Changes in the electromyographic spectrum power distribution caused by a progressive increase in the force level

The purpose of the present study was to determine the specific changes occurring in the power spectrum with an increasing force level during isometric contractions. Surface electromyographic signals of the triceps brachii (TB) and the anconeus (AN) of 29 normal subjects were recorded during isometric ramp contractions performed from 0 to 100% of the maximum voluntary contraction (MVC) in a 5-s period. Power spectra were obtained at 10, 20, 30, 40, 50, 60, 70, 80 and 90% MVC. Changes in the shape of these spectra were evaluated visually and with the calculation of several statistical parameters related to the distribution of power along the frequency axis, such as median frequency and mean power frequency, standard deviation, skewness, first and third quartiles and half-power range. For the AN, the behaviour of the spectrum was relatively similar across subjects, presenting a shift toward higher frequencies without any major change in the shape of the spectrum. For the TB, subjects with a thin skinfold thickness presented similar behaviours. In subjects with a thicker skinfold, however, a loss of power in the high frequency region paralleled the increase in the force level. Significant correlations were obtained between the extent of the change in the value of higher order statistical parameters across force and the thickness of the skin. This points out the importance of the skinfold layer when recording with surface electrodes. Furthermore, the use of a combination of several parameters appears to provide a better appreciation of the changes occurring in the spectrum than any single parameter taken alone.     

Contributions of slow and fast muscles of triceps surae to a cyclic movement

Summary The relative contribution of synergistic muscles has been studied during pedalling on a bicycle. The electromyographic (EMG) activity of the different components of triceps surae (namely soleus or SOL and medial gastrocnemius or MG) has been recorded and analyzed for increasing pedalling speed performed against increasing resistance. The results indicate that SOL IEMG (integrated EMG) increases linearly (y=2x−12.1; r=0.98) with increasing load (10–70 N) at constant speed (60 rpm), whereas no change is noted in MG IEMG below 40 N. In contrast, wehen the pedalling speed is increased (from 30 to 170 rpm) at constant load, MG IEMG shows the largest increase. Furthermore, although in both muscles EMG activity appears earlier in the movement with increases in load and/or speed, the delay between the onset of both EMGs remains unchanged at constant speed and synchronization of MG with SOL is only observed when speed is increased above 140 rpm. These results suggest that the different muscles of the triceps surae make specific contributions to the development of the mechanical tension required to maintain or increase the speed of movement. This work was supported by the Fonds National de la Recherche Scientifique of Belgium and the Conseil de la Recherche of the University of Brussels     

Modulation of EMG power spectrum frequency during motor imagery

To provide evidence that motor imagery (MI) is accompanied by improvement of intramuscular conduction velocity (CV), we investigated surface electromyographic (EMG) activity of 3 muscles during the elbow flexion/extension. Thirty right-handed participants were asked to lift or to imagine lifting a weighted dumbbell under 3 types of muscular contractions, i.e. concentric, isometric and eccentric, taken as independent variables. The EMG activity of the agonist (long and short heads of biceps brachii) and the antagonist (long portion of triceps brachii) muscles was recorded and processed to determine the median frequency (MF) of EMG power spectrum as dependant variable. The MF was significantly higher during the MI sessions than during the resting condition while the participants remained strictly motionless. Moreover, the MF during imagined concentric contraction was significantly higher than during the eccentric. Thus, the MF variation was correlated to the type of contraction the muscle produced. During MI, the EMG patterns corresponding to each type of muscle contraction remained comparable to those observed during actual movement. In conclusion, specific motor programming is hypothesized to be performed as a function of muscle contraction type during MI.     

The mechanical effectiveness of short latency reflexes in human triceps surae muscles revealed by ischaemia and vibration

Summary The resistance to stretch provided by short latency (SL) reflexes in human triceps surae muscles was investigated under three experimental conditions: control, ischaemia, and with 100 Hz vibration applied to the Achilles tendon. Incremental changes in plantar flexion force always showed a strong initial resistance followed by yielding in response to rapid dorsiflexion of the foot about the ankle joint. These changes were attributed to inherent stiffness of the triceps surae muscles. The force curves for each experimental condition diverged during the yield phase some 20 ms after the onset of SL EMG reflexes. During ischaemia, SL EMG reflexes were reduced to 8% of control values and yielding continued until the onset of medium latency EMG activity whereas the yielding was interrupted by SL action in the control situation. The difference between the ischaemia and control force curves was attributed to force recruited by SL reflexes under normal stretch conditions. Vibration reduced the SL EMG reflex amplitude to 20% of control values and produced with it a reduced force response.Supported by the Swiss National Science Foundation grants no. 3.585.79 and 3.505.79, the Dr. Erik Slack-Gyr Foundation, the Sandoz Foundation, and Deutsche Forschungsgemeinschaft grant SFB 70     

Dynamic relationship between EMG and torque at the human ankle: Variation with contraction level and modulation

Stochastic system identification techniques were used to determine the dynamic relationship between the electromyogram (EMG) and torque in the ankle muscles of normal human subjects. EMG and torque were recorded while subjects modulated ankle torque by tracking a computer-generated stochastic waveform. Nonparametric impulse response functions (IRFs) relating EMG to ankle torque were computed and parameterised by determining the parameters of the second-order system which provided the best least-squares fit. Two sets of experiments were carried out. In the first, the mean level of torque was varied from 5 per cent of the maximum voluntary contraction (MVC) to 30 per cent MVC while the depth of modulation was held constant at ±5 per cent of MVC. In the second series of experiments the mean torque was held constant at 25 per cent MVC while the depth of modulation was varied from ±2.5 per cent to ±25 per cent. The major findings were: (1) A second-order, low-pass filter provided a good quasilinear model of the EMG/force dynamics under all conditions; (2) The model parameters depended only weakly on the mean level of torque; (3) In contrast, the model parameters depended strongly on the amplitude with which the contraction was modulated; the natural frequency increased significantly with the depth of modulation.     

A correlogram analysis of the activity in the rostral ventromedial medulla of awake rats and in rats anesthetized with ketamine or pentobarbital following the administration of morphine

The present study was designed to determine the relative contribution of the gastrocnemius muscle to isometric plantar flexor torque production at varying knee angles, while investigating the activation of the gastrocnemius muscle at standardised non-optimal lengths. Voluntary plantar flexor torque, supramaximally stimulated twitch torque and myoelectric activity (EMG) from the triceps surae were measured at different knee angles. Surface and intra-muscular EMG were recorded from the soleus muscle and the medial and lateral heads of the gastrocnemius muscle in 10 male subjects. With the ankle angle held constant, knee angle was changed in steps of 30° ranging from 180° (extended) to 60° (extreme flexion), while voluntary torque from a 5-s contraction was determined at 10 different levels of voluntary effort, ranging from 10% of maximal effort to maximal effort. To assess effort, supramaximal twitches were superimposed on all voluntary contractions, and additionally during rest. Maximal plantar flexor torque and resting twitch torque decreased significantly in a sigmoidal fashion with increasing knee flexion to 60% of the maximum torque at 180° knee angle. For similar levels of voluntary effort, the EMG root mean square (RMS) of gastrocnemius was less with increased knee flexion, whereas soleus RMS remained unchanged. From these data, it is concluded that the contribution of gastrocnemius to plantar flexor torque is at least 40% of the total torque in the straight leg position. The decrease of gastrocnemius EMG RMS with decreasing muscle length may be brought about by a decrease in the number of fibres within the EMG electrode recording volume and/or impaired neuromuscular transmission.     

大腿截肢患者的残肢肌电运动识别

目的研究利用大腿残肢肌电信号进行下肢运动模式识别的方法,探讨肌电信号控制下肢假肢的可能性。方法采集15名大腿截肢者残肢侧股直肌、股外侧肌、阔筋膜张肌、股二头肌、半腱肌、臀大肌6块肌肉的表面肌电信号,提取肌电信号的6种时域、频域特征,利用支持向量机对平地行走、上楼梯、下楼梯、坐下、起立5种下肢运动模式进行识别。结果利用残肢肌电信号可以实现5种下肢运动模式的在线识别,对同一受试者同次测试数据识别率为94%,同一受试者的多次混合数据识别率为85%,对不同受试者混合数据识别率为74%。通过特征优化,仅利用3块肌肉的2个特征,对同一受试者的同次测试数据识别率仍可达92%。对平地行走、上楼梯、下楼梯3种动作的识别,同一受试者同次测试数据识别率为100%,同一受试者的多次混合数据识别率为98.33%,对不同受试者混合数据识别率为93.33%。结论仅仅利用残肢肌电信号能够实现运动意图的在线识别,通过对同一患者使用前的多次数据训练,有望达到较高的识别率。研究结果为肌电运动识别用于下肢假肢控制奠定了基础。     

Interaction of fibre type,potentiation and fatigue in human knee extensor muscles

Aim: To examine the effect of fibre type on potentiation and fatigue. Methods: Young men (n = 4 per group) with a predominance of type I [61.4 ± 6.9% (SD), group I (GI)] or type II [71.8 ± 9.2%, group II (GII)] fibres in vastus lateralis, performed a fatigue protocol of sixteen 5‐s maximal voluntary isometric contractions (MVCs) of the right knee extensors. Maximal twitches and corresponding muscle action potentials (M‐waves) were evoked before the first MVC, during the 3‐s rest period after each MVC and at intervals during the 5‐min recovery period after the last MVC. Results: Group II [49.3 ± 2.6% (SE)] had a greater decrease in MVC force than GI (22.8 ± 6.2%) during the fatigue protocol. Group II (126.4 ± 13.6%) showed greater twitch force potentiation early in the fatigue protocol than GI (38.2 ± 2.3%), but greater depression at the end (33.7 ± 13.7% vs.17.4 ± 3.4%). Twitch time‐to‐peak torque (TPT) and half relaxation time (HRT) initially decreased but then increased as the fatigue protocol progressed; GII had a greater increase in HRT. During a 5‐min recovery period twitch force increased above the prefatigue level and remained so until the end of the recovery period; the pattern was similar in GI and GII. Twitch TPT and HRT remained elevated during recovery. M‐wave area increased throughout the fatigue protocol and the first part of recovery before returning to baseline values in GII, whereas there were no significant changes in GI. The interaction between potentiation and fatigue was amplified in GII early in the fatigue protocol with concurrently greater twitch and M‐wave potentiation, and greater MVC force decrease and HRT increase. Late in the protocol, GII had a greater decrease in twitch and MVC force combined with greater M‐wave potentiation. Conclusion: It is concluded that fibre type distribution influences potentiation and fatigue of the twitch, and potentiation of the M‐wave during fatiguing exercise.     

Relation between EMG power spectrum shifts and muscle fibre action potential conduction velocity changes during local muscular fatigue in man

Summary Muscular fatigue was studied in the human m. biceps brachii by contracting the muscle as long as possible at 50% of maximum voluntary strength. For 8 subjects changes in the muscle fibre action potential conduction velocity could successfully be measured using the cross-correlation technique between two surface EMG signals. Shifts in the EMG power spectrum were quantified using the mean power frequency (MPF) of the EMG power spectral density function.During fatigue the EMG power spectrum gradually shifted to lower frequencies. The mean value and standard deviation of the MPF value decreased from (115±20) Hz at the beginning of the experiment to (60±18) Hz at the end. For 4 of the 8 subjects the decrease in MPF value was accompanied by a substantial decrease in conduction velocity (mean decrease was 33% of the initial velocity value). For the other 4 subjects, despite the great MPF changes, there was hardly any change in conduction velocity (mean decrease was 3%).The present study shows that great EMG power spectral shifts during muscular fatigue may occur without a concomitant change in muscle fibre action potential conduction velocity.     

Modification of myo-electric power spectrum in fatigue from 15% maximal voluntary contraction of human elbow flexor muscles,to limit of endurance: reflection of conduction velocity variation and/or centrally mediated mechanisms?

Summary Isometric flexion of the right elbow at 15% of the maximal voluntary contraction (MVC) was maintained to the limit of endurance (elbow angle 135°). The surface electromyogram (EMG) of the brachioradialis (BR) and biceps brachii (BB) muscles was recorded for calculation of conduction velocity (CV) by the cross-correlation method, and determination of median frequency (fm) and root mean square (rms) amplitude. Perceived exertion was rated for both muscles, and heart rate and blood pressure were measured. The EMG of ten brief 15% MVC contractions distributed over a 30-min recovery period was also recorded. Eleven males in their twenties volunteered for the investigation. The average endurance time was 906 (SD 419) s. Mean CV for the unfatigued muscles was 4.2 (SD 0.41) m·s^–1 (BR), and 4.3 (SD 0.29) m·s^–1 (BB). The contraction caused a significant decrease in CV of BR (12%,P<0.001) whereas CV variation of BB remained insignificant. Concurrently the meanf _m of both muscles dropped to approximately 66% of their initial values and their average rms amplitudes grew by approximately 380% (BR and BB:P<0.001, both parameters). The 1st min of recovery lowered the rms amplitudes by approximately 60% (BR and BB:P<0.01), while thef _m increased to approximately 88% of the initial recording (BR,P<0.01; BB,P<0.05). The accompanying small increases in CV were beyond the level of significance. Over the next 29 min a significant parallel restitution inf _m and CV took place; changes inf _m evidenced a simple one to one reflection of relative CV variation. A similar uncomplicated linear causality between relative changes in CV andf _m was hypothesized for the endurance contraction. Consequently, the 12% CV decrease of the BR accounted for only one-third of the fatigue inducedf _m reduction of 33%, while two-thirds were assumed to be attributable to centrally mediated regulatory interventions in motor unit (MU) performance. Independent of contributions from the virtually unchanged CV, thef _m of the BB muscle decreased by 35%; from one subject exhibiting a remarkably manifest burst-type pattern of MU activity it is argued that synchronization/grouping of MU firing predominantly determined the power redistribution in the BB spectrum.     

Consequences of prolonged total thermoneutral immersion on muscle performance and EMG activity

We hypothesized that the changes in muscle temperature and interstitial pressure during thermoneutral immersion may affect the reflex adaptation of the motor drive during static contraction, assessed by the decrease in median frequency (MF) of electromyogram (EMG) power spectrum. Ten subjects were totally immerged for 6 h at 35°C and repeated maximal voluntary contraction (MVC) and submaximal (60% MVC) leg extensions sustained until exhaustion. In vastus lateralis (VL) and soleus (SOL) muscles, the compound muscle potential evoked by muscle stimulation with single shocks (M-wave) was recorded at rest, and MF of surface EMG was calculated during 60% MVCs. We measured lactic acid and potassium venous blood concentrations and calculated plasma volume changes. Data were compared to those obtained in the same individuals exercising at 35°C under dry conditions where the MF decrease during 60% MVCs was modest (−4 to−5%). During immersion, the rectal temperature remained stable, but the thigh and calf surface temperatures significantly increased. Lactic acid and potassium concentrations did not vary, but plasma volume decreased from the 180th min of immersion. The M-wave did not vary in VL but was prolonged in SOL from the 30th min of immersion. From the 220th min of immersion, the maximal MF decrease was majored in both muscles (−18 to −22%). Thus, compared to the dry condition, total body thermoneutral immersion enhances fatigue-induced EMG changes in leg muscles, perhaps through the activation of warm-sensitive muscle endings and/or the changes in interstitial pressure because of vasodilatation.     

Optimal signal bandwidth for the recording of surface EMG activity of facial, jaw, oral, and neck muscles

Spontaneous pericranial electromyographic (EMG) activity is generally small and is contaminated by strong low-frequency artifacts. High-pass filtering should suppress artifacts but affect EMG signal power only minimally. In 24 subjects who performed a warned simple reaction time task, the optimal high-pass cut-off frequency was examined for nine different pericranial muscles. From four experimental conditions (visual and auditory reaction signals combined with hand and foot responses), 1-min EMG recordings were selected (bandwidth: 0.4-512 Hz) and divided into 60 1-s data segments. These segments were high-pass filtered, the -3-dB cut-off frequency varying from 5 to 90 Hz, and subjected to power spectral analysis. Optimal high-pass filter frequencies were determined for the mean power spectra based on visual estimation or comparison with a theoretical spectrum of the artifact-free EMG signal. The optimal frequencies for the different muscles varied between 15 and 25 Hz and were not influenced by stimulus or response modality. For all muscles, a low-pass filter frequency between 400 and 500 Hz was appropriate.     

EMG power spectra of elbow extensors during ramp and step isometric contractions

Summary The goal of the present study was to compare electromyogram (EMG) power spectra obtained from step (constant force level) and ramp (progressive increase in the force level) isometric contractions. Data windows of different durations were also analysed for the step contractions, in order to evaluate the stability of EMG power spectrum statistics. Fourteen normal subjects performed (1) five ramp elbow extensions ranging from 0 to 100% of the maximum voluntary contraction (MVC) and (2) three stepwise elbow extensions maintained at five different levels of MVC. Spectral analysis of surface EMG signals obtained from triceps brachii and anconeus was performed. The mean power frequency (MPF) and the median frequency (MF) of each power spectrum were obtained from 256-ms windows taken at 10, 20, 40, 60 and 80% MVC for each type of contraction and in addition on 512-, 1024-and 2048-ms windows for the step contractions. No significant differences (P>0.05) were found in the values of both spectral statistics between the different window lengths. Even though no significant differences (P>0.05) were found between the ramp and the step contractions, significant interactions (P<0.05) between these two types of contraction and the force level were found for both the MPF and the MF data. These interactions point out the existence of different behaviours for both the MPF and the MF across force levels between the two types of contraction.     

Consequences of prolonged total body immersion in cold water on muscle performance and EMG activity

The consequences of a prolonged total body immersion in cold water on the muscle function have not been documented yet, and they are the object of this French Navy research program. Ten elite divers were totally immerged and stayed immobile during 6 h in cold (18 and 10°C) water. We measured the maximal voluntary leg extension (maximal voluntary contraction, MVC) and evoked compound muscle potential (M wave) in vastus lateralis and soleus muscles at rest, after a submaximal (60% MVC) isometric extension allowing the measurement of the endurance time (Tlim). The power spectrum of surface electromyograms (EMG) was computed during 60% MVCs. MVCs and 60% MVC maneuvers were repeated four times during the immersion. Data were compared with those obtained in a control group studied in dry air condition during a 6-h session. Total body cooling did not affect MVC nor Tlim. The M wave duration increased in the coolest muscle (soleus), but only at 10°C at rest. There were no further fatigue-induced M wave alterations in both muscles. During 60% the MVCs, a time-dependant increase in the leftward shift of the EMG spectrum occurred at the two temperatures. These EMG changes were absent in the control group of subjects studied in dry air. The plasma lactate concentration was elevated throughout the 18 and mostly the 10°C immersion conditions. Throughout the 18°C immersion study, the resting potassium level did not significantly vary, whereas at 10°C, a significant potassium increase occurred soon and persisted throughout the study. Thus, total body immersion in cold water did not affect the global contractile properties of leg muscles during static efforts but elicited significant alterations in electromyographic events which may be related to the variations of interstitial fluid composition.     

Comparison of the histochemical and contractile properties of human triceps surae

The purpose of the study was to compare the contractile properties determined from an electrically stimulated twitch with histochemically determined fibre type parameters of the human triceps surae. Muscle samples were obtained from the medial head of the gastrocnemius of ten male athletes. Ages ranged from 20 to 29 years. Muscle samples from the belly of the medial gastrocnemius muscle were obtained using the needle biopsy technique. The samples were treated histochemically for myosin ATPase to classify the fibres as either slow twitch (ST) or fast twitch (FT) and to determine fibre areas. Surface electrical stimulation was used to determine muscle twitch parameters. The contractile variables of the muscle twitch were latency (L), time to peak force (TPF), peak force (PF), half-contraction time (1/2 CT) and half-relaxation time (1/2 RT). Backward elimination procedures for dependent variables were used to determine which contractile properties best represented the histochemical profile of the muscles. Prediction formulas were developed for FT and ST percentages (R²=0·98, p<0·001), relative area percentage (R²=0·87, p<0·001), and ST area (R²=0·85, p<0·01). It was concluded that the use of the electrotensiometer (ETM) protocol was a valid testing procedure when studying physiological relationships of histochemical properties in intact human skeletal muscle. Protected by patent no. 4 688 581.     

The co-existence of the gastrocnemius tertius and accessory soleus muscles

A bilateral gastrocnemius tertius muscle and a unilateral accessory soleus muscle were encountered during the routine educational dissection studies. The right gastrocnemius tertius muscle consisted of one belly, but the left one of two bellies. On the left side, the superficial belly of the gastrocnemius tertius muscle had its origin from an area just above the tendon of the plantaris muscle, the deep belly from the tendon of the plantaris muscle. The accessory soleus muscle originated from the posteromedial aspect of the tibia and soleal line of the tibia and inserted to the medial surface of the calcaneus. On the right side, the gastrocnemius tertius muscle had its origin from the lateral condyle of the femur, and inserted to the medial head of the gastrocnemius muscle. The co-existence of both gastrocnemius tertius and accessory soleus muscle has not, to our knowledge, been previously reported.     

Muscle activity and fatigue in the shoulder muscles during repetitive work

Summary 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.     

Surface electromyogram power spectrum changes in human leg muscles following 4 weeks of simulated microgravity

Surface electromyogram (EMG) spectrum changes in human tibialis anterior (TA) and gastroenemius medialis (GM) muscles were studied to investigate the effect of 4-week bed rest (BR) on muscle fatigability. An exhausting isometric test at 50% of the maximal voluntary contraction was performed by 12 clinically healthy men before and after BR. During this test, mean power frequency (MPF) calculated from surface EMG decreased linearly for TA and GM. When changes in MPF were expressed in terms of rate of decrease a significant difference appeared between TA and GM. Furthermore, as a result of BR, the shift in MPF increased significantly for GM (6.1% vs 10.4%) whereas it was not significantly changed for TA (28.6% vs 20.95%). Alterations in maximal torque were also observed with a more pronounced decrease for plantar-flexor (20.5%) compared with dorsiflexor (15.1%) muscles. These results would seem to indicate that simulated microgravity preferentially affects muscles having an antigravity function.     

Influence of motor unit firing statistics on the median frequency of the EMG power spectrum

Summary Changes in the EMG power spectrum during static fatiguing contractions are often attributed to changes in muscle fibre action potential conduction velocity. Mathematical models of the EMG power spectrum, which have been empirically confirmed, predict that under certain conditions a distinct maximum occurs in the low-frequency part of the spectrum, indicating the dominant firing rate of the motor units. The present study investigated the influence of this firing rate peak on the spectral changes during a static fatiguing contraction at 50% of maximum EMG amplitude in the frontalis and corrugator supercilii muscles. An exponential decrease of the median frequency (MF) of the EMG power spectrum was observed when the firing rate peak was absent. When the firing rate peak was present, an exaggerated decrease of MF in the beginning of the contraction was found, which was associated with an increase in firing rate peak magnitude. In later stages of the contraction, a partial recovery of MF occurred, concomitant with a decrease in firing rate peak magnitude.The influence of the firing rate peak on MF was also investigated during nonfatiguing contractions of the frontalis muscle at 20, 40, 60, and 80% of maximum EMG amplitude. A curvilinear relationship between MF and contraction strength was found, whether firing rate peaks were present or absent. The presence of firing rate peaks, however, was associated with a decrease in MF which was inversely related to contraction strength, due to the inverse relationship between firing rate peak magnitude and contraction strength.