Validation of the wavelet spectral estimation technique in biceps brachii and brachioradialis fatigue assessment during prolonged low-level static and dynamic contractions.

An experiment was carried out to investigate the myoelectrical manifestations of fatigue of the Biceps Brachii and Brachioradialis muscles in low-level (15% MVC) prolonged isometric and dynamic contractions. The range of the joint angle was 70-110 degrees and the mean speed of flexion and extension was about 33.33 degrees /s (1.2 s for 40 degrees ). The use of Wavelet transform (IMNF) in weakly stationary dynamic SEMG signals was validated in comparison with the Fourier transform (MPF). The development of fatigue and its myoelectrical manifestations (increase in RMS and decrease in mean frequency) in dynamic contractions show no deviations from what is found in literature for both spectral estimation techniques. The benefit of Wavelets is its future use in non-stationary conditions. Lower IMNF slopes in dynamic compared to isometric contractions for Biceps Brachii might be an indication that wavelets reflect more the changes in muscle fiber propagation velocity. The results of the use of Wavelet transform in detecting frequency modulations in different movement phases of the dynamic tests show that in the eccentric phase a systematic shift towards lower frequencies occurs. It also reveals the great possibilities of phase separation using Wavelets with high resolution and low interaction.     

Alternate activity in the synergistic muscles during prolonged low-level contractions

The purpose ofthis study was to investigate the functional interrelationship betweensynergistic muscle activities during low-level fatiguing contractions.Six human subjects performed static and dynamic contractions at anankle joint angle of 110° plantar flexion and within the range of90-110° (anatomic position = 90°) under constant load(10% maximal voluntary contraction) for 210 min. Surfaceelectromyogram records from lateral gastrocnemius (LG), medialgastrocnemius (MG), and soleus (Sol) muscles showed high and silentactivities alternately in the three muscles and a complementary andalternate activity between muscles in the time course. In the secondhalf of all exercise times, the number of changes in activity increasedsignificantly (P < 0.05) in each muscle. The ratios of active to silent periods of electromyogram activity were significantly higher (P < 0.05) in MG (4.5 ± 2.2) and Sol (4.3 ± 2.8) than in the LG(0.4 ± 0.1), but no significant differences were observed betweenMG and Sol. These results suggest that the relativeactivation of synergistic motor pools are not constant during alow-level fatiguing task.

     

Physiological and psychological indices of fatigue during static contractions

The development of fatigue and the relationship between psychological and physiological indices of fatigue were studied in a group of 18 male subjects during static contractions. Exercise was performed as a static elbow flexion at 25% MVC. Heart rate (HR), intraarterial blood pressure (BP) and surface EMG [mean amplitude (MA) and central frequency (CF)] were studied during contractions sustained until exhaustion. The amount of effort expended (relative to total exhaustion) and the rating of perceived pain were recorded following contractions interrupted after 20, 30 . . . 80, and 100% of endurance time. HR, BP and EMG amplitude responses were similar to those previously recorded. The decline in CF occurred in two phases, possibly related to a change in motor unit recruitment after the initial 70% of endurance time. The subjects overestimated the amount of effort expended and thus underestimated their endurance capacity. The best correlation between perceived effort and physiological responses was obtained using blood pressure data, whereas changes in EMG data did not parallel the psychological responses. It is concluded that the perception of effort during a static contraction is produced through a complex process, in which several influences of peripheral and central origin are integrated.     

Duration of differential activations is functionally related to fatigue prevention during low-level contractions

The aim of this study was to investigate the importance of duration of differential activations between the heads of the biceps brachii on local fatigue during prolonged low-level contractions. Fifteen subjects carried out isometric elbow flexion at 5% of maximal voluntary contraction (MVC) for 30 min. MVCs were performed before and at the end of the prolonged contraction. Surface electromyographic (EMG) signals were recorded from both heads of the biceps brachii. Differential activation was analysed based on the difference in EMG amplitude (activation) between electrodes situated at the two heads. Differential activations were quantified by the power spectral median frequency of the difference in activation between the heads throughout the contraction. The inverse of the median frequency was used to describe the average duration of the differential activations. The relation between average duration of the differential activations and the fatigue-induced reduction in maximal force was explored by linear regression analysis. The main finding was that the average duration of differential activation was positively associated to relative maximal force at the end of the 30 min contraction (R² = 0.5, P < 0.01). The findings of this study highlight the importance of duration of differential activations for local fatigue, and support the hypothesis that long term differential activations prevent fatigue during prolonged low-level contractions.     

Intramuscular pressure and EMG relate during static contractions but dissociate with movement and fatigue.

Intramuscular pressure (IMP) and electromyography (EMG) mirror muscle force in the nonfatigued muscle during static contractions. The present study explores whether the constant IMP-EMG relationship with increased force may be extended to dynamic contractions and to fatigued muscle. IMP and EMG were recorded from shoulder muscles in three sessions: 1). brief static arm abductions at angles from 0 to 90 degrees, with and without 1 kg in the hands; 2). dynamic arm abductions at angular velocities from 9 to 90 degrees /s, with and without 1 kg in the hands; and 3). prolonged static arm abduction at 30 degrees for 30 min followed by recovery. IMP and EMG increased in parallel with increasing shoulder torque during brief static tasks. During dynamic contractions, peak IMP and EMG increased to values higher than those during static contractions, and EMG, but not IMP, increased significantly with speed of abduction. In the nonfatigued supraspinatus muscle, a linear relationship was found between IMP and EMG; in contrast, during fatigue and recovery, significant timewise changes of the IMP-to-EMG ratio occurred. The results indicate that IMP should be included along with EMG when mechanical load sharing between muscles is evaluated during dynamic and fatiguing contractions.     

Electromyography of superficial and deep neck muscles during isometric, voluntary, and reflex contractions

Increasingly complex models of the neck neuromusculature need detailed muscle and kinematic data for proper validation. The goal of this study was to measure the electromyographic activity of superficial and deep neck muscles during tasks involving isometric, voluntary, and reflexively evoked contractions of the neck muscles. Three male subjects (28-41 years) had electromyographic (EMG) fine wires inserted into the left sternocleidomastoid, levator scapulae, trapezius, splenius capitis, semispinalis capitis, semispinalis cervicis, and multifidus muscles. Surface electrodes were placed over the left sternohyoid muscle. Subjects then performed: (i) maximal voluntary contractions (MVCs) in the eight directions (45 deg intervals) from the neutral posture; (ii) 50 N isometric contractions with a slow sweep of the force direction through 720 deg; (iii) voluntary oscillatory head movements in flexion and extension; and (iv) initially relaxed reflex muscle activations to a forward acceleration while seated on a sled. Isometric contractions were performed against an overhead load cell and movement dynamics were measured using six-axis accelerometry on the head and torso. In all three subjects, the two anterior neck muscles had similar preferred activation directions and acted synergistically in both dynamic tasks. With the exception of splenius capitis, the posterior and posterolateral neck muscles also showed consistent activation directions and acted synergistically during the voluntary motions, but not during the sled perturbations. These findings suggest that the common numerical-modeling assumption that all anterior muscles act synergistically as flexors is reasonable, but that the related assumption that all posterior muscles act synergistically as extensors is not. Despite the small number of subjects, the data presented here can be used to inform and validate a neck model at three levels of increasing neuromuscular-kinematic complexity: muscles generating forces with no movement, muscles generating forces and causing movement, and muscles generating forces in response to induced movement. These increasingly complex data sets will allow researchers to incrementally tune their neck models' muscle geometry, physiology, and feedforward/feedback neuromechanics.     

Motor unit recruitment during prolonged isometric contractions

Motor unit recruitment patterns were studied during prolonged isometric contraction using fine wire electrodes. Single motor unit potentials were recorded from the brachial biceps muscle of eight male subjects, during isometric endurance experiments conducted at relative workloads corresponding to 10% and 40% of maximal voluntary contraction (MVC), respectively. The recordings from the 10% MVC experiment demonstrated a characteristic time-dependent recruitment. As the contraction progressed both the mean number of motor unit spikes counted and the mean amplitude of the spikes increased significantly (P<0.01). This progressive increase in spike activity was the result of a discontinuous process with periods of increasing and decreasing activity. The phenomenon in which newly recruited motor units replace previously active units is termed motor unit rotation and appeared to be an important characteristic of motor control during a prolonged low level contraction. In contrast to the 10% MVC experiment, there was no indication of de novo recruitment in the 40% MVC experiment. Near the point of exhaustion a marked change in action potential shape and duration dominated the recordings. These findings demonstrate a conspicuous difference in the patterns of motor unit recruitment during a 10% and a 40% MVC sustained contraction. It is suggested that there is a close relationship between intrinsic muscle properties and central nervous system recruitment strategies which is entirely different in fatiguing high and low level isometric contractions.     

Rectus femoris surface myoelectric signal cross-talk during static contractions

The clinical application of EMG requires that the recorded signal is representative of the muscle of interest and is not contaminated with signals from adjacent muscles. Some authors report that surface EMG is not suitable for obtaining information on a single muscle but rather reflects muscle group function [J. Perry, C.S. Easterday, D.J. Antonelli, Surface versus intramuscular electrodes for electromyography of superficial and deep muscles. Physical Therapy 61 (1981) 7–15]. Other authors report however, that surface EMG is adequate to determine individual muscle function, once guidelines pertaining to data acquisition are followed [D.A. Winter, A.J. Fuglevand, S.E. Archer. Cross-talk in surface electromyography: theoretical and practical estimates. Journal of Electromyography and Kinesiology 4 (1994) 15–26]. The aim of this study was to determine whether surface EMG was suitable for monitoring rectus femoris (RF) activity during static contractions. Five healthy subjects, having given written informed consent, participated in this trial. Surface and fine wire EMG from the rectus femoris and the vastus lateralis (VL) muscles were recorded simultaneously during a protocol of static contractions consisting of knee extensions and hip flexions. Ratios were used to quantify the relationship between the surface EMG amplitude value and the fine wire EMG amplitude value for the same contraction. The results showed that hip flexion contractions elicited RF activation only and that knee extension contractions elicited fine wire activity in VL only. When the relationship between RF surface and RF fine wire electrodes was compared for hip flexion and knee extension contractions, it was observed that for all subjects, there was a tendency for increased RF surface activity in the absence of RF fine wire activity during knee extensions. It was concluded that the activity recorded by the RF surface electrode arrangement during knee extension consisted of EMG from the vastii, i.e., cross-talk and that vastus intermedius was the most likely origin of the erroneous signal. Therefore it is concluded that for accurate EMG information from RF, fine wire electrodes are necessary during a range of static contractions.     

An EMG fractal indicator having different sensitivities to changes in force and muscle fatigue during voluntary static muscle contractions.

During a sustained contraction, electromyographic signals (EMGs) undergo a spectral compression. This fatigue behaviour induces a shift of the mean and the median frequencies to lower frequencies. On the other hand, several studies conclude that the mean/median frequency can increase, decrease or remain constant with an increasing force level. Such inconsistency is embarrassing since the fatigue state may be influenced by the force level. In this paper, we propose a frequency indicator which is sensitive to the force level independently of the fatigue state evaluated at 70% of the maximal voluntary contraction. Ten healthy volunteers participated in the study and both surface EMGs (from the short head of the biceps brachii) and force signals were measured. This study compared force and fatigue effects on the EMGs during short (3-s) isometric contractions at different strength intensities and during a sustained isometric contraction until exhaustion. The EMGs partly show 1/falpha spectral behaviours since their power spectral densities may experimentally fit with two linear segments in a log-log representation. The measured "right" slope produces variations of force as 20 times the variations of fatigue. 1/falpha Behaviour may be related to stochastic fractals. This fractal indicator is a new frequency indicator that is thus complementary to other known classical frequency indicators when studying force during unknown fatigue states.     

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.     

Mechanomyography versus Electromyography,in monitoring the muscular fatigue

Background  

The use of the mechanomyogram (MMG) which detects muscular vibrations generated by fused individual fiber twitches has been refined. The study addresses a comparison of the MMG and surface electromyogram (SEMG) in monitoring muscle fatigue.     

Evidence of long term muscle fatigue following prolonged intermittent contractions based on mechano- and electromyograms.

The focus of the present study is the long term element of muscle fatigue provoked by prolonged intermittent contractions at submaximal force levels and analysed by force, surface electromyography (EMG) and mechanomyogram (MMG). It was hypothesized that fatigue related changes in mechanical performance of the biceps muscle are more strongly reflected in low than in high force test contractions, more prominent in the MMG than in the EMG signal and less pronounced following contractions controlled by visual compared to proprioceptive feedback. Further, it was investigated if fatigue induced by 30 min intermittent contractions at 30% as well as 10% of maximal voluntary contraction (MVC) lasted more than 30 min recovery. In six male subjects the EMG and MMG were recorded from the biceps brachii muscle during three sessions with fatiguing exercise at 10% with visual feedback and at 30% MVC with visual and proprioceptive feedback. EMG, MMG, and force were evaluated during isometric test contractions at 5% and 80% MVC before prolonged contraction and after 10 and 30 min of recovery. MVC decreased significantly after the fatiguing exercise in all three sessions and was still decreased even after 30 min of recovery. In the time domain significant increases after the fatiguing exercise were found only in the 5% MVC tests and most pronounced for the MMG. No consistent changes were found for neither EMG nor MMG in the frequency domain and feedback mode did not modify the results. It is concluded that long term fatigue after intermittent contractions at low force levels can be detected even after 30 min of recovery in a low force test contraction. Since the response was most pronounced in the MMG this may be a valuable variable for detection of impairments in the excitation-contraction coupling.     

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.     

Alternate muscle activity observed between knee extensor synergists during low-level sustained contractions.

To determine quantitatively the features of alternate muscle activity between knee extensor synergists during low-level prolonged contraction, a surface electromyogram (EMG) was recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) in 11 subjects during isometric knee extension exercise at 2.5% of maximal voluntary contraction (MVC) for 60 min (experiment 1). Furthermore, to examine the relation between alternate muscle activity and contraction levels, six of the subjects also performed sustained knee extension at 5.0, 7.5, and 10.0% of MVC (experiment 2). Alternate muscle activity among the three muscles was assessed by quantitative analysis on the basis of the rate of integrated EMG sequences. In experiment 1, the number of alternations was significantly higher between RF and either VL or VM than between VL and VM. Moreover, the frequency of alternate muscle activity increased with time. In experiment 2, alternating muscle activity was found during contractions at 2.5 and 5.0% of MVC, although not at 7.5 and 10.0% of MVC, and the number of alternations was higher at 2.5 than at 5.0% of MVC. Thus the findings of the present study demonstrated that alternate muscle activity in the quadriceps muscle 1) appears only between biarticular RF muscle and monoarticular vasti muscles (VL and VM), and its frequency of alternations progressively increases with time, and 2) emerges under sustained contraction with force production levels < or =5.0% of MVC.     

Factors affecting blood pressure during heavy weight lifting and static contractions.

Brachial arterial pressure was directly recorded in 31 healthy male volunteers through protocols examining the effects of the Valsalva maneuver, muscle size and strength, contraction force, contraction type (concentric, isometric, eccentric), changes in joint angle, and muscle fatigue on the blood pressure response to resistance exercise. Weight lifting at the same relative intensity produced similar increases in blood pressure, regardless of individual differences in muscle size or strength. Concentric, isometric, or eccentric exercise at the same relative intensity caused similar increases despite differences in force production. In weight lifting, the greatest increase in blood pressure occurred at the joint angle corresponding to the weakest point in the strength curve and the least at the angle corresponding to the strongest point. Isometric contractions of the same relative intensity at different joint angles produced identical blood pressures despite differences in absolute force production. When subjects attempted to maintain a maximum isometric contraction for 45 s, the blood pressure increase remained the same despite a marked diminution in force. Thus the magnitude of the blood pressure response depends on the degree of effort or central command and not actual force production. A brief Valsalva maneuver, which exaggerates the increase in blood pressure, is unavoidable when desired force production exceeds approximately 80% maximum voluntary contraction.     

Changes in surface EMG parameters during static and dynamic fatiguing contractions.

The effect of contraction types on muscle fiber conduction velocity (MFCV), median frequency (MDF) and mean amplitude (AMP) of surface electromyography was examined in the vastus lateralis of 19 healthy male adults. The subjects performed knee extension both statically and dynamically until they were exhausted. The static contraction was a sustained isometric extension of the knee at a joint angle of 90 degrees with 50% of the maximum voluntary contraction (MVC) load. The dynamic contraction was a repetitive isotonic extension of the knee between the angles of 90 degrees and 180 degrees with the same 50% MVC load at a frequency of 10 times per minute. MFVC during the static contraction significantly decreased during the exercise (p < 0.01). On the other hand, MFVC during the dynamic contraction did not significantly change throughout the exercise. MDF decreased and AMP increased during both types of contractions (p < 0.01). Because the blood flow within the muscle is maintained during the dynamic contraction by enhanced venous return from the contracting muscle, these results suggested that MFVC is affected by the metabolic state in the muscle and the changes in MDF cannot be explained only by that of MFVC.     

Hyperthermia and central fatigue during prolonged exercise in humans.

The present study investigated the effects of hyperthermia on the contributions of central and peripheral factors to the development of neuromuscular fatigue. Fourteen men exercised at 60% maximal oxygen consumption on a cycle ergometer in hot (40 degrees C; hyperthermia) and thermoneutral (18 degrees C; control) environments. In hyperthermia, the core temperature increased throughout the exercise period and reached a peak value of 40.0 +/- 0.1 degrees C (mean +/- SE) at exhaustion after 50 +/- 3 min of exercise. In control, core temperature stabilized at approximately 38.0 +/- 0.1 degrees C, and exercise was maintained for 1 h without exhausting the subjects. Immediately after the cycle trials, subjects performed 2 min of sustained maximal voluntary contraction (MVC) either with the exercised legs (knee extension) or with a "nonexercised" muscle group (handgrip). The degree of voluntary activation during sustained maximal knee extensions was assessed by superimposing electrical stimulation (EL) to nervus femoralis. Voluntary knee extensor force was similar during the first 5 s of contraction in hyperthermia and control. Thereafter, force declined in both trials, but the reduction in maximal voluntary force was more pronounced in the hyperthermic trial, and, from 30 to 120 s, the force was significantly lower in hyperthermia compared with control. Calculation of the voluntary activation percentage (MVC/MVC + EL) revealed that the degree of central activation was significantly lower in hyperthermia (54 +/- 7%) compared with control (82 +/- 6%). In contrast, total force of the knee extensors (MVC + force from EL) was not different in the two trials. Force development during handgrip contraction followed the same pattern of response as was observed for the knee extensors. In conclusion, these data demonstrate that the ability to generate force during a prolonged MVC is attenuated with hyperthermia, and the impaired performance is associated with a reduction in the voluntary activation percentage.