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PubMed2005

Biphasic effect of extracellular ATP on human and rat airways is due to multiple P2 purinoceptor activation.

BACKGROUND: Extracellular ATP may modulate airway responsiveness. Studies on ATP-induced contraction and [Ca2+]i signalling in airway smooth muscle are rather controversial and discrepancies exist regarding both ATP effects and signalling pathways. We compared the effect of extracellular ATP on rat trachea and extrapulmonary bronchi (EPB) and both human and rat intrapulmonary bronchi (IPB), and investigated the implicated signalling pathways. METHODS: Isometric contraction was measured on rat trachea, EPB and IPB isolated rings and human IPB isolated rings. [Ca2+]i was monitored fluorimetrically using indo 1 in freshly isolated and cultured tracheal myocytes. Statistical comparisons were done with ANOVA or Student's t tests for quantitative variables and chi2 tests for qualitative variables. Results were considered significant at P < 0.05. RESULTS: In rat airways, extracellular ATP (10(-6)-10(-3) M) induced an epithelium-independent and concentration-dependent contraction, which amplitude increased from trachea to IPB. The response was transient and returned to baseline within minutes. Similar responses were obtained with the non-hydrolysable ATP analogous ATP-gamma-S. Successive stimulations at 15 min-intervals decreased the contractile response. In human IPB, the contraction was similar to that of rat IPB but the time needed for the return to baseline was longer. In isolated myocytes, ATP induced a concentration-dependent [Ca2+]i response. The contractile response was not reduced by thapsigargin and RB2, a P2Y receptor inhibitor, except in rat and human IPB. By contrast, removal of external Ca2+, external Na+ and treatment with D600 decreased the ATP-induced response. The contraction induced by alpha-beta-methylene ATP, a P2X agonist, was similar to that induced by ATP, except in IPB where it was lower. Indomethacin and H-89, a PKA inhibitor, delayed the return to baseline in extrapulmonary airways. CONCLUSION: Extracellular ATP induces a transient contractile response in human and rat airways, mainly due to P2X receptors and extracellular Ca2+ influx in addition with, in IPB, P2Y receptors stimulation and Ca2+ release from intracellular Ca2+ stores. Extracellular Ca2+ influx occurs through L-type voltage-dependent channels activated by external Na+ entrance through P2X receptors. The transience of the response cannot be attributed to ATP degradation but to purinoceptor desensitization and, in extrapulmonary airways, prostaglandin-dependent PKA activation.

Adenosine Triphosphate↗

PubMed1984

From activity to rest: gating of excitatory autogenetic afferences from the relaxing muscle in man.

The changes in reflex excitability of the motoneurones to the soleus (Sol) muscle occurring during and after voluntary releases of various duration from a constant plantar-torque level in isometric or isotonic conditions have been investigated in normal humans by means of the H-reflex and T-response. The amplitude of both reflexes during the release phase attains values lower than control (obtained in resting conditions) even in the presence of force and EMG activity; the maximal inhibition is reached at the end of the release; the reflexes recover gradually to control values over several seconds. The more abrupt is the release, the more inhibited is the reflex and the shorter is the time to recovery and vv. These results apply both in isometric and isotonic conditions. Activation of the antagonist muscles, sometimes occurring at the end of the fastest release, does not contribute to the H-reflex inhibition. Tonic isometric contractions and relative releases have also been evoked by the tonic vibration reflex (TVR). The H-reflex during the TVR-induced contractions were lower than control values, at variance with those obtained during the voluntary contractions, but their amplitudes during the releases had similar values and time-courses in both conditions, pointing to a common involved inhibitory mechanism. Any voluntary ballistic or ramp contraction taking place after a preceding release, in the period in which the H-reflexes were still inhibited, was not apparently influenced, despite the fact that H-reflexes evoked during the release-conditioned ramp contractions were significantly lower than when evoked during control ramps of similar characteristics. The results are discussed in terms of a premotoneuronal, possibly presynaptic, inhibitory mechanism.

Adult↗

PubMed2005

Statistical modelling of fatigue-related electromyographic median frequency characteristics of back and hip muscles during a standardized isometric back extension test.

The purpose of the present study was to evaluate which statistical model - linear, logarithmic, quadratic or exponential - best described the fatigue-related electromyographic (EMG) changes of back and hip muscles. Twenty healthy volunteers performed a modified Biering-Sorensen test. The EMG activity of the latissimus dorsi (LD), longissimus thoracis pars thoracis (LTT) and lumborum (LTL), iliocostalis lumborum pars thoracis (ILT) and lumborum (ILL), multifidus (MF), gluteus maximus (GM) and biceps femoris (BF) was measured bilaterally using surface electrodes. Higher R(2) values were found for the quadratic models (p<0.05 for all muscles), and lower R(2) values for the logarithmic models (p<0.05 for LTT, LTL, ILL, MF and GM). The exponential models generated higher R(2) values compared to the linear ones for the LTT, LTL and MF (all p<0.05). Further analyses revealed, however, that these models did not add useful additional information, and therefore would only increase the complexity. The findings of the current study validate the use of simple linear regression techniques when studying fatigue-related EMG median frequency characteristics of back and hip muscles during isometric contractions.

Adult↗

PubMed2000

Reduced muscle vascular resistance in intrauterine growth restricted newborn piglets.

It has been shown that asymmetrical intrauterine growth restriction is denoted by disproportional reduction of muscle mass compared to body weight reduction. However, the effects of IUGR on regional vascular resistance and blood flow of skeletal muscles and their contractile function have not been studied until now. Therefore, muscle blood flow (MBF) and isometric force output of serial stimulated hindlimb plantar flexors was measured in thiopental -anesthetized normal weight (NW; n = 9) and intrauterine growth restricted (IUGR; n = 9) one-day-old piglets. Additionally, muscle vascular resistance (MVR) and thyroid hormones were estimated. MBF was found to be markedly increased in IUGR piglets by 36% with a concomitant MVR reduction of 37% (p < 0.05). Isometric force of the plantar flexors was considerably higher in NW than in IUGR piglets (p < 0.05). However, amount of muscle fatigue was more pronounced in NW piglets (9.1+/-2.8%) than was in IUGR piglets (3.7+/-2.3%) (p < 0.05). Furthermore, specific tension of NW muscles (18.8+/-0.7 N/cm2) was significantly lower than for IUGR muscles (21.2+/-0.9 N/cm2) (P<0.05). IUGR newborn piglets exhibited increased plasma levels of thyroxine (T4) (p < 0.05), whereas triiodothyronine (T3) showed similar values in both animal groups. These data clearly indicate that muscle hemodynamics and contractile function are more developed in newborn IUGR piglets. Furthermore it is suggested that the improved tolerance to fatigue during isometric contractions may indicate an increased oxidative capacity of calf muscles due to intrauterine growth restriction.

Animals↗

PubMed1989

[Muscle fiber conduction velocity and frequency parameters of surface EMG during fatigue of the human masseter muscle. 2. Frequency parameters].

During sustained isometric contraction of the jaw muscles, muscular fatigue causes a shift towards lower frequencies in the EMG frequency power spectrum. This shift is due to a reduction of myoelectric conduction velocity. The purpose of this study was to investigate the effect of surface electrode location on the estimates of frequency components, and to determine the appropriate frequency parameters of surface EMG which provided the best fit when comparing conduction velocity. Studies were performed in the masseter muscles of 8 healthy subjects. The myoelectric potentials were derived using 12 differential amplifiers from electrode arrays. The frequency parameters calculated were as follows: peak frequency, mean power frequency, cumulative power function (10, 25, 50 and 75%), median frequency ratio, low frequency ratio and measure H. As the results, following findings were obtained: 1. The high frequency content occurred at the region of end-plate zone and tendonous insertion of the muscle. 2. EMG spectra obtained during fatigue were shifted to lower frequencies for all regions except adjacent end-plate zone. 3. The relationships between conduction velocity and frequency parameters during fatiguing contractions were almost linear except peak frequency. 4. It was concluded that the location between distal end-plate and the tendon was "stable" zone for studying muscular fatigue.

Electromyography↗

PubMed1987

Effects of sodium and calcium concentration on the barium chloride-induced electrical and contractile responses of the guinea pig vas deferens.

1. The mechanism of 10 mM barium chloride-induced electrical and contractile responses of the guinea pig vas deferens was investigated using calcium channel blockers and by modifying the Na+ and Ca2+ concentrations of the nutritional solution. Isometric contraction and membrane depolarization were measured simultaneously by the sucrose-gap technique. 2. In the absence of added Ca2+ there was a decay of the contractile but not of the depolarizing effect, which was independent of the frequency of 10 mM barium chloride administration. However, both barium-induced contraction and depolarization were reduced and the former more rapidly in a Ca2+-free solution (0.5 mM EGTA). 3. In low-Na+ solution (16 mM) there was a 3-fold increase in depolarization and no change in contraction, but in a low-Na+ Ca2+-free solution (0.5 mM EGTA), only contraction was reduced. 4. Nifedipine blocked both barium-induced contraction and depolarization in a dose-dependent manner when the organs were bathed in regular nutrient solution. In contrast, in low-Na+ solution only 50% of the depolarization was blocked by 10(-6) M nifedipine, which completely inhibited contraction. Both barium-induced contraction and depolarization in low-Na+ Ca2+-free solution were blocked by 10(-3) M LaCl3. 5. These data indicate that in the presence of calcium, barium mobilizes extracellular calcium through voltage-dependent channels and the contraction is due to the entry of calcium. In contrast, in the absence of added Ca2+ or in Ca2+-free solutions (0.5 mM EGTA), barium enters through the voltage-dependent channel and the contraction is reduced because the intracellular stores of calcium are limited.

Animals↗

PubMed1989

Isometric and isokinetic knee extension and flexion torque in men and women aged 20-70.

An investigation was made of the influence of age and sex on peak muscle torque in knee extension and flexion during maximal isokinetic and isometric contraction. The study was performed on both legs of 139 clinically healthy men and 141 clinically healthy women aged 20, 30, 40, 50, 60 or 70 years. Maximum knee extension and flexion muscle torque (Newtonmetre, Nm) was measured isokinetically at various angles of velocity (12, 90 and 150 degrees/s) and isometrically under standardized conditions (Cybex II). No significant differences were found between the right and the left leg in the whole material. Muscle torque was higher in men than in women in all age groups (p less than 0.001). Both isokinetic and isometric torque decreased with age in both sexes. Isokinetic torque decreased significantly (p less than 0.05) between 20 and 30 years of age in men and between 40 and 50 years of age in women (at all velocities studied; p less than 0.05). A significant decrease (p less than 0.05) was found between the ages of 60 and 70 years in both sexes. Maximum isometric torque showed a significant decrease (p less than 0.05) between 60 and 70 years in men and women. There were no significant differences in isokinetic or isometric torque between moderately active and inactive men or women. Significant correlations were found between muscle torque and body weight, height and body surface area.

Adult↗

PubMed1994

A modified force-velocity equation for smooth muscle contraction.

It has been suggested that in skeletal muscle the force-velocity relationship may not be a simple hyperbolic one, as defined by Hill's equation. To determine whether smooth muscle demonstrated the same properties, quick-release force-velocity curves were obtained from canine tracheal smooth muscle. The results showed that the observed data points for tracheal smooth muscle systematically deviated from a hyperbola. Such deviation occurred at values of force (P) approaching maximum isometric force (Po) for curves elicited by quick release at 2 and 10 s in the course of isometric contractions. Shortening velocities under a given afterload were overestimated at the high-force end (P > 75% Po) by Hill's equation; this implied that a relationship more complex than a simple hyperbola was involved at high loads. We next focused on finding an equation to also fit those directly measured data points that did not conform to a hyperbola. Our rationale in developing the equation was that a plot of the linearized transform of Hill's equation should yield a straight line over the entire range of loads at which velocities were measured. The plot demonstrated that, in the low-load high-velocity portion of the curve, a peak value was reached at 70-80% Po, which decreased as load increased in the high-load low-velocity portion.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

The effects of treadmill running on the isometric fatigue of the handgrip muscles.

Twelve male subjects, ages 18-32, performed low, moderate, and high intensity systemic treadmill exercise to examine its effect on isometric strength and fatigue of the handgrip muscles. Baseline handgrip values were recorded, as well as maximal oxygen uptake and ventilatory threshold (VT), to determine the intensity for each of the three exercise conditions. Task A involved a 10-min treadmill run at 20% below VT, task B was given at VT, and task C was 20% above VT. Immediately following the endurance treadmill run the subjects were given a 2-min isometric contraction at maximum intensity. Heart rate and blood pressure were also measured. No significant differences were found between control measures and the strength and endurance variables. Rates of fatigue were similar for all conditions, and heart rate and blood pressure remained elevated during the handgrip fatigue task. It is concluded that isometric strength and endurance of non-involved muscles are not affected by prior endurance exercise.

Adolescent↗

PubMed2003

Activation among the elbow flexor muscles differs when maintaining arm position during a fatiguing contraction.

Twenty-four men (n = 11) and women (n = 13) supported an inertial load equivalent to 20% of the maximum voluntary contraction force with the elbow flexor muscles for as long as possible while maintaining a constant elbow angle at 90 degrees. Endurance time did not differ on the three occasions that the task was performed (320 +/- 149 s; P > 0.05), and there was no difference between women (360 +/- 168 s) and men (273 +/- 108 s; P = 0.11). The rate of increase in average electromyogram (EMG) for the elbow flexor muscles was similar across sessions (P > 0.05). However, average EMG during the fatiguing task increased for the long head of biceps brachii, brachioradialis, and brachialis (P < 0.05) but not for the short head of biceps brachii. Furthermore, the average EMG for the brachialis was greater at the start and end of the contraction compared with the other elbow flexor muscles. The rate of bursts in EMG activity increased during the fatiguing contraction and was greater in brachialis (1.0 +/- 0.2 bursts/min) compared with the other elbow flexor muscles (0.5 +/- 0.1 bursts/min). The changes in the standard deviation of acceleration, mean arterial pressure, and heart rate during the fatiguing contractions were similar across sessions. These findings indicate that the EMG activity, which reflects the net excitatory and inhibitory input received by the motoneurons in the spinal cord, was not adaptable over repeat sessions for the maintain-position task. Furthermore, these results contrast those from a previous study (Hunter SK and Enoka RM. J Appl Physiol 94: 108-118, 2003) when the goal of the isometric contraction was to maintain a constant force. These results, from a series of studies on the elbow flexor muscles, indicate that the type of load supported during the fatiguing contraction influences the extent to which endurance time can change with repeat performances of the task.

Acceleration↗

PubMed1995

Contraction due to microtubule disruption is associated with increased phosphorylation of myosin regulatory light chain.

Microtubules have been proposed to function as rigid struts which oppose cellular contraction. Consistent with this hypothesis, microtubule disruption strengthens the contractile force exerted by many cell types. We have investigated alternative explanation for the mechanical effects of microtubule disruption: that microtubules modulate the mechanochemical activity of myosin by influencing phosphorylation of the myosin regulatory light chain (LC20). We measured the force produced by a population of fibroblasts within a collagen lattice attached to an isometric force transducer. Treatment of cells with nocodazole, an inhibitor of microtubule polymerization, stimulated an isometric contraction that reached its peak level within 30 min and was typically 30-45% of the force increase following maximal stimulation with 30% fetal bovine serum. The contraction following nocodazole treatment was associated with a 2- to 4-fold increase in LC20 phosphorylation. The increases in both force and LC20 phosphorylation, after addition of nocodazole, could be blocked or reversed by stabilizing the microtubules with paclitaxel (former generic name, taxol). Increasing force and LC20 phosphorylation by pretreatment with fetal bovine serum decreased the subsequent additional contraction upon microtubule disruption, a finding that appears inconsistent with a load-shifting mechanism. Our results suggest that phosphorylation of LC20 is a common mechanism for the contractions stimulated both by microtubule poisons and receptor-mediated agonists. The modulation of myosin activity by alterations in microtubule assembly may coordinate the physiological functions of these cytoskeletal components.

Animals↗

PubMed1991

Excitatory pathways in smooth muscle investigated by phase-plot analysis of isometric force development.

Excitatory pathways in the smooth muscle of the pig urinary bladder were investigated using phase-plot analysis of isometric contractions. The phase plots, plots of the rate of change of the force as a function of the force itself, were dominated by a straight line described by the horizontal intercept (Fiso) and the vertical intercept (U). The quotient Fiso/U is a time constant that characterizes the rate-limiting step in isometric force development in the muscle. Bladder strips of 1 mm diameter were activated by electrical field stimuli, acetylcholine, potassium, and ATP in combination with selective pathway inhibitors such as verapamil, atropine, or a calcium-free solution containing ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. When pathways that depended significantly on depolarization or intracellular calcium release were selected, the time constant was significantly smaller, indicating a faster process. The results indicated that the rate-limiting step in force development was determined by the influx of extracellular calcium.

Animals↗

PubMed1992

The effects of resistance training on muscle area and strength in prepubescent age.

The purpose of this study was to determine the effect of strength training in prepubescent boys and girls on muscle strength and cross-sectional area of upper arm. Subjects were ninety-nine healthy elementary school children who belonged to 1st, 3rd and 5th grades and they were assigned to two groups of training (N = 52) and control (N = 47). Training group participated in strength training program for 12 weeks which consisted of maximum sustained isometric contraction of elbow flexion for ten sec, whereas control group did not participate in special training program in this period. In orger to determine the changes due to training, cross-sectional areas of the tissues in upper arm, such as muscle, fat and bone, were measured by the ultrasonic method. Maximum isometric and isokinetic muscle strength of elbow flexion and extension were measured by means of isokinetic cybex dynamometer. In order to assess the development of physiological maturity, TW2 method was used to estimate the skeletal age in each subject by taking the hand-wrist X-ray photograph. After 12-week training period, the whole cross-sectional areas increased in both training and control groups. This increment was due to significant increases in muscle and bone area in the training group and, on the other hand, due to the increase in fat area in the control. The increment of muscle area of training group was about 50% of that derived from the study on adults (Fukunaga, T., 1978). The increment in cross-sectional area of muscle with training was significantly correlated with the skeletal age.(ABSTRACT TRUNCATED AT 250 WORDS)

Arm↗

PubMed2005

In vivo assessment of elbow flexor work and activation during stretch-shortening cycle tasks.

The purpose of this study was to use an electromyography (EMG) based muscle model to investigate the performance enhancement of stretch-shortening cycle (SSC) tasks at different elbow flexion-extension velocities. A torque motor was used to oscillate the forearms of seven healthy male subjects (23-40 years) during SSC and non-SSC contractions at four frequencies of movement (.58, 1.5, 2.4 and 3.3Hz) over a range of 105 degrees -162 degrees of elbow extension. The torque was integrated as a function of joint angle to yield the work produced by the elbow flexors. The elbow flexors were transcutaneously stimulated with a voltage equivalent to 60% maximum voluntary isometric contraction torque for 4s at 50Hz. EMG of the elbow flexors and extensors was recorded from the biceps and triceps respectively. The processed EMG was used to drive a Hill based model to predict the torque of the elbow flexors. Results indicate that muscle work increases from non-SSC to SSC trials. Work decreases for SSC and non-SSC trials with increasing velocity. The simulated constant activation muscle model predicted work well for all trials and conditions, indicating muscle model accuracy. The EMG driven model predicted well for all non-SSC trials, but significantly underestimated the work for SSC tasks, suggesting that the contractile component is directly involved in optimising muscle work during SSC tasks.

Adult↗

PubMed1989

Effect of motor unit firing pattern on twitches obtained by spike-triggered averaging.

The spike-triggered averaging technique (STA) was used to determine the twitch profile of single motor units in human masseter during a voluntary isometric contraction. The effect of the immediate firing pattern of the unit on the twitch was assessed with a computer program which scanned the unit discharge records and selected valid trigger spikes for the averager on the basis of the interspike intervals preceding and following the trigger spike. Successive averages from the same data using different interval parameters revealed progressively more fusion of twitches as the instantaneous firing rate increased. When the data were averaged with interval parameters similar to those used in earlier studies, some fusion of the twitch profile was also evident. It is therefore likely that the degree of fusion of human motor unit twitches obtained by STA has been underestimated in the past. It is further concluded that masseter motor units are sufficiently fast-contracting to allow a relatively unfused twitch profile to be obtained with STA, provided trigger spikes are subjected to appropriate rate control.

Adolescent↗

PubMed1999

Loss of potassium from muscle during moderate exercise in humans: a result of insufficient activation of the Na+-K+-pump?

In this study, we have investigated whether the muscle net potassium (K+) loss, observed during two-legged intermittent static knee-extensions at 30% MVC (n = 9), is caused by an insufficient activation of the Na+-K+-pumps. Furthermore, we have investigated whether the changes in the K+ homeostasis can be causally related to fatigue. K+ loss was calculated from the arterio-venous concentration difference and plasma flow. In three subjects, femoral venous K+ concentration was measured continuously with a K+ selective electrode. Na+-K+-pump activity was estimated from the rate of removal of K+ from the blood during 30-s pauses inserted into the exercise protocol. A large net K+ loss took place during the first minutes of exercise, but diminished quickly and disappeared after 20 min. An increasing net K+ loss reappeared after 30 min. Only 10% of the lost K+ had been regained after the 20-min recovery. A lag in the activation of the Na+-K+-pumps may explain the K+ loss at the beginning of exercise, but gradual pump activation prevented a net K+ loss after 20 min of exercise. The reappearance of the net K+ loss in the later stage of exercise and the subsequent slow recovery of intracellular K+ seemed to be caused by an insufficient further activation of the pumps, rather than by the capacity of the pumps being surpassed. Fatigue was not related to the accumulation of K+ in the interstitium. However, during exercise, the decrease in intracellular K+ content was linearly related to the fall of maximal force. We conclude that during repeated isometric contractions, insufficient activation of the Na+-K+-pumps causes a continuous muscle K+ loss which was associated with fatigue.

Adult↗

PubMed1997

Comparison of surface EMG signals between electrode types, interelectrode distances and electrode orientations in isometric exercise of the erector spinae muscle.

The influence of electrode type, interelectrode distance (IED) and electrode orientation on EMG signals from the paraspinal muscles was investigated. Bipolar electrodes were placed at distances 2, 3, 4, 6 and 8 cm over the erector spinae in the cranio-caudal direction ("in series") as well as in the direction perpendicular to it ("in parallel"). Ten subjects performed 5 s isometric contractions of the erector spinae at 20, 40, 60, 80 and 100% MVC by pulling upward on a handlebar attached to the floor. RMS EMG signals were analyzed for mean average amplitude (AA). Mean total power (TP) and mean median frequency (MF) of the raw EMG signal were determined using fast Fourier transform. In addition to graded loading, sustained fatiguing contractions were performed from which TP and MF were obtained. With increasing IED the AA and TP increased while MF decreased. Although a trend towards higher AA, TP and MF was found for electrodes "in series", as compared to those "in parallel", the difference never reached significance. It is concluded that consistent information about muscle activity was obtained with Miniature Biopotential Skin Electrodes and 14445C Hewlett-Packard electrodes independently from IED or orientation. Orientation "in parallel" prevented the electrodes from sliding during muscle contraction. The third tested type, electrodes developed in the Neuromuscular Research Center, Boston, proved extremely sensitive to movement.

Adult↗

PubMed2004

Effect of multiple-load training on the force-velocity relationship.

The effect of training with a combination of different loads (multiple-load training) on the force-velocity and force-power relationships was examined with training programs that included maximal isometric contraction (Fmax) and concentric contraction of the elbow flexor muscles. Twenty-one male college students were placed into 3 equal training groups (G(30 + 60), G(30 + 100), and G(30 + 60 + 100)) and performed multiple-load training 3 days per week for 8 weeks. The training load was a set fraction of the maximal isometric strength (% Fmax). The G(30 + 60) group performed 6 repetitions of elbow flexion at 30 and 60% Fmax. The G(30 + 100) group performed 6 repetitions at 30% Fmax and six 5-second Fmax loads. The G(30 + 60 + 100) group performed 4 repetitions at 30 and 60% Fmax and four 5-second Fmax loads. After training, Fmax and maximal velocity significantly increased (p < 0.05) in all 3 training groups. The increases in maximal power were significantly (p < 0.05) different between the G(30 + 60 + 100) group (52.9%) and the G(30 + 100) group (24.2%). These results suggest that multiple-load training programs with 4-6 repetitions are effective for improving muscle power and velocity of the elbow flexors.

Adult↗