Time-frequency analysis of surface electromyographic signals during fatiguing isokinetic muscle actions

The purpose of this study was to use a wavelet analysis designed specifically for electromyography (EMG) signals in combination with a trend plot to examine changes in EMG intensity patterns during maximal, fatiguing isokinetic muscle actions. Eleven men (mean ± SD age = 22.4 ± 1.1 years) and 7 women (mean ± SD age = 22.7 ± 2.1 years) performed 50 consecutive maximal concentric isokinetic muscle actions of the dominant leg extensors at a velocity of 180°·s(-1). During each muscle action, a bipolar surface EMG signal was detected from the vastus lateralis. All signals were then processed with a wavelet analysis designed specifically for EMG signals, which resulted in EMG intensity patterns. The patterns for each subject were then analyzed with a trend plot, which provided information regarding the changes that occurred because of fatigue. The results indicated that for all the 18 subjects, the EMG intensity patterns moved in a predictable manner in pattern space, but the changes to the patterns were different for each subject. These findings reflect the complex changes that occur in the EMG signal during fatigue. These changes cannot be characterized fully with a single amplitude and center frequency parameter and can be useful for athletes and coaches who need to track the fatigue status of individual muscles.     

Muscular fatigue during repeated isokinetic shoulder forward flexions in young females

Peak torque, work, mean power and electromyographic (EMG) activity were recorded for each of 150 repeated isokinetic maximal shoulder flexions (45 degrees-90 degrees) in 23 healthy females. From the EMG signals of trapezius, deltoid, infraspinatus and biceps brachii the mean power frequency and the signal amplitude were determined in real time. The mechanical output showed a steep decrease during the first 40 contractions, followed by a plateau maintained until the end. In all muscles, except the biceps brachii, significant decreases in mean power frequency occurred during the first 40 contractions, showing a tendency to stabilize around the same absolute frequency value. Signal amplitude increased in the trapezius, the deltoid and the infraspinatus, but was constant in the biceps brachii. For some individuals rather high EMG activity was recorded in the muscles during the time the arm was supposed to be passively extended to the starting position, and this was found to be associated with lower strength and endurance levels. Longitudinal analyses showed that the mean power frequencies correlated better than the signal amplitudes with the three mechanical variables. The results suggest that the initial steep decrease in mechanical performance and mean power frequency is caused by fatiguing of type 2 motor units.     

Muscle activity and fatigue in the shoulder muscles during repetitive work. An electromyographic study

The amplitude distribution probability function (ADPF) and the power spectrum of the surface electromyogram from m. deltoideus anterior, m. infraspinatus and m. trapezius pars descendens were analyzed from 7 persons working at a pillar drill. Recordings were performed 6 times during a working day. The ADPF was analyzed from 2-3 work-cycles from each recording. The static contraction level was 11.0% MCV in m. deltoideus anterior, 8.5% MVC in m. infraspinatus, and 20.5% MCV in m. trapezius, without any change occuring throughout the day. When compared to previous suggested upper limits of ADPF levels, both the static and the medium contraction levels were too high in the performance of this particular task. The power spectrum of the EMG was analyzed during isometric contractions of the shoulder muscles. The mean power frequency decreased during the day in m. trapezius only, suggesting muscular fatigue in this area.     

Comparison of Fourier and wavelet transform procedures for examining the mechanomyographic and electromyographic frequency domain responses during fatiguing isokinetic muscle actions of the biceps brachii.

The primary purpose of the present study was to compare the fast Fourier transform (FFT) with the discrete wavelet transform (DWT) for determining the mechanomyographic (MMG) and electromyographic (EMG) center frequency [mean power frequency (mpf), median frequency (mdf), or wavelet center frequency (cf)] patterns during fatiguing isokinetic muscle actions of the biceps brachii. Seven men (mean+/-SD age=23+/-3 years) volunteered to perform 50 consecutive maximal, concentric isokinetic muscle actions of the dominant forearm flexors at a velocity of 180 degrees s(-1). Non-parametric "run" tests indicated significant (p<0.05) trends in the MMG and EMG signals for the 5th, 25th, and 45th muscle actions for all subjects, thereby confirming non-stationarity of the MMG and EMG signals. There were significant (p<0.05) correlations among the average normalized mpf, mdf, and cf values for contractions 1-50 for both MMG (r=0.671-0.935) and EMG (r=0.956-0.987). Polynomial regression analyses demonstrated quadratic decreases in normalized MMG mpf (R2=0.439), MMG mdf (R2=0.258), MMG cf (R2=0.359), EMG mpf (R2=0.952), EMG mdf (R2=0.914) and EMG cf (R2=0.888) across repetitions. The primary finding of this study was the similarity in the mpf, mdf, and cf patterns for both MMG and EMG, which suggested that, despite the concerns over non-stationarity, Fourier based methods are acceptable for determining the patterns for normalized MMG and EMG center frequency during fatiguing dynamic muscle actions at moderate velocities.     

Spinal reflexes and coactivation of ankle muscles during a submaximal fatiguing contraction.

This study examined the involvement of spinal mechanisms in the control of coactivation during a sustained contraction of the ankle dorsiflexors at 50% of maximal voluntary contraction. Changes in the surface electromyogram (EMG) of the tibialis anterior and of two antagonist muscles, the soleus and lateral gastrocnemius, were investigated during and after the fatigue task. Concurrently, the compound action potential (M-wave) and the Hoffmann reflex of the soleus and lateral gastrocnemius were recorded. The results showed that the torque of the ankle dorsiflexors and the average EMG of the tibialis anterior during maximal voluntary contraction declined by 40.9 +/- 17.7% (mean +/- SD; P < 0.01) and 37.0 +/- 19.9% (P < 0.01), respectively, at task failure. During the submaximal fatiguing contraction, the average EMG of both the agonist and antagonist muscles increased, leading to a nearly constant ratio at the end of the contraction when normalized to postfatigue values. In contrast to the monotonic increase in average EMG of the antagonist muscles, the excitability of their spinal reflex pathways exhibited a biphasic modulation. The amplitude of the Hoffman reflexes in the soleus and lateral gastrocnemius increased to 147.5 +/- 52.9% (P < 0.05) and 166.7 +/- 74.9% (P < 0.01), respectively, during the first 20% of the contraction and then subsequently declined to 66.3 +/- 44.8 and 74.4 +/- 44.2% of their initial values. In conclusion, the results show that antagonist coactivation did not contribute to task failure. The different changes in voluntary EMG activity and spinal reflex excitability in the antagonist muscles during the fatiguing contraction support the concept that the level of coactivation is controlled by supraspinal rather than spinal mechanisms. The findings indicate, however, that antagonist coactivation cannot simply be mediated by a central descending "common drive" to the motor neuron pools of the agonist-antagonist muscle pairs. Rather, they suggest a more subtle regulation of the drive, possibly through presynaptic mechanisms, to the motoneurons that innervate the antagonist muscles.     

The dynamic control ratio and its equilibrium point: A preliminary study of isokinetic fatiguing internal-external rotational effort of the shoulder joint in healthy subjects

The shoulder joint-related dynamic control ratio (DCR) is derived by dividing the peak eccentric moment of the external rotators (ER) by the peak concentric moment of the internal rotators (IR). However, given the inherent limitation associated with a single value DCR, an alternative approach is to calculate it at fixed angular intervals. This preliminary study aimed at exploring the variation in the DCR at a resolution of 1° and under fatiguing external and internal rotation exertions. Eighteen young men, 10 experienced and 8 without experience in overhead sporting activities completed two separate series of 45 ER eccentric and 45 IR concentric isokinetic repetitions at 120°/s. The analysis focused on repetitions 1–3 (TR1), 21–23 (TR2) and 41–43 (TR3). The relative fatigue values for both muscle groups and for both E and NE participants were 25–40% with significantly higher fatigue resistance in eccentric compared with concentric exertions. The DCR traces varied substantially linearly for most of the internal rotation range of motion while significant differences (p < 0.001) were found in their values within group between TR1, TR2 and TR3 and between experienced and non-experienced participants. An antagonistic moment equilibrium (DCR = 1) was reached in all instances and for both groups only during TR3 with a significant progressive reduction in this moment as fatigue increased. Thus, considering the DCR as an angle-based variable rather than a single value isokinetic parameter, may add new insight regarding the interplay between the rotatory muscles of the shoulder joint.     

The effect of handedness on electromyographic activity of human shoulder muscles during movement.

The aim of the study was to investigate whether there was a difference in the electromyographic (EMG) activity of human shoulder muscles between the dominant and nondominant side during movement and to explore whether a possible side-difference depends on the specific task. We compared the EMG activity with surface and intramuscular electrodes in eight muscles of both shoulders in 20 healthy subjects whose hand preference was evaluated using a standard questionnaire. EMG signals were recorded during abduction and external rotation. During abduction, the normalized EMG activity was significantly smaller on the dominant side compared to the nondominant side for all the muscles except for infraspinatus and lower trapezius (P 相似文献   

Reliability of burst superimposed technique to assess central activation failure during fatiguing contraction.

Recording a superimposed electrically-induced contraction at the limit of endurance during voluntary contraction is used as an indicator of failure of muscle activation by the central nervous system and discards the existence of peripheral muscle fatigue. We questioned on the reliability of this method by using other means to explore peripheral muscle failure. Fifteen normal subjects sustained handgrip at 60% of maximal voluntary contraction (MVC) until exhaustion. During sustained contraction, the power spectrum analysis of the flexor digitorum surface electromyogram allowed us to calculate the leftward shift of median frequency (MF). A superimposed 60 Hz 3 s pulse train (burst superimposition) was delivered to the muscle when force levelled off close to the preset value. Immediately after the fatigue trial had ended, the subject was asked to perform a 5 s 60% MVC and we measured the peak contractile response to a 60 Hz 3 s burst stimulation. Recordings of the compound evoked muscle action potential (M-wave) allowed us to explore an impairment of neuromuscular propagation. A superimposed contraction was measured in 7 subjects in their two forearms, whereas it was absent in the 8 others. Despite these discrepancies, all subjects were able to reproduce a 3 s 60% MVC immediately after the fatigue trial ended and there was no post-fatigue decrease of contraction elicited by the 60 Hz 3 s burst stimulation, as well as no M-wave decrease in amplitude and conduction time. Thus, there was no indication of peripheral muscle fatigue. MF decrease was present in all individuals throughout the fatiguing contraction and it was not correlated with the magnitude of superimposed force. These observations indicate that an absence of superimposed electrically-induced muscle contraction does not allow us to conclude the existence of a sole peripheral muscle fatigue in these circumstances.     

An electromyographic study of elbow motion during postexercise muscle soreness

Postexercise muscle soreness was induced in the elbow flexors of human volunteer subjects through the use of a regimen of eccentric contractions. Physical examination before and 48 h after the exercise included measurements of range of elbow motion at the elbow and of arm diameter. Electromyographic (EMG) observations, utilizing fine wire electrodes introduced into each of the elbow flexors, were carried out before and 48 h after the exercise. These observations included resting EMG levels, EMG activity under various isometric loads, and activity during active and passive extensions. The soreness induced was accompanied by a reduction in resting elbow angle while the subjects stood with arms relaxed at their sides, decreased range of both flexion and extension of the elbow, and swelling of the arm. EMG measurements showed no increase in EMG activity as the sore arms were extended passively at the elbow, indicating that the extra resistance to extension associated with the soreness was not a result of stretch receptor-induced activity in the flexors. The results rule out the possibility that neuromuscular activity is responsible for the restriction of motion and are consistent with the idea that edematous changes within the perimuscular connective tissue alter the elastic behavior of the muscles and cause restriction of motion.     

Activation among the elbow flexor muscles differs when maintaining arm position during a fatiguing contraction.

Twenty-four men (n = 11) and women (n = 13) supported an inertial load equivalent to 20% of the maximum voluntary contraction force with the elbow flexor muscles for as long as possible while maintaining a constant elbow angle at 90 degrees. Endurance time did not differ on the three occasions that the task was performed (320 +/- 149 s; P > 0.05), and there was no difference between women (360 +/- 168 s) and men (273 +/- 108 s; P = 0.11). The rate of increase in average electromyogram (EMG) for the elbow flexor muscles was similar across sessions (P > 0.05). However, average EMG during the fatiguing task increased for the long head of biceps brachii, brachioradialis, and brachialis (P < 0.05) but not for the short head of biceps brachii. Furthermore, the average EMG for the brachialis was greater at the start and end of the contraction compared with the other elbow flexor muscles. The rate of bursts in EMG activity increased during the fatiguing contraction and was greater in brachialis (1.0 +/- 0.2 bursts/min) compared with the other elbow flexor muscles (0.5 +/- 0.1 bursts/min). The changes in the standard deviation of acceleration, mean arterial pressure, and heart rate during the fatiguing contractions were similar across sessions. These findings indicate that the EMG activity, which reflects the net excitatory and inhibitory input received by the motoneurons in the spinal cord, was not adaptable over repeat sessions for the maintain-position task. Furthermore, these results contrast those from a previous study (Hunter SK and Enoka RM. J Appl Physiol 94: 108-118, 2003) when the goal of the isometric contraction was to maintain a constant force. These results, from a series of studies on the elbow flexor muscles, indicate that the type of load supported during the fatiguing contraction influences the extent to which endurance time can change with repeat performances of the task.     

Myonemal contraction of Spirostomum. I. Kinetics of contraction and relaxation

A microphotometric method is introduced that allows measurement of the contraction-relaxation kinetics of Spirostomum in response to electrical stimulation. The time course of contraction includes a rapidly contracting phase of some 4–5 mS during which cells shorten at a rate in excess of 100 cell lengths sec^?1. While a stimulus strength-duration curve determines the threshold of the response, the response to above threshold stimuli of different strengths and to trains of stimuli suggest that contraction of Spirostomum may not be an all-or-none event. The kinetics of relaxation following high stimulating voltages and repetitive after contractions also induced by high voltages are explained by excitation-contraction coupling through a stimulus-dependent intermediate effector, possibly the release of calcium ions. Changes in resting membrane potential detected by intracellular recording do not influence the initiation of contraction, while microinjection of calcium buffers above 10^?5 M Ca²⁺ invariably induces contraction.     

The relationship between anaerobic threshold and electromyographic fatigue threshold in college women

The purpose of this study was to investigate the relationship between anaerobic threshold (Th(an)) and muscle fatigue threshold (EMGFT) as estimated from electromyographic (EMG) data taken from the quadriceps muscles (vastus lateralis) during exercise on a cycle ergometer. The subjects in this study were 20 female college students, including highly trained endurance athletes and untrained sedentary individuals, whose fitness levels derived from their maximal oxygen consumption ranged from 24.9 to 62.2 ml.kg-1.min-1. The rate of increase in integrated EMG (iEMG) activity as a function of time (iEMG slope) was calculated at each of four constant power outputs (350, 300, 250, 200 W), sufficiently high to bring about muscle fatigue. The iEMG slopes so obtained were plotted against the exercise intensities imposed, resulting in linear plots which were extrapolated to zero slope to give an intercept on the power axis which was in turn interpreted as the highest exercise intensity sustainable without electromyographic evidence of neuromuscular fatigue (EMGFT). The Th(an) was estimated from gas exchange parameters during an incremental exercise test on the same cycle ergometer. The mean results indicated that oxygen uptake (VO2) at Than was 1.39 l.min-1, SD 0.44 and VO2 at EMGFT was 1.33 l.min-1, SD 0.57. There was no significant difference between these mean values (P greater than 0.05) and there was a highly significant correlation between VO2 at Than and VO2 at EMGFT (r = 0.823, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Distribution of calcium during contraction and relaxation of crayfish skeletal muscle fibre.

A model of activation of muscle contraction has been applied to the crayfish isolated skeletal muscle fibre. The model is based on calcium diffusion and binding to specific regulatory sites in a sarcomere. Calcium ions activate interactions of contractile proteins and thus the generation of force. The model quantifies the relation between calcium released from intracellular stores and force elicited. Experimental tension records from isolated crayfish skeletal muscle fibres under voltage clamp conditions are analyzed. Model parameters were determined either via approximation of the onset of tension by the model solution or from the model based relations between the tension maximum, and depolarizing pulse length and amplitude. This allowed to determine time changes of free and bound calcium distribution in the sarcomere and the calcium release from terminal cisternae. The steady state calcium concentration at terminal cisternae showed S-shaped voltage dependence with saturation below approx. 10 mumol/l at positive membrane potentials.     

The effects of interelectrode distance on electromyographic amplitude and mean power frequency during isokinetic and isometric muscle actions of the biceps brachii.

The purpose of this study was to examine the effects of interelectrode distance (IED) on the absolute and normalized electromyographic (EMG) amplitude and mean power frequency (MPF) versus isokinetic and isometric torque relationships for the biceps brachii muscle. Ten adults [mean+/-SD age=22.0+/-3.4 years] performed submaximal to maximal, isokinetic and isometric muscle actions of the dominant forearm flexors. Following determination of isokinetic peak torque (PT) and the isometric maximum voluntary contraction (MVC), the subjects performed randomly ordered, submaximal step muscle actions in 10% increments from 10% to 90% PT and MVC. Surface EMG signals were recorded simultaneously from bipolar electrode arrangements placed over the biceps brachii muscle with IEDs of 20, 40, and 60mm. Absolute and normalized EMG amplitude (muVrms and %max) increased linearly with torque during the isokinetic and isometric muscle actions (r(2) range=0.988-0.998), but there were no significant changes for absolute or normalized EMG MPF (Hz or %max) from 10% to 100% PT and MVC. In some cases, there were significant (p<0.05) differences among the three IED arrangements for absolute EMG amplitude and MPF values, but not for the normalized values. These findings suggested that for the biceps brachii muscle, IEDs between 20 and 60mm resulted in similar patterns for the EMG amplitude or MPF versus dynamic and isometric torque relationships. Furthermore, unlike the absolute EMG amplitude and MPF values, the normalized EMG data were not influenced by changes in IED between 20 and 60mm. Thus, normalized EMG data can be compared among previous studies that have utilized different IED arrangements.     

Quadriceps and hamstring isokinetic strength and electromyographic activity measured at different ranges of motion: a reproducibility study.

Isokinetic strength measurements of the quadriceps and hamstring that are commonly conducted using a 90 degrees range of motion (RoM) may involve some risk to specific knee patient groups. Testing these muscles at a much shorter RoM may reduce the risk but in order to render this method clinically acceptable the reproducibility of the derived test findings has to be established. Therefore the main objective of this study was to assess the reproducibility of isokinetic peak torque and normalized EMG scores of these muscles based on 90 degrees (0-90 degrees flexion, LR) and three successive short RoMs: 0-30 degrees (SR1), 30-60 degrees (SR2) and 60-90 degrees (SR3). Eight healthy subjects were tested three times with a 2 week between-session interval. All tests were performed on the dominant limb and consisted of maximal concentric and eccentric exertions. The velocities applied were 90 degrees /s for LR and 30 degrees /s for each of the SRs. Findings indicated no between-session improvement in strength. Based on the coefficient of variation the measurement error for all isokinetic strength scores remained stable throughout the testing sessions ranging 0.6-13.9% with the absolute majority of instances less than 10%. The reproducibility of the EMG scores was poorer ranging 1.5-25% and 0.5-19% for the quadriceps and hamstring, respectively. It is concluded that testing of knee muscles at short (30 degrees ) RoMs does not compromise the reproducibility of the strength or EMG scores derived from the commonly used RoM of 90 degrees . However, whereas strength was reproducible to within the accepted clinical standards, the corresponding EMG scores were characterized by a wider error band.     

Myonemal contraction of Spirostomum. III. The thermal dependence of contraction, relaxation and excitation-contraction coupling.

A microphotometric technique that displays rapid length changes of Spirostomum has been used to follow the variation with temperature of these kinetic parameters of myonemal contraction: contraction rate, relaxation rate and stimulus duration at threshold. In each case the exponential form of the relationship indicated that the gross rate constant might be equated with the limiting rate constant, k, of a driving chemical reaction, and from standard expressions of chemical kinetics the change in activation free energy appropriate to this reaction has been computed.     

Activation varies among the knee extensor muscles during a submaximal fatiguing contraction in the seated and supine postures.

Ten young men sustained an isometric contraction of the knee extensor muscles at 20% of the maximum voluntary contraction (MVC) torque on three separate occasions in a seated posture. Subjects performed an isometric knee extension contraction on a fourth occasion in a supine posture. The time to task failure for the seated posture was similar across sessions (291 +/- 84 s; P > 0.05), and the MVC torque was similarly reduced across sessions after the fatiguing contraction (42 +/- 12%). The rate of increase in electromyograph (EMG) activity (%MVC) and torque fluctuations during the fatiguing contractions were similar across sessions. However, the rate of increase in EMG differed among the knee extensor muscles: the rectus femoris began at a greater amplitude (31.5 +/- 11.0%) compared with the vastus lateralis and vastus medialis muscles (18.8 +/- 5.3%), but it ended at a similar value (45.4 +/- 3.1%). The time to task failure and increase in EMG activity were similar for the seated and supine tasks; however, the reduction in MVC torque was greater for the seated posture. These findings indicate that the time to task failure for the knee extensor muscles that have a common tendon insertion did not alter over repeat sessions as had been observed for the elbow flexor muscles (Hunter SK and Enoka RM. J Appl Physiol 94: 108-118, 2003).