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At least 307 records · Page 17Linked to original sources

PubMed2000

Effect of muscle mass and intensity of isometric contraction on heart rate.

The purpose of this study was to determine the effect of muscle mass and the level of force on the contraction-induced rise in heart rate. We conducted an experimental study in a sample of 28 healthy men between 20 and 30 yr of age (power: 95%, alpha: 5%). Smokers, obese subjects, and those who performed regular physical activity over a certain amount of energetic expenditure were excluded from the study. The participants exerted two types of isometric contractions: handgrip and turning a 40-cm-diameter wheel. Both were sustained to exhaustion at 20 and 50% of maximal force. Twenty-five subjects finished the experiment. Heart rate increased a mean of 15.1 beats/min [95% confidence interval (CI): 5.5-24.6] from 20 to 50% handgrip contractions, and 20.7 beats/min (95% CI: 11.9-29.5) from 20 to 50% wheel-turn contractions. Heart rate also increased a mean of 13.3 beats/min (95% CI: 10.4-16.1) from handgrip to wheel-turn contractions at 20% maximal force, and 18.9 beats/min (95% CI: 9. 8-28.0) from handgrip to wheel-turn contractions at 50% maximal force. We conclude that the magnitude of the heart rate increase during isometric exercise is related to the intensity of the contraction and the mass of the contracted muscle.

Adult↗

PubMed1994

The relation between the direction dependence of electromyographic amplitude and motor unit recruitment thresholds during isometric contractions.

The activation of muscles can be studied by measuring the activity of a representative set of single motor units in a muscle or by measuring the surface electromyographic (EMG) activity of a muscle that results from the contribution of a large number of motor units. In this study we have developed a model showing how the direction dependence of the amplitude of the EMG activity during isometric contractions can be understood from the recruitment thresholds of single motor units when force is applied in various directions within a plane. The model predicts that the direction with the largest EMG activity (called the "preferred direction") corresponds to the direction in which the largest number of motor units is recruited. If one assumes homogeneous activation of a population of motor units, this preferred direction can be shown to be equivalent to the direction in which the recruitment threshold of the motor units is smallest. The experimental data show that, for most muscles in the human arm, the amplitude of surface EMG activity for a constant, isometric force at the wrist was proportional to the cosine of the angle between the muscle's preferred direction and the direction of force. As predicted by the model, the preferred direction coincided with the direction in which the recruitment threshold of motor units was smallest. Musculus deltoideus anterior had a more complicated directional sensitivity for surface EMG activity that could not be well fit by a single cosine function. This effect could be explained by the finding of two subpopulations of motor units within that muscle, each with a different recruitment behavior.

Adult↗

PubMed1999

Cold pressor stimulation effect on hemodynamic changes following sustained isometric contraction in human jaw-closure muscles.

It is postulated that an altered adrenergic response pattern may be associated with chronic muscle pain states. To evaluate this hypothesis, one must fully understand the effect of an adrenergic activation on masticatory muscle blood flow under various conditions. This study evaluated the effect of a 12 degrees C cold pressor stimulation (a mild adrenergic activator), applied to the hand-forearm area, on intramuscular hemodynamics in the human masseter and temporalis muscles following a sustained isometric contraction. We assessed hemodynamics by measuring intramuscular hemoglobin concentration repeatedly, using a non-invasive near-infrared spectroscopy device. Measurements were taken before, during, and after a 30-second sustained 50% maximum voluntary contraction task. Fourteen healthy subjects, seven males and seven females, with no history of muscle pain in the masticatory system participated in this study. This protocol was repeated three times, but in the second trial, the cold pressor stimulation was applied to the subject during and for 5 min after the sustained contraction task. Repeated-measure analysis of variance performed on these data revealed that the peak hemoglobin concentration levels in the post-contraction recovery period were significantly reduced (between 13 and 14%) with cold pressor stimulation, both in the masseter (p < 0.001) and in the temporalis (p < 0.001) muscles. The results suggest that cold pressor stimulation produced a reduced intramuscular vasodilative response in these muscles during the immediate post-contraction period. One explanation for these results is that altering the local chemical environment of the muscle affects the adrenergic response pattern typically induced by a cold pressor stimulation.

Adult↗

PubMed2006

Physiological characteristics of motor units in the brachioradialis muscle across fatiguing low-level isometric contractions.

The purpose of this study was to determine (i) if decomposition-based quantitative electromyography (DQEMG) could detect changes in motor unit potential (MUP) morphology and motor unit (MU) firing pattern statistics associated with muscle fatigue, (ii) if any detected changes are correlated with surface electromyographic (SEMG) signs of fatigue, and (iii) if significant fatigue-dependent changes are repeatable within individuals. Mean MU firing rates and the morphology of MUPs detected using needle and surface electrodes during constant-torque isometric contractions held until exhaustion were investigated in the brachioradialis (BR) muscle in 10 healthy volunteers (mean age=28.6 yr, SD+/-3.9). Time dependant changes were investigated using an analysis of variance with normalized time as a main effect. Partial correlation coefficients were computed using a repeated measures analysis of covariance to determine if changes in MU firing rates, needle-detected MUPs and surface-detected MUPs (SMUPs) were related to changes in SEMG signal amplitude and frequency parameters. Intraclass correlation coefficients (ICCs) were used to determine the within-subject repeatability of changes in MU firing rates, and MUP and SMUP parameters. Significant decreases in mean MU firing rates were found along with significant increases in various duration and area related parameters in both MUPs and SMUPs across the fatiguing contraction. The SEMG signal demonstrated the expected changes with fatigue: an increase in amplitude and a decrease in frequency content. SEMG amplitude was significantly positively correlated with SMUP peak-to-peak voltage (r=0.85, p<0.05), and SMUP area (r=0.86, p<0.05). Mean power frequency was significantly negatively correlated with SMUP negative peak duration (r=-0.74, p<0.05). The significant time-dependent changes were reliably observed (ICCs were 0.94 for MUP peak to peak amplitude, 0.97 for MUP area and 0.95 for MUP area to amplitude ratio, 0.95 for SMUP peak-to-peak voltage, 0.83 for SMUP area, 0.99 for SMUP negative peak amplitude and 0.88 for SMUP negative peak area). The decreases in mean MU firing rates measured along with the increases in amplitude, duration and area parameters of MUPs and SMUPs and their partial correlation with SEMG amplitude during submaximal fatiguing contractions of the BR, suggest that recruitment is a main cause of increased SEMG amplitude parameters with fatigue. We conclude that DQEMG can be effectively and reliably used to detect changes in physiological characteristics of MUs that accompany fatigue.

Adult↗

PubMed1978

Effects of certain antibiotics on isometric contractions of isolated rat heart muscle.

The direct effects of potassium-penicillin-G, kanamycin, streptomycin and chloramphenicol on isometric contraction of isolated rat heart muscle were examined. Potassium-penicillin-G did not depress myocardial contractility but rather increased it. Those increases are not due to penicillin itself but due to a small amount of K+ in potassium-penicillin-G. Kanamycin and streptomycin did show not only direct myocardial depressants effects but concentration-dependent depressions. The depression produced by kanamycin could be restored to normal by adding Ca++ to the bath solution. Chloramphenicol did not show any significant concentration-dependent depression in our studies. We conclude that it is important to be aware of the potential depression of cardiac function by antibiotics, particularly in patients who have diminished cardiac reserve and who are undergoing surgical procedures under anesthesia which may also depress cardiac function.

Animals↗

PubMed1987

Clinical comparison of bladder contractility parameters calculated from isometric contractions and pressure-flow studies.

Parameters describing the contractility of the urinary bladder can be calculated from both isometric contractions and pressure-flow studies. The first method has the advantage of making very little demand either on the patient or on the urodynamicist, and the disadvantage of yielding a parameter that is not clearly related to muscle physiology. The second method demands more from both patient and experimenter but yields straightforward parameters. For a group of 86 patients with mixed pathologic findings, a correlation between the two types of parameters was demonstrated, showing that both methods test, at least partly, the same mechanism.

Adult↗

PubMed1967

The relation between heat produced and phosphorylcreatine split during isometric contraction of frog's muscle.

1. Heat production, tension development and phosphorylcreatine (PC) splitting have been measured simultaneously during isometric contractions of iodoacetate-poisoned frogs' sartorii at 0 degrees C. The muscles were stimulated to produce a series of 30 twitches or a 10 sec tetanus or a 30 sec tetanus.2. Of the several possible methods of calculating PC breakdown the best appears to be the one based on the assumption that PC/(total creatine) was originally the same in the two muscles of a pair.3. The difficulty of expressing PC break-down, heat production etc. in terms of the muscle's size is demonstrated.4. Several artifacts are discussed, including the heat produced by the stimulus and the possible role of reactions other than PC splitting.5. Even when these have been substantially eliminated there remains the rather intractable statistical problem of establishing a functional relationship between heat produced and PC split when both of these variables contain errors, so that ordinary regression analysis is misleading. It is shown that this problem can be dealt with satisfactorily by introducing other instrumental variables that can be determined experimentally.6. The final conclusion is that the in vivo enthalpy of hydrolysis of PC is about 10.6 kcal/mole, and is the same in twitches and in tetani.

Animals↗

PubMed1980

Recording and evaluation of isotonic and isometric contractions of skeletal muscle in situ.

A procedure is described for obtaining records and identifying curves of isotonic and isometric contractions (both tetanic and twitch) of the gastrocnemius muscle of the rabbit in situ. The experimental conditions have made it possible to follow the above mentioned phenomena and to measure further parameters in series at sequentially altered muscle lengths and loads acting in the course of isotonic contractions.

Animals↗

PubMed2001

Axial disposition of myosin heads in isometrically contracting muscles.

Meridional x-ray diffraction diagrams, recorded with high angular resolution, from muscles contracting at the plateau of isometric tension show that the myosin diffraction orders are clusters of peaks. These clusters are due to pronounced interference effects between the myosin diffracting units on either side of the M-line. A theoretical analysis based on the polarity of the myosin (and actin) filaments shows that it is possible to extract phase information from which the axial disposition of the myosin heads can be determined. The results show that each head in a crown pair has a distinct structural disposition. It appears that only one of the heads in the pair stereospecifically interacts with the thin filament at any one time.

Actins↗

PubMed2001

Blood flow in the triceps brachii muscle in humans during sustained submaximal isometric contractions.

The main purpose of this study was to determine the extent to which blood flow through the profunda artery within the triceps brachii muscle may be compromised during maintained low-force isometric fatiguing contractions. Doppler ultrasound techniques were used to record mean blood velocity and arterial diameter of the profunda brachii artery during sustained isometric contractions of 20% maximal voluntary contraction. The arterial diameter did not change throughout the contraction. Thus, blood velocity was considered to be an indicator of blood flow. The mean blood velocity increased initially and then remained constant during the contraction period. When compared to rest [0.06 (SD 0.03) m s-1] mean blood velocity was significantly larger at the start of the contraction [0.13 (SD 0.07) m s-1] and larger yet during recovery following the contraction [0.30 (SD 0.14) m s-1]. Although blood flow through the conduit artery did not drop during the contraction, the post-contraction hyperaemia suggested that circulatory compromise might have occurred at the level of the capillary beds.

Adult↗

PubMed1989

Muscle architecture and fibre characteristics of rat gastrocnemius and semimembranosus muscles during isometric contractions.

Rat gastrocnemius medialis (GM) and semimembranosus (SM) muscles have a very different morphology. GM is a very pennate muscle, combining relatively short muscle fibre length with sizable fibre angles and long muscle and aponeurosis lengths. SM is a more parallel-fibred muscle, combining a relatively long fibre length with a small fibre angle and short aponeurosis length. The mechanisms of fibre shortening as well as angle increase are operational in GM as well as SM. However, as a consequence of isometric contraction, changes of fibre length and angle are greater for GM than for SM at any relative muscle length. These differences are particularly notable at short muscle lengths: at 80% of optimum muscle length, fibre length changes of approximately 30% are coupled to fibre angle changes of 15 degrees in GM, while for SM these changes are 4% and 0.6 degrees, respectively. A considerable difference was found for normalized active slack muscle length (GM approximately 80 and SM approximately 45%). This is explained by differences of degree of pennation as well as factors related to differences found for estimated fibre length-force characteristics. Estimated normalized active fibre slack length was considerably smaller for SM than for GM (approximately 40 and 60%, respectively). The most likely explanation of these findings are differences of distribution of optimum fibre lengths, possibly in combination with differences of myofilament lengths and/or fibre length distributions.

Animals↗

PubMed2003

Mapping of movement in the isometrically contracting human soleus muscle reveals details of its structural and functional complexity.

It is becoming increasingly apparent that precise knowledge of the anatomic features of muscle, aponeurosis, and tendons is necessary for understanding how a muscle-tendon complex generates force and accomplishes length changes. This report presents both anatomic and functional data from the human soleus muscle acquired by using magnetic resonance imaging. The results show a strong relationship between the complex three-dimensional structure of the muscle-tendon system and the intramuscular distribution of tissue velocities during in vivo isometric contractions. The proximal region of the muscle is unipennate, whereas the midregion has a radially bipennate hemicylindrical structure, and the distal region is quadripennate. Tissue velocity mapping shows that the highest velocity regions overlay the aponeuroses connected to the Achilles tendon. These are located on the anterior and posterior surfaces of the muscle. The lowest velocities overlay the aponeuroses connected to the origin of the muscle and are generally located intramuscularly.

Adult↗

PubMed1984

Effects of topical cooling on isometric contractions of the human masseter muscle.

Six adult males performed tooth clenching for 10 and 80 s at maximum voluntary contraction strength (MVC). Motor activity in the right and left masseter muscles was monitored by surface electromyography. Local temperatures of both cheeks were monitored by a thermocouple. MVC activity for 80 s induced pain and fatigue in both muscles; 10 s of clenching caused no muscle discomfort. Ice, subsequently applied to the right cheek for 30 min, lowered the temperature of the cheek and masseter muscle. The isometric MVC exercises were then repeated. During 10 s of isometric contraction, MVC motor activity in the cooled masseter muscle was significantly increased by 29 per cent; that of the non-cooled contralateral muscle was insignificantly reduced by 12 per cent. During 80 s of clenching, MVC motor activity in the cooled muscle was significantly increased by 30 per cent; that of the non-cooled muscle was insignificantly reduced by 4 per cent. MVC activity for 80 s by the cooled muscle caused no pain and fatigue, but the discomforts continued in the non-cooled muscle. Increased MVC alpha motor activity in the cooled masseter muscle might have resulted from motor facilitation that was mediated by cutaneous, muscular or mucosal cold receptors, or it might have been due to the absence of pain and fatigue.

Adult↗

PubMed2005

Strain and elongation of the vastus lateralis aponeurosis and tendon in vivo during maximal isometric contraction.

The strain and elongation of the vastus lateralis (VL) tendon, tendon plus aponeurosis, and aponeurosis were examined during maximal voluntary contractions on a Biodex-dynamometer (knee angle 115 degrees , hip angle 140 degrees ) in 12 sprinters. Following a warm-up phase, the subjects were instructed to perform a gradual maximal knee extension and hold it for about 3 s. The kinematics of the leg were recorded using a Vicon 512 system with eight cameras operating at 120 Hz. Ultrasonography images were taken simultaneously from the VL myotendinous junction and the mid lateral part of the VL muscle belly. During the maximal isometric knee extensions, the knee joint rotated (13.6+/-5.9 degrees ), leading to an overestimation of the elongation of the tendinous tissues. After correcting for this, the maximal elongation of the VL tendon examined at the myotendinous junction was lower (P<0.05) than the maximal elongation of the VL tendon plus aponeurosis examined at the muscle belly (15 vs. 27 mm, respectively). The maximal estimated strains of the tendon, tendon plus aponeurosis, and aponeurosis showed no statistical differences (8+/-2%, 8+/-1%, and 7+/-2%, respectively, P>0.05). It is concluded that the strains of the human VL tendon, VL tendon plus aponeurosis, and VL aponeurosis, as estimated in vivo by two dimensional ultrasound during maximal isometric contractions, do not differ from each other. The displacement measured at a cross point in the VL muscle belly is significantly greater than that measured at the VL myotendinous junction.

Adult↗

PubMed1989

Supporting and membrane structures of human outer hair cells: evidence for an isometric contraction.

The supporting elements of the human organ of Corti express a number of cytokeratins, and it is obvious that the presence of cytokeratins provides mechanical stability to the framework of the entire organ of Corti. In the basal part of the outer hair cells and in the apical part of the Deiters cells, there is a significant expression of actin. In the human outer hair cells, the system of subsurface cisterns is not only restricted to the lateral cell wall but is also continuous to the basal region where the hair cells are closely attached to the supporting Deiters cells. In contrast to the well-known organization of the subsurface cisterns and pillar structures at the lateral outer hair cell wall of rodents, the pillars at the base of human outer hair cells can form channels. These channels allow a direct communication between the lumen of the subsurface cisterns and the intercellular space which separates outer hair cells and Deiters cells. At the apical part of the Deiters cells, a condensation of microfilaments is present which displays morphological similarities to actin fibers. These well-organized structural configurations between outer hair cells and Deiters cells imply that this region has special contractile properties. The human outer hair cells are therefore firmly stabilized not merely apically but also at the cell base, a situation which would permit a mainly isometric contraction under in vivo physiological conditions.

Actin Cytoskeleton↗

PubMed1991

Decreased ATP cost of isometric contractions in ATP-depleted rat fast-twitch muscle.

ATP was depleted to inosine 5'-monophosphate (IMP) in gastrocnemius muscles of anesthetized rats during two consecutive bouts of intermittent tetanic stimulation (each 1 Hz at 100 Hz, 100-ms duration, for 3 min) separated by 5 min. During a subsequent 75-min recovery period, resynthesis of adenine nucleotide from IMP was inhibited in one group of muscles with hadacidin (N-formyl-N-hydroxyaminoacetate, total dose 300 mg/kg ip), an inhibitor of adenylosuccinate synthase. In vivo 31P-nuclear magnetic resonance spectra and analysis of muscle extracts confirmed that ATP was reduced by over 30% in hadacidin-treated muscles compared both with nonstimulated muscles and with identically stimulated and rested control muscles in saline-injected rats. After the recovery period, calculated [ADP] in ATP-depleted muscles was half that in controls, while phosphorylation potential [PP = ln([ATP]/[ADP] [Pi]), where Pi is inorganic phosphate] and twitch force were similar. During a final 8 min of 0.75-Hz twitch stimulation, [ADP] and the initial rate of phosphocreatine (PCr) hydrolysis were less in ATP-depleted compared with control muscles, while changes in PP and twitch force were similar in the two groups. During a final 3 min of tetanic stimulation, ATP, PCr, and pH decreased less in ATP-depleted compared with control muscles. The results indicate that the ATP cost of isometric contraction is decreased by acute ATP depletion.

Adenosine Triphosphate↗

PubMed1978

The effect of alcohol on active and passive stiffness, and on isometric contractions of glycerinated heart muscle in rats.

The response of the contractile and the series elastic elements to ethanol was studied isometric contraction and quick release methods after measuring the passive length-tension relationship in glycerinated heart muscle fibers of rats at resting state. In rats consuming 30% ethanol for an average of 5 weeks, maximal developed tension (P0), the maximal rate of tension development (dp/dtmax) and Vmax were significantly depressed, the time to peak tension (t0) was not changed. As in fresh papillary muscle the modulus of elasticity of active glycerinated muscle increased in proportion to load. The stiffness of the series elastic element showed significant elevation in rats receiving ethanol. The passive stiffness in resting state revealed no significant difference between control and alcohol exposed rats. Therefore, increased stiffness of the series elastic element and diminished contractility are present following chronic alcohol consumption.

Animals↗

PubMed1998

Shortening versus isometric contractions in isolated human failing and non-failing left ventricular myocardium: dependency of external work and force on muscle length, heart rate and inotropic stimulation.

BACKGROUND: For reasons of simplicity, studies on isolated human myocardium have been conducted using exclusively isometric contractions, although positive inotropic interventions may differently influence force development, extent of shortening and myocardial work performance. We investigated human left ventricular failing and non-failing preparations comparing isometric versus isotonic, i.e., shortening contractions. RESULTS: (1) When muscle length is increased from 90% to 100% lMAX, peak developed force increases by 36% and 43% (p < 0.05) in non-failing and failing human left ventricular myocardium, respectively. Maximum performed work increases similarly in non-failing but decreases in failing myocardium. It can be shown that this discrepancy is due to significantly higher resting tension and does not present an insufficient intrinsic shortening capacity in failing myocardium. (2) When stimulation rate is increased from 0.5 to 2.0 Hz, isometric force increases significantly by 59% in non-failing and decreases by 27% in failing myocardium, whereas maximum performed work increases by 98% and decreases by 46%, respectively. (3) Pharmacological positive inotropic interventions by 7.2 mM calcium (n = 9), 3 x 10(-8) M isoproterenol (n = 7), 3 x 10(-8) M ouabain (n = 5), and 10(-5) M EMD 57033 (n = 3) equally increased force development and extent of shortening: When the fractional effect on shortening (y) was correlated to the fractional effect on force (x), the following linear regression equation was obtained: y = 0.91x + 0.26 (r = 0.86; p < 0.001). CONCLUSIONS: The data presented are of clinical and pharmacological importance: (1) The Frank-Starling mechanism is demonstrated to be existent in the failing human myocardium regarding both isometric force developed and maximum work performed. (2) Both force-frequency relations and--to a greater extent--work-frequency relations are reversed in failing human myocardium. (3) Independent of the pharmacological mode of action, positive inotropic compounds increase developed isometric force to the same extent as isotonic shortening and therefore potentiate maximum performed work.

Adolescent↗