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Results for “Isometric Contraction”

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PubMed1993

Identification of time-varying dynamics of the human triceps surae stretch reflex. I. Rapid isometric contraction.

We have examined the time variations of stretch reflex dynamics throughout rapid voluntary changes in the isometric contraction level of the human triceps surae muscles. This was achieved by superimposing a small stochastic displacement upon many such changing contractions and then identifying the time-varying relationship between the perturbation and the evoked electromyograms (EMGs). An "ensemble" time-varying system identification technique was used to estimate these input-output dynamics as a set of impulse response functions, one for each time before, during, and after the change in contraction level, with a temporal resolution equal to the data acquisition rate. Three main findings resulted. First, stretch reflex gain (relating joint velocity to EMG) was significantly modulated during changes in voluntary contraction level, increasing as the subject contracted the muscles and decreasing as the subject relaxed. Second, stretch reflex dynamics did not change with contraction level, even when its gain varied substantially. Third, the time course of the gain changes closely followed the level of the EMG, even though the subjects used rather different activation and deactivation patterns. These results suggest that, for the behavior studied (i.e., rapid changes in isometric contraction level), stretch reflex gain and motoneuron pool activation level were controlled by a common descending command rather than being independently specified.

Adult↗

PubMed1990

Effects of hypoxemia with and without acidemia on the isometric contraction time and the electromechanical delay of the fetal myocardium: an experimental study on the ovine fetus.

This study assessed the response of the preejection period during hypoxemia with and without acidemia. In five pregnant ewes, hypoxemia was created during 1 hour followed by fetal infusion of lactic acid during 2 hours. A micromanometer catheter positioned above the fetal aortic valve, an endocavitary phonocardiogram, and a fetal electrocardiogram allowed measurements of the two components of the preejection period--the isometric contraction time and the electromechanical delay. At the onset of hypoxemia, because of changes in isometric contraction time, the preejection period began to shorten. When acidemia was induced, the preejection period modified slowly in the opposite direction and lengthened, initially because of a prolongation of electromechanical delay and later because of an increase in the already shortened isometric contraction time. This process developed slowly and at the end of 2 hours of acidemia, preejection period were back to preexperimental values. It can be concluded that systolic time intervals can be normal and misleading when acidosis complicates hypoxemia.

Acids↗

PubMed1999

Skeletal muscle transverse strain during isometric contraction at different lengths.

An important assumption in 2D numerical models of skeletal muscle contraction involves deformation in the third dimension of the included muscle section. The present paper studies the often used plane strain description. Therefore, 3D muscle surface deformation is measured from marker displacements during isometric contractions at various muscle lengths. Longitudinal strains at superficial muscle fibers ( - 14 +/- 2.6% at L0, n = 57) and aponeurosis (0.8 +/- 0.9% at L0) decrease with increasing muscle length. The same holds for transverse muscle surface strains in superficial muscle fibers and aponeurosis, which are comparable at intermediate muscle length, but differ at long and short muscle length. Because transverse strains during isometric contraction change with initial muscle length, it is concluded that the effect of muscle length on muscle deformation cannot be studied in plane strain models. These results do not counteract the use of these models to study deformation in contractions with approximately - 9 % longitudinal muscle fiber strain, as transverse strain in superficial muscle fibers and in aponeurosis tissue is minimal in that case. Aponeurosis surface area change decreases with increasing initial muscle length, but muscle fiber surface area change is - 11%, independent of muscle length. Assuming incompressible muscle material, this means that strain perpendicular to the muscle surface equals 11%. Taking the relationship between transverse and longitudinal muscle fiber strain into account, it is hypothesized that superficial muscle fibers flatten during isometric contractions.

Animals↗

PubMed1993

Motor unit recruitment in human medial gastrocnemius muscle during combined knee flexion and plantarflexion isometric contractions.

Previous work on multifunctional muscle has suggested that motor unit recruitment during a combined force task is the result of an interactive effect of weighted inputs acting simultaneously on the motoneuron pool. The present study shows that a similar effect describes motor unit activation in a two-joint muscle as forces are combined at both proximal and distal attachments. The recruitment thresholds of single motor units in medial gastrocnemius muscle were determined during combined knee flexion and plantarflexion isometric contractions. Slow isometric ramp contractions in knee flexion were produced while maintaining various background levels of plantarflexion force. The combination of knee flexion and plantarflexion forces at which a motor unit initially discharged was used to characterize recruitment as represented by the slope of the regression line fit to the individual data points. Each subject completed two experiments; one at each of two knee joint angles, with the ankle joint fixed at 90 degrees. The effect of knee angle was assessed by comparing the slopes of the regression lines that characterized motor unit recruitment at each knee angle. Motor units in medial gastrocnemius were recruited when the linear sum of the forces exerted in plantarflexion and knee flexion exceeded a certain threshold of combined force. Specifically, the apparent force threshold of recruitment in knee flexion decreased as the level of force maintained in plantarflexion increased. Further, evidence is provided indicating that the linear relationship describing recruitment in two-joint muscle is dependent upon joint angle.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1986

The effect of forskolin on the isometric contraction of the isolated hemidiaphragm of the rat.

The effects of forskolin on the parameters of the isometric contraction alone and in combination with aminophylline and isoprenaline were studied on the isolated hemidiaphragm of the rat during direct electrical stimulation. Forskolin (2.6 - 18.2 mumol l-1) produced a concentration-dependent increase in tension developed (Td) and, to a lesser extent, in the maximum rate of rise in tension (dT/dt max). The dose-response curve for the action of forskolin (2.6 - 18.2 mumol l-1) on Td was shifted to the left in the presence of a standard concentration of aminophylline (0.32 mmol l-1). Forskolin (5.2 mumol l-1) produced a further and significant increase in both Td and dT/dt max in the presence of isoprenaline (0.24 mumol l-1) in the bath. In a calcium-free medium, the effects of forskolin (7.80 and 18.2 mumol l-1) on Td and dT/dt max were significantly weaker than in a medium containing calcium. These data indicate that forskolin increases the isometric contraction of the isolated hemidiaphragm probably by activating the adenosine 3':5'-cyclic monophosphate (cyclic AMP) generating system. These effects are possible only in the presence of calcium.

Aminophylline↗

PubMed1978

Energy metabolism in different human skeletal muscles during voluntary isometric contractions.

The energy turnover in contracting skeletal muscle was studied by measuring the rate of temperature rise during voluntary, isometric contractions and circulatory arrest in M. soleus, M. sacrospinalis and M. biceps brachii in 14 males, by thermoelements inserted in the muscles. A linear relationship between rate of temperature rise and force intensity given as per cent of maximal voluntary contraction (MVC) was demonstrated in biceps (r = 0.95), but not so clearly confirmed in soleus (r = 0.73). Muscle biopsies were taken from the same muscles and fibre type distribution was determined histochemically by staining for ATPase. The rate of heat production at MVC showed positive correlation to the percentage of fast twitch (FT) fibres in the muscles (r = 0.90). Linear extrapolation indicates that the maximal energy turnover in human FT fibres is approximately six times that of slow twitch (ST) fibres during voluntary isometric contractions.

Adult↗

PubMed1999

Isometric contraction induces the Ca2+-independent activation of the endothelial nitric oxide synthase.

Shear stress and tyrosine phosphatase inhibitors have been shown to activate the endothelial NO synthase (eNOS) in a Ca2+/calmodulin-independent manner. We report here that isometric contraction of rabbit aorta activates eNOS by a pharmacologically identical pathway. Endothelium-intact aortic rings were precontracted under isometric conditions up to 60% of the maximal phenylephrine-induced tone. The NO synthase inhibitor NGnitro-L-arginine (L-NA) and the soluble guanylyl cyclase inhibitor NS 2028 induced an additional contraction, the amplitude of which depended on the level of precontraction. The maximal production of NO by isometrically contracted aortic rings (as estimated by the increase in cGMP in detector smooth muscle cells in a superfusion bioassay) was observed during the initial phase of isometric contraction and was greater than that detected following the application of acetylcholine. The supplementary L-NA-induced increase in vascular tone was inhibited by the nonselective kinase inhibitor staurosporine and the tyrosine kinase inhibitors erbstatin A and herbimycin A. Another tyrosine kinase inhibitor, genistein, the calmodulin antagonist calmidazolium, and the selective protein kinase C inhibitor, Ro 31-8220, had no effect. Coincident with the enhanced NO formation during isometric contraction was an increase in the tyrosine phosphorylation of endothelial proteins, which also correlated with the level of precontraction. Thus, isometric contraction activates eNOS via a Ca2+-independent, tyrosine kinase inhibitor-sensitive pathway and, like shear stress, seems to be an independent determinant of mechanically induced NO formation.

1-Methyl-3-isobutylxanthine↗

PubMed1986

Muscle ATP turnover rate during isometric contraction in humans.

ATP turnover and glycolytic rates during isometric contraction in humans have been investigated. Subjects contracted the knee extensor muscles at two-thirds maximal voluntary force to fatigue (mean +/- SE, 53 +/- 4 s). Biopsies were obtained before and after exercise and analyzed for high-energy phosphates and glycogenolytic-glycolytic intermediates. Total ATP turnover was 190 +/- 7 mmol/kg dry muscle, whereas the average turnover rate was 3.7 +/- 0.2 mmol . kg dry muscle-1 . S-1. The average ATP turnover rate was positively correlated with the percentage of fast-twitch fibers in the postexercise biopsy (r = 0.71; P less than 0.05) and negatively correlated with contraction duration to fatigue (r = -0.88; P less than 0.05). At fatigue, phosphocreatine ranged from 1 to 11 mmol/kg dry muscle (86-99% depletion of value at rest), whereas lactate ranged from 59 to 101. The mean glycolytic rate was 0.83 +/- 0.05 mmol . kg dry muscle-1 . S-1 and was positively correlated with the rate of glucose 6-phosphate accumulation (r = 0.83; P less than 0.05). It is concluded that a major determinant of the ATP turnover rate is the muscle fiber composition, which is probably explained by a higher turnover rate in fast-twitch fibers; fatigue is more closely related to a low phosphocreatine content than to a high lactate content; and the increase in prephosphofructokinase intermediates is important for stimulating glycolysis during contraction.

Adenosine Triphosphate↗

PubMed1988

Effect of topical cheek surface anesthesia on isometric contractions of the human masseter muscle.

To study the possibility of interactions between buccal cutaneous sensory receptors and voluntary maximum isometric contractions of the masseter muscles, six adult subjects exercised maximum teeth clenching before and after spraying the right cheek surface with aerosol containing 20% benzocaine. The right cheek and masseter muscle served as the experimental side, the left cheek and masseter muscle as the control side. Isometric motor outputs, on the right and left sides, were monitored by integrated surface electromyography over periods of 10 seconds. Topical surface anesthesia provided no evidence of motor modulation by cutaneous tactile receptors. Before and after anesthesia, the two muscles showed nearly identical and well-coordinated motor innervation patterns. It is suggested that the cortical motor commands of maximum isometric contractions, with recruitment of practically all available motor units, overrule all modulatory inputs except those of fatigue.

Adult↗

PubMed1988

Heat production of rat anococcygeus muscle during isometric contraction.

Heat production, unloaded shortening velocity (Vus), and load-bearing capacity (LBC) were studied in the isolated rat anococcygeus muscle during isometric contractions at 27 degrees C. The relation between the total suprabasal heat produced and the stress-time integral for isometric contractions of various durations was curvilinear, demonstrating a decreasing slope as contractile duration increased. The rate of heat production at 600 s was approximately 68% of the peak value of 6.55 mW/g that occurred at 10 s. At the same time, force rose from a mean of 92 mN/mm2 at 10 s to a value of 140 mN/mm2 at 600 s. This produced a nearly threefold increase in the economy of force maintenance. The decline in the rate of heat production was accompanied by a decline in Vus from 0.56 Lo/s at 10 s to 0.28 Lo/s at 600 s, where Lo is the length for optimal force development. This suggests the fall in the rate of heat production was caused, at least in part, by a slowing of cross-bridge kinetics. The ratio of LBC to developed tension at 10 s was not significantly different from the ratio at 600 s, suggesting that the increase in tension was due to an increased number of attached cross bridges. The decline in heat production, therefore, appears contradictory, since an increased number of attached cross bridges would predict an increased rate of energy expenditure. The observations can be reconciled if either 1) the increase in force is caused by a progressive increase in the attachment time of a constant number of cross bridges that cycle at a lower frequency or 2) the decline in energy expenditure caused by the slowing of cross-bridge cycling is sufficient to mask the increase caused by the recruitment of additional cross bridges.

Animals↗

PubMedDate not supplied

Amplitude of the surface electromyogram during fatiguing isometric contractions.

Five voluntee subjects held isometric handgrip contractions at specific submaximal tensions until the required tension could no longer be maintained. At the start of those contractions, the amplitude of the surface electromyogram (EMG) was linearly related to the tension exerted; the amplitude of the EMG increased linearly throughout these substained contractions by a constant amount--about 30% of the maximum. During sustained contractions, brief, intermittent maximal efforts showed that strength declined linearly at all tensions. At 25% maximal voluntary contraction (MVC), there was a linear fall in the EMG amplitude associated with the brief maximal efforts, but the fall in strength was more rapid than the fall in EMG amplitude. At 70% MVC, there was no fall in the EMG amplitude in response to the brief maximal efforts, while the muscle strength fell linearly.

Adult↗

PubMed1985

The effect of adrenaline infusion on the regulation of glycogenolysis in human muscle during isometric contraction.

The regulation of glycogenolysis in human muscle during isometric contraction without and with adrenaline infusion has been investigated. The content of cAMP in muscle increased three-fold during the infusion. Total glycogen phosphorylase and synthetase activities were unchanged during contraction without and with adrenaline infusion. The fraction of phosphorylase in the a form was in resting muscle 26% and at the end of contraction 24%. During adrenaline infusion phosphorylase a increased to 80%. Contraction during continued infusion resulted in a decrease of phosphorylase a to 42%, despite persistently increased cAMP content in muscle. The activity of synthetase I decreased to about half of the initial value during adrenaline infusion and contraction both without and with the infusion. The rate of glycogenolysis in muscle during contraction was not significantly changed by the infusion. Phosphocreatine (PCr) decreased during the contraction and the decrease was similar without and with adrenaline infusion. The amount of inorganic phosphate (Pi) accumulated in muscle during contraction was lower when adrenaline was given due to a greater accumulation of hexose-monophosphates. It is concluded that the rate of glycogenolysis in muscle during contraction without and with adrenaline infusion is a function both of phosphorylase in the form a form and the availability of Pi at the active site of the enzyme.

Adult↗

PubMed1988

G-1,6-P2 in human skeletal muscle after isometric contraction.

The content of glucose 1,6-bisphosphate (G-1,6-P2), an in vitro activator of phosphofructokinase (a rate-limiting enzyme for glycolysis), and the glycolytic rate in skeletal muscle during isometric contraction have been determined. Subjects contracted the knee extensor muscles at two-thirds maximal voluntary force to fatigue. Biopsies from the quadriceps femoris muscle were obtained before and immediately after contraction. G-1,6-P2 increased in all subjects from a mean of 101 +/- 15 (SE) mumol/kg dry wt at rest to 128 +/- 24 at fatigue (P less than 0.05). Muscle glucose did not change significantly, whereas hexosemonophosphates were significantly increased after contraction. The glycogenolytic and glycolytic rate averaged 70.0 +/- 13.8 and 47.3 +/- 6.7 mmol.kg dry wt-1.min-1, respectively, and the glycolytic rate was positively correlated with the accumulation rates of fructose 6-phosphate (F-6-P) (r = 0.95, P less than 0.01) and G-6-P (r = 0.96, P less than 0.01). Phosphocreatine and ATP decreased by 87 and 17%, respectively, whereas ADP increased by 31% after contraction. These data demonstrate that intense, short-term isometric contraction results in an elevation of the muscle content of G-1,6-P2. The increase in G-1,6-P2 could not be accounted for by the side reactions of phosphoglucomutase or phosphofructokinase. It remains to be determined whether the observed increase in G-1,6-P2 is sufficient to account for the high glycolytic rate during intense exercise. The lack of increase in muscle glucose while G-6-P increased (which will inhibit hexokinase) suggests that the debranching enzyme complex was not active during contraction.

Adenine Nucleotides↗

PubMed1993

Changes in the heart rate and electromyogram beyond the limit time of an isotonic isometric contraction.

Nine men [24.6 (SEM 1.1) years] carried out isometric contractions (IC) of the right elbow flexors at 50% and 100% of the maximal voluntary contraction (MVC). At 50% MVC they had to maintain IC until the limit time (isotonic IC: IIC50) and beyond for as long as possible (anisotonic IC: AIC50). At 100% MVC, IC was anisotonic since the decrease in force was immediate (AIC100). Measurements of the force, the integrated electromyogram (iEMG) and the heart rate (fc) were made during the entire period of contraction. There was a linear relationship between the iEMG increase and the fc increase for IIC50 and AIC100. This relationship was not found for AIC50. The role played by the peripheral information would seem to have become more important in fc regulation when the isotonic IC preceding the anisotonic IC was sufficiently long (submaximal IIC). It would seem that the idea of muscle exhaustion at the limit time was only relative, and depended greatly on the subject's motivation and his capacity to endure a certain degree of pain.

Adult↗

PubMed1981

Comparative actions of substances affecting cyclic AMP metabolism on the isometric contractions of fast and slow skeletal muscle during single-pulse and subtetanic stimulation.

Increasing concentrations of isoprenaline (0.5-4 muM) produced a dose-dependent increase in both TD and dT/dtmax during direct single-pulse stimulation of hemidiaphragm of the rat. The same drug during the same type of stimulation produced an insignificant change in these parameters of the isometric contraction of the isolated guinea-pig soleus muscle. On the contrary, isoprenaline produced a dose-dependent decrease of the isometric contraction during subtetanic stimulation of the soleus muscle. Contrary to the results obtained on hemidiaphragm, there was no interaction between halothane and aminophylline on the soleus muscle. In the soleus muscle, aminophylline (0.3-3.2 mM) produced a dose-dependent increase in TD and dT/dtmax during single-pulse stimulation, whereas isoprenaline failed to do so under the same experimental conditions, in spite of the fact that both substances are activators of cyclic AMP system. The beta2-selective adrenoceptor agonist terbutaline acted in the same way as isoprenaline. During subtetanic stimulation aminophylline (0.3-3.2 mM) produced a dose-dependent decrease of both parameters of the isometric contraction of hemidiaphragm, which is opposite to the results obtained during single-pulse stimulation. It is concluded that various types of electrical stimulation can produce different responses in slow and fast-contracting muscles, depending on the fundamental biochemical differences of two types of muscle, but some of these responses are the same irrespective of the method of muscle activation.

4-(3-Butoxy-4-methoxybenzyl)-2-imidazolidinone↗

PubMed1991

Sustained isometric contraction of skeletal muscle results in release of immunoreactive neurokinins in the spinal cord of the anaesthetized cat.

Antibody microprobes were used to study release of immunoreactive neurokinins in the dorsal horn of the anaesthetized spinal cat following sustained isometric contraction of ipsilateral hindlimb muscles. Microprobes had immobilized antibodies to neurokinin A (NKA) on their outer surfaces and bound a proportion of released molecules when inserted in the central nervous system. Bound molecules were detected in autoradiographs as zones of reduced binding of 125I-NKA in which microprobes were incubated after withdrawal from the spinal cord. The left hindlimb was immobilized using an epoxy bandage splint and isometric contraction of muscles induced by intermittent tetanic stimulation of a ventral root. A basal presence of immunoreactive neurokinins was detected and this was increased by sustained isometric muscle contraction. It is probable that ergoreceptors contain and release neurokinins.

Anesthesia, General↗

PubMed1993

Influence of active muscle size on sympathetic nerve discharge during isometric contractions in humans.

We tested the hypothesis that sympathetic nerve discharge to nonactive skeletal muscle (MSNA) is influenced by active muscle size during isometric contractions performed at a constant submaximal force in humans. In six subjects, MSNA (peroneal microneurography), arterial pressure, heart rate, and ratings of perceived effort were recorded before (resting control) and during isometric contractions of either a small hand muscle [1st dorsal interosseus (FDI); 2 trials (FDI1 and FDI2)] or the fore- and upper arm muscles (handgrip; 1 trial) sustained to the point of exhaustion and normalized to endurance time (te). There were no differences in resting control levels. MSNA, heart rate, arterial pressure, and perceived effort all increased similarly during the two FDI contractions. During handgrip, the rates of rise and peak increases MSNA, heart rate, and arterial pressure were much greater than during FDI (all P < 0.05), but peak levels of perceived effort were not different. These findings indicate that during voluntary isometric efforts sustained to the same performance and perceptual end points the rates of increase and final levels of MSNA, heart rate, and arterial pressure are greater during contraction of a larger muscle mass and that these responses appear to be similar during successive trials of a task when normalized to te. Thus, active muscle size can be an important factor in the regulation of sympathetic nervous system discharge and cardiovascular function during isometric muscle activity in the human.

Adult↗

PubMed1994

Muscular fatigue and recovery following alternating isometric contractions at different levels of force.

The purpose of this study was to document the amount and rate of muscular fatigue during alternating levels of isometric contraction similar to that found during the Simulated Aerial Combat Maneuver (SACM). In addition, the time needed to recover from such an exercise was examined. Twenty males between the ages of 22 and 35 years performed an isometric contraction of their right quadriceps muscle at alternating levels of tension (20 and 50% maximum voluntary contraction) until exhaustion. The time at each contraction level was 10 s. After each exhaustive exercise bout, subjects were assigned to one of six recovery intervals (10, 20, 40, 60, 120, and 240 min) followed by a repeat of the exhaustive exercise. All subjects were tested under each of the six recovery intervals. Results showed that the amplitude (RMS) of the myoelectric signal increased while the frequency content of the signal (MPF) decreased over the course of the fatiguing activity. Endurance time (ET) was found to be significantly (p < 0.05) recovered (90.96%) within 60 min after stopping the exercise. Although MPF returned to its prefatigue value within 10 min of rest, the RMS value had still not recovered after 4 h.

Adult↗