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PubMed1991

Effect of muscle mass on the pressor response in man during isometric contractions.

1. Changes in blood pressure and heart rate were measured in six healthy male subjects during voluntary isometric contractions of the forearm and quadriceps muscles. Arterial pressure was measured directly via a catheter inserted into the radial artery of the non-contracting arm. Each subject exerted two types of contractions: (a) sustained contractions at 70% of the maximum voluntary contraction (MVC) until fatigue occurred and (b) sustained contractions starting at maximum tension (100% MVC), held for a total duration of 1 min. 2. During fatiguing contractions at 70% MVC, there was a progressive increase in blood pressure, reaching a peak level at fatigue. The same level of mean arterial pressure was achieved during contractions of the same relative tension, regardless of the muscle mass. The same trend was observed for the changes in heart rate. 3. During contractions which started with the maximum tension, where tension fell continuously during the 60 s of maximal effort, mean arterial pressure rapidly increased to high levels within a few seconds, and then increased further by 20-30 mmHg during the sustained maximal effort. There was no difference in the initial rapid increases in mean arterial pressure, nor in the final mean arterial pressures reached between contractions of the forearm or quadriceps muscles. There were no differences in the heart rates achieved during these contractions either. 4. There was no significant difference between the mean arterial pressures observed at fatigue of a 70% MVC contraction or at the end of the 60 s maximum effort during handgrip contractions, or during contractions with the quadriceps muscles. 5. These results support the view that muscle mass is not a determinant of the magnitude of the cardiovascular reflexes during fatiguing isometric contractions in man.

Adult↗
PubMed2004

Strategies that improve human skeletal muscle performance during repetitive, non-isometric contractions.

Previous studies have suggested that during sustained maximal voluntary and evoked contractions, decreasing activation rates may minimize fatigue. The idea of gradually decreasing stimulation frequency to preserve force during fatiguing isometric contractions has, however, recently been challenged. The primary purpose of this study was to test the effectiveness of decreasing or increasing electrical stimulation rates during evoked, repetitive, submaximal, non-isometric contractions of healthy human quadriceps femoris muscles. The ability of the muscles to produce a 50 degrees knee excursion repetitively was evaluated using low-frequency trains, high-frequency trains, and a combination of these trains. Results showed that stimulating the muscles with high-frequency trains followed by low-frequency trains produced the worst performance and starting with low-frequency trains followed by high-frequency trains produced the best performance. Present results cast doubt on the applicability of decreasing stimulation rates during non-isometric contractions and suggest that a combination of trains that begin at a low frequency and then switch to a higher-frequency may improve performance during functional electrical stimulation.

Adult↗
PubMed1983

Muscular pain caused by isometric contraction: evaluation of pain through visual analog scale.

Pain caused by muscular isometric contraction was investigated through use of the visual analog scale. The results demonstrate that the latency time and the time to reach maximum intensity are inversely proportional to the amount of the load; therefore increase of pain is linear. The high statistical significance of the results suggests that pain by isometric contraction could be a comparison test for somatic pain evaluation in spontaneous pathological conditions.

Adult↗
PubMed1995

Rapid activation of glycogen synthase and protein phosphatase in human skeletal muscle after isometric contraction requires an intact circulation.

The effects of isometric contraction (66% of maximal force) and recovery on glycogen synthase fractional activity (GSF) in human skeletal muscle have been studied. Biopsies were taken from the quadriceps femoris muscle at rest, at fatigue and 5 min postexercise on two occasions: after one of the contractions, the circulation to the thigh was occluded during the 5 min recovery (OCC), and after the other contraction, the circulation was intact (control, CON). During CON, GSF decreased from (mean +/- SE) 0.34 +/- 0.05 at rest to 0.24 +/- 0.02 at fatigue and then increased to 0.74 +/- 0.04 at 5 min postexercise; corresponding values for OCC were 0.37 +/- 0.04, 0.25 +/- 0.04 and 0.48 +/- 0.05 (P < 0.001 vs. CON for 5 min postexercise only). Compared with the value at fatigue, protein phosphatase activity (PP) increased by 79 +/- 16% during CON recovery (P < 0.01), whereas no change was observed during OCC recovery. Uridine diphosphate glucose increased by approximately 2.5-fold at fatigue, remained elevated during OCC recovery, but reverted to the preexercise level during CON recovery (P < 0.001 vs. OCC recovery). Glucose 6-P increased approximately 5-fold at fatigue and was higher at 5 min postexercise in OCC vs. CON recovery (8.6 +/- 1.5 vs. 4.1 +/- 0.9 mmol/kg dry wt; P < 0.01). It is concluded that the rapid increase in GSF after intense exercise with an intact circulation may be at least partly attributed to an increase in the specific activity of PP. The increase in GSF during recovery in OCC may be at least partly attributed to the high glucose 6-P content in vivo, which enhances the substrate suitability of GS for PP. Thus, separate mechanisms exist for the activation of PP and GS during recovery from intense short term exercise.

Adult↗
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↗
PubMed2004

Muscle kinematics during isometric contraction: development of phase contrast and spin tag techniques to study healthy and atrophied muscles.

PURPOSE: To develop and compare phase-contrast (PC) and spin-tag (ST) MR imaging techniques for accurate quantification of velocity and displacement distribution in the muscle tendon complex of the lower leg during isometric contractions under in vivo conditions, in healthy subjects and subjects with atrophy. MATERIALS AND METHODS: Techniques were developed to acquire PC and ST dynamic images, gated to the force exerted by a subject during isometric contraction. Algorithms were optimized for correction of phase shading errors. Flow velocity quantification was validated in phantoms and ex vivo rabbit muscles. Trajectories of pixels calculated from PC images were compared with those in ST images. Velocity distributions were determined in healthy muscles, those atrophied by four weeks of suspension, and during physiotherapy-induced recovery. RESULTS: The technique developed allowed acquisition of images retrospectively gated to the isometric contraction performed with the subject in the scanner. Significant phase shading errors in PC images (approximately 3 cm/second over the field of view) were reduced to the background noise level by the correction algorithm. Tissue trajectories calculated from PC images agreed very well with those from ST images both in human and excised animal tissues. Peak velocities in atrophied muscles were significantly lower compared to the preatrophy state but recovered to baseline values after six weeks of therapy. CONCLUSION: We show the feasibility of monitoring muscle velocity and tissue displacement during voluntary contractions in humans under in vivo conditions using MR tissue motion mapping methods. The clinical feasibility of this technique in monitoring atrophied muscle is also demonstrated.

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↗
PubMed2006

[Effects of isometric contraction training by electrostimulation on Type I and II hindlimb muscles in cerebral ischemia model rats].

PURPOSE: The purpose of this study was to examine the effects of cerebral ischemia on Type I(soleus) and Type II(plantaris, gastrocnemius) muscles, and to determine the effects of isometric contraction training by electrostimulation on Type I and II muscles in cerebral ischemia model rats. METHOD: Twenty-five male Sprague-Dawley rats were randomly divided into four groups: ST(stroke), STES(stroke+electrostimulation), SH(sham) and SHES (sham+electrostimulation). The ST and STES groups received a transient right middle cerebral artery occlusion operation. The SH and SHES groups received a sham operation. The STES and SHES groups had daily isometric contraction training by electrostimulation(100 Hz, 45 mA, 7.5 V) on hindlimb muscles for 7 days. RESULT: Plantaris and gastrocenmius muscle weight, myofibrillar protein contents of soleus and gastrocnemius, and the muscle fiber cross-sectional area of gastrocnemius in the ST group significantly decreased compared with the SH group. Soleus, plantaris, gastrocnemius muscle weight, myofibrillar protein contents of soleus and gastrocnemius, and the Type I muscle fiber cross-sectional area of soleus and the Type II muscle fiber cross-sectional area of gastrocnemius in the STES group significantly increased compared with the ST group. CONCLUSION: Hindlimb muscle atrophy occurs after acute stroke and isometric contraction training by electrostimulation during early stages of a stroke attenuates muscle atrophy of Type I and Type II muscles.

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↗
PubMed2002

Differences in the oxygenation of the forearm muscle during isometric contraction in trained and untrained subjects.

The aim of this study was to test a hypothesis that a longer duration of isometric contraction is related to an increased oxygenation status of the muscle. A group of trained rock climbers and untrained subjects performed 15 kp sustained isometric contraction until fatigue set in. The oxygenation status was assessed using near infrared spectroscopy (ISS, USA). The results support the hypothesis. The concentration of relative oxygenated hemoglobin was higher in the rock climbers than in the untrained subjects. The relative total hemoglobin did not differentiate the two groups enough to show that blood volume also strongly influences contraction time.

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↗
PubMed2001

Oscillatory interaction between human motor cortex and trunk muscles during isometric contraction.

We investigated oscillatory interaction between magnetoencephalographic signals of the human motor cortex and surface electromyogram from the paraspinal (PS) and abdominal (ABD) muscles. The results were compared with data obtained during contraction of the first dorsal interosseus (FDI) and tibialis anterior (TA) muscles. Significant coherence at 15-35 Hz was observed for both PS and ABD muscles in all subjects but the coherence was weak compared with that for FDI and TA. The cortical sources for both the PS and the ABD coherences were located in the motor cortex between the source areas for the FDI and TA coherences, thereby agreeing with the classical trunk area of the motor homunculus previously determined by invasive studies. The sources were strictly contralateral for PS but bilateral for ABD contractions. Our results indicate that during isometric contractions descending motor commands are modulated by cortical oscillations for both limb and trunk muscles, although the modulation is weaker and may be bilateral for trunk muscles.

Abdominal Muscles↗
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↗
PubMed2002

Modeling variability of force during isometric contractions of the quadriceps femoris.

The authors modeled variability of force during continuous isometric contractions of the quadriceps femoris. Twenty adults (aged 25 +/- 6 years old) performed isometric leg extensions. Target forces were 11 percentages of maximum voluntary contraction (%MVC), ranging from 2 to 95%, and 5 absolute levels, from 25 to 225 N. The authors used standard deviation of absolute force, coefficient of variation, and signal-to-noise ratio as measures of variability. The results suggested a nonlinear relationship between variability and level of force, which could best be expressed as %MVC and not as absolute force. Variability for continuous isometric contractions was described best by a sigmoidal logistic function. The sigmoidal pattern of variability as a function of %MVC is consistent with physiological mechanisms.

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↗
PubMed2001

Muscle fatigue assessment during sustained high isometric contractions.

The myoelectrical fatigue manifestation was assessed during sustained isometric maximal and submaximal (75% of maximal (MVC)) isometric contractions by means of a level-trigger averaging of the surface electromyogram (EMG) recorded from m. biceps brachii by branched bipolar electrodes. The conduction velocity (CV) of excitation was calculated from the time shift of the negative peaks of the averaged potentials (APs). The amplitude and the duration of the negative phase of APs were also measured. The conduction velocity was the most appropriate EMG-derived parameter for muscle fatigue assessment. The duration of the endurance time and the time pattern of CV changes during submaximal sustained contractions were found to resemble very closely those during maximal contraction efforts sustained to 30 percent reduction of the initial torque. Thus the recruitment of additional motor units (MUs) during sustained high contractions does not affect essentially the calculated values of CV.

Action Potentials↗