Mechanically corrected EMG for the continuous estimation of erector spinae muscle loading during repetitive lifting

Few studies have been carried out on the changes in biomechanical loading on low-back tissues during prolonged lifting. The purpose of this paper was to develop a model for continuously estimating erector spinae muscle loads during repetitive lifting and lowering tasks. The model was based on spine kinematics and bilateral lumbar and thoracic erector spinae electromyogram (EMG) signals and was developed with the data from eight male subjects. Each subject performed a series of isometric contractions to develop extensor moments about the low back. Maximum voluntary contractions (MVCs) were used to normalize all recorded EMG and moment time-histories. Ramp contractions were used to determine the non-linear relationship between extensor moments and EMG amplitudes. In addition, the most appropriate low-pass filter cut-off frequencies were calculated for matching the rectified EMG signals with the moment patterns. The mean low-pass cut-off frequency was 2.7 (0.4) Hz. The accuracy of the non-linear EMG-based estimates of isometric extensor moment were tested with data from a series of six rapid contractions by each subject. The mean error over the duration of these contractions was 9.2 (2.6)% MVC. During prolonged lifting sessions of 20 min and of 2 h, a model was used to calculate changes in muscle length based on monitored spine kinematics. EMG signals were first processed according to the parameters determined from the isometric contractions and then further processed to account for the effects of instantaneous muscle length and velocity. Simple EMG estimates were found to underestimate peak loading by 9.1 (4.0) and 25.7 (11.6)% MVC for eccentric and concentric phases of lifting respectively, when compared to load estimates based on the mechanically corrected EMG. To date, the model has been used to analyze over 5300 lifts.     

Topography of the lumbar part of the erector spinae muscle

Summary The topography of different muscles in the erector spinae muscle at different lumbar levels was studied by serial cutting as well as by dissections in 21 subjects. The lumbar part of the erector spinae muscle is subdivided into a medial muscle compartment, the transversospinalis compartment, and a lateral muscle compartment, the sacrospinalis compartment. The former contains mainly the multifidi muscles, the latter chiefly being made up of the longissimus and the iliocostalis muscles. The different muscles and parts of muscles within the transversospinalis compartment are interwoven in such a way that it is difficult to separate them from each other. The longissimus and the iliocostalis muscles are posteriorly but usually not anteriorly separated from each other in the lumbar spine.It is assumed that it is possible to study the multifidi, the longissimus and the iliocostalis muscles at different lumbar levels with present electromyographic methods. Two techniques for insertion of EMG electrodes into these muscles at different lumbar levels are suggested. The techniques are based on the assumption that the insertion can be guided either by fluoroscopy or by palpation only.     

The influence of torque and velocity on erector spinae muscle fatigue and its relationship to changes of electromyogram spectrum density

The influence of contraction force and velocity during isokinetic contractions on the development of fatigue in the erector spinae muscle was studied. Seven male subjects performed a series of 250 contractions at 25% and 50% of their isometric maximal voluntary contraction (MVC) at 40 and 80°·s^–1. Fatigue defined as a decrease of the contractile capacity of the muscles was studied by means of a 15-s maximal test-contraction following the exercise. Both the initial force and the force decrement during the test-contraction were studied. Surface electromyogram (EMG) signals of the main tracts of the erector spinae muscle were recorded. The frequency content was studied by calculating the zero-crossing rate for the signals obtained during dynamic contractions and by means of fast Fourier transformation for the test contraction. After the 50% MVC dynamic contractions the initial force during the postexercise test-contraction was significantly lower than after the 25% MVC contractions. No significant influence of contraction velocity on fatigue development was found. The force decrement during the test-contraction did not depend on the experimental conditions. The EMG amplitude indicated that the subjects were better able to relax their muscles during the counter movement (flexion) at high forces and high velocities compared to the other experimental conditions. The frequency content of the EMG signals during the dynamic contractions and the postexercise test-contraction showed only very weak relationships with fatigue. Therefore, spectrum EMG parameters as determined in the present study do not seem suitable as indicators of muscle fatigue as a consequence of dynamic contractions of trunk extensor muscles.     

Electromyography and fatigue during prolonged,low-level static contractions

Summary 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 thesustained 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% MVCsustained 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 transcutanous electrical stimulation of the knee extensors showed that low frequency fatigue was present after the contraction. Withintermittent 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.     

Radiographic control in the insertion of EMG electrodes in the lumbar part of the erector spinae muscle

Summary A method for control of the location of EMG electrodes in the multifidi, the longissimus and the iliocostalis muscles in the lumbar spine is presented. The insertion of the electrodes is guided radiographically with TV-fluoroscopy as well as electromyographically. The chosen site of the electrodes is then confirmed radiographically after injection of carbon dioxide through the electrode. The technique is discussed and the radiographic criteria for the location of the electrodes in the individual muscles is described.     

Habitual level of physical activity and muscle fatigue of the elbow flexor muscles in older men

Reduced muscle performance, related to the loss of muscle mass and strength, is a common and natural part of ageing. Nevertheless, it is generally believed that regular participation in activities of moderate intensity may slow down these age-related changes. This study investigated the relationship between the habitual level of physical activity (PA), assessed by the modified Baecke Questionnaire, and the mechanical and fatigue characteristics of the right elbow flexor muscles, m. biceps brachii and m. brachioradialis, in men over the age of 55 years. Muscle fatigue was quantified both by measuring the maximal voluntary contraction (MVC) torque before and after a sustained isometric contraction at 25% MVC until exhaustion, and also by the temporal changes observed in the surface electromyographic (EMG) signal recorded during the fatigue task. Results showed a decreased MVC torque at the end of the fatiguing contraction. After 20 min of recovery, the MVC force was still significantly lower than the pre-fatigue value, except for the most active subjects. Typical myoelectrical indications of fatigue were also observed: a shift in the frequency spectrum of the signal towards lower frequencies accompanied by an increase in the EMG amplitude. We concluded from this study that the level of PA was related to the absolute isometric MVC values and the measurement of neuromuscular efficiency after 20 min of recovery, but did not influence the indications of muscle fatigue during an isometric fatigue task. Electronic Publication     

Trunk muscle activity during the simultaneous performance of two motor tasks

A unique feature of trunk muscles is that they can be activated to meet functional requirements for combined behaviors, including those related to posture and breathing. Trunk muscles therefore may have developed mechanisms for dealing with simultaneous inputs for different task requirements. This study was designed to test the hypothesis that a linear addition in trunk muscle activities would occur when an isometric trunk task and a pulsed expiration task was performed simultaneously. Surface electromyograms (EMG) were recorded from four trunk regions (medial and lateral back, upper and lower lateral abdomen) in sitting during the performance of the individual isometric trunk task, the individual pressure task, and the combined task (isometric trunk and pressure task). The direction of static holding for the isometric trunk task was varied between flexion and extension positions. For the pressure task subjects produced two consecutive pressure pulses (2/s) to a target oral pressure. For each muscle recording, a linear prediction was calculated from the mathematical addition of the EMG recorded from the individual trunk and pressure tasks. This linear prediction was compared to the actual muscle activity recorded during the combined task. Typically the EMG from two muscles showed linear addition, such that the relative contribution of muscle activity did not change for the combined task. This suggests that the motor commands for each task reached these motor neuron pools essentially unmodified. The other two muscles showed nonlinear combination of two EMG patterns. That is, qualitatively both EMG patterns, specific to each command, were evident in the measured EMG traces for the combined task, but quantitatively the muscle did not meet all criteria for linear addition. Linear addition may provide a simple mechanism for combining breathing-related behaviors (expiratory efforts) with other trunk behaviors (holding against gravity). This suggests that some muscles can be shared for two different voluntary tasks without changing their contribution to either component task. At the same time, nonlinear combination suggests that some muscles are shared, but their contribution to either component task may be modulated, thus avoiding the construction of a third new and unique plan.     

Relationship between work and electromyographic activity during repeated leg muscle contractions in orienteers

Summary Contraction work (CW) was recorded for each of 200 repetitive isokinetic plantar flexions (1.05 rad · s^–1) and knee extensions (1.57 rad · s^–1) in 14 elite male orienteers. Simultaneous recordings of integrated electromyograms (iEMG) were obtained from the 3 parts of triceps surae and from 3 superficial portions of quadriceps femoris. CW in both muscle groups decreased significantly during the first 30 contractions (the fatigue phase), followed by a steady state level. The relative steady state level was higher for the plantar flexors (70±17%) than for the knee extensors (56±12%). For quadriceps a significant increase in iEMG occurred during the first 10 contractions followed by a decrease, whereas the iEMG of the plantar flexors showed a gradual decrease to the steady state level, which was similar for the two muscle groups (71–72%). The chosen expression of output/input balance (CW/iEMG) was constant throughout the plantarflexion test but decreased during the initial 20 knee extensions down to 82%. Thus, the fatigue phase of the knee extensions appeared to be divided into two; the first part had decreases in both CW and CW/iEMG and the second part with a decrease in CW alone. In contrast the plantar flexors only showed the characteristics of the second part.     

The relationships among peak torque, mean power output, mechanomyography, and electromyography in men and women during maximal, eccentric isokinetic muscle actions

The purpose of the present investigation was to examine the velocity-related patterns of peak torque (PT), mean power output (MP), mechanomyographic (MMG) amplitude, and electromyographic (EMG) amplitude of the vastus lateralis (VL) muscle in men and women during maximal, eccentric isokinetic muscle actions of the leg extensors. Eight women [mean (SD)] [aged 23 (3) years] and seven men [aged 23 (2) years] volunteered for this investigation. Their PT was measured on a calibrated dynamometer at randomly ordered velocities of 30, 90, and 150°·s^–1. A piezoelectric MMG recording sensor was placed between bipolar surface EMG electrodes (Ag-AgCl) over the VL muscle. Their MP was determined using the dynamometer software. The results indicated no sex-related differences (P>0.05) for the patterns of PT, MP, MMG amplitude, or EMG amplitude across velocity. The normalized values for MP and MMG amplitude increased (P<0.05) from 30 to 150°·s^–1 (30°·s^–1<90°·s^–1<150°·s^–1), while PT and EMG amplitudes remained unchanged across velocity. The results indicated close associations between the velocity-related patterns of MP and MMG amplitudes as well as PT and EMG amplitudes. Thus, MMG amplitude reflected the velocity-related changes in muscle power output, but not torque production, while EMG amplitude reflected torque production, but not muscle power output. During maximal, eccentric isokinetic muscle actions, EMG and MMG signals provided unique information regarding the electrical and mechanical aspects of muscle strength and power. Electronic Publication     

The influence of muscle pain and fatigue on the activity of synergistic muscles of the leg

The purpose of this study was to investigate the effect of experimentally induced muscle pain on the motor-control strategies of synergistic muscles during submaximal fatiguing isometric contractions. The root mean square (RMS) and median frequency (MF) of the surface electromyographic (EMG) signal from synergistic plantarflexors and dorsiflexors were assessed to exhaustion. Ten subjects performed sustained dorsiflexions and plantarflexions at two contraction levels, 50% and 80% of maximum voluntary contraction, with or without muscle pain, induced by injection of 6% hypertonic saline in one synergist. In the painful contractions, the RMS of the EMG signal was decreased compared to the control condition in the initial phase of the contraction, in the muscles where pain was induced as well as in the nonpainful synergists. Moreover, the EMG signal MF decreased faster during muscle pain than in the control condition. The endurance time was shorter during muscle pain, and some of the nonpainful synergists showed increased compensatory activity at the end of the contractions to maintain the target force. The decreased EMG activation during pain was coupled with significantly decreased torque levels during the painful condition that would partly explain the results. However, the ratio between force and EMG amplitude was decreased for both the painful and nonpainful synergists, so other mechanisms might explain the present findings. This study shows that localized muscle pain can reorganize the EMG activity of synergists where no pain is present. These findings may have implications for the understanding of manifestations seen in relation to painful musculoskeletal disorders.     

Effect of prior isometric muscle action on concentric torque output during plantar flexion

The influence of different levels of prior isometric muscle action on the concentric torque output during plantar flexion was examined at two angular velocities (60°·s^–1 and 120°·s^–1) in ten healthy female subjects. The levels of the prior muscle actions were 25%, 50%, 75% and 100% of the maximal voluntary isometric contraction (MVIC). A KINetic-COMmunicator II dynamometer was used to measure torque output during plantar flexion within a range of motion of 78°-120° of the ankle joint. Simultaneous recordings of electromyograms (low-pass filtered and rectified) were obtained from the gastrocnemius medialis muscle and the soleus muscle. Torque-angle curves were made for the plantar flexions using different prior muscle actions. Up to 75% of MVIC, the torque output in the first part of the range of motion increased with the level of the prior isometric muscle action; at higher levels of MVIC the torque did not appear to increase any further. Later in the range of motion, after 24° in the plantar flexion at a velocity of 60°·s^–1 and 31° at 120°·s^–1, the prior muscle actions had no further influence. No increase was found in the electromyograms, with one exception, during the concentric movements when preceded by higher levels of MVIC. It would seem therefore that the increase in torque output early in the range of motion cannot be explained on the basis of differences in electrical muscle activation in this study.     

Muscle blood flow during isometric activity and its relation to muscle fatigue

Summary The effect of isometric exercise on blood flow, blood pressure, intramuscular pressure as well as lactate and potassium efflux from exercising muscle was examined. The contractions performed were continuous or intermittent (5 s on, 5 s off) and varied between 5% and 50% maximal voluntary contraction (MVC). A knee-extensor and a hand-grip protocol were used. Evidence is presented that blood flow through the muscle is sufficient during low-level sustained contractions (<10% MVC). Despite this muscle fatigue occurs during prolonged contractions. One mechanism for this fatigue may be the disturbance of the potassium homeostasis. Such changes may also play a role in the development of fatigue during intermittent isometric contractions and even more so in the recovery from such exercise. In addition the role of impaired transport of substances within the muscle, due to longlasting daily oedema formation, is discussed in relation to fatigue in highly repetitive, monotonous jobs.     

Electrically evoked contractions of the triceps surae during and following 21 days of voluntary leg immobilization

Summary The effects of 21 days voluntary leg (plaster) immobilization on the mechanical properties of the triceps surae have been studied in 11 young female subjects, mean age 19.4 years. The results show that during the period of immobilization the mean time to peak tension (TPT) and half relaxation time (1/2RT) and tension (P_t) of the maximal twitch increased significantly (p<0.001) but the effects were short lived. Maximal tension and contraction times of the twitch recovered within 2–14 days following the removal of the plaster cast. The electrically evoked tetanic tensions at 10 Hz and 20 Hz did not change significantly (p>0.1) during immobilization, but the 50 Hz tetanic tension (P_°50) and maximal voluntary contraction (MVC) were reduced (p<0.05). The fall in P_°50 and MVC was associated with 10% decrease in the estimated muscle (plus bone) cross-sectional area. The relative (%) change in P_°50 and MVC following immobilization was related to the initial physiological status (as indicated by the response of the triceps surae to a standard fatigue test prior to immobilization) of the muscle. The rate of rise and recovery fall of the tetanus were slightly but significantly (p<0.01) reduced on day 7 of immobilization, but thereafter remained constant. The isokinetic properties of the triceps surae as reflected in the measured torque/velocity relation of the muscle in 4 subjects did not change significantly if account was taken of the slight degree of atrophy present following immobilization. It was concluded that short term voluntary leg immobilization produces atrophy and some loss of isometric twitch and tetanic function, but has little effect on the isokinetic properties of the triceps surae. The changes in the twitch characteristics during and immediately following immobilization may be indicative of a prolongation of the active state of the muscle.     

Acoustic myography reflects force changes during dynamic concentric and eccentric contractions of the human biceps brachii muscle

Summary The relationship between acoustic myography (AMG), electromyography (EMG) and force during submaximal dynamic contractions was examined in the biceps brachii muscles of eight healthy males (aged 17–26 years). Different weights were lifted and lowered at a constant speed, using a wall pulley system, to perform concentric and eccentric contractions, respectively. Integrated AMG (iAMG) and integrated EMG (iEMG) activity both increased linearly with force during concentric (iAMGr=0.94; iEMGr=0.99) and eccentric (iAMGr=0.90; iEMGr=0.94) contractions. The slopes of the concentric regression lines were significantly different from the eccentric slopes (P<0.01) for both iAMG and iEMG with concentric contractions showing greater levels of activity. The results indicated that AMG can be used to detect changes in force during dynamic contractions which has important implications for the use of AMG in rehabilitation. The differences in iAMG activity between concentric and eccentric contractions are discussed in relationship to the origin of the AMG signal.     

An investigation into the relevance of the pattern of temporal activation with respect to erector spinae muscle endurance

Summary The aim of the present study was to evaluate the viability of a relationship between the temporal activation pattern of parts of the erector spinae muscle and endurance. Seven subjects performed intermittent isometric contractions [4 s at 7007o maximal voluntary contraction (MVC), 2 s rest] until exhaustion, during which the electromyographical (EMG) activity of the multifidus, iliocostalis thoracis and longissimus muscle segments was recorded. Endurance was defined as the time until exhaustion. Subjects were divided into a high and a low endurance group. The high endurance group showed significantly more variability of EMG amplitude over succeeding contractions. This group demonstrated significantly more alternations of EMG activity between parts of the muscle also. Variability of the EMG amplitude within the contractions did not differ between the groups, nor did MVC. The results indicated that alternating activity between different parts of the erector spinae muscle may function to postpone exhaustion of this muscle as a whole.     

Strategies that improve human skeletal muscle performance during repetitive, non-isometric contractions

Previous studies have suggested that during sustained maximal voluntary and evoked contractions, decreasing activation rates may minimize fatigue. The idea of gradually decreasing stimulation frequency to preserve force during fatiguing isometric contractions has, however, recently been challenged. The primary purpose of this study was to test the effectiveness of decreasing or increasing electrical stimulation rates during evoked, repetitive, submaximal, non-isometric contractions of healthy human quadriceps femoris muscles. The ability of the muscles to produce a 50° knee excursion repetitively was evaluated using low-frequency trains, high-frequency trains, and a combination of these trains. Results showed that stimulating the muscles with high-frequency trains followed by low-frequency trains produced the worst performance and starting with low-frequency trains followed by high-frequency trains produced the best performance. Present results cast doubt on the applicability of decreasing stimulation rates during non-isometric contractions and suggest that a combination of trains that begin at a low frequency and then switch to a higher-frequency may improve performance during functional electrical stimulation.     

Contractile properties of the human triceps surae muscle during simulated weightlessness

The effect of a 120-day period of bed rest on the mechanical properties of human triceps surae muscle was studied in a group of male volunteers (n = 6, mean age 38 years). The results shows that the contractile properties of skeletal muscle in response to disuse change considerably. Time to isometric peak tension of the triceps surae muscle increased from 120 (SEM 3.0) ms to 136 (SEM 2.9) ms (P < 0.01), half relaxation time from 92 (SEM 2.1) ms to 100 (SEM 1.6) ms (P < 0.05) and total contraction time from 440 (SEM 9.9) ms to 540 (SEM 18.7) ms (P < 0.001). Isometric twitch force (F _t) decreased by a mean of 36.7% (P < 0.05), maximal voluntary contraction (MVC) and maximal force (F _max) by a mean of 45.5% and 33.7%, respectively (P < 0.05-0.01). The valueF _max:F _t ratio increased by 3.6% (nonsignificant). The difference betweenF _max and MVC, expressed as a percentage ofF _max and referred to as force deficiency, has also been calculated. Force deficit increased by a mean of 60% (P < 0.001) after bed rest. Force-velocity properties of the triceps surae muscle calculated according to an absolute scale of voluntary and electrically evoked contraction development decreased considerably. The calculations of the same properties on a relative scale did not differ substantially from the initial physiological state. The results would suggest that muscle disuse is associated with both atrophy and a reduction in contractility in the development ofF _max and decreased central (motor) drive. The change in the triceps surae muscle contractile velocity properties may indicate changes in the kinetically active state in the muscles.     

Effect of antagonist muscle fatigue on knee extension torque

The effect of hamstring fatigue on knee extension torque was examined at different knee angles for seven male subjects. Before and after a dynamic flexion fatigue protocol (180° s^–1, until dynamic torque had declined by 50%), maximal voluntary contraction extension torque was measured at four knee flexion angles (90°, 70°, 50° and 30°). Maximal torque generating capacity and voluntary activation of the quadriceps muscle were determined using electrical stimulation. Average rectified EMG of the biceps femoris was determined. Mean dynamic flexion torque declined by 48±11%. Extensor maximal voluntary contraction torque, maximal torque generating capacity, voluntary activation and average rectified EMG at the four knee angles were unaffected by the hamstring fatigue protocol. Only at 50° knee angle was voluntary activation significantly lower (15.7%) after fatigue (P<0.05). In addition, average rectified EMG before fatigue was not significantly influenced by knee angle. It was concluded that a fatigued hamstring muscle did not increase the maximal voluntary contraction extension torque and knee angle did not change coactivation. Three possible mechanisms may explain the results: a potential difference in recruited fibre populations in antagonist activity compared with the fibres which were fatigued in the protocol, a smaller loss in isometric torque generating capacity of the hamstring muscle than was expected from the dynamic measurements and/or a reduction in voluntary activation.     

Microcirculation in the upper trapezius muscle during varying levels of static contraction,fatigue and recovery in healthy women —a study using percutaneous laser-Doppler flowmetry and surface electromyography

Summary Microcirculation in the upper portion of the trapezius muscle was measured percutaneously by continuous laser-Doppler flowmetry (LDF) during two 10-min series of alternating 1-min periods of static contraction and rest determined electromyographically (EMG). Stepwise increased contraction was induced by keeping the arms straight and elevated at 30, 60, 90 and 135°, which was repeated with a 1-kg load carried in each hand. Thereafter, fatigue and recovery were recorded while the subject kept her arms straight and elevated at 45° carrying the 1-kg hand load as long as possible, followed by rest with arms hanging and no load. A group of 16 healthy women of different ages was studied. Signal processing was done on line using a 386 SX computer. The LDF- and root-mean-square (rms) EMG signals were normalized. Spectrum analyses of EMG mean power frequency (MPF) and median spectrum frequency were performed. The rms-EMG increased significantly with an increase in the calculated shoulder torque (r=0.75). Accumulated local fatigue was indicated by a decrease in MPF with increased shoulder angle and added load (r = –0.54). Blood flow increased with increased shoulder angle (r=0.82, with hand loadr=0.62) and with increased shoulder torque (r=0.72), and also showed a significant increase with increased EMG activity (r=0.74). The LDF showed a negative correlation to MPF (r= –0.67), with increased values when MPF was lowered. During the endurance test, a moderate increase of LDF occurred which reached its maximum during the 1st min of recovery. Then, a slow return to the base level was recorded. The ability to increase the flow in the microcirculation with increasing muscle load was not diminished with age.