Mean Power Frequency Shift Research Articles

Jaw-Clenching Intensity Effects on Masseter Oxygen Dynamics and Fatigue: A NIRS Oximetry Study.

The purpose of this study was to clarify the effects of jaw-clenching intensity on masseter muscle oxygen dynamics during clenching and recovery and masseter muscle fatigue using the spatially resolved method of near-infrared spectroscopy. Pulse rate, mean power frequency from electromyography in the masseter and visual analogue scale for masseter fatigue were also examined as related items. The 25% and 50% maximum voluntary contractions were determined using electromyography before the experiment and used as visual feedback on the screen. Twenty-three healthy adult male subjects volunteered for this study. Clenching decreased oxygen and oxygenated haemoglobin, and increased deoxygenated haemoglobin in the masseter muscle. The higher the intensity of clenching, the more prominent the effect. The oxygen dynamics tended to return to normal after clenching, but the change was slower with higher clenching intensity. Pulse rate increased with clenching, and the increment was more prominent with higher clenching intensity. Clenching caused a shift of mean power frequency to a lower range, an increase in subjective fatigue, an early appearance of a breakpoint appearance time and a prolongation of a 1/2 recovery time. All of these effects were more evident with increasing clenching intensity. In conclusion, clenching intensity influenced the oxygen dynamics of the masseter muscle and fatigue state during clenching and recovery. The higher the intensity, the greater the impact.

Backpack load affects lower limb muscle activity patterns of female hikers during prolonged load carriage

This study investigated the effect of prolonged load carriage on lower limb muscle activity displayed by female recreational hikers. Electromyography (EMG) signals from vastus lateralis (VL), biceps femoris (BF), semitendinosus (ST), tibialis anterior (TA) and gastrocnemius (GM) were recorded for fifteen female hikers carrying four loads (0%, 20%, 30% and 40% body weight (BW)) over 8 km. Muscle burst duration, muscle burst onset relative to initial contact and integrated EMG signals (iEMG) were calculated to evaluate muscle activity, whereas the shift in mean power frequency (MPF) was used to evaluate muscle fatigue. Increased walking distance significantly decreased the MPF of TA; decreased the iEMG for VL, ST and GM; and shortened VL muscle burst duration. Furthermore, carrying 20–40% BW loads significantly increased VL and GM iEMG and increased BF muscle burst duration, whereas a 40% BW load caused a later VL muscle burst onset. The differences observed in muscle activity with increased load mass seem to be adjustments aimed at maintaining balance and attenuating the increased loads placed on the lower limbs during gait. Based on the changes in muscle activity, a backpack load limit of 30% BW may reduce the risk of lower limb injury for female hikers during prolonged walking.

Changes in sEMG parameters among trunk and thigh muscles during a fatiguing bilateral isometric multi-joint task in trained and untrained subjects

This study aimed to explore changes in the electrical activity distribution among synergist muscles involved in the maintenance of this bilateral multi-joint task. It also tested relations between changes in surface electromyographic (sEMG) parameters with endurance time. Eighteen subjects, trained and untrained in hiking, performed a submaximal (50% of maximal contraction) isometric hiking test until exhaustion. The electrical activity of main superficial muscles implicated in this posture was recorded bilaterally. Trained subjects sustained the hiking position for 315 ± 82 s, versus 225 ± 68 s for untrained subjects. Patterns of electrical activity and mean power frequency (MPF) were different between populations. MPF shift in abdominal muscles was higher than in other synergists for both groups. Although typical changes in sEMG parameters were observed, few relations with endurance time were found, and for untrained subjects only. Changes in the relative contribution among synergists were observed, mainly for trained subjects. It is hypothesized that the task (a complex multi-joint posture involving numerous joints and muscles) may allow some variability in the contribution of synergist muscles during fatigue especially for the trained group. This probably explains the absence of relationship between endurance time and sEMG changes for trained subjects.

작업형태 및 최대 수의적 수축에 따른 등척성 근지구력에 관한 연구

This study was performed to investigate the isometric endurance time as percentages of maximum voluntary contraction. Electromyogram(EMG) and Borg's CR-I0 value were measured by push-pull-up-down tasks for 10 healthy males. The normalized EMG value and the MPF(mean power frequency) were used to estimate the muscle recruitment pattern and the development of muscle fatigue. The subjects exerted and maintained 5 levels of %MVC(maximum voluntary contraction) in 90oshoulder flexion/ 180oelbow extension at sitting posture. The up-task showed the lower endurance time and higher Borg's CR-I0 value than the other task types. Comparing Rohmert's curve with the endurance time of task types. Rohmert's curve overestimated the endurance time of up-task and underestimated the endurance time of push-pull-down tasks. The normalized EMG value showed that muscles recruitment patterns were different from task types. The 4 muscles(biceps brachii muscle, tricep brachii muscle. middle deltoid muscle. trapezius muscle) recruitment patterns of up-task were higher than those of other tasks. The MPF value decreased with the endurance time, and the shift of MPF at up-task was larger than that of the other task types.

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.

Mean power frequency shift during fatigue and recovery in patients with craniomandibular disorders

EMG spectral shifts of anterior temporal and medial masseter muscles were evaluated. Mean power frequency (MPF) shift during fatigue and recovery of 46 healthy subjects and 46 patients with craniomandibular disorder were recorded at the beginning and the end of fatiguing clenching, then 3, 8, 13 and 18 min following the fatiguing clenching. The reference clenching force was 80% of each subject's maximal voluntary contraction (MVC). Recording was stopped when subjects felt pain or discomfort. Significance was found between the healthy group (N) and the diseases group (CMD) in the three following points: (i) the mean of MPF values of the masseter muscles at the end of fatiguing clenching; (ii) the recovery pattern of the temporal muscles; and (iii) MPF shift induced by fatiguing clenching. Our results therefore suggest that MPF reflects several aspects of craniomandibular disorders. Thus, measuring fatigue and recovery MPF could be useful in the screening of CMD.

The temporal occurrence of the mean power frequency shift of the electromyogram during maximum prolonged dynamic and static working cycles.

The study investigated the temporal occurrence of the mean power frequency shift (MPF) of the EMG during prolonged (dynamic and static) working cycles. Ten healthy subjects took part in the experiment. The experimental protocol required each subject to first complete 70 maximum isokinetic knee extensions at 90 degrees s-1). Without any rest, the subject then performed a maximum static knee extension with the dynamometer positioned at 45 degrees of flexion until subjective exhaustion. This sequence of dynamic and static exercise was repeated two more times with a 5-min rest interval preceding each sequence. The EMG signal of the rectus femoris was obtained simultaneously with the torque signal (PT) from the dynamometer. Dynamic contractions PT and MPF decreased throughout the three dynamic periods. Patterns composed of two phases were found; first an initial steep decrease during the first 40 contractions followed by a less steep decrease. Static contractions PT decreased significantly during the three static contractions. Significant (small) decreases in MPF occurred only during the second and third static contractions. Since the major part of the MPF shift occurred early in the exhaustive performance, it is questioned whether the MPF shift detects peripheral fatigue of the type 1 fibers.

Fatigue in the shoulder muscles during static work at two different torque levels

The present study aimed to investigate whether and when a shift in the mean power frequency (MPF) of the electromyogram (EMG) occurs at low torque levels during a maximum endurance test of three shoulder flexors. Twelve clinically healthy women performed two endurance tests of the shoulder flexors (at 50% MVC--the high torque level, and at approximately 18% MVC--the low torque level) until maximum exhaustion. Surface EMG were obtained and MPF and signal amplitude (RMS) were computed for the trapezius descendens, the anterior part of the deltoid and the infraspinatus. The subjects also rated the perception of fatigue in the shoulder muscles throughout the two tests using a 10-graded scale. A significantly higher degree of perceived fatigue was found at the low rather than at the high torque level. In contrast significantly lower MPF were found at the end of the endurance times in the three muscles at the high torque level when compared to the low torque level. At the low torque level MPF of the trapezius was constant throughout the test. In the deltoid the most prominent decrease occurred during the initial 30-40% of the endurance time at the low torque level. It is suggested that the MPF shift mainly reflects peripheral fatigue of the type-2 fibres. The results of the present study question the use of the MPF shift to monitor peripheral fatigue in the fibres active (mainly type-1) at low torque levels.

Is the mean power frequency shift of the EMG a selective indicator of fatigue of the fast twitch motor units?

During repeated maximum isokinetic contractions parallel decreases in output and the mean power frequency (MPF) of the electromyogram (EMG) occur, followed by stable levels (the endurance level). This study aimed to investigate if hypoxia of the plantar flexors will result in a shift in MPF during the endurance level. Mechanical performance and simultaneous surface EMG activity of three plantar flexors (m. soleus, m. gastrocnemius medialis and m. gastrocnemius lateralis) were recorded during repeated isokinetic plantar flexions at 1.05 rad s-1 in 10 healthy women. The study consisted of two tests with 14 days in between: (1) a maximum test consisting of 125 maximum plantar flexions. After 100 contractions, using a tourniquet, a relative ischaemia (hypoxia) was induced. (2) a submaximum test performed at the level established between contractions 70-100 of the maximum test, consisting of 125 plantar flexions. During the maximum test mechanical performance output, the signal amplitude (RMS) of the EMG and the MPF decreased in parallel during the initial 50 contractions followed by stable levels during the subsequent 50 contractions. When hypoxia was induced decreases in mechanical performance and RMS appeared without significant decreases in MPF. The present study indicates that the MPF shift is mainly sensitive to biochemical changes in the type 2 fibres. Hypoxia mainly located to the type 1 fibres will not be detected using the MPF variable.

Electromyographic evidence of local muscle fatigue in a subgroup of patients with myogenous craniomandibular disorders

Ten female patients with myogenous craniomandibular disorders who had complete dentitions and mainly unilateral muscle pain and tenderness to palpation, and 10 healthy female controls, participated in the study. The surface electromyographic signals of masseter and anterior temporal muscles were recorded during 30-s test contractions at 50% of the maximum voluntary clenching effort. A force transducer placed between the central incisors recorded the maximum voluntary bite force. The patients exerted a lower maximum voluntary bite force than the controls ( p < 0.05). In the patients the electromyographic signals of the painful muscles were weaker than those of the painless muscles ( p < 0.01). The signals of the control subjects were stronger than those of the painless muscles of the patients ( p < 0.01). The mean power frequency of the signals, recorded at the start of the 30-s contractions, showed no differences between the painful and painless muscles and between the muscles of the patients and of the controls. The rate of mean power frequency shift in the electromyographic signal, as a response to the 30-s test contraction, was normalized with respect to the amplitude of that signal to account for its amplitude dependency. The painful muscles showed a higher normalized rate of mean power frequency shift than the painless muscles and the muscles of the control subjects ( p < 0.01). These results confirm the presence of an impaired condition in the painful muscles of this group of patients with myogenous craniomandibular disorders.

Effects of partial occlusion of circulation on frequency and amplitude of surface electromyography

We examined two distinct questions. First, would the amplitude and frequency changes characteristic of fatigue produced by an isometric exercise be produced by fatiguing isotonic exercise? Second, are the frequency shifts produced during fatiguing exercise the result of increased metabolite concentration? Ten subjects (five men and five women, aged 22–43 years) performed two sets of 25 biceps curls at between 50 and 60% of their maximal voluntary contraction (MVC). One set was performed with the circulation to the arm unimpaired (NO); during the other set, a blood pressure (BP) cuff held midway between systolic and diastolic pressure was used to partially occlude (PO) circulation. Both the NO and PO conditions produced significant changes in root mean square (rmsEMG) of the amplitude and the mean power frequency (MPF) of the signal. Both the increase in amplitude and the downward shift in MPF of the frequency power spectrum characteristic of fatigue were significantly greater for the PO versus NO condition (PO = 29.47 Hz, NO = 7.39 Hz). These results indicate that the changes in amplitude and frequency characteristic of fatigue observed during isometric exercise can be produced to a lesser degree during dynamic exercise. In addition, these changes can be amplified by addition of occlusion caused by external compression. Because the compressive force used for occlusion allowed blood flow into the working muscle while impairing venous return, this degree of change appears to be directly related to reduced metabolite washout.

Recovery of the human biceps electromyogram after heavy eccentric, concentric or isometric exercise

Five men performed submaximal isometric, concentric or eccentric contractions until exhaustion with the left arm elbow flexors at respectively 50%, 40% and 40% of the prefatigued maximal voluntary contraction force (MVC). Subsequently, and at regular intervals, the surface electromyogram (EMG) during 30-s isometric test contractions at 40% of the prefatigued MVC and the muscle performance parameters (MVC and the endurance time of an isometric endurance test at 40% prefatigued MVC) were recorded. Large differences in the surface EMG response were found after isometric or concentric exercise on the one hand and eccentric exercise on the other. Eccentric exercise evoked in two of the three EMG parameters [the EMG amplitude (root mean square) and the rate of shift of the EMG mean power frequency (MPF)] the greatest (P less than 0.001) and longest lasting (up to 7 days) response. The EMG response after isometric or concentric exercise was smaller and of shorter duration (1-2 days). The third EMG parameter, the initial MPF, had already returned to its prefatigued value at the time of the first measurement, 0.75 h after exercise. The responses of EMG amplitude and of rate of MPF shift were similar to the responses observed in the muscle performance parameters (MVC and the endurance time). Complaints of muscle soreness were most frequent and severe after the eccentric contractions. Thus, eccentric exercise evoked the greatest and longest lasting response both in the surface EMG signal and in the muscle performance parameters.

Electromyographic power-spectrum changes during repeated fatiguing contractions of the human masseter muscle

During 7 sessions at weekly intervals, 10 healthy males performed the same endurance test three times by isometrically contracting the masseter muscles at 20, 30, 40, 50, 60, 75 or 90 per cent of maximal electromyographic activity. The interval between the tests in any one session was 30 s. Changes in the surface electromyogram were monitored by calculating the changes in mean power frequency (MPF) of the signals. With the higher contraction levels, endurance times were shorter, the MPF shifted to lower frequencies more rapidly and the MPF value was lower at the end of the test. At these levels, the subjects reported lack of power as the sole reason for stopping the test but at low levels, pain was the important reason. The endurance time and the rate of MPF shift for the second and third test of each experiment were the same. For the first test, the endurance time was longer ( p < 0.025) and the rate of MPF shift was lower ( p < 0.05). Thus the analysis of specific electromyographic (MPF) indicators provides information about the development of muscle fatigue.

EMG Power Spectrum Analysis to Assess Muscle Fatigue under Isometric Endurance Work

A research experimental study was conducted to make use of EMG power spectrum shift to assess the local muscle fatigue induced by isometric work. Sixteen male volunteer subjects participated in the study. The results indicate that the shift in mean power frequency (MPF) and the time constant of the straight line relationship between in (MPF) and elapsed time are reliable objective measures of local muscular fatigue.