Electromyographic mapping of the erector spinae muscle with varying load and during sustained contraction

The purpose of the study was to explore changes in the spatial distribution of erector spinae electromyography amplitude during static, sustained contractions and during contractions of increasing load. Surface electromyographic (EMG) signals were detected from nine healthy subjects using a grid of 13 × 5 electrodes placed unilaterally over the lumbar erector spinae musculature. Subjects stood in a 20° forward flexed position and performed: (1) six 20-s long contractions with loads ranging from 2.5 kg to 12.5 kg (2.5 kg increments) and (2) a 6 min sustained contraction with 7.5 kg load. Root mean square (RMS) and mean power spectral frequency (MNF) were computed from the recorded EMG signals. EMG RMS increased (P < 0.0001) and MNF remained constant during contractions of increased load. During the sustained contraction, MNF decreased (P < 0.0001) and RMS did not change over time. The centroid (center of activity) of the RMS map shifted caudally (P < 0.0001) with time during the sustained contraction but did not change with varying load. These results suggest a change in the distribution of erector spinae muscle activity with fatigue and a uniform distribution of muscle activation across loads.     

Pain-induced changes in cervical muscle activation do not affect muscle fatigability during sustained isometric contraction

This study investigated whether pain-induced changes in cervical muscle activation affect myoelectric manifestations of cervical muscle fatigue. Surface EMG signals were detected from the sternocleidomastoid and splenius capitis muscles bilaterally from 14 healthy subjects during 20-s cervical flexion contractions at 25% of the maximal force. Measurements were performed before and after the injection of 0.5 ml of hypertonic (painful) or isotonic (control) saline into either the sternocleidomastoid or splenius capitis in two experimental sessions. EMG average rectified value and mean power spectral frequency were estimated throughout the sustained contraction. Sternocleidomastoid or splenius capitis muscle pain resulted in lower sternocleidomastoid EMG average rectified value on the side of pain (P < 0.01). However, changes over time of sternocleidomastoid EMG average rectified value and mean frequency (myoelectric manifestations of fatigue) during sustained flexion were not changed during muscle pain. These results demonstrate that pain-induced modifications of cervical muscle activity do not change myoelectric manifestations of fatigue. This finding has implications for interpreting the mechanisms underlying greater cervical muscle fatigue in people with neck pain disorders.     

Electromyogram and mechanomyogram changes in fresh and fatigued muscle during sustained contraction in men

In surface electromyogram (EMG) and mechanomyogram (MMG) the electrical and mechanical activities of recruited motor units (MU) are summated. Muscle fatigue influences the electrical and mechanical properties of the active MU. The aim of this study was to evaluate fatigue-induced changes in the electrical and mechanical properties of MU after a short recovery period, using an analysis of force, surface EMG and MMG. In seven subjects the EMG and MMG were recorded from the biceps brachii muscle during sustained isometric effort at 80% of the maximal voluntary contraction (MVC), before (test 1) and 10 min after (test 2) a fatiguing exercise. From the time and frequency domain analysis of the signals, the root mean square (rms) and the mean frequency (fˉ) of the power spectrum were calculated. The results were that the mean MVC was 412 (SEM 90) N and 304 (SEM 85) N in fresh and fatigued muscle, respectively; during tests 1 and 2 the mean EMG rms increased from 0.403 (SEM 0.07) mV to 0.566 (SEM 0.09) mV and from 0.476 (SEM 0.07) mV to 0.63 (SEM 0.09) mV, respectively; during test 1 the mean MMG rms decreased from 9.4 (SEM 0.8) mV to 5.7 (SEM 0.9) mV; in contrast, during test 2 constantly lower values were observed throughout contraction; during tests 1 and 2 the EMG fˉ declined from 122 (SEM 7) Hz to 74 (SEM 7) Hz and from 106 (SEM 8) Hz to 60 (SEM 7) Hz, respectively; during test 1 the MMG fˉ increased in the first 6 s from 19.3 (SEM 1.4) Hz to 23.9 (SEM 2.9) Hz, falling to 13.9 (SEM 1.3) Hz at the end of contraction; in contrast, during test 2 the MMG fˉ declined continuously from 18.7 (SEM 1) Hz to 12.4 (SEM 0.8) Hz. The lower MVC after the fatiguing exercise and the changes in the EMG parameters confirmed that 10 min after the fatiguing exercise, the mechanical and electrical activities of MU were altered. In addition, the MMG results suggested that after a 10-min recovery, some highly fatigable MU might not be recruitable. Accepted: 9 June 1998     

Exchange of 20-kDa myosin light chain-bound phosphate during sustained contraction of arterial smooth muscle

K(+)-contracted porcine carotid arterial muscles containing phosphorylated 20-kDa myosin light chains (LC) were exposed to carrier-free [32P]orthophosphate in K(+)-stimulating solution during sustained contraction. The covalently bound LC phosphate was completely replaced by [32P]phosphate, indicating that myosin light chain phosphatase and kinase have ready access to the bound phosphate during the sustained contraction. On average, 0.38 mol [32P]phosphate was incorporated per mole LC during the sustained K+ contraction. This value was about half of the maximal value for [32P]phosphate incorporation into LC, 0.74 mol/mol, in muscles contracted with K+ for 1 min. Assuming that sustained contraction involves the maximal number of cross-bridges attached to actin, the data suggest that half of the attached cross-bridges contain phosphorylated LC.     

The effect of exercise induced hyperthermia on muscle fibre conduction velocity during sustained isometric contraction

This study investigated the effect of dynamic exercise in a hot environment on muscle fibre conduction velocity (MFCV) of the knee extensors during a sustained isometric contraction. Seven trained male cyclists (mean [±SD], age, and were 35 ± 9.9 and 57.4 ± 6.6 ml kg⁻¹ min⁻¹) cycled for 50 min at 60% of peak power output in either: (1) 40 °C (HOT); or (2) 19 °C (NEUTRO); and (3) remained passive in 40 °C (PASS). Post-intervention a 100 s maximal sustained isometric contraction (SMC) of the knee extensors was performed. Rectal temperature increased (p < 0.01) for both HOT and NEUTRO with PASS unchanged and with HOT rising higher (p < 0.01) than NEUTRO (38.6 ± 0.4 vs. 37.6 ± 0.4 °C). Muscle temperature increased (p < 0.01) for all three conditions with HOT rising the highest (p < 0.01) (40.3 ± 0.5 vs. 38.3 ± 0.3 and 37.6 ± 1.3 °C for NEUTRO and PASS, respectively). Lactate showed higher accumulation (p < 0.01) for HOT than NEUTRO (6.9 ± 2.3 vs. 4.2 ± 2.1 mmol l⁻¹). During SMC the torque, electromyography root mean squared (RMS) and MFCV all significantly (p < 0.01) declined. Only in HOT did MFCV decline significantly (p < 0.01) less than torque and RMS (9.9 ± 6.2% vs. 37.5 ± 17.8% and 37.6 ± 21.4%, respectively). In conclusion, during exercise induced hyperthermia, reduced motor unit recruitment as opposed to slower conducting properties of the muscle fibre appears to be responsible for the greater reduction in torque output.     

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.     

Inspiratory muscle function with restrictive chest wall loading during exercise in normal humans

The effects of selective restriction of rib cage (Res,rc) and abdominal wall (Res,ab) movements on endurance of short-term constant-load heavy exercise and on diaphragmatic function during such exercise were examined in five normal young men. An inelastic surgical corset was used to achieve Res,rc and Res,ab. Subjects exercised on a cycle ergometer at 80% of their maximum power output to exhaustion on three occasions: with Res,rc, with Res,ab, and without restriction of chest wall movements (control). Transdiaphragmatic (Pdi), esophageal, and gastric pressures were measured. Electromyogram of the diaphragm was recorded by an esophageal electrode, and the ratio of the power content of a high-frequency to low-frequency band (H/L ratio) was measured. In addition, maximum Pdi (Pdimax) pre- and immediately postexercise was recorded. Res,rc was associated with a shorter endurance time, a progressive decline of the H/L ratio, and a significant reduction of Pdimax postexercise, whereas no such changes were found with Res,ab. We conclude that diaphragmatic function was well defended with abdominal wall loading, whereas limitation of rib cage expansion reduced diaphragmatic endurance during exercise. The diaphragmatic tension-time index (TTdi) in exercise was always less than the critical value of 0.15 found by Bellemare and Grassino (J. Appl. Physiol. 53: 1190-1195, 1982) when subjects inspired against large resistive loads at normal minute ventilations. We suggest that the higher inspiratory flow rate (P less than 0.05) and breathing frequency (P less than 0.05) account for the occurrence of diaphragmatic fatigue in exercise with Res,rc when the TTdi was 0.06 +/- 0.02.     

Impedance and relative displacements of relaxed chest wall up to 4 Hz

We measured the effective resistance (Reff) and elastance (Eeff) of the chest wall in four subjects, relaxed at functional residual capacity (FRC), during sinusoidal volume changes (5% vital capacity up to 4 Hz) delivered at the mouth. Subjects sat in a head-out body plethysmograph, and transthoracic pressure was measured with an esophageal balloon. Changes in Reff and in Eeff with frequency were nearly the same in all subjects. Reff (in cmH2O X l-1 X s) was 2.9 +/- 0.8 at 0.2 Hz and fell sharply to minimum values (0.5-0.9) at 1-4 Hz. Eeff (in cmH2O X l-1) increased from approximately 10 at the lowest frequency to a plateau of about 15 at 1-3 Hz and decreased above 3 Hz. In the same subjects, we measured the relative magnitude and phase between the displacements of different parts of the chest wall with magnetometers during identical sinusoidal forcing. Results indicate that the chest wall expands and deflates uniformly at frequencies up to 1 Hz. Thereafter the abdomen makes relatively larger excursions, and the relative magnitude and phase of displacement at different points on the chest wall show complex changes. We conclude that the frequency dependence of Reff and Eeff below 1 Hz is not due to nonuniformities in displacement of different parts of the chest wall. The frequency dependency of Reff is consistent with an increasing contribution of rate-independent plastic dissipation to the pressure difference in phase with flow as breathing frequency decreases.     

Low frequency sounds from sustained contraction of human skeletal muscle.

Low frequency audible vibrations are produced by human skeletal muscles undergoing sustained contraction. The effect is easily demonstrable with an electronic stethoscope which amplifies sound below 50 Hz. Autocorrelation analysis of the signal shows that it is periodic with a frequency 25 +/- 2.5 Hz. The quality of the sound is the same for all the skeletal muscles tested and is unaffected by changes in tension, ambient temperature, and blood flow. Electrically-stimulated contraction produces a sound which is indistinguishable from voluntary contraction. The amplitude of the sound increases linearly with tension. The sound signals are uncorrelated both in frequency and phase with electromyographic signals obtained simultaneously while the muscle is contacted. Arguments are presented to show that the sounds may be an intrinsic property of muscle contraction.     

Kinin peptides in human trapezius muscle during sustained isometric contraction and their relation to pain.

To determine the muscular concentration of bradykinin and kallidin during static contraction, microdialysis probes were implanted bilaterally in the trapezius muscles of healthy women. Three hours after probe implantation, 200 microM of the angiotensin-converting enzyme (ACE) inhibitor enalaprilat were added to the perfusion solution in one of the sides for 30 min. Thirty minutes later, the subjects performed a sustained bilateral shoulder abduction at 10% of the maximal voluntary contraction until exhaustion. This protocol was repeated twice, with an interval of at least 17 days. High intersession repeatability was observed in the concentration of bradykinin but not of kallidin. Enalaprilat induced a significant increase in bradykinin levels in the dialysate, without affecting kallidin levels. The sustained contraction induced a significant increase in dialysate levels of both kinin peptides. The contraction also induced a significant increase in pain ratings, as measured by a visual analog scale. During contraction, positive correlations were found between pain ratings and levels of kinin peptides in dialysate, predominantly in the side previously perfused with enalaprilat. Subjects with the higher pain ratings also showed larger increases in kinin peptides in the side previously perfused with enalaprilat. The present results show that both plasma and tissue kinin-kallikrein are activated during muscle contraction, but that their metabolic pathways are differently regulated during rest and contraction, because they showed a different response to ACE inhibition. They also indicate that intramuscular kinin peptides levels, and ACE activity, may contribute to muscle pain.     

Relationship between innervation zone width and mean muscle fiber conduction velocity during a sustained isometric contraction

Objectives:

To examine the relationship between the biceps brachii muscle innervation zone (IZ) width and the mean muscle fiber conduction velocity (MFCV) during a sustained isometric contraction.

Methods:

Fifteen healthy men performed a sustained isometric elbow flexion exercise at their 60% maximal voluntary contraction (MVC) until they could not maintain the target force. Mean MFCV was estimated through multichannel surface electromyographic recordings from a linear electrode array. Before exercise, IZ width was quantified. Separate non-parametric one-way analyses of variance (ANOVAs) were used to examine whether there was a difference in each mean MFCV variable among groups with different IZ width. In addition, separate bivariate correlations were also performed to examine the relationships between the IZ width and the mean MFCV variables during the fatiguing exercise.

Results:

There was a significant difference in the percent decline of mean MFCV (%ΔMFCV) among groups with different IZ width (χ² (3)=11.571, p=0.009). In addition, there was also a significant positive relationship between the IZ width and the %ΔMFCV (Kendall’s tau= 0.807; p<0.001).

Conclusions:

We believe that such relationship is likely influenced by both muscle fiber size and the muscle fiber type composition.     

Force fluctuations are modulated by alternate muscle activity of knee extensor synergists during low-level sustained contraction.

The study examined the hypothesis that altered synergistic activation of the knee extensors leads to cyclic modulation of the force fluctuations. To test this hypothesis, the force fluctuations were investigated during sustained knee extension at 2.5% of maximal voluntary contraction force for 60 min in 11 men. Surface electromyograms (EMG) were recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles. The SD of force and average EMG (AEMG) of each muscle were calculated for 30-s periods during alternate muscle activity. Power spectrum of force was calculated for the low- (< or =3 Hz), middle- (4-6 Hz), and high-frequency (8-12 Hz) components. Alternate muscle activity was observed between RF and the set of VL and VM muscles. The SD of force was not constant but variable due to the alternate muscle activity. The SD was significantly greater during high RF activity compared with high VL and VM activity (P < 0.05), and the correlation coefficient between the SD and AEMG was significantly greater in RF [0.736 (SD 0.095), P < 0.05] compared with VL and VM. Large changes were found in the high-frequency component. During high RF activity, the correlation coefficient between the SD and high-frequency component [0.832 (SD 0.087)] was significantly (P < 0.05) greater compared with other frequency components. It is suggested that modulations in knee extension force fluctuations are caused by the unique muscle activity in RF during the alternate muscle activity, which augments the high-frequency component of the fluctuations.     

Rib cage deformities alter respiratory muscle action and chest wall function in patients with severe osteogenesis imperfecta

Background

Osteogenesis imperfecta (OI) is an inherited connective tissue disorder characterized by bone fragility, multiple fractures and significant chest wall deformities. Cardiopulmonary insufficiency is the leading cause of death in these patients.

Methods

Seven patients with severe OI type III, 15 with moderate OI type IV and 26 healthy subjects were studied. In addition to standard spirometry, rib cage geometry, breathing pattern and regional chest wall volume changes at rest in seated and supine position were assessed by opto-electronic plethysmography to investigate if structural modifications of the rib cage in OI have consequences on ventilatory pattern. One-way or two-way analysis of variance was performed to compare the results between the three groups and the two postures.

Results

Both OI type III and IV patients showed reduced FVC and FEV₁ compared to predicted values, on condition that updated reference equations are considered. In both positions, ventilation was lower in OI patients than control because of lower tidal volume (p<0.01). In contrast to OI type IV patients, whose chest wall geometry and function was normal, OI type III patients were characterized by reduced (p<0.01) angle at the sternum (pectus carinatum), paradoxical inspiratory inward motion of the pulmonary rib cage, significant thoraco-abdominal asynchronies and rib cage distortions in supine position (p<0.001).

Conclusions

In conclusion, the restrictive respiratory pattern of Osteogenesis Imperfecta is closely related to the severity of the disease and to the sternal deformities. Pectus carinatum characterizes OI type III patients and alters respiratory muscles coordination, leading to chest wall and rib cage distortions and an inefficient ventilator pattern. OI type IV is characterized by lower alterations in the respiratory function. These findings suggest that functional assessment and treatment of OI should be differentiated in these two forms of the disease.     

Regional chest wall impedance during nonrespiratory maneuvers

To assess changes in total and regional chest wall properties during nonrespiratory maneuvers, we measured electromyographic activity of various chest wall muscles, esophageal pressure, and rib cage and abdominal surface displacements in six subjects before and during various static tasks. Subjects were seated at functional residual capacity, and quasi-sinusoidal forcing at the mouth (0.4 Hz, 500 ml) was imposed during the maneuver in the absence of active breathing. Magnitude of total chest wall impedance (magnitude of Zw) increased with effort during all maneuvers; changes in phase were small. Maneuvers involving primarily muscles of the neck and rib cage--holding a 10-kg weight, 10 kg of isometric tension between the arms, and isometric neck flexion--roughly doubled the magnitude of rib cage impedance (magnitude of Zrc) and, to a lesser degree, increased magnitude of diaphragm-abdomen impedance (magnitude of Zd-a). Unilateral and bilateral leg lifts, in addition to increasing magnitude of Zd-a, increased magnitude of Zrc. Passive 90 degrees rotation of the torso caused approximately 25% increases in magnitude of Zrc and magnitude of Zd-a; if the rotation was actively maintained by the trunk muscles, both regional impedances increased over 100%. Increases in magnitude of regional impedance were correlated to increases in regional electromyographic activity; changes in phase were small. Passive restriction of rib cage displacement by strapping increased magnitude of Zrc and magnitude of Zw but not magnitude of Zd-a, whereas abdominal strapping increased magnitude of Zd-a but did not affect magnitude of Zrc or magnitude of Zw.(ABSTRACT TRUNCATED AT 250 WORDS)