Electromyogram Median Frequency Research Articles

Myoelectric, Myo-Oxygenation, and Myotonometry Changes during Robot-Assisted Bilateral Arm Exercises with Varying Resistances.

Robot-assisted bilateral arm training has demonstrated its effectiveness in improving motor function in individuals post-stroke, showing significant enhancements with increased repetitions. However, prolonged training sessions may lead to both mental and muscle fatigue. We conducted two types of robot-assisted bimanual wrist exercises on 16 healthy adults, separated by one week: long-duration, low-resistance workouts and short-duration, high-resistance exercises. Various measures, including surface electromyograms, near-infrared spectroscopy, heart rate, and the Borg Rating of Perceived Exertion scale, were employed to assess fatigue levels and the impacts of exercise intensity. High-resistance exercise resulted in a more pronounced decline in electromyogram median frequency and recruited a greater amount of hemoglobin, indicating increased muscle fatigue and a higher metabolic demand to cope with the intensified workload. Additionally, high-resistance exercise led to increased sympathetic activation and a greater sense of exertion. Conversely, engaging in low-resistance exercises proved beneficial for reducing post-exercise muscle stiffness and enhancing muscle elasticity. Choosing a low-resistance setting for robot-assisted wrist movements offers advantages by alleviating mental and physiological loads. The reduced training intensity can be further optimized by enabling extended exercise periods while maintaining an approximate dosage compared to high-resistance exercises.

Muscular forearm activation in hand-grip tasks with superimposition of mechanical vibrations

The purpose of this paper is to evaluate the muscular activation of the forearm, with or without vibration stimuli at different frequencies while performing a grip tasks of 45s at various level of exerted force. In 16 individuals, 9 females and 7 males, the surface electromyogram (EMG) of extensor carpi radialis longus and the flexor carpi ulnari muscles were assessed. At a short latency from onset EMG, RMS and the level of MU synchronization were assessed to evaluate the muscular adaptations. Whilst a trend of decay of EMG Median frequency (MDFd) was employed as an index of muscular fatigue. Muscular tasks consists of the grip of an instrumented handle at a force level of 20%, 30%, 40%, 60% of the maximum voluntary force. Vibration was supplied by a shaker to the hand in mono-frequential waves at 20, 30, 33 and 40Hz. In relation to EMG, RMS and MU synchronization, the muscular activation does not seem to change with the superimposition of the mechanical vibrations, on the contrary a lower MDFd was observed at 33Hz than in absence of vibration. This suggests an early muscular fatigue induced by vibration due to the fact that 33Hz is a resonance frequency for the hand-arm system.

Fatigue in the Superficial and Deep Digital Flexor Muscles During Exercise in Thoroughbred Horses

IntroductionOne of the causes of injury to the superficial digital flexor tendon (SDFT) is assumed to be repeated high intensity loading of the tendon. Earlier fatigue of the deep digital flexor muscle (DDFM) in comparison to the time of fatigue of the superficial digital flexor muscle (SDFM) is also assumed to increase load on the SDFT during long distance running. This study evaluated the fatigue of SDFM and DDFM with maximal exercise.MethodsTwo wire electrodes were inserted into the SDFM and DDFM of 6 Thoroughbred horses (461–557 kg). Electromyogram (EMG) and hoof strain gauge signals were recorded at 2,500 Hz with horses on an inclined treadmill (6%), and filter processing was applied (high‐pass 10 Hz for EMG and low‐pass 200 Hz for strain gauge). Integrated EMG (iEMG), the median frequency (MF) of EMG, and the stride frequency (SF) were compared using paired t‐test during 100–105% heart rate max exercise and trotting before and after maximal exercise.ResultsThe SF significantly decreased and iEMG of SDFM tended to decrease during maximal exercise (P = 0.055), while MF of EMG in SDFM and DDFM did not change. The iEMG of SDFM and DDFM significantly decreased and MF of EMG in DDFM tended to decrease at a trot after maximal exercise (P = 0.063).ConclusionsThe DDFM may fatigue earlier than SDFM during long distance running. Further study is needed to evaluate the relationship between the decrease in iEMG of SDFM and DDFM during maximal exercise and the injury to SDFT.Ethical Animal ResearchThe study was approved by the Animal Welfare and Ethics Committee of the Japan Racing Association Equine Research Institute. Sources of funding: Japan Racing Association. Competing interests: none.

Does the fast Fourier transformation window length affect the slope of an electromyogram's median frequency plot during a fatiguing isometric contraction?

Reduced median frequencies (MDFs) are the hallmark of muscle fatigue in an electromyogram (EMG), the magnitude of which may be influenced by window lengths in fast Fourier transformation (FFT) algorithms used to compute power spectra. This study examined whether MDF reductions in fatigued muscles differed across varying FFT windows. EMG data were acquired from the gluteus maximus and semitendinosus muscles during a modified Biering–Sørensen test. Data were processed through varying FFT windows (0.1-, 0.5-, 1.0-, 2.0- and 5.0-s) and MDF slopes were compared with repeated measures analyses of variance. While FFT windows influenced variability in MDFs, the slopes did not differ across window lengths in either muscle group. When muscle fatigue is assessed via MDF slopes during submaximal isometric contractions, the FFT windows through which spectral characteristics are processed may have little bearing on results.

Counterpoint: Spectral properties of the surface emg do not provide information about motor unit recruitment and muscle fiber type

The surface electromyogram (EMG) comprises the sum of the electrical contributions made by the active motor units to the interference signal detected by electrodes placed on the skin overlying the muscle. Because it provides a global measure of motor unit activity, this signal is a valuable tool for

Contraction mode shift in quadriceps femoris muscle activation during dynamic knee extensor exercise with increasing loads

The objective of the present study was to examine the superficial quadriceps femoris (QF) muscle electromyogram (EMG) during dynamic sub-maximal knee extension exercise between young adult men and women. Thirty subjects completed, in a random order, 2 sub-maximal repetitions of single-leg knee extensions at 20–90% of their one-repetition maximum (1RM). Vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) muscle integrated EMG (IEMG) during each sub-maximal lift was normalized to the respective 1RM for concentric, isometric and eccentric modes. The EMG median frequency ( f med) was determined over the isometric mode. Men attained a significantly ( p<0.05) greater knee angular velocity than the women during the concentric mode (83.6±19.1°/s and 67.4±19.8°/s, respectively). RF IEMG was significantly lesser than the VM ( p=0.014) and VL ( p<0.001) muscles, when collapsed across all contraction modes, loads, and sex. Overall IEMG was significantly greater during the concentric ( p<0.001) and isometric ( p<0.001) modes, than the eccentric mode. Men generated significantly ( p=0.03) greater VL muscle IEMG than the women, while the opposite pattern emerged for the RF muscle. VM f med (105.1±11.1 Hz) was significantly lesser than the VL (180.3±19.5 Hz) and RF (127.7±13.9 Hz) muscles across all lifting intensities, while the men (137.7±10.7 Hz) generated greater values than the women (129.0±11.4 Hz). The findings demonstrate a reduction in QF muscle activation across the concentric to eccentric transition, which may be related to the mode-specific velocity pattern.

Fatigue Responses during Repeated Sprints Matched for Initial Mechanical Output

To compare muscle fatigability during two sets of repeated cycling sprints matched for initial mechanical output in a nonfatigued and fatigued state. Eight young men performed 10, 6-s all-out sprints on a cycle ergometer interspersed with 30 s of recovery, followed, after 6 min of passive recovery, by five 6-s sprints, again interspersed by 30 s of recovery. On the basis of total work (TW), performance in sprint 11 (79.8 +/- 4.8 J.kg) was not significantly different to performance in sprint 4 (80.3 +/- 5.3 J.kg; P = 0.81). The decrease in TW for the five sprints after sprint 4 (i.e., sprints 4 to 8) averaged 14.5% (P < 0.001), which was significantly less than the decrement in TW from sprints 11 to 15 (20.3%; P < 0.05). Despite no significant differences in TW values achieved in sprints 4 and 11, the amplitude of the electromyogram (EMG) signal (i.e., root mean square (RMS)) recorded during sprint 11 (0.398 +/- 0.03 V) was 12.0% lower (P < 0.05) than in sprint 4 (0.452 +/- 0.02 V). In contrast, values of EMG median frequency (MF) recorded during sprint 4 (85.5 +/- 5.5 Hz) and 11 (89.3 +/- 7.2 Hz) were not significantly different (P = 0.33). However, the rate of decrease in EMG activity (i.e., RMS and MF) was similar for the two set of sprints. These findings suggest that previous fatiguing repeated-sprint exercise, followed by a rest period, induces greater fatigability during subsequent repeated-sprint exercise, regardless of the initial mechanical output, and that these changes are associated with acute neuromuscular adjustments.

Changes in muscle strength and EMG median frequency after anterior cruciate ligament reconstruction

The purpose of this study was to monitor neuromuscular changes in quadriceps femoris muscle at 1 and 3 months after anterior cruciate ligament reconstruction (ACLR). Changes in isometric muscle strength (MVC), voluntary activation and surface electromyogram (EMG) parameters were examined in relation to knee stability, pain and swelling in 31 patients (25 M, 6 F) mean (SD) 30(8) years. Physically inactive (RC) and sports participants (SC) acted as controls. Median frequency (Hz) and amplitude (mV) of rectus femoris using Fast Fourier Transform (FFT) 2(11) was calculated during 5-s isometric contractions at 100, 75, 50 and 25% of MVC. One month after surgery, a significant correlation (P < 0.01) was found between activation (%) and MVC of injured knee extensors. By 3 months, most patients were pain free and had achieved full activation but still had muscle weakness. At 1 and 3 months post-surgery and for all levels of MVC contraction, the median frequencies of the injured limbs were significantly lower (P < 0.05) compared to the SC group as were those of the RC group. There was a significant lowering of the median frequencies of the uninjured limbs compared to the SC group at 75 and 100% of MVC. The EMG amplitude of the uninjured and injured limbs mirrored those of the SC and RC groups, respectively. These results support the view that muscle activation patterns were altered following ACL injury and surgical repair and may contribute to subsequent changes in muscle fibre properties during detraining and subsequent retraining.

Arm Abduction Angle and Contraction Intensity Effects on EMG Median Frequency During Abduction Isometric Contractions

The electromyogram (EMG) median frequency (MDF) has been observed to be muscle/length and contraction intensity (CI) dependent in particular muscle groups. However, the interacting effect of these aforementioned variables on the EMG MDF of the scapular stabilizer and deltoid muscles has yet to be conclusively established. PURPOSE: The purpose of this investigation was to determine the effect of arm abduction (AA) angle and CI on the EMG MDF of the deltoid and scapular rotator muscles. METHODS: Twelve healthy, young adults (8 men, 4 women) performed maximal and sub-maximal isometric arm abduction contractions in the scapular plane on a Biodex isokinetic dynamometer. Subjects performed five isometric maximal voluntary contractions (MVC), followed by randomly ordered ten-second sub-maximal contractions at 10–90% MVC (10% increments), at the following AA angles: 15, 30, 45, 60, 75 and 90 degrees. During each contraction EMG was obtained from the deltoid (DEL), upper trapezium (UT), lower trapezium (LT) and serratus anterior (SA) muscles. EMG signals were collected at 2000 Hz and bandpass filtered (20–500 Hz). A 3 second analysis window was chosen during the point of maximal torque for the MVC's and during the point at which the subject was producing the target torque. The power spectrum MDF was calculated during 0.5 sec overlapping windows throughout the three second contraction window. RESULTS: The results demonstrated a significant muscle (P<0.001), and CI (P<0.001) main effects on MDF. The highest MDF was calculated for DEL (98.01 Hz) and LT (91.36 Hz), while a lower MDF was calculated for UT (82.52 Hz) and SA (74.19 Hz). The MDF for all muscles decreased with increasing CI. The AA by CI interaction was statistically significant (P<0.001). The 15 degree AA position showed the greater decrease (77.93–106.02 Hz) in MDF than the other AA positions (30 deg: 82.5–94.9 Hz, 45 deg: 94.3–81. 1Hz, 60 deg: 80.9–94.2Hz, 75 deg: 82.2–93.1 Hz, 90 deg: 85.8–96.0 Hz). A significant three way AAby CI by muscle interaction (P<0.001) was found. DEL and UT MDF increased as AA increased at equal CI, LT and SA MDF were unaffected by increasing AA at equal CI. CONCLUSIONS: The major findings of this investigation demonstrate a decrease in the MDF with increasing CI for each of the four muscles. Increasing AA increased the MDF of the DEL and UT but did not affect the MDF of the LT and SA.

Relationships Among Muscle Fiber Type, Electromyography, and Mechanomyography During Fatigue In Resistance- vs. Aerobically-Trained Subjects

PURPOSE: Previous studies have suggested that the mechanomyogram (MMG) may be used as a noninvasive technique to globally estimate skeletal muscle fiber type composition. The purpose of this study was to examine the relationships among muscle fiber type and the time and frequency domains of the MMG and surface electromyogram (EMG). METHODS: Five resistance-trained (RT; age ± SD = 23 ± 4 years; stature =176 ± 8 cm; body mass = 102 ± 38 kg) and five aerobically-trained (AT; 33 ± 5 years; 176 ± 3 cm; 68 ± 4 kg) men volunteered to perform a 30-second isometric leg extension at 50% of the maximal voluntary contraction (MVC) force. MMG and EMG signals were recorded from the vastus lateralis muscle with an accelerometer and bipolar surface electrodes, respectively. MMG and EMG amplitude (mVrms and uVrms, respectively) and median frequency (Hz, discrete Fourier transform) were determined for each consecutive 1-sec epoch. Muscle samples from the vastus lateralis were extracted via needle biopsy, and analyzed for myosin heavy chain (MHC) isoform content. RESULTS: Differences (p <0.05) were observed for percent MHC IIa (RT=59.0 ± 4.2%, AT=27.4 ± 7.8%) and percent MHC I (RT=40.9 ± 4.3; AT=72.6 ± 7.8%), but not (p>0.05) for percent MHC IIb (RT=0.1±0.1 %, AT=0.0 ± 0.0%). MMG amplitude and median frequency were greater (p<0.05) for the RT group than the AT group, however, there were no differences (p>0.05) between groups for EMG amplitude or median frequency. MMG and EMG amplitude increased (p <0.05), EMG median frequency decreased (p <0.05), and MMG median frequency did not change (p>0.05) across time. The patterns of response across time for MMG and EMG were the same for the RT and AT groups. CONCLUSIONS: Surface EMG was unable to distinguish between RT and AT groups. Greater MMG amplitudes for the RT may have been due to greater muscle mass for the RT than the AT group, whereas higher MMG median frequencies may have been due to greater type IIa fiber type and consequent higher motor unit firing rates for the RT group. These findings suggested that the MMG signal may be useful for identifying global differences in fiber type composition.

Fingertip force and electromyography of finger flexor muscles during a prolonged intermittent exercise in elite climbers and sedentary individuals

The aim of this study was to characterize forearm muscle fatigue identified by the decrease in electromyogram median frequency and/or fingertip force during intermittent exercise. Nine elite climbers (international competitive level, USA 5.14a on sight) and ten non-climbers were instructed to maintain a fingertip force of 80% of their maximal voluntary contraction force on a dynamometer mimicking a rock climbing grip during a 5 s effort/5 s rest cycle for 36 repetitions (i.e. 6 min of exercise). Elite climbers lasted twice as long as non-climbers (climbers: 3 min; non-climbers: 1 min 30 s) before the force could no longer be maintained (i.e. the failure point). After this moment, fingertip force decreased and stabilized until the end of the exercise around 50% maximum voluntary contraction force in non-climbers and 63% in elite climbers. Electromyogram median frequency showed a greater decrease in non-climbers than in elite climbers before the failure point. No change in median frequency was observed after the failure point in elite climbers or in non-climbers. These results confirm that elite climbers are better adapted than non-climbers for performing the intermittent fingertip effort before the failure point. After this point, the better fingertip force of elite climbers suggests different forearm muscle properties, while the electromyography results do not provide any indication about the fatigue process.

Quadriceps femoris electromyogram during concentric, isometric and eccentric phases of fatiguing dynamic knee extensions

The objective of this study was to examine the superficial quadriceps femoris (QF) muscle electromyogram (EMG) during fatiguing knee extensions. Thirty young adults were evaluated for their one-repetition maximum (1RM) during a seated, right-leg, inertial knee extension. All subjects then completed a single set of repeated knee extensions at 50% 1RM, to failure. Subjects performed a knee extension (concentric phase), held the weight with the knee extended for 2 s (isometric phase), and lowered the weight in a controlled manner (eccentric phase). Raw EMG of the vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) muscles were full-wave rectified, integrated and normalized to the 1RM EMG, for each respective phase and repetition. The EMG median frequency ( f med) was computed during the isometric phase. An increase in QF muscle EMG was observed during the concentric phase across the exercise duration. VL EMG was greater than the VM and RF muscles during the isometric phase, in which no significant changes occurred in any of the muscles across the exercise duration. A significant decrease in EMG across the exercise duration was observed during the eccentric phase, with the VL EMG greater than the VM and RF muscles. A greater decrease in VL and RF muscle f med during the isometric phase, than the VM muscle, was observed with no gender differences. The findings demonstrated differential recruitment of the superficial QF muscle, depending on the contraction mode during dynamic knee extension exercise, where VL muscle dominance appears to manifest across the concentric–isometric–eccentric transition.

Tongue mechanical characteristics and genioglossus muscle EMG in obstructive sleep apnoea patients

The increased genioglossus muscle (GGm) activity seen in obstructive sleep apnoea syndrome (OSAS) may lead to increased fatigability or longer recovery time of the tongue. Maximal force, endurance, and recovery times of the tongue, electromyogram (EMG) absolute value, and EMG spectral analysis of the GGm obtained during submaximal contractions were compared in eight individuals without chronic snoring and eight OSAS patients. Endurance time values were not significantly different between the two groups (P = 0.40). Time to recovery of initial maximal force was significantly greater in the OSAS group (P = 0.01). Final EMG median frequency was significantly higher (P = 0.01) and the final low-frequency EMG component smaller in the OSAS patients (P = 0.02). Patients did not have changes in endurance time or fatigability but had longer recovery times and changes in spectral analysis variations. This functional investigation may be helpful in determining the presence of OSAS and the potential contribution of the tongue to pharyngeal obstruction.

Electromyogram median frequency, spectral compression and muscle fibre conduction velocity during sustained sub-maximal contraction of the brachioradialis muscle

Changes in the median frequency of the power spectrum of the surface electromyogram (EMG) are commonly used to detect muscle fatigue. Previous research has indicated that changes in the median frequency are related to decreases in muscle fibre conduction velocity (MFCV) during sustained fatiguing contractions. However, in experimental studies the median frequency has been consistently observed to decrease by a relatively greater amount than MFCV. In this paper, a new estimate of EMG frequency compression, the Spectral Compression Estimate (SCE), is compared with the median frequency of the EMG power spectrum, the median frequency of the EMG amplitude spectrum and MFCV measured during sustained, isometric, fatiguing contractions of the brachioradialis muscle at 30, 50 and 80% maximum voluntary contraction (MVC). The SCE is found to provide a better estimate of the observed changes in MFCV than the median frequency of either the EMG power spectrum or EMG amplitude spectrum.

The effects of voluntary contraction effort on quadriceps femoris electromyogram median frequency in humans: a muscle and sex comparison.

The purpose of this study was to examine the effect of voluntary contraction efforts on the median frequency (f(med)) of the electromyogram (EMG) recorded from the quadriceps femoris muscle in healthy men and women. A group of 30 healthy volunteers (15 men, 15 women) were assessed for EMG activity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles during isometric contractions with the knee at 60 degrees flexion. Subjects performed a series of 5 s maximal voluntary isometric contractions that anchored the perceptual range with a "10" on a 10-point scale. Sub-maximal isometric contractions were then separately performed at the following perceived effort levels on the 10-point scale: 1, 2, 3, 4, 5, 6, 7, 8 and 9, in a random order. Subjects were instructed to maintain the contraction at each perceived level of effort for 5 s. The f(med) of the three muscles was assessed using a power spectrum analysis performed over 11 consecutive, 512 ms, epochs overlapping each other by half their length during the middle 3 s of each contraction. The f(med) for each of the 11 epochs was then determined for each muscle, followed by calculation of the means and normalized coefficients of variation [(standard deviation/mean)x100%] for each contraction. The results demonstrated that the mean f(med) of VL was significantly greater than those of the other two muscles, and that f(med) of RF was significantly greater than that of VM. The VL muscle demonstrated a significant increase in mean f(med) across the contraction efforts, compared to the VM and RF muscles that displayed a significant decrease. The men displayed significantly higher f(med) values for the VM muscle than did the women, as well as showing a significantly greater increase across the contraction efforts for the VL muscle. The variability of f(med) was shown to be significantly higher for the VM muscle, compared to the VL and RF muscles. The findings of this study suggest that the f(med) statistic is most sensitive to contraction intensity efforts for the VL muscle, and that men display significantly higher values for the VL and VM muscles, compared to women.

EMG-BASED FATIGUE MEASUREMENTS DURING A DYNAMIC HIGH-INTENSITY EXERCISE

Changes in electromyogram (EMG) median frequency during isometric contractions have been used extensively to quantify muscle fatigue in human subjects. Unfortunately, the application of this technique to dynamic high-intensity activities may result in misleading fatigue measurements. This is due, in part, to muscle temperature changes and the documented influence of these changes on the EMG signal. The purpose of this study was to establish an EMG-temperature compensation technique that could correct for muscle temperature changes during the performance of dynamic high-intensity activities. In phase one of this study, the relationship between muscle temperature and EMG median frequency was determined for the Vastus Lateralis muscle (VL). Subjects were 4 healthy males between the ages of 27–42. The muscle was heated with diathermy to a temperature of approximately 40°C. Isometric EMG data were collected during the performance of a non-fatiguing protocol as the muscle cooled (39–34°C). The results revealed a linear relationship between EMG median frequency and muscle temperature (n = 7, mean slope = 2.85 ± 60 Hz/degree Celsius, R2 > .79). This was consistent with previous works. The mean slope was used as a muscle temperature correction factor that quantified the influence of muscle temperature on EMG median frequency. In phase two of this study, the subjects rode a lower extremity ergometer at workloads of 25%, 50%, and 75% of their VO2 max. Dynamic EMG data was collected as the subjects exercised. In addition, isometric EMG and VL intramuscular temperature data were collected at 10-minute intervals. The muscle temperature correction factor established in phase one was used with the measured change in muscle temperature to compensate for the effects of temperature on the EMG median frequency. To date, the successful use of this temperature correction technique with isometric muscle contractions has been demonstrated with some consistency. Efforts are now directed toward improving the performance of this method and extending the temperature correction methodology for use with dynamic EMG data.

Median power frequency of the surface electromyogram and blood lactate concentration in incremental cycle ergometry.

The electromyogram (EMG) median power frequency of the vastus lateralis and flexor digitorum superficialis muscles was measured in 12 subjects during cycle ergometry with step-wise increasing exercise intensities up to 100% of VO2max. Blood lactate concentration was measured to investigate the relationship between changes in lactate concentration and shifts in the EMG median power frequency of exercising vastus lateralis and non-exercising flexor digitorum superficialis muscles. The results indicated that lactate concentration did not systematically affect median frequency: in spite of a considerable increase in blood lactate concentration, no systematic decrease of the median frequency during exercise was found, either for the vastus lateralis or for the flexor digitorum superficialis muscles. Instead of a decrease of the median frequency during exercise, as seen in isometrical protocols, an increase was seen in most subjects. An interesting finding was a decrease of the median frequency of vastus lateralis muscle during recovery in 8 subjects. The present findings showed that the relationship between EMG frequency decrease, lactate accumulation and fatigue, as observed in isometric protocols, cannot be simply applied to dynamic exercise.

Functional linkages between motor cortical cells and elbow flexor muscles. Evidence for and characteristics of postspike facilitation.

1. Two monkeys (Macaca fascicularis) making high-level but submaximal isometric flexions of the elbow were investigated for the output effect of motor cortical cells on the electromyogram (EMG) activity of two main elbow flexors using the method of spike-triggered averaging of rectified EMGs (STAs). 2. Monkeys were trained to perform individual isometric contractions for > 2 s, and two series of > or = 20 contractions, the second series being at a greater force. EMG electrodes pairs were implanted in the biceps brachii and brachioradialis. A total of 257 cortical cells were found that discharged with the active and passive movements of the elbow. We examined the EMG postspike facilitations (PSFs) produced in either one or the two flexors for only those cells that discharged during the isometric contraction, and provoked PSFs in the two series of contractions. 3. The main characteristics of the EMG isometric contractions in the agonists were analyzed. Spectral analysis showed that the increases in the EMG median frequency with force stabilized at the force levels performed by monkeys. Cross correlation methods showed no cross talk between agonists. 4. The 26 selected cortical cells had a regular discharge frequency. Ten cells did not change frequency with a 22-30% force increase, 14 cells discharged at a higher frequency, and 2 cells discharged at a lower frequency. For single-cell frequencies of 5-65 Hz, interspike intervals < 10 ms were rare: the median and modal intervals were 20-30 ms. 5. The significance of PSFs with respect to the EMG background noise was estimated statistically. STAs from successive epochs under identical load conditions, and STAs performed at a distance from the trigger, showed that PSFs were authentic postspike effects and not sudden EMG changes synchronized by chance with the triggering cell. The features distinguishing PSF from secondary postspike EMG changes or coactivation and task-related effects were studied in simultaneous STAs of flexors and autocorrelogram of cortical spikes. 6. The magnitude of the PSF was expressed as the percent peak amplitude above the mean EMG baseline. The mean percent amplitude of the 90 PSFs produced in both muscles and series was 4.0 +/- 2.4% (mean +/- SD). There was no difference in the average amplitude of PSFs in the two flexors, although the baseline voltages in the biceps brachii were higher. Neither was there any significant change with force while the baseline level increased by 29 +/- 10%, indicating that the absolute PSF amplitude increased in the same proportion as baseline.(ABSTRACT TRUNCATED AT 400 WORDS)

Electromyogram power spectra frequencies associated with motor unit recruitment strategies

The isolated contributions of motor unit recruitment and firing rate variations to the median frequency of the electromyogram's power density spectrum were determined. Orderly stimulation of the cat gastrocnemius motor units via nerve electrodes gave rise to linearly increasing median frequency regardless of the action potential firing rate of the active motor units. Increase in the discharge rate of all the motor units resulted in nearly constant median frequency. It was concluded that the increasing average conduction velocity during motor unit recruitment is the major contributor to variations in the electromyogram median frequency. The possibility of using the median frequency as the index to identify the recruitment control strategies employed by various muscles during increasing force contraction is suggested.