Changes In Mean Power Frequency Research Articles

Linear spectrum and non-linear complexity features of lumbar muscle surface electromyography between people with and without non-specific chronic low back pain during Biering-Sorensen test.

This study aimed to investigate the electromyographic parameters of lumbar muscles during the Biering-Sorensen test (BST) in people with and without non-specific chronic low back pain (NCLBP). Thirteen healthy controls and thirteen NCLBP patients participated in the current study, where they performed the 90s-BST, while the surface electromyography (sEMG) was recorded from the erector spinae (ES) at L1 and L3 level and lumbar multifidus (LM) at L5 level, bilaterally. Spectral and nonlinear analyses were applied by calculating mean power frequency (MPF), fractal dimension (FD) and the percentage of determinism (%DET) in the 10-second non-overlapping time-windows and EMG-EMG coherence during the first half and second half of the BST. Also, the slopes of the linear fitting curves of MPF, FD and %DET were calculated. NCLBP group had significantly lower rates of changes in MPF, FD and %DET compared to asymptomatic controls in the ES(L3) and LM. Coherence in left-right LM and in the right ES-LM increased significantly in the gamma band in the Control group with no increase in the NCLBP group. Our findings indicated that compared to people with NCLBP, the sEMG signals of lumbar muscles of people without NCLBP were more regular and less complex during the 90s-BST.

Differences of Surface Electromyography during Incremental Cycling Exercise in Hypoxia and Normoxia Using Wavelet Transform Analysis

Previous studies indicated that there was a difference between hypoxia and normoxia for amplitude parameters of surface electromyography (EMG). However, there are few reports on frequency parameters of EMG because of limitations of traditional analytical methods. PURPOSE: The purpose of this study was to investigate the differences of surface electromyography during incremental cycling exercise in hypoxia and normoxia using wavelet transform analysis. METHODS: Subjects were fourteen active and healthy men. (mean±SD; age: 21.08 ± 1.50 years; height: 1.69 ± 0.06 m; weight: 60.16 ± 7.77 kg; maximal oxygen consumption: 51.11. ±9.54 ml / min / kg). The subjects performed incremental cycling exercise test to exhaustion in hypoxia (FIO2: 13.4%) and normoxia conditions (FIO2: 20.9%). EMG activities of vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF) and biceps femoris (BF) were recorded during cycling exercise test at a sampling rate of 2000 Hz. Integrated electromyogram (iEMG) and mean power frequency (MPF) for each exercise intensity were calculated from the EMG data. Contentious wavelet analysis was used to calculate MPF, and mother wavelet was set at morlet wavelet. The differences between exercise intensities and FIO2 conditions were compared using a two-way analysis of variance. The significance level was set at p< 0.05. RESULTS: iEMG of each muscle significantly increased with increasing exercise intensities (all: p < 0.01). However, there were no significant differences between FIO2 conditions for each muscle (VL: p = 0.37; VM: p = 0.49; RF: p = 0.37; BF: p = 0.64). MPF of VL significantly decreased with increasing exercise intensities (p < 0.01), and MPF in hypoxia was higher than that in normoxia (p < 0.01). MPF of VM significantly decreased with increasing exercise intensities (p<0.01), but there were no significant differences between FIO2 conditions (p = 0.37). In addition, there were no significant differences between exercise intensities (RF: p = 0.49; BF: p = 0.57) and FIO2 conditions (RF: p = 0.49; BF: p = 0.19) for RF and BF. CONCLUSION: The current results demonstrated that the change in MPF was different for each muscle, and MPF of VL in hypoxia was only higher than that in normoxia, and suggested hypoxia exposure affects neuromuscular activity agonist muscle during exercise.

Dynamic Postural Instability in Individuals with High Body Mass Index

Objective(s): To evaluate and compare static and dynamic balance between individuals with high body mass index (BMI) and normal BMI.Methods: This case-control study included 19 individuals with high BMI and 7 individuals with normal BMI. It was conducted at a regional level university setting. The center of pressure (COP) changes during double limb stance and single limb stance were obtained using a force plate (Brazil Do). The main COP outcome measures included total displacement, area of sway, amplitude, velocity, and mean power frequency changes in anteroposterior (AP) and mediolateral (ML) directions.Results: During single limb stance, there was a significant difference in COP amplitude (cm) (p=0.015) and velocity (cm/s) (p=0.012) between the 2 groups in the AP direction. However, there were no significant differences found in the ML direction for amplitude (cm) (p=0.155) or velocity (cm/s) (p=0.186). There was no significant difference in the COP outcome measure for double limb stability for both the eyes closed and eyes opened conditions.Conclusion: Individuals with high BMI have impaired dynamic balance control in the AP direction reflected through higher COP amplitude and velocity. They have preserved stability in the ML direction during single limb stance stability. The COP changes during the double limb stance are unimpaired, both with eyes opened and closed.

Oxygenation and EMG in the proximal and distal vastus lateralis during submaximal isometric knee extension

Muscle oxygenation responses are reportedly greater in the distal muscle region than in the proximal muscle region. We combined near infrared spectroscopy and electromyography (EMG) to determine whether regional differences in oxygenation are associated with differences in (1) muscle activation and/or (2) fatigue development. Nine males performed 2-min sustained isometric knee extensions at 15% and 30% maximum voluntary contraction during which oxygenation and EMG were recorded simultaneously from proximal and distal locations of the vastus lateralis muscle. Near infrared spectroscopy variables for oxygen saturation (StO2%) were initial slope at contraction onset, peak drop, and recovery slope at contraction end. Electromyography produced the root mean square to indicate muscle activation and mean power frequency changes over time (decreasing slope) to indicate fatigue development. For StO2%, significantly greater peak drop and steeper recovery slope were found for the distal muscle region than for the proximal muscle region. Root mean square, however, was not different between locations. Mean power frequency decreased throughout the contractions but changes were not different between locations. Our results indicate that for modest submaximal contractions, regional differences in oxygenation are not associated with differences in muscle activation or with fatigue development (as interpreted by changes in mean power frequency over time).

Experimental and modeling investigation of spectral compression of biceps brachii SEMG activity with increasing force levels

This study investigated possible motor unit (MU) firing patterns underlying changes in biceps brachii (BB) surface electromyographic (SEMG) activity in 96 participants who performed isometric actions of the elbow flexors at 40%, 60%, 80%, and 100% of maximum voluntary contraction (MVC). We also conducted a modeling investigation to determine the extent to which a model would fit the experimental results. Experimentally, there was a linear increase (277%; p < 0.01) in root-mean-square (RMS) amplitude with increasing force. The mean power frequency (MNF) remained stable from 40% to 80% of MVC, but there was a decrease (8.2%; p < 0.01) between 80% and 100% of MVC. A modeling approach was taken wherein well-known recruitment and rate-coding schemes activated MUs whose basic building block was the muscle fibre action potential. Two conditions were investigated: (1) an increase in firing rate (rate-coding) and (2) synchronization. The levels of rate-coding and synchronization were selected to produce a linear RMS–force relationship as observed in the experimental data. Then, the impact of these two strategies on changes in MNF was assessed. The MNF remained stable from 40% to 80% of maximum excitation for both the rate-coding and synchronization conditions. There was a decrease in MNF between 80% and 100% of maximum excitation for both modeling conditions, similar to that observed for the experimental data. Thus, at these high forces at which experimental data are technically difficult to obtain, the model supports the idea that both rate-coding and synchronization are responsible for the changes observed in surface EMG amplitude and frequency characteristics.

Age-Related Differences in the Effect of a Perceived Threat to Stability on Postural Control

Studies indicate that the strategy for postural control may be affected by psychological factors, and that young adults respond to perceived threat to stability by increasing their ankle stiffness. The objective of this study was to compare the postural control strategy adopted by young and old adults when faced with postural threat induced by manipulating surface height. Sixty elderly (mean age 77.5+/-4.4 years) and 20 young volunteers (mean age 21.5+/-3.7 years) participated in the study. Movement of the center of pressure (COP) was recorded for 60 seconds with a portable force-plate, as participants stood with eyes open and closed, both at ground level and 85 cm above ground level. Analysis of variance and Tukey-Kramer tests were conducted to determine the effects of age, postural threat, and vision on mean power frequency (MPF), on amplitude variability both in the anterior-posterior (AP) and medio-lateral (ML) directions, and on mean COP sway velocity, with the significance level set at p=.05. Postural threat did not affect COP measures for the young adults, whereas it induced greater changes in MPF in the AP and ML directions in the elderly participants. A similar trend was observed in the elderly group for amplitude variability in the ML direction and for mean COP sway velocity. In comparison with young adults, elderly adults tend to have an exaggerated postural response to conditions that are perceived as threatening to stability. This response involves primarily increases in MPF, with limited compensatory decreases in amplitude variability observed only in the ML direction.

Net caloric cost and electromyographic responses to seated leg presses performed against inertial resistance

An inertia-based ergometer is among the latest devices suggested to abate space flight-induced muscle deconditioning. The current study uses separate sets of subjects to assess net caloric cost (n = 14) and electromyography (EMG) (n = 46) responses to leg presses done on the ergometer. All subjects performed the eight-repetition protocol three times over a two-week period. Net caloric cost measurements entailed indirect calorimetry from O 2 uptakes resulting from exercise on the ergometer. EMG subjects provided first and eighth repetition work, root mean square (RMS) and mean power frequency (MPF) data. Work output was more correlated to net caloric cost (r = 0.83, p < 0.05) than were anthropometric variables. EMG subject data showed work had gender-by-muscle-action and repetition-by-muscle-action effects, with male concentric and first repetition concentric data responsible the respective interactions. From the first to the eighth repetition, eccentric work incurred insignificant changes. RMS had a repetition-by-muscle-action effect, with eighth eccentric repetition data the source of the interaction. MPF showed an effect for repetition (eighth < first). In lieu of significant eighth-repetition RMS and MPF changes, high-threshold lengthening motor unit recruitment and/or greater series elastic element activity may explain the lack of eccentric work losses.

Faster Rate Of Tissue Oxygen Extraction During Contraction After Short Period Of Resistance Training

PURPOSE This study was carried out to examine the effect of short period of resistance exercise training on the muscle oxygen utilization capability at onset of muscle contraction using near infrared spectroscopy (NIRS). METHODS Twelve healthy male adults were divided into training group (TR: n=6, 27.7+/− 5.0 yrs) and untrained group (CON: n=6, 24.5+/− 4.5 yrs). The TR took part in the resistance training, which consisted of 4 sessions, 7 sets of 10 repetitions of isokinetic knee extension at 120 degree/sec. All subjects performed isometric knee extension at two different workloads for 1 minute before and after the training period. The workloads corresponded to 30% and 50% of their maximum voluntary contraction force (MVC) obtained prior to the training. The [deoxy-Hb/Mb] signal of the NIRS recorded from vastus lateralis muscle, was fitted as an exponential equation to determine the time constant (t), time delay (TD) and amplitude (AP). Electromyography (EMG) also analyzed in terms of average amplitude EMG (AMP) and mean power frequency (MPF). RESULTS The isometric MVC in the TR increased significantly, but not in CON. The t of NIRS kinetics significantly reduced in the TR at 30%MVC after the training (13.53+/− 2.30 sec to 9.20+/− 2.11 sec, p<0.01), but not at 50%MVC (7.35+/− 2.93 sec to 6.34+/− 2.98 sec, ns). The TD and AP of NIRS kinetics did not change significantly in the TR. The AMP and MPF at 30%MVC tended to reduce in the TR after the training. Besides, there was a significant relationship between change in the t and change in MPF among the TR at 30%MVC (r=0.833, p<0.05, n=6). On the other hand, there was no significant change in NIRS and EMG parameters in the CON. CONCLUSIONS The short period of resistance training (total effort was only 3.5 min) induced a facilitation of oxygen utilization in mitochondria even though a reduction of the muscle activation level at same workload as a result of an improvement of the MVC.

Masseter Muscle Fatigue Induced by Experimental Sustained Light Clenching

Purpose: The aim of this study was to clarify the characteristics of masseter muscle fatigue induced by experimental sustained light clenching with rest intervals between clenching events.Materials and Methods: The subjects consisted of 18 healthy young adults. For each recorded task, the subjects were asked to perform experimental tooth clenching 3 times, with a duration of 60s for each event and an interval (30s or 60s) between successive clenching events. The experimental clenching forces selected were 10% and 20% of maximum voluntary clenching (MVC). The electromyographic activity during the experimental procedure was measured. The mean power frequency (MPF) of muscle activity was used as an indicator of muscle fatigue. Changes to lower frequencies of the MPF were then analyzed.Results: Despite the light clenching forces used (10% and 20% of MVC), masseter muscle fatigue occurred. When the subjects performed experimental clenching at 10% of MVC with intervals of 60s between clenching events, the MPF changes were gradually larger depending on the events. The regression line had the best fit to data when the subjects were asked to perform experimental clenching under the conditions mentioned above.Conclusion: It was possible to obtain a regression line that represented the relationship between clenching events and masseter muscle fatigue.

Evaluation of Shoulder Muscular Fatigue Induced During VDT Tasks

This study was designed to evaluate localized muscular fatigue induced during visual display terminal (VDT) tasks. In the experimental paradigm used, electromyography (EMG) signals were not recorded during the VDT task but during isometric contractions in which the load imposed on the shoulder muscle was kept constant. The change in mean power frequency (MPF) and the root mean square values of EMG signals with time were explored. The correspondence between these measures and the psychological rating of localized muscular fatigue also were examined. The effectiveness of the experimental paradigm and of the measures used for evaluation of localized muscular fatigue are discussed. MPF measured during isometric contraction was found to be a sensitive measure of localized muscular fatigue.

Paraspinal muscle denervation, paradoxically good lumbar endurance, and an abnormal flexion-extension cycle in lumbar spinal stenosis.

A descriptive study was conducted to investigate the paraspinal muscle function in patients with lumbar spinal stenosis. To evaluate paraspinal muscle innervation and endurance in lumbar spinal stenosis. Abnormal lumbar paraspinal muscle function is associated with chronic low back pain. Abnormal neurophysiologic findings of the lower limbs often are observed in lumbar spinal stenosis, and abnormal lumbar function also can be expected. However, paraspinal muscle function and innervation have not been studied in lumbar spinal stenosis. The study evaluated 25 patients with clinically and radiologically diagnosed lumbar spinal stenosis. Electromyography of the paraspinal muscles was performed from L3 to S1 bilaterally using a concentric needle. At least 20 insertions were analyzed from each muscle. The aim of the examination was to detect abnormal spontaneous activity associated with axonal damage (fibrillation potentials, positive sharp waves, and complex repetitive discharges). Paraspinal muscle activity during trunk flexion-extension movement and muscle endurance during the dynamic isoinertial back endurance test were assessed by surface electromyography. Muscle fatigue was calculated using mean power frequency analysis. Abnormal findings in needle electromyography of the paraspinal muscles were observed in 18 of the 22 (81.8%) examined patients. Abnormal flexion-extension activation of the paraspinal muscles was observed in all the examined patients. The change in mean power frequency was significantly smaller than in previously evaluated healthy subjects and patients with nonspecific chronic low back pain (P < 0.001) who were not experiencing symptoms of lumbar spinal stenosis. Paraspinal muscle fatigability was not associated with the denervation of the muscles. CONCLUSIONS Denervation and abnormal activation of lumbar paraspinal muscles are frequent findings in patients with lumbar spinal stenosis who have not undergone surgery. The paraspinal muscle endurance of the patients was unexpectedly good.

Mechanomyogram and electromyogram responses of upper limb during sustained isometric fatigue with varying shoulder and elbow postures.

To investigate the behavior of mechanomyogram (MMG) and electromyogram (EMG) signals in the time and frequency domains during sustained isometric contraction, MMG and surface EMG were obtained simultaneously from four muscles: upper trapezius (TP), anterior deltoid (DL), biceps brachii (BB), and brachioradialis (BR) of 10 healthy male subjects. Experimental conditions consisted of 27 combinations of 9 postures [3 shoulder angles (SA): 0 degree, 30 degrees, 60 degrees and 3 elbow angles (EA): 120 degrees, 90 degrees, 60 degrees] and 3 contraction levels: 20%, 40%, and 60% of maximum voluntary contraction (MVC). Subjective evaluations of fatigue were also assessed using the Borg scale at intervals of 60, 30, and 10 sec at 20%, 40%, and 60% MVC tests, respectively. The mean power frequency (MPF) and root mean square (RMS) of both signals were calculated. The current study found clear and significant relationships among physiological and psychological parameters on the one hand and SA and EA on the other. EA's effect on MVC was found to be significant. SA had a highly significant effect on both endurance time and Borg scale. In all experimental conditions, significant correlations were found between the changes in MPF and RMS of EMG in BB with SA and EA (or muscle length). In all four muscles, MMG frequency content was two or three times lower than EMG frequency content. During sustained isometric contraction, the EMG signal showed the well-known shift to lower frequencies (a continuous decrease from onset to completion of the contraction). In contrast, the MMG spectra did not show any shift, although its form changed (generally remaining about constant). Throughout the contraction, increased RMS of EMG was found for all tests, whereas in the MMG signal, a significant progressive increase in RMS was observed only at 20% MVC in all four muscles. This supports the hypothesis that the RMS amplitude of the MMG signal produced during contraction is highly correlated with force production. Possible explanations for this behavioral difference between the MMG and EMG signals are discussed.

Effects of dehydration and rehydration on EMG changes during fatiguing contractions.

This study measured the effects of sauna-induced dehydration (Dhy) and the effectiveness of rapid rehydration on muscle performance and EMG frequency spectrum changes associated with fatigue during isometric contractions. Knee extensor muscle strength during isometric maximal voluntary contraction (MVC) and endurance time at 25% and 70% of MVC (ET25 and ET70, respectively) were measured three times in 11 healthy male subjects, under euhydration conditions (Eu), after Dhy, and after rehydration following Dhy (Rhy). Dhy led to a decrease in body weight by 2.95 +/- 0.05%. No significant effect of the hydration status was shown on MVC values. A 23% decrease in ET25 was recorded during Dhy (P < 0.01), whereas ET70 only tended to decrease (-13%, P = 0.06). ET25 was higher during Rhy than Dhy (8%, P < 0.05) but remained lower than during Eu (-17%, P < 0.05). The EMG root mean square (RMS) increased earlier during Dhy than Eu. Opposite changes were shown for the mean power frequency (MPF) of EMG, and Dhy resulted in an accelerated fall in MPF. However, because ET25 decreased with dehydration, RMS and MPF changes were similar during Eu and Dhy when reported to normalized contraction time, exhaustion was thus associated with similar values of RMS and MPF for all conditions. RMS and MPF changes during Rhy showed an intermediate pattern between Eu and Dhy. Dhy induced an increase in muscle fatigue, associated with early changes in EMG spectral parameters. It is not clear whether these alterations could be attributed to biochemical modifications, and the role of increased perception of effort when subjects were dehydrated should be clarified.

Influence of electrode position on changes in electromyograph parameters of the upper trapezius muscle during submaximal sustained contractions.

The purpose of the present study was to evaluate whether differences could be found in the changes in mean power frequency (MPF) and root-mean-square (rms) due to electrode positions on the upper trapezius muscle during a sustained submaximal task. A group of 25 healthy subjects performed a continuous forward flexion of the right arm at 20% of their maximal voluntary contraction (MVC). Three pairs of bipolar surface electrodes were positioned on a straight line between the spine of the seventh cervical vertebra (C7) and the lateral edge of the acromion. The fourth pair (the caudal position) was placed 2 cm below the midpoint of this line. The analysis of variance revealed significant differences between the electrode positions during the short 20% MVC and during MVC (P < or = 0.01). Furthermore, lower rms values were found when the electrodes were placed symmetrically around the midpoint of the reference line, which confirms previous studies. A statistical model has been developed to discriminate additionally the very small changes in the electromyogram parameters due to electrode position during the sustained 20% MVC. It was found that the slope coefficients of rms and MPF were significantly different from 0 (P < or = 0.01), although to a lesser degree for rms and more clearly for MPF of the caudal position (P < or = 0.05). Furthermore, significant differences were found between several combinations of the uppermost electrodes and the caudal position (P < or = 0.01). From this study, it is concluded that it is important to evaluate several electrode positions on the upper trapezius muscle to be able to represent the behaviour of this muscle accurately during a sustained contraction.

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 gastrocnemius 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 seen: to indicate that simulated microgravity preferentially affects muscles having an antigravity function.

Trunk extensor endurance and its relationship to electromyogram parameters.

The present study was designed to investigate the relationship between muscle performance and electromyogram (EMG) parameters of the trunk extensor muscles in the development of fatigue. Nine subjects performed continuous isometric trunk extensions at 25% and 40% maximal voluntary contraction. The EMG signals of the longissimus thoracis, iliocostalis lumborum, multifidus and latissimus dorsi muscles were recorded. The EMG amplitude (RA-EMG) appeared to increase consistently during the contractions in all muscles, whereas the mean power frequency (MPF) showed a fairly consistent decrease during the contractions. The time constants of the exponential change of the RA-EMG and of the MPF were related to the endurance time. The prediction of endurance based on both EMG parameters appeared to yield better results than the prediction based on the relative force. In particular the time constants of the MPF changes of the multifidus and longissimus muscles appeared to be good predictors of endurance time. The consistency of the spectrum shift of EMG appeared to coincide with a reduced variability of the activation of the muscle involved.

Electromyogram spectrum changes during sustained contraction related to proton and diprotonated inorganic phosphate accumulation: a 31P nuclear magnetic resonance study on human calf muscles.

The calf muscles of five clinically healthy men were submitted to isometric exercise and examined by 31P nuclear magnetic resonance (NMR) spectroscopy and electromyography (EMG) to evaluate the influence of proton (H+) and diprotonated forms of inorganic phosphate (H2PO4-) accumulation on EMG spectrum changes. The experiments were performed in a supra-conducting magnet (2.35 Tesla, 35-cm effective diameter) using a surface coil (7-cm diameter) positioned against the calf muscles. The EMG surface electrodes were applied on the gastrocnemius medialis muscle and acquisition of both NMR and EMG signals was synchronized. The exercise consisted of a sustained isometric contraction at 70% of the maximal voluntary contraction until exhaustion. A continuous decrease in phosphocreatine content and a large concomitant increase in H2PO4- was observed in the calf muscles of each subject. A significant increase in H+ concentration was also found when considering the whole population but intracellular acidosis was low for two subjects. Moreover, a quasilinear decrease in mean power frequency (MPF) was found during the test. Changes in MPF were correlated with variations in H+ and H2PO4- concentration but a more significant relationship was found when MPF changes were correlated with H2PO4- concentration. An interpretation of EMG spectrum changes in terms of an accumulation of by-products of anaerobic metabolism and an increase in the relative number of activated slow fibres is proposed.

The fatigability of two agonistic muscles in human isometric voluntary submaximal contraction: an EMG study. I. Assessment of muscular fatigue by means of surface EMG.

During an external isometric constant torque (25% of the maximal voluntary contraction) maintained until the maximal endurance time (limit time), we analysed and compared the changes in electromyographic (EMG) activity illustrating muscular fatigue simultaneously with mechanical activity (the tangential acceleration theta") related to physiological tremor. The EMG activities recorded were of two agonistic flexors, the biceps brachii (BB) and the brachioradialis (BR) muscles and one of the main extensors, the triceps brachii (TB). The integrated EMG increase and the mean power frequency (MPF) of the power spectrum density function (PSDF) decrease were larger for BR than for BB activity. These two findings suggested a greater BR fatigability. However, it is shown that differences between BB and BR MPF changes could be related to differences in the PSDF upper frequency limit of the two muscles and also to the relative magnitude of their tremor component.

Quantitative surface EMG during sustained and intermittent submaximal contractions

In an earlier occupational study the EMG was found to be unchanged during a whole working day. The purpose of this study was therefore to examine EMG changes during experiments in a laboratory environment comparable to occupational work. The surface EMG of the quadriceps muscle of 16 subjects was analysed during 3 experiments: (I) 10% maximal voluntary contraction (MVC) sustained for 1 h, and with a constant work intensity of the muscle; III) 20% MVC for 5 sec alternating with a rest of 5 sec for 1 h; (III) 20% MVC was sustained until exhaustion. In the latter the endurance time decreased with increasing MVC. The results indicated that the methods used in this study can be trusted to reveal changes in EMG due to fatigue in a group of subjects but not in the individual subject. The decrease in mean power frequency (MPF) and the increase in root mean square amplitude (RMS) were most pronounced during the first 10 min of the 10% sustained contraction, indicating a decrease in the conduction velocity along the muscle fibres or synchronization of motor unit firing. In contrast, at 20% intermittent contraction the MPF was unchanged. Between 10 and 60 min of contraction time the increase in RMS amplitudes associated with none or only slight changes in MPF indicated recruitment of new motor units for both types of contraction. The lack of EMG fatigue during a whole day in an earlier occupational study may be because the work performed required varying degrees of muscle force. Finally, the EMG findings indicated that 10 min rest after 1 h of contraction was not enough to restore the muscle.

Surface EMG recordings during maximum static shoulder forward flexion in different positions.

This study investigated how position in the range of motion influences the power spectral density function during static shoulder forward flexion. 23 healthy females (20-30 years) volunteered as subjects. They performed maximum static shoulder forward flexions in three positions: 45, 65 and 90 degrees of shoulder flexion. An isokinetic dynamometer was used and the subjects were seated in a specially constructed chair to enable adequate fixation. The elbow was extended and the hand pronated. Electromyographic (EMG) signals (using surface electrodes) were obtained from the descending part of the right trapezius, the anterior portion of the right deltoid, the right infraspinatus and the common belly of the right biceps brachii. The four EMG-signals and the torque and shoulder angle were analyzed by computer. For each 256 ms, mean power frequency, root mean square value and mean torque were calculated. At each of the three positions four 256 ms periods were analyzed and the data are presented as their means. In the trapezius and the biceps brachii the mean power frequency did not change between the three positions. Deltoid and infraspinatus had significantly higher mean power frequencies at 90 degrees than at 45 degrees of flexion. Different factors behind the change in mean power frequency are discussed. The need to standardize the range of motion when studying dynamic fatiguing contractions is emphasised.