force versus slope coefficient relationship greater than 0.85, and (3) a standard error for the EMG(FT) below 5% of maximal voluntary contraction. The EMG(FT) could only be determined for one muscle (the long head of Biceps brachii) and only in three out of the eight subjects (mean value = 24.9 +/- 1.1% of maximal voluntary contraction). The lack of EMG(FT) in most of the subjects (5/8) could be explained by putative compensations between elbow muscles which were indirectly observed in some subjects. In this way, EMG(FT) should be studied from a more simple movement i.e., ideally a movement implying mainly one muscle.

Relationships between surface EMG variables and motor unit firing rates

Although surface electromyography (sEMG) is a widely used electrophysiological technique, its physiological interpretation remains somewhat controversial. This study examined the relationship between motor unit firing rates (MUFR) and the root mean square (RMS) amplitude and mean power frequency (MPF) of the sEMG signal in the biceps brachii. Eleven subjects performed maximal isometric elbow flexion while indwelling and sEMG recordings were obtained from the biceps. The RMS amplitude and MPF of the surface signal, and the mean MUFR from the indwelling signal, were calculated over 500 ms epochs. Group means showed a strong MUFR-RMS amplitude relationship (r (2) = 0.91), but a weak MUFR-MPF relationship (r (2) = 0.20). Using all trials, the MUFR-RMS amplitude (r (2) = 0.19) and MUFR-MPF (r (2) = 0.0037) relationships were much weaker. Within individual subjects, the MUFR-RMS amplitude (mean r (2) = 0.13 +/- 0.17) and the MUFR-MPF (mean r (2) = 0.040 +/- 0.041) relationships were also weak. These results suggest that MUFR cannot be predicted from the characteristics of the sEMG signal.

Removal of visual feedback alters muscle activity and reduces force variability during constant isometric contractions

The purpose of this study was to compare force accuracy, force variability and muscle activity during constant isometric contractions at different force levels with and without visual feedback and at different feedback gains. In experiment 1, subjects were instructed to accurately match the target force at 2, 15, 30, 50, and 70% of their maximal isometric force with abduction of the index finger and maintain their force even in the absence of visual feedback. Each trial lasted 22 s and visual feedback was removed from 8-12 to 16-20 s. Each subject performed 6 trials at each target force, half with visual gain of 51.2 pixels/N and the rest with a visual gain of 12.8 pixels/N. Force error was calculated as the root mean square error of the force trace from the target line. Force variability was quantified as the standard deviation and coefficient of variation (CVF) of the force trace. The EMG activity of the agonist (first dorsal interosseus; FDI) was measured with bipolar surface electrodes placed distal to the innervation zone. Independent of visual gain and force level, subjects exhibited lower force error with the visual feedback condition (2.53 +/- 2.95 vs. 2.71 +/- 2.97 N; P < 0.01); whereas, force variability was lower when visual feedback was removed (CVF: 4.06 +/- 3.11 vs. 4.47 +/- 3.14, P < 0.01). The EMG activity of the FDI muscle was higher during the visual feedback condition and this difference increased especially at higher force levels (70%: 370 +/- 149 vs. 350 +/- 143 microV, P < 0.01). Experiment 2 examined whether the findings of experiment 1 were driven by the higher force levels and proximity in the gain of visual feedback. Subjects performed constant isometric contractions with the abduction of the index finger at an absolute force of 2 N, with two distinct feedback gains of 15 and 3,000 pixels/N. In agreement with the findings of experiment 1, subjects exhibited lower force error in the presence of visual feedback especially when the feedback gain was high (0.057 +/- 0.03 vs. 0.095 +/- 0.05 N). However, force variability was not affected by the vastly distinct feedback gains at this force, which supported and extended the findings from experiment 1. Our findings demonstrate that although removal of visual feedback amplifies force error, it can reduce force variability during constant isometric contractions due to an altered activation of the primary agonist muscle most likely at moderate force levels in young adults.

The reliability of surface EMG recorded from the pelvic floor muscles

The neuromuscular function of the pelvic floor muscles (PFMs) is frequently evaluated using surface electrodes embedded on vaginal probes. The purpose of this study was to determine the between-trial and between-day reliability of EMG data recorded from the PFM using two different vaginal probes while subjects performed PFM maximum voluntary contractions and a coughing task. The Femiscan and the Periform vaginal probes were used to acquire EMG data while the subjects performed the tasks. Peak RMS amplitudes were computed for each instrument, task, and side of the pelvic floor using a sliding window technique. The between-trial reliability was evaluated using intraclass correlation coefficients (ICCs) and coefficients of variation (CV). Between-trial reliability was determined using ICCs, Pearson's correlation coefficients, computing the mean absolute difference between days, and calculating the standard error the measurement (SEM) for each instrument and task. EMG amplitude differences were detected between the left and right PFM (p<0.05), therefore all of the analyses were performed separately for each side. Overall, between-trial reliability was fair to high for the Femiscan (ICC((3,1))=0.58-0.98, CV=8.5-20.7%) and good to high for the Periform (ICC((3,1))=0.80-0.98, CV=9.6-19.5%), however between-day reliability was generally poor for both vaginal probes (ICC((3,1))=0.08-0.84). The results suggest that although it is acceptable to use PFM surface EMG as a biofeedback tool for training purposes, it is not recommended for use to make between-subject comparisons or to use as an outcome measure between-days when evaluating PFM function.

Running versus strength-based warm-up: acute effects on isometric knee extension function

This study investigated the influence of two warm-up protocols on neural and contractile parameters of knee extensors. A series of neuromuscular tests including voluntary and electrically evoked contractions were performed before and after running- (R(WU); slow running, athletic drills, and sprints) and strength-based (S(WU); bilateral 90 degrees back squats, Olympic lifting movements and reactivity exercises) warm ups (duration approximately 40 min) in ten-trained subjects. The estimated overall mechanical work was comparable between protocols. Maximal voluntary contraction torque (+15.6%; P < 0.01 and +10.9%; P < 0.05) and muscle activation (+10.9 and +12.9%; P < 0.05) increased to the same extent after R(WU) and S(WU), respectively. Both protocols caused a significant shortening of time to contract (-12.8 and -11.8% after R(WU) and S(WU); P < 0.05), while the other twitch parameters did not change significantly. Running- and strength-based warm ups induce similar increase in knee extensors force-generating capacity by improving the muscle activation. Both protocols have similar effects on M-wave and isometric twitch characteristics.

High-density surface EMG study on the time course of central nervous and peripheral neuromuscular changes during 8weeks of bed rest with or without resistive vibration exercise

The aim of the present study was to assess the time course and the origin of adaptations in neuromuscular function as a consequence of prolonged bed rest with or without countermeasure. Twenty healthy males volunteered to participate in the present study and were randomly assigned to either an inactive control group (Ctrl) or to a resistive vibration exercise (RVE) group. Prior to, and seven times during bed rest, we recorded high-density surface electromyogram (sEMG) signals from the vastus lateralis muscle during isometric knee extension exercise at a range of contraction intensities (5-100% of maximal voluntary isometric torque). The high-density sEMG signals were analyzed for amplitude (root mean square, RMS), frequency content (median frequency, F(med)) and muscle fiber conduction velocity (MFCV) in an attempt to describe bed rest-induced changes in neural activation properties at the levels of the motor control and muscle fibers. Without countermeasures, bed rest resulted in a significant progressive decline in maximal isometric knee extension strength, whereas RMS remained unaltered throughout the bed rest period. In line with observed muscle atrophy, both F(med) and MFCV declined during bed rest. RVE training during bed rest resulted in maintained maximal isometric knee extension strength, and a strong increase ( approximately 30%) in maximal EMG amplitude, from 10 days of bed rest on. Exclusion of other factors led to the conclusion that the RVE training increased motor unit firing rates as a consequence of an increased excitability of motor neurons. An increased firing rate might have been essential under training sessions, but it did not affect isometric voluntary torque capacity.

The effects of interelectrode distance over the innervation zone and normalization on the electromyographic amplitude and mean power frequency versus concentric, eccentric, and isometric torque relationships for the vastus lateralis muscle

The purpose of this study was to examine the influence of interelectrode distance (IED) over the estimated innervation zone (IZ) for the vastus lateralis muscle and normalization on the torque-related patterns of responses for electromyographic (EMG) amplitude and mean power frequency (MPF) during concentric isokinetic, eccentric isokinetic, and isometric muscle actions of the leg extensors. Eight men performed submaximal to maximal concentric isokinetic, eccentric isokinetic, and isometric muscle actions of the dominant leg extensors. Surface EMG signals were recorded simultaneously with two bipolar electrode arrangements in single differential configuration (20 and 40mm IEDs) placed over the estimated IZ for the vastus lateralis muscle and a third electrode arrangement in single differential configuration (20mm IED) placed distal to the estimated IZ. The results indicated that there were only a few (six of 90 statistical comparisons) significant (p<0.05) mean differences among the three electrode arrangements for absolute EMG amplitude. There were no mean differences among the three electrode arrangements for absolute or normalized EMG MPF values or normalized EMG amplitude for the three types of muscle actions. Thus, it may be possible to reduce the potential influence of the IZ on amplitude and spectral parameters of the EMG signal through normalization.

Adaptation of motor behavior to preserve task success in the presence of muscle fatigue

To achieve task goals in the various contexts of everyday life, the CNS has to adapt to short time scale changes in the properties of the neuromuscular system, such as those induced by fatigue. Here we investigated how humans preserve task success despite fatigue-induced changes within the neuromuscular system, when they have to aim at a target as fast and as accurately as possible. In such a task, subjects generally choose a compromise between speed and accuracy that has been formalized as Fitts's law. We first characterized the effect of fatigue on Fitts's law in an experiment where participants had to perform fast but accurate elbow movements aimed at targets of different sizes, before and after a fatiguing exercise that reduced maximal voluntary force by approximately 30%. We found that movements were slower to guarantee task success in the presence of fatigue. We then used an optimal control model to determine how fatigue-induced changes in variables such as noise in motor commands, muscle contraction and relaxation times, and the gain between neural activation and muscle force may contribute to changes in Fitts's law with fatigue. We concluded that the observed behavior was not due to the lack of available force, but very likely reflected the fact that the CNS uses the same optimal strategy with a fatigued neuromuscular plant that notably exhibits increased signal-dependent noise in motor commands. This strategy appears necessary to preserve task success in the presence of acute changes in the neuromuscular system.

Sex differences in time to task failure and blood flow for an intermittent isometric fatiguing contraction

The purpose of this study was to compare the time to task failure, postcontraction hyperemia, and vascular conductance of young men and women for a submaximal intermittent fatiguing contraction performed with the handgrip muscles. Twenty men and 20 women (mean +/- SD: 22 +/- 4 years) performed an isometric contraction at 50% of maximal voluntary contraction (MVC) (6-s contraction, 4-s rest) until task failure. Forearm venous occlusion plethysmography was used to estimate the peak blood flow (after 10-min occlusion) and blood flow at rest after 6-s submaximal contractions of varying intensities, and during an intermittent fatiguing contraction at 1-min intervals and task failure. The time to task failure was longer for the women compared with the men (408 +/- 205 s vs. 297 +/- 57 s, P < 0.05). Postcontraction hyperemia and vascular conductance were greater for men than for women after nonfatiguing 6-s submaximal contractions performed at 20%, 40%, 50%, 60%, and 80% of MVC force (P < 0.05). In contrast, hyperemia and vascular conductance were similar for both genders when measured at 50 s into the fatiguing contraction, at each minute thereafter, and at task failure. Regression analysis indicated that the rate of electromyographic activity and perceived exertion were the significant predictors of the time to task failure. The longer time to task failure for women compared with men for an intermittent fatiguing contraction with handgrip muscles was not explained by postcontraction hyperemia or vascular conductance with fatigue.

Neuromuscular efficiency of the rectus abdominis differs with gender and sport practice

The purpose of this investigation was to distinguish the abilities of the rectus abdominis (RA) muscle according to gender and sport training by means of neuromuscular parameters extracted from electromyography (EMG)-torque relationships. Thirty-eight healthy students, divided into 4 groups (i.e., 8 male runners, 10 female gymnasts, 12 male controls, and 8 female controls) were asked to perform 6 seconds of isometric trunk flexions at 20%, 25%, 75%, and 100% of their maximal voluntary contraction. Flexion torque and surface EMG of the RA muscle were recorded simultaneously to construct a EMG-torque relationship. Under maximal and submaximal conditions, an index of neuromuscular efficiency (NME) was determined to characterize the capacity of the RA muscle to develop a torque. At each level of contraction, the area of data scattering (ADS), reflecting torque and EMG fluctuations, was computed to express the capacity to maintain a constant target torque. Flexion torque, NME, and ADS values differed significantly between genders, but when data were related to anthropometric characteristics, no difference was observed. Although runners were not distinguished from male controls, gymnasts had higher flexion torque, higher NME, and lower ADS values than female controls had. These differences should reflect neural and muscular adaptations linked to the specificity of gymnastic training. These findings revealed different functional abilities of the RA muscle, according to gender and sport practices. The indices of neuromuscular capacities used in this study could constitute complementary tools to athletic trainers and professionals in sports medicine for evaluating and following, during sport-specific training programs, the abdominal muscle performance implied in force transfers with a lower cost and lower risks of back pain.

A method for better positioning bipolar electrodes for lower limb EMG recordings during dynamic contractions

To obtain a high quality EMG acquisition, the signal must be recorded as far away as possible from muscle innervations and tendon zones, which are known to shift during dynamic contractions. This study describes a methodology, using commercial bipolar electrodes, to identify better electrode positions for superficial EMG of lower limb muscles during dynamic contractions. Eight female volunteers participated in this study. Myoelectric signals of the vastus lateralis, gastrocnemius medialis, peroneus longus and tibialis anterior muscles were acquired during maximum isometric contractions using bipolar electrodes. The electrode positions of each muscle were selected assessing SENIAM and then, other positions were located along the length of muscle up and down the SENIAM site. The raw signal (density), the linear envelopes, the RMS value, the motor point site, the position of the IZ and its shift during dynamic contractions were taken into account to select and compare electrode positions. For vastus lateralis and peroneus longus, the best sites were 66% and 25% of muscle length, respectively (similar to SENIAM location). The position of the tibialis anterior electrodes presented the best signal at 47.5% of its length (different from SENIAM location). The position of the gastrocnemius medialis electrodes was at 38% of its length and SENIAM does not specify a precise location for signal acquisition. The proposed method should be considered as another methodological step in every EMG study to guarantee the quality of the signal and subsequent human movement interpretations.

Alteration of neuromuscular function in squash

The alteration in neuromuscular function of knee extensor muscles was characterised after a squash match in 10 trained players. Maximal voluntary contraction (MVC) and surface EMG activity of vastus lateralis (VL) and vastus medialis (VM) muscles were measured before and immediately after a 1-h squash match. M-wave and twitch contractile properties were analysed following single stimuli. MVC declined (280.5+/-46.8 vs. 233.6+/-35.4 Nm, -16%; P<0.001) after the exercise and this was accompanied by an impairment of central activation, as attested by decline in voluntary activation (76.7+/-10.4 vs. 71.3+/-9.6%, -7%; P<0.05) and raw EMG activity of the two vastii (-17%; P<0.05), whereas RMS/M decrease was lesser (VL: -5%; NS and VM: -12%; P=0.10). In the fatigued state, no significant changes in M-wave amplitude (VL: -9%; VM: -5%) or duration were observed. Following exercise, the single twitch was characterised by lower peak torque (-20%; P<0.001) as well as shorter half-relaxation time (-13%; P<0.001) and reduced maximal rate of twitch tension development (-23%; P<0.001) and relaxation (-17%; P<0.05). A 1-h squash match play caused peripheral fatigue by impairing excitation-contraction coupling, whereas sarcolemmal excitability seems well preserved. Our results also emphasise the role of central activation failure as a possible mechanism contributing to the torque loss observed in knee extensors. Physical conditioners should consider these effects when defining their training programs for squash players.

Interpretation of EMG integral or RMS and estimates of "neuromuscular efficiency" can be misleading in fatiguing contraction

In occupational and sports physiology, reduction of neuromuscular efficiency (NME) and elevation of amplitude characteristics, such as root mean square (RMS) or integral of surface electromyographic (EMG) signals detected during fatiguing submaximal contraction are often related to changes in neural drive. However, there is data showing changes in the EMG integral (I(EMG)) and RMS due to peripheral factors. Causes for these changes are not fully understood. On the basis of computer simulation, we demonstrate that lengthening of intracellular action potential (IAP) profile typical for fatiguing contraction could affect EMG amplitude characteristics stronger than alteration in neural drive (central factors) defined by number of active motor units (MUs) and their firing rates. Thus, relation of these EMG amplitude characteristics only to central mechanisms can be misleading. It was also found that to discriminate between changes in RMS or I(EMG) due to alterations in neural drive from changes due to alterations in peripheral factors it is better to normalize RMS of EMG signals to the RMS of M-wave. In massive muscles, such normalization is more appropriate than normalization to either peak-to-peak amplitude or area of M-wave proposed in literature.

Discharge Rate of Sternohyoid Motor Units Activated With Surface EMG Feedback

We analyzed individual motor units of the sternohyoid muscle with the aim of characterizing their minimum and maximum discharge rates and their variability in discharge during voluntary contractions. Surface EMG signals were recorded with an array of eight electrodes from the sternohyoid muscle of seven healthy men (age: 30.2 ± 3.5 yr). The multichannel surface EMG signals were displayed as feedback for the subjects who identified and modulated the activity of one target motor unit in 30-s contractions during which the discharge rate was increased from minimum to maximum (ramp contraction), sustained at maximum level (sustained), or increased in brief bursts (burst). During the ramp contractions, the minimum average discharge rate over epochs of 1 s was 11.6 ± 1.5 pulses per second (pps) and the maximum 57.0 ± 5.7 pps ( P < 0.001). During the sustained contractions, the motor unit discharge rate decreased from 65.5 ± 8.4 pps at the beginning to 52.9 ± 7.6 pps at the end of the contraction ( P < 0.05). The coefficient of variation for the interspike interval during the sustained contractions was 40.2 ± 9.8% and a large percentage of discharges had instantaneous rates >50 pps (52.2 ± 12.5%) and >100 pps (8.0 ± 1.2%), with peak values >150 pps. During the burst contractions, the instantaneous discharge rate reached average maximum values of 97.6 ± 36.8 pps. The observed discharge rates and their variability are higher than those reported for limb muscles, which may be due to large synaptic input and noise received by these motor neurons.

A comparison of critical force and electromyographic fatigue threshold for isometric muscle actions of the forearm flexors

The purpose of this study was twofold: (1) to determine if the mathematical model used for estimating the EMG(FT) during cycle ergometry was applicable to isometric muscle actions; and (2) to compare the mean torque level from the CF test to that of the EMG(FT) test. The CF was defined as the slope coefficient of the linear relationship between total "isometric work" (W (lim) in N m s) and time to exhaustion (T (lim)). The EMG(FT) was defined as the y-intercept of the isometric torque versus EMG fatigue curve slope coefficient relationship. There was a significant (p < 0.05) mean difference between CF (6.6 +/- 3.2 N m) and EMG(FT) (10.9 +/- 4.7 N m). The results of the present study suggested that, during isometric muscle actions of the forearm flexors, fatigue thresholds estimated from the W (lim) versus T (lim) relationship (CF) are different from those estimated from electromyographic fatigue curves (EMG(FT)).