Electromyography and fatigue during prolonged, low-level static contractions

Findings from five separate studies of EMG changes and muscle fatigue during prolonged low-level static contractions are summarized, and the possible mechanisms behind the changes are briefly discussed. Sustained static contractions (10%, 7% and 5% MVC) of up to 1 h duration were performed by finger flexors, elbow flexors and extensors, and knee extensors. In one experiment, intermittent static arm pulling (triceps) (10 s contraction and 5 s rest, average work load 14% and 10% MVC) was performed for 7 h. The endurance time for the sustained contractions was around one hour for 10% MVC, and it was shown--all in all--that the concept of "indefinite" endurance times at contractions below 15-20% MVC cannot be maintained. After 5% MVC sustained contractions for one hour a 12% reduction in MVC was seen, and significant increases in EMG amplitude and decreases in the mean spectral frequency of the EMG-power spectrum were found. Marked differences were also seen in the EMG changes in the elbow flexors and extensors, and transcutaneous electrical stimulation of the knee extensors showed that low frequency fatigue was present after the contraction. With intermittent contractions similar changes in the EMG parameters were seen after 2-3 h of contractions at 14% MVC. On average, during contractions of 10% MVC no EMG changes were detected. Increased extracellular potassium concentration in the contracting muscles is suggested as a possible explanation of these findings.     

Human motor unit recruitment and derecruitment during long lasting intermittent contractions.

Seven healthy subjects were investigated in cyclic ramp-and-hold long lasting isometric contractions. Wire branched electrodes were used for selective recording of single motor unit (MU) potentials from m. biceps brachii. MU behaviour was defined in terms of recruitment/derecruitment thresholds (RT and DT) and the duration of interspike intervals (ISI). A total of 63 MUs was investigated: 40 units were active from the beginning of the task performance and another 23 were recruited later. There were no changes in the recruitment pattern of MUs with fatigue development - a short first ISI followed by a very long second one and an almost constant firing rate after this transient phase. The tendency of RT to gradually decrease dominates the results. Thus, the required constant rate of force increase with fatigue development was maintained mostly by the mechanisms of space coding (i.e., decrease of RT and recruitment of additional MUs). Oppositely, the time behaviour of the DT changes was not uniform and rate coding was an essential mechanism in the adaptation of MU activity to muscle fatigue during relaxation phases. The recruitment pattern and fatigue related behaviour of the additionally recruited MUs were similar to those of MUs active from the first cycle of the motor task performance.     

The assessment of back muscle capacity using intermittent static contractions. Part II: Validity and reliability of biomechanical correlates of muscle fatigue

IntroductionIn a previous paper, standard surface electromyographic (EMG) indices of muscle fatigue, which are based on the lowering of the median or mean frequencies of the EMG power spectrum in time, were applied during an intermittent absolute endurance test and were evaluated relative to criterion validity and test–retest reliability. The aims of this study were to assess mechanical and alternative EMG correlates of muscle fatigue.MethodsHealthy subjects (44 males and 29 females; age: 20–55 yrs) performed three maximal voluntary contractions (MVC) and an endurance test while standing in a static dynamometer. Surface EMG signals were collected from four pairs of back muscles (multifidus at the L5 level, iliocostalis lumborum at L3, and longissimus at L1 and T10). The test, assessing absolute endurance (90 N m torque), consisted of performing an intermittent extension task to exhaustion. Strength was defined as the peak MVC whereas our endurance criterion was defined as the time to reach exhaustion (Tend) during the endurance test. Mechanical indices quantifying physiological tremor and steadiness were computed from the dynamometer signals (L5/S1 extension moments) along with EMG indices presumably sensitive to variable load sharing between back muscle synergists during the endurance test.ResultsMechanical indices were significantly correlated to Tend (r range: −0.47 to –0.53) but showed deceiving reliability results. Conversely, the EMG indices were correlated to Tend (r range: −0.43 to –0.63) with some of them particularly correlated to Strength (r = − 0.72 to –0.81). In addition, their reliability results were acceptable (intra-class correlation coefficient >0.75; standard error of measurement <10% of the mean) in many cases. Finally, several analyses substantiated their physiological relevance. These findings imply that these new EMG indices could be used to predict absolute endurance as well as strength with the use of a single intermittent and time-limited (5–10 min) absolute endurance test, a practical way to assess the back capacity of chronic low back pain subjects.     

Supraspinal fatigue during intermittent maximal voluntary contractions of the human elbow flexors.

Responses to transcranial magnetic stimulation in human subjects (n = 9) were studied during series of intermittent isometric maximal voluntary contractions (MVCs) of the elbow. Stimuli were given during MVCs in four fatigue protocols with different duty cycles. As maximal voluntary torque fell during each protocol, the torque increment evoked by cortical stimulation increased from approximately 1.5 to 7% of ongoing torque. Thus "supraspinal" fatigue developed in each protocol. The motor evoked potential (MEP) and silent period in the elbow flexor muscles also changed. The silent period lengthened by 20-75 ms (lowest to highest duty cycle protocol) and recovered significantly with a 5-s rest. The MEP increased in area by >50% in all protocols and recovered significantly with 10 s, but not 5 s, of rest. These changes are similar to those during sustained MVC. The central fatigue demonstrated by the torque increments evoked by the stimuli did not parallel the changes in the electromyogram responses. This suggests that part of the fatigue developed during intermittent exercise is "upstream" of the motor cortex.     

The effect of prolonged isometric contractions on muscle fluid balance

Ultrasound scanning was performed at three sites above the fossa supraspinata on nine healthy subjects and five patients with myofascial shoulder pain. This method produced a well-defined depiction of the soft tissue layers above the fossa supraspinata and reproducible muscle thickness measurements. In the healthy subjects the average distance from the skin surface to the trapezius muscle was 7.7 mm and the average thickness of the trapezius muscle was 5.3 mm, and the average thickness of supraspinatus muscle was 20.0 mm. The supraspinatus muscle was thinner at the medial measuring site than at the other two sites. In contrast, a tendency towards a larger distance was seen from the skin to trapezius muscle at the medial measuring site than at the other two sites. No statistical differences were found between the two groups of subjects either at rest or during brief shoulder abductions. All the subjects performed a 30° unilateral isometric shoulder abduction test to exhaustion. The median endurance time was 33 min for the healthy subjects and only 5 min for the patients. The ratings of perceived exertion (RPE) were in line with this, since the increment in RPE with time was larger for the patients than for the healthy group. The reduced shoulder abduction endurance time in the patient group may have been related to impaired muscle function and/or pain development. During the 33-min shoulder abduction in the healthy subjects, the thickness of supraspinatus muscle increased by 14%, indicating muscle swelling, whereas the thickness of trapezius muscle remained constant. The fluid imbalance in the supraspinatus muscle compartment may well play a role in the development of muscle fatigue and the disorders found in industry resulting from prolonged work with arms elevated.     

ATP utilization and force during intermittent and continuous muscle contractions

Energy utilization and force generation under anaerobic conditions were studied in electrically stimulated quadriceps femoris muscle of four volunteers. To investigate the effects of intermittent vs. continuous stimulation one leg was stimulated intermittently and the other continuously during 50 s. The same initial force was produced, and biopsy samples were obtained before the stimulation and after 10, 20, and 50 s and analyzed for energy-rich phosphagens, glycolytic intermediates, and phosphorylase. The ATP utilization and glycolysis were greater during intermittent contraction, but glycogenolysis was equal. ATP content decreased to lower values after intermittent contraction (16.4 compared with 19.6 mmol/kg dry muscle after continuous contraction). Force generation was well preserved during continuous contraction but successively decreased after 20 s of intermittent stimulation down to 50% of initial at end of work. The energy cost per unit work was greater during intermittent stimulation and increased with contraction time, whereas it decreased with time during continuous stimulation. The decrease in force generation in intermittent exercise is suggested to be due to the higher energy cost for contraction resulting in greater changes in the intracellular environment with lower ATP and increased H+ and Pi. These changes would decrease both activation of the contractile system and the cross-bridge turnover rate resulting from activation.     

Time-frequency methods applied to muscle fatigue assessment during dynamic contractions.

This paper discusses the assessment of the electrical manifestations of muscle fatigue during dynamic contractions. In the past, the study of muscle fatigue was restricted to isometric constant force contractions because, in this contraction paradigm, the myoelectric signal may be considered as wide sense stationary over epochs lasting up to two or three seconds, and hence classic spectral estimation techniques may be applied. Recently, the availability of spectral estimation techniques specifically designed for nonstationary signal analysis made it possible to extend the employment of muscle fatigue assessment to cyclic dynamic contractions, thus increasing noticeably its possible clinical applications. After presenting the basics of time-frequency distributions, we introduce instantaneous spectral parameters well suited to tracking spectral changes due to muscle fatigue, discuss the issues of quasi-stationarity and quasi-cyclostationarity, and present different strategies of signal analysis to be utilized with cyclic dynamic contractions. We present preliminary results obtained by analyzing data collected from paraspinal muscles during repetitive lift movements, from the first dorsal interosseus during abduction-adduction movements of the index finger, and from knee flexors and extensors during isokinetic exercise. In conclusion, data herein reported demonstrate that the described techniques allow for evidencing the electrical manifestations of muscle fatigue in different paradigms of cyclic dynamic contractions. We believe that the extension of the objective assessment of the electrical manifestations of muscle fatigue from static to dynamic contractions may increase considerably the interest of researchers and clinicians and open new application fields, as ergonomics and sports medicine.     

Supraspinal fatigue does not explain the sex difference in muscle fatigue of maximal contractions.

Young women are less fatigable than young men for maximal and submaximal contractions, but the contribution of supraspinal fatigue to the sex difference is not known. This study used cortical stimulation to compare the magnitude of supraspinal fatigue during sustained isometric maximal voluntary contractions (MVCs) performed with the elbow flexor muscles of young men and women. Eight women (25.6 +/- 3.6 yr, mean +/- SD) and 9 men (25.4 +/- 3.8 yr) performed six sustained MVCs (22-s duration each, separated by 10 s). Before the fatiguing contractions, the men were stronger than the women (75.9 +/- 9.2 vs. 42.7 +/- 8.0 N.m; P < 0.05) in control MVCs. Voluntary activation measured with cortical stimulation before fatigue was similar for the men and women during the final control MVC (95.7 +/- 3.0 vs. 93.3 +/- 3.6%; P > 0.05) and at the start of the fatiguing task (P > 0.05). By the end of the six sustained fatiguing MVCs, the men exhibited greater absolute and relative reductions in torque (65 +/- 3% of initial MVC) than the women (52 +/- 9%; P < 0.05). The increments in torque (superimposed twitch) generated by motor cortex stimulation during each 22-s maximal effort increased with fatigue (P < 0.05). Superimposed twitches were similar for men and women throughout the fatiguing task (5.5 +/- 4.1 vs. 7.3 +/- 4.7%; P > 0.05), as well as in the last sustained contraction (7.8 +/- 5.9 vs. 10.5 +/- 5.5%) and in brief recovery MVCs. Voluntary activation determined using an estimated control twitch was similar for the men and women at the start of the sustained maximal contractions (91.4 +/- 7.4 vs. 90.4 +/- 6.8%, n = 13) and end of the sixth contraction (77.2 +/- 13.3% vs. 73.1 +/- 19.6%, n = 10). The increase in the area of the motor-evoked potential and duration of the silent period did not differ for men and women during the fatiguing task. However, estimated resting twitch amplitude and the peak rates of muscle relaxation showed greater relative reductions at the end of the fatiguing task for the men than the women. These results indicate that the sex difference in fatigue of the elbow flexor muscles is not explained by a difference in supraspinal fatigue in men and women but is largely due to a sex difference of mechanisms located within the elbow flexor muscles.     

Rate of fatigue during repeated submaximal contractions of human quadriceps muscle.

Our purpose was to determine the effect of eight different combinations of contraction intensity, duration, and rest on the rate of fatigue in vastus lateralis muscle. A single combination consisted of contractions at 30 or 70% maximal voluntary contraction (MVC), held for 3 or 7 s with 3- or 7-s rest intervals. Contractions were repeated until the subject could not hold the force for the requisite duration. At regular intervals during each experiment, a brief MVC, a single twitch, and the response to eight stimulation pulses at 50 Hz were elicited. The rate of fatigue was the rate of decline of MVC calculated from regression analysis. Mean rate of fatigue (n = 8) ranged from 0.3 to 25% MVC/min and was closely related (r = 0.98) to the product of the relative force and the duty cycle. Force from 50 Hz stimulation fell linearly and in parallel with MVC. Twitch force was first potentiated and then fell twice as fast as 50 Hz stimulation and MVC (p less than 0.05). Differentiated twitch contraction and relaxation rates were higher at potentiation and lower at the limit of endurance, compared with control values (p less than 0.05). The maximal electromyogram decreased 25% and the submaximal EMG increased to maximal by the end of the protocol, indicating that the entire motor unit pool had been recruited. The close relation between rate of fatigue and the force x time product probably reflects the off-setting interaction of contraction amplitude, duration, and rest interval. This occurs despite the changes in twitch characteristics and the apparent recruitment of fast fatiguing motor units.     

Selective fatigue of fast motor units after electrically elicited muscle contractions.

The aim of the present study was to elucidate the electrophysiological manifestations of selective fast motor unit (MU) activation by electrical stimulation (ES) of knee extensor muscles. In six male subjects, test contraction measurement at 40% maximal voluntary contraction (MVC) was performed before and at every 5 min (5, 10, 15 and 20 min) during 20-min low intensity intermittent exercise of either ES or voluntary contractions (VC) at 10% MVC (5-s isometric contraction and 5-s rest cycles). Both isolated intramuscular MU spikes obtained from three sets of bipolar fine-wire electrodes and surface electromyogram (EMG) were simultaneously recorded and were analyzed by means of a computer-aided intramuscular spike amplitude-frequency analysis and frequency power spectral analysis, respectively. Results indicated that mean MU spike amplitude, particularly those MUs with relatively large amplitude, was significantly reduced while those MUs with small spike amplitude increased their firing rate during the 40% MVC test contraction after the ES. This was accompanied by the increased amplitude of surface EMG (rmsEMG). However, no such significant changes in the intramuscular and surface EMGs were observed after VC. These findings indicated differential MU activation patterns in terms of MU recruitment and rate coding characteristics during ES and VC, respectively. Our data strongly suggest the possibility of "an inverse size principle" of MU recruitment during ES.     

EMG spectral indices and muscle power fatigue during dynamic contractions

The purpose of this study was to examine acute exercise-induced changes on muscle power output and surface electromyography (sEMG) parameters (amplitude and spectral indices of muscle fatigue) during a dynamic fatiguing protocol. Fifteen trained subjects performed five sets consisting of 10 leg presses (10RM), with 2 min rest between sets. Surface electromyography was recorded from vastus medialis (VM) and lateralis (VL) and biceps femoris (BF) muscles. A number of EMG-based parameters were compared for estimation accuracy and sensitivity to detect peripheral muscle fatigue. These were: Mean Average Voltage, median spectral frequency, Dimitrov spectral index of muscle fatigue (FI_nsm5), as well as other parameters obtained from a time–frequency analysis (Choi–Williams distributions) such as mean and variance of the instantaneous frequency and frequency variance. The log FI_nsm5 as a single parameter predictor accounted for 37% of the performance variance of changes in muscle power and the log FI_nsm5 and MFM as a two factor combination predictor accounted for 44%. Peripheral impairments assessed by sEMG spectral index FI_nsm5 may be a relevant factor involved in the loss of power output after dynamic high-loading fatiguing task.     

Impulse propagation and muscle activation in long maximal voluntary contractions

With fatigue, force generation may be limited by several factors, including impaired impulse transmission and/or reduced motor drive. In 5-min isometric maximal voluntary contraction, no decline was seen in the peak amplitude of the tibialis anterior compound muscle mass action potential (M wave) either during or immediately after the voluntary effort, provided maximal nerve stimulation was retained. For first dorsal interosseous (FDI) muscle, M wave amplitudes declined by 19.4 +/- 1.6% during the first 2 min but did not change significantly thereafter, despite the continued force reduction (up to 94% in 5 min for both muscles). The duration of the FDI M waves increased (greater than 30%), suggesting that the small decline in amplitude was the result of increased dispersion between the responses of different motor units. Some subjects kept FDI maximally activated throughout, but when they used tibialis anterior, twitch occlusion and tetanic muscle stimulation showed that most subjects were usually only able to do so for the first 60 s and thereafter only during brief "extra efforts." Thus force loss during isometric voluntary contractions sustained at the highest intensities results mainly from failure of processes within the muscle fibers.     

A comparison of EMG-based muscle fatigue assessments during dynamic contractions

Several EMG-based approaches to muscle fatigue assessment have recently been proposed in the literature. In this work, two multivariate fatigue indices developed by the authors: a generalized mapping index (GMI) and the first component of principal component analysis (PCA) were compared to three univariate indices: Dimitrov’s normalized spectral moments (NSM), Gonzalez-Izal’s waveletbased indices (WI), and Talebinejad’s fractal-based Hurst Exponent (HE). Nine healthy participants completed two repetitions of fatigue tests during isometric, cyclic and random fatiguing contractions of the biceps brachii. The fatigue assessments were evaluated in terms of a modified sensitivity to variability ratio yielding the following scores (mean ± std.dev.): PCA: (12.6 ± 5.6), GMI: (11.5 ± 5.4), NSM: (10.3 ± 5.4), WI: (8.9 ± 4.6), HE: (8.0 ± 3.3). It was shown that PCA statistically outperformed WI and HE (p < 0.01) and that GMI outperformed HE (p < 0.02). There was no statistical difference among NSM, WI and HE (p > 0.2). It was found that taking the natural logarithm of NSM and WI, although reducing the parameters’ sensitivity to fatigue, increased SVR scores by reducing variability.     

The short-time Fourier transform and muscle fatigue assessment in dynamic contractions

The mean frequency of the power spectrum of an electromyographic signal is an accepted index for monitoring fatigue in static contractions. There is however, indication that it may be a useful index even in dynamic contractions in which muscle length and/or force may vary. The objective of this investigation was to explore this possibility. An examination of the effects of amplitude modulation on modeled electromyographic signals revealed that changes in variance created in this way do not sufficiently affect characteristic frequency data to obscure a trend with fatigue. This validated the contention that not all non-stationarities in signals necessarily manifest in power spectral parameters. While an investigation of the nature and effects of non-stationarities in real electromyographic signals produced from dynamic contractions indicated that a more complex model is warranted, the results also indicated that averaging associated with estimating spectral parameters with the short-time Fourier transform can control the effects of the more complex non-stationarities. Finally, a fatigue test involving dynamic contractions at a force level under 30% of peak voluntary dynamic range, validated that it was possible to track fatigue in dynamic contractions using a traditional short-time Fourier transform methodology.     

Activation of human quadriceps femoris muscle during dynamic contractions: effects of load on fatigue.

Muscle fatigue is both multifactorial and task dependent. Electrical stimulation may assist individuals with paralysis to perform functional activities [functional electrical stimulation (FES), e.g., standing or walking], but muscle fatigue is a limiting factor. One method of optimizing force is to use stimulation patterns that exploit the catchlike property of skeletal muscle [catchlike-inducing trains (CITs)]. Although nonisometric (dynamic) contractions are important parts of both normal physiological activation of skeletal muscles and FES, no previous studies have attempted to identify the effect that the load being lifted by a muscle has on the fatigue produced. This study examined the effects of load on fatigue during dynamic contractions and the augmentation produced by CITs as a function of load. Knee extension in healthy subjects was electrically elicited against three different loads. The highest load produced the least excursion, work, and average power, but it produced the greatest fatigue. CIT augmentation was greatest at the highest load and increased with fatigue. Because CITs were effective during shortening contractions for a variety of loads, they may be of benefit during FES applications.     

Reversal of muscle fatigue during 16 h of heavy intermittent cycle exercise.

This study examined the effects of extended sessions of heavy intermittent exercise on quadriceps muscle fatigue and weakness. Twelve untrained volunteers (10 men and 2 women), with a peak oxygen consumption of 44.3 +/- 2.3 ml.kg(-1).min(-1), exercised at approximately 91% peak oxygen consumption for 6 min once per hour for 16 h. Muscle isometric properties assessed before and after selected repetitions (R1, R2, R4, R7, R12, and R15) were used to quantitate fatigue (before vs. after repetitions) and weakness (before vs. before repetitions). Muscle fatigue at R1 was indicated by reductions (P < 0.05) in peak twitch force (135 +/- 13 vs. 106 +/- 11 N) and by a reduction (P < 0.05) in the force-frequency response, which ranged between approximately 53% at 10 Hz (113 +/- 12 vs. 52.6 +/- 7.4 N) and approximately 17% at 50 Hz (324 +/- 27 vs. 270 +/- 30 N). No recovery of force, regardless of stimulation frequency, was observed during the 54 min between R1 and R2. At R2 and for all subsequent repetitions, no reduction in force, regardless of stimulation frequency, was generally found after the exercise. The only exception was for R2, where, at 20 Hz, force was reduced (P < 0.05) by 18%. At R15, force before repetitions for high frequencies (i.e., 100 Hz) returned to R1 (333 +/- 29 vs. 324 +/- 27 N), whereas force at low frequency (i.e., 10 Hz) was only partially (P < 0.05) recovered (113 +/- 12 vs. 70 +/- 6.6 N). It is concluded that multiple sessions of heavy exercise can reverse the fatigue noted early and reduce or eliminate weakness depending on the frequency of stimulation.     

Development of muscle fatigue during intermittent submaximal static contraction in an agonist heterogeneous muscle group.

A comparison of the mean power frequency (MPF) and the root mean square amplitude (rms) of the myo-electric signal of two agonist muscles [triceps brachii (fast; TB) and anconeus (slow; ANC)] has been made during repeated intermittent static contractions. Subjects were asked to maintain different extension torques at 50% of maximal voluntary contraction until this could no longer be maintained (endurance time). The interval between successive contractions was kept constant at 3 min. During the first six successive contractions, a decrease in MPF and an increase in rms were most pronounced, ANC and TB MPF recovered with subsequent overshoot. A marked decline in endurance time was also seen. The increase in rms was greater for TB than for ANC when the decrease in MPF was greater for ANC than for TB. The differences in power spectrum density function upper frequencies of the two muscles could explain the greater decrease of MPF in ANC. Our data would suggest a greater fatigability in TB relative to ANC. On and after the seventh contraction, a steady-state in duration, muscle temperature, MPF and rms was reached. These results suggested that a slow (ANC) and a fast (TB) muscle acted in a similar way during intermittent static contractions, when the intervening rest was not long enough to allow full recovery of the muscles.     

Mechanical behavior of skeletal muscle during intermittent voluntary isometric contractions in humans

Vøllestad, N. K., I. Sejersted, and E. Saugen. Mechanical behavior of skeletal muscle duringintermittent voluntary isometric contractions in humans.J. Appl. Physiol. 83(5):1557-1565, 1997.Changes in contractile speed and force-fusionproperties were examined during repetitive isometric contractions withthe knee extensors at three different target force levels. Sevenhealthy subjects were studied at target force levels of 30, 45, and60% of their maximal voluntary contraction (MVC) force. Repeated 6-s contractions followed by 4-s rest were continued until exhaustion. Contractile speed was determined for contractions elicited by electrical stimulation at 1-50 Hz given during exercise and a subsequent 27-min recovery period. Contraction time remained unchanged during exercise and recovery, except for an initial rapid shift in thetwitch properties. Half relaxation time(RT_1/2) decreased gradually by 20-40% during exercise at 30 and 45% of MVC. In the recovery period, RT_1/2 values werenot fully restored to preexercise levels. During exercise at 60% MVC,the RT_1/2 decreased for twitches and increased for the 50-Hz stimulation. In the recovery period after60% MVC, RT_1/2 values declinedtoward those seen after the 30 and 45% MVC exercise. The forceoscillation amplitude in unfused tetani relative to the mean forceincreased during exercise at 30 and 45% MVC but remained unalteredduring the 60% MVC exercise. This altered force-fusion was closelyassociated with the changes inRT_1/2. The faster relaxation mayat least partly explain the increased energy cost of contractionreported previously for the same type of exercise.