Changes In Median Frequency Research Articles

Gender Differences after Downhill Running for Voluntary Isometric Contractions of Knee Extensor Muscles using Surface EMG

Introduction: We examined gender differences for performance and surface electromyograms (sEMG) of knee extensors 1hr after downhill running. Methods: Maximal voluntary isometric force (MVIF) and sEMG of vastus lateralis (VL) and vastus medialis (VM) muscles during 50%MVIF were measured in 8 males (22±4yr, 75.8±6.1kg, 179.2.0±5.0cm) and 8 females (27±8yr, 60.8±7.2kg, 166.3±5.0cm) before and 1 hr after downhill running [(5x8 min, -12%, 60%V max (females: 7.7±0.9km·h -1 , males: 10.4±0.5km·h -1 )]. Results: Males had higher MVIF values (674.3±59.8N versus 480.6±49.9N, P<0.001) and longer endurance times during 50%MVIF than females (78.7±24.1s versus 57.2±12.6s, P<0.05) before downhill running. After downhill running, MVIF deficits were similar (males: 11.7±7.2%, females: 7.5±7.7%) with no changes in endurance times during 50%MVIF. After the downhill run, there was a trend in females, but not males, of an increased ratio for the root mean square (RMS) values of the VL and VM muscles during MVIF testing by 23% (P=0.054). After the downhill run, females, but not males, lowered during 50%MVIF the change in RMS for VM from 48±32% to 30±31% and for VL from 36±23% to 21±18% (P<0.05). For males, a trend was observed for the change in median frequency in VL during 50%MVIF (P=0.05). After downhill running, females, altered the vastus medialis and vastus lateralis contributions to maximal isometric force production and the activity pattern of these muscles during submaximal isometric fatigue. Conclusion: Acute neuromuscular adaptations after downhill running occur during voluntary isometric contractions of knee extensor muscles in females, but not in males.

Improved surface EMG electrode for measuring genioglossus muscle activity

Activation of the genioglossus (GG) muscles is necessary to maintain the patency of the upper airway. In the condition of obstructive sleep apnea (OSA) this mechanism fails and the possible role of fatigue in its pathogenesis is still not fully understood. In this paper, a new electrode design for recording the genioglossus surface electromyogram (sEMG) is presented. The new design differs from a widely used GG surface electrode in both electrode configuration (unilateral rather than bilateral) and electrode material (Ag-AgCl rather than stainless steel (SS)). The separate effects of these factors were evaluated during force-varying and fatiguing contractions on normal human subjects and using GG sEMG model simulations. Unilateral sEMG was found to have lower amplitude, lower frequency content and a different rate of change of median frequency during fatiguing contractions. It was shown to overcome several disadvantages posed by the bilateral configuration and be more selective. Ag-AgCl has more favorable impedance characteristics and resulted in greater signal amplitudes. It was concluded that the new design is more suitable for detecting GG sEMG and allows more reliable interpretation of changes in sEMG due to physiological mechanisms, thus providing a new methodology for studying GG function and the role of fatigue in OSA.

Differential changes in myoelectric characteristics of slow and fast fatigable frog muscle fibres during long-lasting activity

The electrical activity of different muscle fibre types during fatigue at varying stimulation frequency and fibre stretch was studied. Extracellular action potentials (ECAPs) were recorded from isolated frog muscle fibres at initial length and stretched by 15%, 25% and 35% and stimulated for 180 s by suprathreshold pulses with frequencies of 5, 6.7 and 10 Hz. The changes in ECAP negative phase duration ( T 0), propagation velocity of excitation (PV), potential power spectrum and its median frequency (MDF) were analysed for the period of uninterrupted activity (endurance time, ET). Slow (SMF) and fast (FMF) fatigable muscle fibre types were distinguished by the rate of PV decrease during ET. With the increase of stimulation frequency and fibre stretch, the rate of ECAP parameter changes increased and was larger in FMF, but this proportion was reversed with stretching over 25% and 10 Hz stimulation. In both fibre types the power spectrum shift to lower frequencies during continuous activity was more pronounced with higher stimulation frequency. In FMFs the rates of MDF changes were positively and more strongly correlated with the rates of PV changes, whilst in SMFs the inverse correlation between the rates of changes of MDF and T 0 was stronger. The results indicate specific adaptation of slow and fast fatigable muscle fibres to stretch and activation frequency due to the differences in their membrane processes.

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.

Neuromuscular fatigue development during maximal concentric and isometric knee extensions

This study aimed to investigate mechanisms of neuromuscular fatigue during maximal concentric and isometric leg extensions inducing similar torque decrements. Nine physically active men performed two separate fatiguing sessions maintained until similar torque decreases were obtained. The first session, only conducted under isokinetic concentric conditions (CON), consisted of three series of 30 maximal voluntary concentric knee extensions (60 degrees/s). The second session, exclusively isometric (ISO), mimicked the torque decreases registered during the CON session while performing three long-lasting ISO contractions. Maximal voluntary torque, activation level (twitch interpolation technique), electromyographic activity (root mean square and median frequency) of the vastus lateralis muscle, and electrically evoked doublet-twitch mechanical properties were measured before and at the end of each of the three series. After the three series, similar torque decrements were obtained for both fatiguing procedures. The total fatiguing contraction durations were not different among procedures. With equivalent voluntary torque decrements, the doublet-twitch amplitude reduction was significantly greater (P<0.01) during the two first series of the CON procedure compared with ISO. No difference was observed for the third series. Although no difference was recorded with fatigue for median frequency changes between CON and ISO, activation levels and root mean square values demonstrated greater reductions (P<0.05) for all three series during the ISO procedure compared with CON. Performing CON or ISO fatiguing exercises demonstrated different fatigue origins. With CON exercises, peripheral fatigue developed first, followed by central fatigue, whereas with ISO exercises the fatigue pattern was inverted.

EMG median frequency changes in the neck–shoulder stabilizers of symptomatic office workers when challenged by different physical stressors

The problem of work-related neck and upper limb disorders among computer users has been reported extensively in the literature, and commonly cited risk factors include static posture, speed and force of keyboard operation. The present study examined changes in median frequency (MF) of the neck–shoulder muscles in symptomatic and asymptomatic office workers when they were exposed to these three physical stressors. A quasi-experimental Case–Control design was used to examine MF changes in two groups of female office workers when they were subjected to controlled doses of computer work involving prolonged static posture, increased typing speed and increased typing force. The MF of four major neck–shoulder muscles were examined bilaterally and compared between groups. The MF changes over time-at-task did not clearly illustrate any muscle fatigue mechanism. However, Case Group consistently showed trends for higher MF than the Control Group, and this pattern was observed in response to all three physical stressors. The consistent group differences in MF suggest different muscle recruitment strategies between symptomatic and asymptomatic office workers. These results implied that symptomatic individuals had altered motor control, which may have important implications in understanding the etiology of work-related musculoskeletal disorders.

Intra-rater reliability of electromyographic recordings and subjective evaluation of neck muscle fatigue among helicopter pilots

The aim was to evaluate the reliability of a method of measuring neck muscle fatigue among helicopter pilots. Method: Surface EMG from three areas in the neck region, bilaterally, was recorded among 10 male helicopter pilots while they were performing isometric contractions in flexion and extension for 45 s, sustaining a force representing 75% of maximum strength in a seated position. Perceived fatigue was rated using the Borg CR-10 scale. The test was repeated twice the first day and then two additional times with one-week intervals. Variables analyzed were the slope of the median frequency change, the normalized slope, and the ratings after 15, 30 and 45 s; and also the initial median frequency (IMDF). The intra-class correlation (ICC) and the measurement error ( S w), intra- and inter-day were calculated statistically. Results: The best reliability for the slope was found for the 45 s intra-day analysis taking all measurements into account (ICC 0.65–0.83). The reliability after 30 s was poorer but still acceptable (ICC 0.52–0.71). For the subjective ratings, the highest reliability was found after 30 s inter-day (ICC 0.86–0.88). IMDF showed generally high reliability for the intra-day analyses (ICC 0.63–0.80). Conclusion: The method is reliable for use in further research. Since performing a contraction of 75% of maximum was quite strenuous, we recommend that the protocol be shortened to 30 s.

Effect of lumbar protective belt on prevention of low back fatigue in personnel during simulated driving

To observe low back fatigue of subjects before and after simulated driving, and to study the role of the belt used in prevention of low back fatigue of drivers during driving. To assess the changes in median frequency (MF), mean power frequency (MPF) of surface electromyogram (SEMG), and flicker frequency, visuognosis persistence of drivers with and without protective belt in simulated driving. There were significant differences in MF [(47.35 +/- 6.07), (39.26 +/- 5.79), (47.21 +/- 6.02), (43.44 +/- 6.26) Hz respectively], MPF [(69.86 +/- 7.08), (59.12 +/- 7.19), (69.86 +/- 7.08), (63.88 +/- 7.49) Hz respectively] between before and after simulated driving without or with the special belt (P < 0.01). But no differences in MF or MPF between without and with the belt before simulated driving were found (P > 0.05). The flicker frequency [(35.64 +/- 2.82), (42.31 +/- 4.68), (35.96 +/- 3.05), (39.79 +/- 3.36) Hz], visuognosis persistence (65.77% +/- 3.94%, 56.83% +/- 5.68%, 65.88% +/- 3.92%, 62.27% +/- 2.91%) had some changes too. Simulated diving could induce fatigue of back muscle in drivers. However, the protective belt can effectively prevent low back fatigue of drivers in simulated driving.

Spectral analysis of the electromyograph of the erector spinae muscle before and after a dynamic manual load-lifting test.

The aim of the present study was to assess the spectral behavior of the erector spinae muscle during isometric contractions performed before and after a dynamic manual load-lifting test carried out by the trunk in order to determine the capacity of muscle to perform this task. Nine healthy female students participated in the experiment. Their average age, height, and body mass (+/- SD) were 20 +/- 1 years, 1.6 +/- 0.03 m, and 53 +/- 4 kg, respectively. The development of muscle fatigue was assessed by spectral analysis (median frequency) and root mean square with time. The test consisted of repeated bending movements from the trunk, starting from a 45 masculine angle of flexion, with the application of approximately 15, 25 and 50% of maximum individual load, to the stand up position. The protocol used proved to be more reliable with loads exceeding 50% of the maximum for the identification of muscle fatigue by electromyography as a function of time. Most of the volunteers showed an increase in root mean square versus time on both the right (N = 7) and the left (N = 6) side, indicating a tendency to become fatigued. With respect to the changes in median frequency of the electromyographic signal, the loads used in this study had no significant effect on either the right or the left side of the erector spinae muscle at this frequency, suggesting that a higher amount and percentage of loads would produce more substantial results in the study of isotonic contractions.

Muscle fatigue and fatigue-related biomechanical changes during a cyclic lifting task.

Electromyographic and biomechanical methods were utilized to investigate correlations between indexes of localized muscle fatigue and changes in the kinematics and kinetics of motion during a cyclic lifting task. Recent advances in time-frequency analysis procedures for electromyographicic signal processing provide a new way of studying localized muscle fatigue during dynamic contractions. These methods provide a means to investigate fatigue-related functional impairments in patients with low back pain. To study the relationship between localized muscle fatigue and the biomechanics of lifting and lowering a weighted box. Fatigue-related changes in the electromyographicic signal of trunk and limb muscles were evaluated and compared to kinematic and kinetic measures in order to determine whether lifting strategy is modified with fatigue. A total of 14 healthy male subjects (26 +/- 5 years) cyclically lifted and lowered a 13 kg box (12 lifts/min) for 4.5 minutes. A 5-second static maximum lifting task was included immediately before and after the cyclic lifting task to measure changes in lifting strength and static electromyographicic fatigue indexes. Electromyographic signals from 14 muscle sites (including paravertebral and limb muscles) were measured. Changes in the electromyographicic Instantaneous Median Frequency, a fatigue index, were computed using time-frequency analysis methods. This index was compared with more standardized measures of fatigue, such as those based on electromyographicic median frequency acquired during a static trunk extension test, subjective fatigue measures, and maximal static lifting strength. Biomechanical measures were gathered using a motion analysis system to study kinematic and kinetic changes during the lifting task. During the cyclic lifting task, the electromyographic Instantaneous Median Frequency significantly decreased over time in the paravertebral muscles, but not in the limb muscles. Paravertebral electromyographicic Instantaneous Median Frequency changes were consistent with self-reports of fatigue as well as decreases in trunk extension strength. The magnitude of muscle-specific changes in electromyographicic Instantaneous Median Frequency was not significantly correlated with electromyographicic median frequency changes from the static trunk extension task. The load of the box relative to the maximal static lifting strength significantly affected the electromyographicic Instantaneous Median Frequency changes of paravertebral back muscles. Significant changes with fatigue during the task were found in the angular displacements at the knee, hip, trunk, and elbow. These biomechanical changes were associated with increased peak torque and forces at the L4-L5 vertebral segment. Our results demonstrate correlation between localized muscle fatigue and biomechanical adaptations that occur during a cyclic lifting task. This new technique may provide researchers and clinicians with a means to investigate fatigue-related effects of repetitive work tasks or assessment procedures that might be useful in improving education, lifting ergonomy, and back school programs. Although both the dynamic and static tasks resulted in spectral shifts in the electromyographicic data, the fact that these methods led to different muscle-specific findings indicates that they should not be considered as equivalent assessment procedures.

The relationship between EMG median frequency and low frequency band amplitude changes at different levels of muscle capacity

Objective. To test the validity of high and low frequency band amplitudes of the surface electromyography (EMG) profile as representation of muscle fatigue. Design. A within subjects ( n=10) repeated measures design was used to collect surface EMG signals from the biceps during an isometric contraction under two levels of fatigue status. Background. The use of the shift in the median frequency of the surface EMG power spectrum is a well known method of assessing muscle fatigue. Fatigue also results in amplitude changes of the specific frequency bands. The use of frequency band analysis may be an alternative option for the assessment of muscle fatigue in specific experimental settings. Methods. Surface EMG profiles of the biceps were recorded at 1024 Hz during a sustained isometric hold at 60% of the individuals fresh and fatigued maximal voluntary isometric torque. The median frequency of the power spectrum was compared with changes in the low frequency (15–45 Hz) and high frequency (>95 Hz) bands. Results. There was a close association between median frequency shift and the amplitude of the 15–45 Hz bandwidth and the high–low frequency amplitude ratio. The association was similar for performance under different muscle capacity states. Conclusions. Frequency band amplitude analysis provides similar information to median frequency shift under isometric conditions and may be suited to specific experimental protocols in workplace fatigue studies. Relevance The use of amplitude band analysis that closely approximates the standard median frequency changes allows greater possibility of assessing muscle fatigue in different experimental settings and the use of lower sampling rates.

Trunk muscle fatigue during a back extension task in standing

There is some evidence that the fatiguing characteristics during isometric back extension tasks may assist in identifying differences between individuals with and without low back pain (LBP). During these tasks, especially in standing, other abdominal trunk muscles are also active. The abdominal trunk muscles acting across multiple segments of the lumbar spine function in isolation or in synergy to create flexion torques. It is suggested that co-activation patterns of the trunk muscles are able to control the axis of rotation of the extension torque and also provide multi-segmental stability of the spine. The purpose of this study was to examine the fatigue responses in 4 asymptomatic individuals to a sustained isometric extension task of the trunk muscles evaluating the shifts in the median frequency of the electromyographic (EMG) signal. This study suggests that in asymptomatic subjects, the more superficial abdominal muscles (External Oblique and Rectus Abdominis) increased in activity as the test progressed. There was large inter-individual variation in both amplitude and median frequency changes. Rectus abdominis and the back extensors demonstrated characteristics of fatigue during the task. Studies to test for any characteristic trends in whether specific trunk muscles fatigue in standing is a feature in chronic LBP, invites a formal investigation.

Effects of prolonged hypobaric hypoxia on human skeletal muscle function and electromyographic events

This study tested the hypothesis that a prolonged decrease in arterial oxygen pressure in resting or contracting skeletal muscles alters their ability to develop force through an impairment of energy-dependent metabolic processes and also through an alteration of electrophysiological events. The experiment was conducted during a 32-day simulated ascent of Mt. Everest (8848 m altitude) (Everest III Comex '97), which also allowed testing of the effects of re-oxygenation on muscle function. Maximal voluntary contractions (MVCs) of the flexor digitorum, and static handgrips sustained at 60% of MVC, were performed by eight subjects before the ascent (control), then during the stays at simulated altitudes of 5000 m, 6000 m and 7000 m, and finally 1 day after the return to 0 m. The evoked muscle compound action potential (M-wave) was recorded at rest and during the manoeuvres at 60% of MVC. The changes in median frequency of electromyographic (EMG) power spectra were also studied during the contraction at 60% of MVC. In four individuals, transient re-oxygenation during the ascent allowed us to test the reversibility of hypoxia-induced MVC and M-wave changes. At rest, a significant decrease in M-wave amplitude was noted at 5000 m. This effect was associated with a prolonged M-wave conduction time at 6000 m and an increased M-wave duration at 7000 m, and persisted after the return to 0 m. Re-oxygenation did not modify the changes in M-wave characteristics. A significant decrease in MVC was measured only during the ascent (-10 to -24%) in the non-dominant forearm of subjects who underwent re-oxygenation; this intervention slightly improved muscle strength at 6000 m and 7000 m. During the ascent and after the return to 0 m, there was a significant reduction of the median frequency decrease throughout contraction at 60% of MVC compared with the EMG changes measured before the ascent. It is concluded that prolonged exposure to hypoxia slows the propagation of myopotentials and alters sensorimotor control during sustained effort. Re-oxygenation did not affect the hypoxia-induced EMG changes and had a modest influence on muscle strength.

Wavelet and short-time Fourier transform analysis of electromyography for detection of back muscle fatigue.

Measurement of the time-varying characteristics of the frequency content of trunk muscle electromyography is a method to quantify the amount of fatigue endured by workers during industrial tasks, as well as a tool that may guide the training and rehabilitation of healthy and injured workers. Quantification of the change of signal power within specific frequency ranges may shed greater insight into the fatigue process. Sixteen healthy male subjects performed isometric trunk extension at 70% of their maximum voluntary contraction. Surface electromyography from medial and lateral erector spinae, and latissimus dorsi locations were processed using the short-time Fourier transform (STFT) and wavelet transform. Linear regression quantified the time rate of change of median frequency as well as frequency specific STFT filter and wavelet scale measures. The median frequency from the short-time Fourier transform declined by 22 Hz/min from an initial value of 77 Hz on average. The wavelet and STFT filter measures demonstrated this decline to be caused by a reduction in 209-349 Hz signal power in addition to an increase in 7-88 Hz signal power. A significant reduction in median frequency and significant elevation in 13-22 Hz wavelet signal component was detected in about 90% of the cases, indicating their use for detecting and quantifying fatigue.

Reliability of EMG spectral parameters in repeated measurements of back muscle fatigue

The change in median frequency of the power spectrum of the electromyographic (EMG) signal may be used as a measure of muscle fatigue. The reliability of the median frequency parameters was investigated for EMG-recording sites at L1 and L5 right and left on the erector spinae. The reliability of subjective fatigue ratings of the back muscles (Borg CR-10 scale) and of maximal trunk extension torque (MVC) was also investigated. Eleven subjects with healthy backs performed a 45-s isometric trunk extension at 80% of MVC twice a day, on three different days. Two-factor analysis of variance was made to obtain the different variances from which the standard error of measurement (SEM) and the intra class correlation coefficient (ICC) were calculated. The SEM within-day was somewhat lower than that between-days. Both were about the same at all four electrode sites. The 95% confidence interval for the studied variables was for the initial median frequency ±10 Hz, for the slope ±0.4–0.5%/s, for the MVC ±36 Nm and for the Borg ratings ±1.6. We conclude that, with the presently used method, changes or differences within these limits should be regarded as normal variability. The slope may be of limited value because of its large variability. Whether the low intra-class correlation coefficient for the EMG parameters in the presently studied test group implies a low potential in discriminating subjects with back pain can not be decisively concluded.

Changes in surface EMG parameters during static and dynamic fatiguing contractions

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

Masseter muscle relaxation rate in volunteers with a myogenous craniomandibular disorder.

Twelve volunteers were investigated to determine their masseter muscle relaxation rate following voluntary contractions. Four of these volunteers were patients diagnosed as having a myogenous craniomandibular disorder. Electromyograms were recorded from the left and right masseter muscles and maximum bite force was recorded in the midline between the incisor teeth. A sustained contraction was maintained at 50% maximum voluntary bite force for 90 s, during which there was a brief relaxation every 10 s. Recordings were continued for a 3 min recovery period. This sequence was then repeated at 25% of maximum bite force. Median power frequencies were calculated from the power spectra for the first and last 3 s of the sustained contractions. Relaxation rates were measured for each brief relaxation during the sustained contraction and for the relaxation from each brief clench during the recovery period. It was found that maximum bite force values were very similar for volunteers in both the patient and control groups. Relaxation rates slowed more and percentage changes in median frequency were greater in the controls than in the patients during the sustained contractions. However, relaxation rates returned to initial levels more quickly in the controls than in the patients.

Fatigue of the erector spinae muscles. A quantitative assessment using "frequency banding" of the surface electromyography signal.

The authors investigated fatigue-induced changes in the frequency content of the surface electromyographic (EMG) signal from the erector spinae muscles. The objective of the study was to understand the EMG changes in fatiguing muscle and to obtain a reliable index of fatigue. Power spectral analysis has been used increasingly in recent years to monitor muscle fatigue, but parameters other than the mean or median frequency have received little attention. Thirty-five healthy volunteers participated. They pulled upward with constant force on a handlebar attached to a floor-mounted load cell while the EMG signal from the erector spinae was recorded at the levels of T10 and L3 at 1024 Hz; 1.0-sec "windows" of the signal were analyzed using fast Fourier transforms, and the resulting power spectra were divided into 10 frequency bands between 5 Hz and 300 Hz. The median frequency, total power, and peak amplitude of the spectra were also calculated. Changes in the frequency content of the EMG signal were examined during submaximal contractions of different intensity and duration. Median frequency decreased steadily during the contractions, whereas total power and peak amplitude increased. The most repeatable and linear index of change was the increase in the EMG signal in the 5-30 Hz frequency band. The middle-to-high frequency component of the EMG signal increased during the early stages of the contractions, but decreased as the endurance limit was approached. Changes in the 5-30 Hz band of the EMG power spectrum provide a more reliable and linear index of fatigue in the erector spinae muscles than do changes in median frequency. In the erector spinae, the early effects of fatigue appear to be delayed by the recruitment of additional motor units.

Frequency distribution of breath sounds as an indicator of bronchoconstriction during histamine challenge test in asthmatic children.

In order to study changes in respiratory sounds associated with acute bronchoconstriction and -dilatation, breath sounds of 11 children with asthma (age range, 10-14 years) were recorded at the chest and at the trachea during histamine challenge test and after subsequent bronchodilatation. The changes in frequency spectra of breath sounds were compared with simultaneous changes in forced expiratory volume in 1 second (FEV1). In seven children who responded to histamine with a decrease in FEV1 of more than 15%, there was a significant relationship between percentage change in FEV1 (delta FEV1) and percentage change in median frequency (delta F50) of expiratory breath sounds recorded at the chest (r = 0.865; beta = -0.706, P = 0.0001) and at the trachea (r = 0.888; beta = -1.12, P = 0.0001). The association between breath sound intensity and FEV1 was weaker. Based on ANOVA, the increase of F50 during the challenge test was significantly larger in children who responded to histamine than in those who were non-responsive (P = 0.0016). At the chest, a decrease of 15% in FEV1 corresponded to an increase of 8% in expiratory F50. The provocative dose of histamine inducing a decrease of 15% in FEV1 (PD15FEV1) and the provocative dose causing an increase of 8% in F50 (PD8F50) were significantly related (r = 0.927, P = 0.003). We conclude that spectral analysis of breath sounds can be used to indicate airway obstruction during bronchial challenge tests in children, and may be adapted for tests in pre-school children. The results suggest that the same mechanisms that induce airflow limitation due to inhaled histamine may generate an increase in frequency content of breath sounds in children with asthma.

Changes in Frequency Spectra of Breath Sounds During Histamine Challenge Test in Adult Asthmatics and Healthy Control Subjects

Air-flow standardized breath sounds were recorded at the chest and at the trachea during histamine challenge test and after subsequent bronchodilation in 12 asthmatics and 6 healthy controls for spectral analysis, to be compared with simultaneous changes in spirometric variables. Of all the lung sound variables measured, the changes in median frequency of the power spectrum (F50) of tracheal expiratory sounds were found to correlate best (r = 0.853, p < 0.0001) with changes in FEV1. The increase of F50 during histamine challenge was significantly larger in asthmatics than in healthy control subjects (p < 0.005). The provocative dose of histamine inducing a decrease of 15 percent in FEV1 (PD15FEV1) and the provocative dose causing an increase of 30 percent in tracheal expiratory F50 (PD30F50) were significantly related (r = 0.754, p = 0.012). In asthmatics, the breath sound frequency distribution in terms of median frequency reflected acute changes in airways obstruction with high sensitivity and specificity. The present method for breath sound analysis can be applied for patients with limited cooperation during bronchial challenge tests.