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PubMed1975

Myocardial mechanics and oxygen consumption in experimental hyperthyroidism.

Myocardial mechanics and oxygen consumption were studied in right ventricular papillary muscles taken from cats (N = 15), pretreated with cristaline L-thyroxine (1 mg/kg/day, i.p.) 8-18 days prior to the examination. Isotonic afterload and isometric contractions were employed. Oxygen consumption was determined polarographically. Data obtained were compared with control studies on papillary muscles taken from euthyroid cats (N = 15). In isotonic afterloaded contractions the extent of shortening was nearly identical in both groups. However, maximal rate of isometric tension development and velocity of isotonic shortening were considerably increased in hyperthyroid myocardium (by 48 and 42%, respectively). Myocardial oxygen consumption was significantly increased in hyperthyroidism (by 47%), primarily because of an increased maximal rate of isometric tension development and-to a small extent-increased isotonic contraction velocity.

Animals↗
PubMed2007

Fatigue of the plantar intrinsic foot muscles increases navicular drop.

Our purpose was to assess the effect of foot intrinsic muscle fatigue on pronation, as assessed with navicular drop, during static stance. Twenty-one healthy young adults participated. Navicular drop was measured before and after fatiguing exercise of the plantar foot intrinsic muscles. Surface electromyography of the abductor hallucis muscle was recorded during maximum voluntary isometric contractions (MVIC) in order to find the baseline median frequency (MedF). Subjects then performed sets of 75 repetitions of isotonic flexion contractions of the intrinsic foot muscles against a 4.55 kg weight on a custom pulley system. After each set an MVIC was performed to track shifts in MedF. After a MedF shift of at least 10%, navicular drop measurements were repeated. Subjects exhibited 10.0+/-3.8mm of navicular drop at baseline and 11.8+/-3.8mm after fatigue (p<0.0005). The change in navicular drop was significantly correlated with change in MedF (r=.47, p=.03). The intrinsic foot muscles play a role in support of the medial longitudinal arch in static stance. Disrupting the function of these muscles through fatigue resulted in an increase in pronation as assessed by navicular drop.

Adult↗
PubMed2001

Information flow from the sensorimotor cortex to muscle in humans.

OBJECTIVES: To investigate the physiologic mechanism of human electroencephalogram-electromyogram (EEG-EMG) coherence, the directed transfer function (DTF) based on a multivariate autoregressive (MVAR) model was computed. METHODS: Fifty-six channel EEG and EMG of the right abductor pollicis brevis muscle during a weak tonic contraction were recorded in 6 normal volunteers. The EEG over the left sensorimotor area and the rectified EMG were used to compute coherence and DTF. RESULTS: EEG-EMG coherence was observed at the peak frequency of 15-29 Hz (mean 18.5 Hz). The peak frequency of DTF from EEG to EMG was 12-27 Hz (mean 17.8 Hz). DTF from EEG to EMG was significantly larger than that from EMG to EEG at 19-30 and 45-50 Hz (P<0.05). CONCLUSIONS: The present findings suggest that the EEG-EMG coupling mechanism for the 19 Hz or higher frequency might differ from that for the lower frequency. Directional information flow from EEG to EMG in the former frequency range likely reflects the motor control command. The finding of the directional information flow from EEG to EMG within the gamma band indicates that 40 Hz EEG-EMG coherence is not specific to the muscle Piper rhythm which is seen only with strong contraction.

Adult↗
PubMed1995

Contribution of P2-purinoceptors to neurogenic contraction of rat urinary bladder smooth muscle.

1. The contribution of P2-purinoceptors to neurogenic contraction was investigated in rat urinary bladder smooth muscle by measurement of isotonic tension. 2. Contraction of rat urinary bladder smooth muscle induced by electrical stimulation was decreased to 84.19 +/- 3.90% of the control (n = 16) in the presence of atropine (1 microM), which was further decreased to 38.80 +/- 2.75% of the control (n = 49) in the presence of both atropine and 10 microM alpha, beta-methylene adenosine 5'-triphosphate (alpha, beta-Me ATP). 3. The contractile response induced by electrical stimulation in the presence of atropine and alpha, beta-Me ATP was decreased to 27.81 +/- 4.07% (n = 23) and 26.63 +/- 5.01% (n = 15) of the control, by the addition of 100 microM cibacron blue 3GA and 100 microM suramin, respectively. The application of 100 microM adenosine 5'-o-2-thiodiphosphate (ADP beta S) in the presence of atropine and alpha, beta-Me ATP decreased the contractile response induced by electrical stimulations to 17.15 +/- 3.71% (n = 15) of the control. 4. Pretreatment of muscle strips with 100 microM ADP beta S significantly reduced the response to either 200 microM alpha, beta-methylene adenosine 5'-diphosphate or 200 microM ADP beta S. 5. Uridine 5'-triphosphate (100 microM to 1 mM) concentration-dependently contracted muscle strips, and this contraction was significantly antagonized by desensitization of P2-receptors with alpha, beta-Me ATP (10 microM), and completely antagonized by pretreatment of muscle strips with both alpha, beta-Me ATP and ADP beta S (100 microM). 6. Di(adenosine-5') tetraphosphate (30 and 100 microM) contracted muscle strips, whereas it failed to contract after desensitization of P2-receptors.7. It is suggested that about 20% of the neurogenic contraction of rat urinary bladder smooth muscle is mediated via ADP beta S-sensitive purinoceptors.

Acetylcholine↗
PubMed1988

Relationship between smooth muscle volume and contractile response in airway tissue. Isometric versus isotonic measurement.

Two different methods of recording contractile response (isometric and isotonic) were used in both spirally cut and intact ring preparations of rabbit airways. The volume of smooth muscle in each airway preparation was compared with the amount of contraction induced by histamine and carbachol. There were significant relationships between smooth muscle volume and carbachol-induced contraction measured isometrically in spiral and ring preparations. There also was a significant relationship between the proportion of airway smooth muscle and the histamine-induced contraction measured isometrically on ring preparations. There were no significant relationships between smooth muscle volume or proportion of airway smooth muscle and the carbachol or histamine contraction measured isotonically. It is concluded that isometric measurements may provide a more accurate representation of smooth muscle changes in response to agonists in vitro. This may indicate that hyper-responsiveness of airway smooth muscle in vitro is more likely to be detected by isometric rather than isotonic measurements.

Animals↗
PubMed1984

Net O2, CO2, lactate, and acid exchange by muscle during progressive working contractions.

The net O2 uptake (VO2), CO2 output (VCO2), lactate output (L), and non-CO2 acid output (HA) by the gastrocnemius-plantaris muscle group of the dog were measured during progressively loaded isotonic tetanic contractions. Shortening during the 1/s contractions was maintained constant as load was increased by raising the stimulus voltage applied to the motor nerve. Contractions at each load continued for 5 min with two arterial and four venous blood samples obtained during the last minute at each load. Work rate (W) during the contractions was calculated from the load and the shortening. The VO2 increased linearly with time and W. The VCO2 generally followed VO2 with a modest lag during the first two work periods. L increased with time, W, and VO2. Maximal L was lower than that seen during repetitive maximal twitch contractions. HA also increased with time, W, and VO2 and was much larger than L at the higher work rates. It is concluded that L and HA are independent variables during progressive working contractions, as they were during repetitive twitch contractions. Both L and HA patterns may be explained as summations of the respective exchanges of L and HA with time by sequentially recruited groups of muscle fibers.

Acid-Base Equilibrium↗
PubMed1993

Blood flow elevation increases VO2 maximum during repetitive tetanic contractions of dog muscle in situ.

Blood flow through the gastrocnemius-plantaris muscle of the dog in situ was increased by a pump in the arterial supply during a 30-min period of 1/s isotonic tetanic contractions. Compared with a control series of experiments with normoxemia and spontaneous flow, the pump increased flow 84%, from 1.51 +/- 0.08 to 2.78 +/- 0.15 ml.g-1.min-1. The perfusion pressure was increased from 125 to 196 mmHg. The pump hyperemia increased maximal O2 uptake (VO2) at 5 min of contractions by 31%, from 8.97 +/- 0.44 to 12.89 +/- 0.30 mumol.g-1.min-1. The extraction was decreased, and venous PO2 (PVO2) was increased. Fatigue, measured as a drop in power production from the highest level at 10 s to 30 min, was 49% during pump hyperemia and 54% in the control conditions. VO2 decreased 30% from the 5-min value to the 30-min value with pump hyperemia and 28% over the same time in the control conditions. At maximal VO2, the ratio VO2/PVO2 was increased by pump hyperemia compared with control conditions, suggesting an increased O2 diffusing conductance of the muscles. We conclude that the elevated perfusion pressure of pump hyperemia increased flow to raise maximal VO2 mainly in areas of the muscle that had restricted flow under control conditions.

Animals↗
PubMed2000

Adaptive neural network control of cyclic movements using functional neuromuscular stimulation.

In this study, we evaluated the performance of an adaptive feedforward controller and its ability to automatically develop and customize stimulation patterns for use in functional neuromuscular stimulation (FNS) systems. Results from previous experiments using the pattern generator/pattern shaper (PG/PS) controller to generate isometric contractions demonstrated its ability to adjust stimulation patterns to account for recruitment nonlinearities and muscle dynamics. In this study, the PG/PS controller was tested under isotonic conditions. This evaluation required the PG/PS controller to account for muscle length-tension and force-velocity properties as well as limb dynamics. The performance of the adaptive controller was also compared with that of a proportional-derivative (PD) feedback controller. The PG/PS controller is composed of a neural network system that adaptively filters a periodic signal to produce a muscle stimulation pattern for generating cyclic movements. We used computer-simulated models to determine controller parameters for the PG/PS and PD controller that perform well across a variety of musculoskeletal systems. The controllers were then experimentally evaluated on both legs of two subjects with spinal cord injury. Results indicated that the PG/PS controller was able to achieve and maintain better tracking performance than the PD controller. This study indicates that the PG/PS control system may provide an effective mechanism for automatically customizing stimulation patterns for individuals using FNS systems.

Algorithms↗
PubMed1991

Tension and instantaneous stiffness of single muscle fibers immersed in Ringer solution of decreased tonicity.

Isometric tension and instantaneous stiffness were measured in frog semitendinosus single muscle fibers in both isotonic and hypotonic Ringer solution. In 0.7 and 0.5 x normal Ringer tension increased 17 and 20%, respectively. There was no corresponding increase in the measured stiffness. The increase in tension in hypotonic Ringer could be reversed by the addition of an osmotic equivalent of sucrose to the bathing solution. These findings suggest that the potentiated tension observed in hypotonic Ringer is due to an increased tension per cross-bridge and not to an increase in the number of attached cross-bridges.

Amphibians↗
PubMed1976

Effects of nerve stimulation on human muscle.

A phasic program of femoral nerve stimulation was used to reduce refractory knee flexion contractures in five patients. In one, rectus femoris was tenotomized before starting stimulation to reduce hip flexion contracture. Muscle biopsies were performed before and after 5 to 12 weeks of intermittent stimulation at separated sites on the same muscle. In the four patients where muscle contraction was isometric, type I fibers increased 3.7%, 6.4%, 48.4%, anand 30.4%, respectively. Both fiber types in each patient showed an increase in size ( p less than .001). In the tenotomized muscle, where contraction was isotonic at a shortened length, the proportion of type I fibers decreased from 40.2% to 25.4%, and their mean diameter also decreased (p less than .001). These observations suggest that the physical conditions of contraction may be more important than the pattern of neuronal discharge in determining the metabolic profile of human muscle fibers.

Axons↗
PubMed1978

A comparison of drug-induced responses on rat tracheal, bronchial and lung strip in vitro preparations.

1. A preparation of rat trachea and a new preparation of rat bronchi are described. Both preparations are fluid-filled and the intraluminal pressure is monitored. 2. A preparation of rat peripheral airways, the lung strip, is described. The preparation consists of a thin strip of lung parenchyma which is superfused, and contractions monitored isotonically. 3. The rat trachea and bronchi have no intrinsic tone but increases in pressure are elicited in response to methacholine. The preparations relax in response to isoprenaline and aminophylline in the presence of a methacholine-induced contraction. Both preparations respond weakly and show tachyphylaxis to 5-hydroxytryptamine (5HT). 4. The lung strip contracts with equal magnitude to methacholine and 5-HT. It exhibits intrinsic tone which is inhibited by indomethacin and relaxed by isoprenaline.

Airway Resistance↗
PubMed1988

Transducing chemical energy into mechanical function: a comparative view.

The energetics of muscle contraction can be understood in terms of the major cellular ATPases. The twitch isometric transduction efficiency is relatively constant across muscle types and species. Although many of the factors that alter the shape of the enthalpy:load relation in isotonic twitch contractions have been identified our molecular understanding is unsatisfactory and more studies are needed of mammalian muscles working closer to 37 degrees C. The thermodynamic efficiency of CB activity seems quite high, probably in excess of 70%. During maintained (tetanic) force there can be greater than a 1000 fold difference in energy usage across muscle types and there are factors that can down regulate CB activity: these factors remain to be fully identified in both skeletal and smooth muscles. The very diversity of muscle types and the different biochemical solutions that have emerged to match energy supply and demand should lead to important insights into the contractile mechanism. The corollary however also applies, it may be dangerous to take results obtained in one muscle type under a particular set of conditions, and extrapolate those findings to muscles in general.

Adenosine Triphosphatases↗
PubMed1999

Muscle O(2) consumption by NIRS: a theoretical model.

In the past, the measurement of O(2) consumption ((2)) by the muscle could be carried out noninvasively by near-infrared spectroscopy from oxyhemoglobin and/or deoxyhemoglobin measurements only at rest or during steady isometric contractions. In the present study, a mathematical model is developed allowing calculation, together with steady-state levels of (2), of the kinetics of (2) readjustment in the muscle from the onset of ischemic but aerobic constant-load isotonic exercises. The model, which is based on the known sequence of exoergonic metabolic pathways involved in muscle energetics, allows simultaneous fitting of batched data obtained during exercises performed at different workloads. A Monte Carlo simulation has been carried out to test the quality of the model and to define the most appropriate experimental approach to obtain the best results. The use of a series of experimental protocols obtained at different levels of mechanical power, rather than repetitions of the same load, appears to be the most suitable procedure.

Adenosine Triphosphate↗
PubMed2005

Strategies that improve paralyzed human quadriceps femoris muscle performance during repetitive, nonisometric contractions.

OBJECTIVE: To determine the effect of combining different stimulation frequencies on the ability of paralyzed human quadriceps muscle to produce a 50 degrees knee excursion repetitively when starting at 90 degrees of flexion. DESIGN: Repeated-measures design. SETTING: Clinical research laboratory. PARTICIPANTS: Complete data were collected from 9 subjects aged 11 to 25 years (mean +/- standard deviation, 17.1+/-4.5y) with spinal cord injury (SCI). INTERVENTION: Three protocols were each tested during separate sessions: 20-Hz trains of pulses followed by 66-Hz trains (C20+66), 33-Hz trains followed by 66-Hz trains (C33+66), and 66-Hz trains alone (C66). For each frequency, stimulation was repeated until the knee failed to produce a 50 degrees excursion. This approach allowed us to evaluate the response to stimulation with 20-, 33-, and 66-Hz and combinations of 20- and 66-Hz and 33- and 66-Hz trains. MAIN OUTCOME MEASURE: Number of successful contractions. RESULTS: The C20 and C33 did not differ (mean, 41.0+/-12.6 excursions and 42.0+/-12.3 excursions, respectively), and each produced more excursions than the C66 protocol. The C20+66 and C33+66 protocols produced 51.4+/-15.0 and 44.9+/-13.6 excursions, respectively, and the C20+66 was the best protocol overall (all P<or=.05). CONCLUSIONS: This study showed that stimulation strategies that start with low frequencies and switch to higher frequencies as the muscle fatigues could improve the ability of functional electric stimulation applications to perform repetitive, nonisometric contractions in subjects with SCI.

Adult↗
PubMed1995

Regulation of vascular contraction by ionic matrix surfaces of the smooth muscle cell: changes in SHR?

1. KCl-induced contractions in aortic rings of spontaneously hypertensive rats (SHR) and two normotensive strains, Sprague-Dawley (SD) and Wistar-Kyoto (WKY) rats, were studied. 2. The contraction elicited by isotonic Na-free, KCl solution containing 2.5 mmol/L Ca and 1 mmol/L Mg was concentration-dependently inhibited by Na and K. Maximal inhibition was induced by 20 mmol/L NaCl in WKY and SHR (65 and 85%, respectively) and by 60 mmol/L NaCl in SD (50%). KCl (10-40 mmol/L) had no effect on SD but produced almost full relaxation in WKY and about 50% relaxation in SHR. 3. Isotonic Ca-free, Na-free high Mg (2 mmol/L), K-solution caused strong phasic contractions in WKY and SHR, but weak responses in SD. 4. Aortic rings of SD, WKY and SHR developed strong, sustained and reproducible contractions in isotonic Ca-free, Na-free, K-solution containing EDTA, which were fully relaxed by 1.2, 1.6 and 8 mmol/L Na, respectively. 5. We suggest that the primary mechanism of KCl-contraction is potential-independent competition between monovalent and divalent cations for binding sites on both surfaces of the plasma membrane (e.g. K with Mg and Na with Ca) and that differences in aortic contractility in SHR, WKY and SD merely reflect strain differences in the composition of the ionic matrix, independent of hypertension.

Animals↗
PubMed2000

The myoneural effects of lithium chloride on the nerve-muscle preparations of rats. Role of adenosine triphosphate-sensitive potassium channels.

The potential effects of lithium chloride on the neuromuscular transmission and muscular contraction were studied using in vitro and in vivo nerve-muscle preparations of rats. Addition of lithium chloride to the fluid bathing the isolated rat diaphragm produced a concentration-dependent inhibition of diaphragmatic contractions elicited by either indirect or direct electrical stimulation. The threshold concentrations were 1 mmol l(-1)and 3 mmol l(-1), respectively. Similarly, the intravenous administration of lithium chloride as bolus injections, produced a dose-dependent progressive inhibition of the indirectly- and directly-induced gastrocnemius muscle contractions during the 2-h period of investigation. The indirectly-induced contractions were much more sensitive to the inhibitory effect of lithium chloride than directly-induced contractions. Also, lithium chloride was found to be capable of enhancing the paralysis of the indirectly stimulated rat diaphragm in vitro and gastrocnemius muscle in vivo induced by either pipecuronium or succinylcholine. The combination of lithium chloride and pipecuronium led to a synergistic inhibition of the neuromuscular transmission, while the combination of lithium chloride and succinylcholine led to additive inhibition. Pretreatment with lithium chloride at the threshold concentrations enhanced the inhibitory effects of verapamil on diaphragmatic contractions elicited either indirectly or directly. The inhibitory effects of verapamil on the indirectly- and directly-induced rat gastrocnemius muscle contractions were potentiated by lithium chloride administration to rats. Glibenclamide was found to be capable of inhibiting the relaxant effects of lithium chloride on the indirectly- and directly-elicited contractions of rat diaphragm in vitro and rat gastrocnemius muscle in vivo, in a concentration- and dose-dependent manner, respectively. Doubling the concentration of magnesium in the bathing fluid potentiated the inhibitory effects of the threshold concentrations of lithium chloride on the diaphragmatic contractions induced either indirectly or directly. Pretreatment with 4-aminopyridine or barium chloride inhibited the relaxant effects of lithium chloride on the indirectly- and directly-elicited diaphragmatic contractions. The inhibitory effects of diazoxide on the indirectly-evoked contractions of rat diaphragm in vitro and rat gastrocnemius muscle in vivo were potentiated by lithium chloride. Pretreatment with glibenclamide inhibited markedly the combined effects of lithium chloride and diazoxide on the contractions of the diaphragm and gastrocnemius muscles induced indirectly. Additionally, the intravenous administration of lithium chloride into rats as bolus injections produced a dose-dependent progressive increase in plasma potassium level and a dose-dependent progressive decrease in the intracellular levels of adenosine triphosphate in the sciatic nerve and gastrocnemius muscle. It is concluded that lithium chloride, via activation of adenosine triphosphate- sensitive potassium channels, acts presynaptically to inhibit the neuromuscular transmission and acts at the muscle membrane to inhibit the muscular contraction.

ATP-Binding Cassette Transporters↗
PubMed2000

Cardiac mechanoenergetics replicated by cross-bridge model.

The aim of this work is to reproduce the experimentally measured linear dependence of cardiac muscle oxygen consumption on stress-strain area using a model, composed of a three-state Huxley-type model for cross-bridge interaction and a phenomenological model of Ca2+-induced activation. By selecting particular cross-bridge cycling rate constants and modifying the cross-bridge activation model, we replicated the linear dependence between oxygen consumption and stress-strain area together with other important mechanical properties of cardiac muscle such as developed stress dependence on the sarcomere length and force-velocity relationship. The model predicts that (1) the amount of the "passenger" cross bridges, i.e., cross bridges that detach without hydrolyzing ATP molecule, is relatively small and (2) ATP consumption rate profile within a beat and the amount of the passenger cross bridges depend on the contraction protocol.

Actomyosin↗
PubMed1994

Storage and release of mechanical energy by contracting frog muscle fibres.

1. Stretching a contracting muscle leads to greater mechanical work being done during subsequent shortening by its contractile component; the mechanism of this enhancement is not known. 2. This mechanism has been investigated here by subjecting tetanized frog muscle fibres to ramp stretches followed by an isotonic release against a load equal to the maximum isometric tension, T(o). Shortening against T(o) was taken as direct evidence of an absolute increase in the ability to do work as a consequence of the previous stretch. 3. Ramp stretches (0.5-8.6% sarcomere strain, confined to the plateau of the isometric tension-length relationship) were given at different velocities of lengthening (0.03-1.8 sarcomere lengths s-1). Isotonic release to T(o) took place immediately after the end of the ramp, or 5-800 ms after the end of the largest ramp stretches. The length changes taking place after release were measured both at the fibre end and on a tendon-free segment of the fibre. The experiments were carried out at 4 and 14 degrees C. 4. After the elastic recoil of the undamped elastic elements, taking place during the fall in tension at the instant of the isotonic release, a well-defined shortening took place against T(o) (transient shortening against T(o)). 5. The amplitude and time course of transient shortening against T(o) were similar at the fibre end and in the segment, indicating that it is due to a properly of the sarcomeres and not due to stress relaxation of the tendons. 6. Transient shortening against T(o) increased with sarcomere stretch amplitude up to about 8 nm per half-sarcomere independent of stretch velocity. 7. When a short delay (5-20 ms) was introduced between the end of the stretch and the isotonic release, the transient shortening against T(o) did not change; after longer time delays, the transient shortening against T(o) decreased in amplitude. 8. The velocity of transient shortening against T(o) increased with temperature with a temperature coefficient, Q10, of approximately 2.5. 9. It is suggested that transient shortening against T(o) results from the release of mechanical energy stored within the damped element of the cross-bridges. The cross-bridges are brought into a state of greater potential energy not only during the ramp stretch, but also immediately afterwards, during the first phase of stress relaxation.

Animals↗