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PubMed1989

Force, endurance time, and cardiovascular responses in voluntary isometric contractions of different muscle groups.

A study was made to reinvestigate the general validity of the force/endurance relation of isometric contractions. Relative force development, mean bioelectrical muscle activity and circulatory responses (blood pressure, interbeat interval) during isometric contractions of different muscle groups were compared. 6 men performed maximum isometric contractions of foot plantar- and dorsalflexors and hand flexors lasting 10 minutes. The hydrostatic blood pressure was varied by placing subject's arms and legs in two different positions, high and low. 9 Women performed plantarflexion and dorsalflexion with maximum strength and at constant load of 60% MVC to exhaustion. The results confirm that the force/endurance relationship during isometric exercise is not a biological constant, applicable for all muscles. The fibre composition of the muscle and the blood pressure varied by different hydrostatic components are discussed as factors causing differences in force/endurance relationship.

Adult↗

PubMed1988

Muscle stiffness changes during enhancement and deficit of isometric force in response to slow length changes.

The mechanism of the enhancement and the deficit of isometric force in response to slow length changes in tetanized frog muscle fibers was studied by recording the stiffness changes with sinusoidal vibrations (0.5-1.9 kHz, peak-to-peak amplitude 0.1% of L0). When a tetanized fiber was slowly stretched, the fiber stiffness first rose abruptly and then decreased linearly while the stretch went on; after the completion of stretch, the stiffness decreased towards a steady value which was equal to that during the ordinary isometric tetanus at the same fiber length, though the force decayed towards a steady level higher than that of the ordinary isometric tetanus at the same fiber length. This indicates that the enhancement of isometric force after stretch is associated with decreased stiffness. If, on the other hand, a tetanized fiber was slowly released, the force and the stiffness changed in parallel with each other. Recordings of the segmental length changes along the fiber with a high-speed video system (200 frames/s) indicated that all the segments lengthened in response to slow stretch, while the segmental length changes in response to slow release was markedly nonuniform. These results are discussed in connection with the cross-bridge performance and the filament-lattice structures.

Animals↗

PubMed1987

Assessment of exercise tolerance of cardiac patients by bicycle, treadmill and treadmill plus isometric exercise with and without nifedipine.

Seventeen post-myocardial infarction patients experiencing angina on effort performed 6 different exercise tests until they reached symptom-limited maximal level, 3 after placebo and 3 after oral administration of 10 mg of the Calcium antagonist, nifedipine, in a randomized, double blind, cross-over controlled study. Four of the tests were conventional bicycle and treadmill tests with stepwise increasing load. In 2 of the tests an isometric exercise of carrying a weight averaging 6 kg and corresponding to about 30% of maximal grip strength was added to the treadmill walking. When the exercise was stopped because of moderately severe angina, the product of heart rate and systolic blood pressure did not show any statistically significant difference between the tests. However, in the treadmill plus isometric test the work time was shorter and the slope of the treadmill was less than in the treadmill test. The difference was caused partly by non-cardiac factors, namely fatigue of the hand muscles. In routine exercise tests of coronary patients the addition of an isometric to a dynamic load did not give substantially more information than dynamic exercise alone. Nifedipine caused a modest increase of exercise tolerance in all tests, the increase being greatest in the treadmill plus isometric test. The increase in exercise tolerance was seen also in patients receiving beta-blocking agent.

Aged↗

PubMed1987

Cardiovascular changes during isometric and dynamic exercise in chronic lung disease.

To compare circulatory changes during dynamic and isometric exercise in patients with chronic lung disease, vascular pressures and cardiac output were measured in pulmonary and systemic circulation. The patients, mainly chronic bronchitics, were divided according to pulmonary vascular resistance (PVR): six patients with PVR below 120 dyn.s.cm-5, and 17 with PVR above this value. Exercise was performed using the legs, in a supine position, at low load. Oxygen consumption increased, on average, from 270 ml/min at rest, to 340 ml/min during isometric exercise, and 585 ml/min during dynamic exercise. Isometric exercise produced an increase in systemic artery pressure similar to that of dynamic exercise, but other vascular changes were of much smaller magnitude: pulmonary artery pressure increased by 4 mmHg with isometric exercise, and by 14 mmHg during dynamic exercise, on average; for cardiac output, these changes were 0.8 and 3.3 l/min, respectively. The pressure/flow relationship in the pulmonary circulation was comparable for both types of exercise. Changes in filling pressures in relation to stroke volume were more marked in the group of patients with elevated pulmonary vascular resistance. Changes in wedge pressure and in right atrial pressure were remarkably parallel for patients with normal as well as with high pulmonary vascular resistance.

Blood Pressure↗

PubMed1986

Bovine longissimus muscle glycogen concentration in response to isometric contraction and exogenous epinephrine.

Isometric contraction was elicited in cattle by electrical stimulation (electrical immobilization) and was used as a means of investigating the interaction between muscle contraction and epinephrine-induced muscle glycogen degradation. At 0.5 and 24 hours after a 15-minute period of continuous muscle contraction, glycogen content of longissimus muscle specimens collected via needle biopsy was not different from precontraction concentrations. Epinephrine (13.2 mg/kg of body weight) given subcutaneously resulted in a 30% to 35% reduction of muscle glycogen. Reduction of muscle glycogen was slightly greater when epinephrine was used in conjunction with isometric contraction, compared with epinephrine treatment alone. Muscle glycogen increased, and free glucose, lactate, and glucose-6-phosphate decreased, with increases in body weight. In younger, lighter cattle (370 kg), epinephrine decreased muscle glycogen and lactate concentrations and generally decreased muscle glucose concentrations. Muscle-free glucose and lactate concentrations were increased in older, heavier cattle (446 kg) by contraction and epinephrine injection when the animals were immobilized for 30 minutes, with intermittent periods of muscle relaxation. In these cattle, muscle glycogen concentration slightly decreased with isometric contraction. Data indicate that reports of increased glycogenolysis observed in cattle subjected to stress by mixing strange animals or exercise is due to dynamic muscle contraction and not due to isometric muscle contraction.

Animals↗

PubMed1984

Isometric exercise before and after beta-receptor blockade with propranolol and mepindolol sulfate.

The effect of isometric exercise on hemodynamic parameters before and after beta blockade were investigated in two groups of six male volunteers, ages 25-28 years. On the basis of a randomized scheme, the subjects received either 15 mg propranolol i.v. or 0.5 mg mepindolol sulfate i.v. Cardiac output (CO) was determined by means of the Swan-Ganz thermodilution technique, and blood pressure was measured invasively in the radial artery. Analysis of the results pointed to the following conclusions. Isometric contraction brings about a reflex constriction of arterial and venous vessels, and thus an increase in blood pressure, pulmonary artery wedge pressures (PAWP) and CO, as well as a reduction in stroke volume (SV). The circulatory reflex mechanism operative during isometric exercise cannot be significantly influenced by beta blockade with propranolol or mepindolol sulfate. The increase in heart rate during isometric exercise is therefore unlikely to be the result of sympathetic activation, but rather of other (vagal?) reflex mechanisms.

Adrenergic beta-Antagonists↗

PubMed1984

Isometric exercise before and after nadolol treatment in uncomplicated essential hypertension.

Systolic, diastolic and mean arterial pressure (SAP, DAP, MAP), heart rate (HR) and tension time index (TTI) were evaluated in 20 uncomplicated hypertensives and in 20 normotensives at rest and during isometric exercise, performed by handgrip (HG). In the hypertensive population the same parameters were evaluated also after two and six weeks of treatment with a beta-blocking agent, nadolol, given in a single daily dose. In all cases isometric exercise induced a significant increase of SAP, DAP, MAP, HR and TTI and this increase was more relevant in hypertensives than in controls. In the hypertensive population, nadolol was effective in lowering arterial pressure at rest, and significantly reduced the levels of SAP, DAP, MAP, HR and TTI reached at the end of the HG test. Moreover, the increments of the investigated parameters induced by isometric exercise were significantly reduced after nadolol. The results suggest that nadolol, unlike other beta-blocking agents, exerts a "protective" action against the hazardous increments of blood pressure which may occur in the daily life of hypertensive subjects during involuntary isometric exercises, and which could trigger dangerous cardiovascular complications.

Adult↗

PubMed1984

Cardiovascular responses to isometric neck muscle contractions: results after dynamic exercise with various headgear loading configurations.

Experiments were conducted to quantify the cardiovascular response (blood pressure and heart rate) elicited by sustained isometric contractions of the neck muscles. The response was secondary to dynamic exercise with various headgear loading combinations. The neck muscles were loaded by the head itself (CON), the standard U.S. Army SPH-4 helmet (HEL), and a combination of the SPH-4 helmet with Night Vision Goggles (H/NVG). During two exercise periods of 5 min and 35 min, each of the five subjects would rotate the head from side-to-side in the CON, HEL, or H/NVG configuration. Immediately thereafter, the subject would position his head in an isometric head dynamometer and exert a sustained right lateral (LAT) neck contraction or forward (FOR) neck contraction at 70% of a maximal voluntary contraction (MVC). During this isometric neck muscle contraction, the subject's endurance time to fatigue was recorded, the blood pressure was manually recorded, and the heart rate was continuously recorded. Characteristic increases in the systolic and diastolic blood pressure and heart rate occurred with sustained isometric neck muscle contractions. There was an average 40% increase in the systolic blood pressure, an average 50% increase in the diastolic blood pressure, and an average 45% increase in the heart rate from resting to the end of a fatiguing 70% MVC (p less than 0.05). These responses appear to be relatively independent of the duration of the exercise period, the loading during the exercise period, and the specific muscle mass involved. The mechanisms for the pressor response and the heart rate response are reviewed.

Adult↗

PubMed1980

[Isometric load tolerance in patients with postinfarct cardiosclerosis].

The work deals with the comparative study of the tolerance of 80 patients with post-infarction cardiosclerosis with a different clinical picture of the disease to isometric and dynamic loads. It was established that the strength of the hands was weaker in patients with the cardiac pain syndrome than in healthy individuals. In addition to this in patients with cardialgia, the tolerance to isometric load was diminished. In isometric loading the systolic arterial pressure in all patients increases much more than in individuals of the control group, and in patients with severe post-infarction angina pectoris there are also changes in the pulse. Cardialgia has a determining effect on the tolerance to isometric load, whereas the dynamic test tolerance is mainly limited by the severity of chronic coronary insufficiency.

Adult↗

PubMed1994

Effects of short-term isotonic & isometric training on cardiovascular & pulmonary function.

A randomised control trial of short-term exercises on specific cardiovascular and respiratory parameters was undertaken in normal male college students. The effects of isotonic training (5BX programme) and isometric training (a programme of isometric exercises working all major groups of muscles) were compared with a control group with no specific workout. Both isotonic and isometric training resulted in significant cardiovascular improvement but seemed inadequate to improve vital capacity and flow rates. Isotonic training in addition, improved ventilatory efficiency. It is concluded that such isotonic or isometric training of thrice a week for ten weeks, requiring no equipment, less time and space can be promoted to improve physical fitness.

Adolescent↗

PubMed1992

Lowered cutaneous sensitivity to nonpainful electrical stimulation during isometric exercise in humans.

The effect of isometric exercise on cutaneous sensitivity to nonpainful electric stimulation was studied in human subjects. The exercises consisted of brief (duration: 1-10 s) palmar flexions of the hand or foot against varying loads (10-30% of the maximal force). A visual "go" cue was used to indicate the start and end of the exercise. Isometric hand exercise produced a load-dependent increase of electrotactile thresholds of the fingers. The threshold elevation was rapidly attenuated with prolonged duration of the exercise. The hand exercise-induced threshold elevation was of equal magnitude in the glabrous and hairy skin of the fingers. Thresholds were not changed for the hand contralateral to the exercising hand. Passive static pressure of the hand did not produce threshold changes, whereas activation of afferent inhibition by a vibrotactile stimulus (100 Hz, 0.1 mm) did produce a significant threshold elevation. Exercise-induced threshold elevation was also significant immediately prior to the EMG response of the arm but not at the time of the visual "go" signal, or before it. The threshold increase found during the EMG response was not significantly stronger than that found prior to the EMG response. These results suggest that isometric exercise load-dependently produces a phasic, rapidly attenuating increase in cutaneous tactile thresholds in the exercising limb but not multisegmentally. Corollary efferent barrage from motor to sensory structures of the brain could be underlying the threshold changes produced by isometric exercise, whereas afferent inhibitory mechanisms seem to have only a minor role.

Adult↗

PubMed1996

Continuous measurement of blood pressure, heart rate and left ventricular performance during and after isometric exercise in head-out water immersion.

Experiments were performed to determine the changes in blood pressure (BP), heart rate (HR) and left ventricular function during and after isometric knee extension during thermoneutral (35 degrees C) head-out water immersion (HWI) or in air. Seven healthy male subjects mean age 24 (SD 3) years kept their knees extended (60% maximal voluntary extension) until they reached exhaustion. The mean BP at rest was 80 (SD 10) and 78 (SD 8) mmHg [10.7 (SD 1.33) and 10.4 (SD 1.07)kPa] in air and during HWI, respectively, (NS). They increased progressively (P < 0.01) during contraction and reached maximal values of 148 (SD 22) and 143 (SD 26) mmHg [19.7 (SD 2.93) and 19.1 (SD 3.47)kPa] in air and in HWI, respectively, (NS). The mean HR at rest was 74 (SD 8) and 70 (SD 11) beats.min-1 in air and in HWI, respectively, (NS). They also increased progressively, (P < 0.01) and reached 126 (SD 14) and 118 (SD 17) beats.min-1 in air and in HWI, respectively, (NS). The changes in BP and HR during contraction in HWI tended to be smaller than those in air (NS). Left ventricular end diastolic diameters (dd) at rest in HWI were greater than those in air and were maintained at higher values during and after isometric contraction. In contrast, dd decreased during isometric contraction in air (P < 0.01). The change of left ventricular systolic diameters (ds) in HWI was no different to those in air. From these findings, isometric exercise in thermoneutral HWI would seem to be characterized by a greater dd than in air and this could be useful for patients with deconditioning effects such as orthostatic hypotension.

Adult↗

PubMed1987

Differential effects of reoxygenation on intracellular calcium and isometric tension.

We used the bioluminescent Ca2+ indicator, aequorin to record intracellular calcium transients during reoxygenation of hypoxic ferret ventricular muscle in order to determine whether alterations in the amplitude and time course of isometric contraction are mediated by changes in [Ca2+]i. Papillary muscles less than or equal to 1 mm in diameter were removed from the hearts of male ferrets and perfused with a bicarbonate-buffered physiologic salt solution at 30 degrees C. Muscles were stimulated to contract isometrically at 0.33 Hz and were loaded with aequorin by a chemical procedure. Hypoxia was induced by changing the gas mixture bubbling the perfusate to 95% N2, 5% CO2; reoxygenation was accomplished by switching the gas mixture to 95% O2, 5% CO2. Hypoxia produced a decrease in peak Ca2+ and tension that was reversed by reoxygenation. However, the effects on tension of changes in oxygenation were greater than expected from the degree of change in [Ca2+]i. The time courses of the Ca2+ transient and isometric twitch moved in opposite directions and were respectively prolonged/abbreviated by hypoxia and abbreviated/prolonged by reoxygenation. These results indicate that changes in the amplitude and time course of the isometric twitch induced by hypoxia and reoxygenation cannot be attributed alone to changes in intracellular Ca2+ availability and are caused in part by a significant decrease in the calcium sensitivity of the contractile apparatus.

Aequorin↗

PubMed1995

Influence of central command and ergoreceptors on the splanchnic circulation during isometric exercise.

The splanchnic circulation can make a major contribution to blood flow changes. However, the role of the splanchnic circulation in the reflex adjustments to the blood pressure increased during isometric exercise is not well documented. The central command and the muscle chemoreflex are the two major mechanisms involved in the blood pressure response to isometric exercise. This study aimed to examine the behaviour of the superior mesenteric artery during isometric handgrip (IHG) at 30% maximal voluntary contraction (MVC). The pulsatility index (PI) of the blood velocity waveform of the superior mesenteric artery was taken as the study parameter. A total of ten healthy subjects [mean age, 21.1 (SEM 0.3) years] performed an IHG at 30% MVC for 90 s. At 5 s prior to the end of the exercise, muscle circulation was arrested for 90 s to study the effect of the muscle chemoreflex (post exercise arterial occlusion, PEAO). The IHG at 30% MVC caused a decrease in superior mesenteric artery PI, from 4.84 (SEM 1.57) at control level to 3.90 (SEM 1.07) (P = 0.015). The PI further decreased to 3.17 (SEM 0.70) (P = 0.01) during PEAO. Our results indicated that ergoreceptors may be involved in the superior mesenteric artery vasodilatation during isometric exercise.

Adult↗

PubMedDate not supplied

The effect of preload and Ca++-ions on the time-course of the isometric force and on the force-velocity relation: is Vmax dependent on the number of activated cross-bridges?

The maximum shortening velocity of the isolated, externally unloaded papillary muscle of cats, as can be obtained by quick-releases in the initial phase of isometric contractions, is clearly dependent on the Ca++-concentration of the bath solution as well as on the preload: In the range of 0.55 m-mole/1 to 4.4 m-mole/1 a doubling of the Ca++-concentration induces a mean increase of 23% in Vmax; in a range of muscle length assumed to be of physiological relevance, a rise in the extension of 10% of Lmax results in a mean augmentation of 20% in Vmax. In order to reveal possible differences in the effect of calcium and various degrees of filament overlap, firstly, isometric mechanograms were measured under low Ca++-concentration at the muscle length of Lmax. By correspondingly lowering the muscle extension under increased Ca++-concentration the reproduction of almost the entire rising phase of these isometric mechanograms was obtained. Both these experimental conditions result in congruent force-velocity relations and consequently in identical values for Vmax. It must therefore be concluded that there exists an inner relationship between the initial time-course of the isometric force and the force-velocity relations measured during this period. The interpretation of this fact involves weighing the hypothesis of an inner frictional force dependent on velocity against that of a maximum shortening velocity dependent on the number of activated cross-bridges. The resultant consequences for the estimation of the contractile qualities of the myocardium are discussed.

Animals↗

PubMed1993

The hemodynamic effects of isometric exercise during late normal pregnancy.

OBJECTIVE: Our study was designed to evaluate the hemodynamic effects of isometric exercise in late normal pregnancy. STUDY DESIGN: Study subjects were 10 healthy pregnant volunteers with uncomplicated singleton gestations between 25 and 36 weeks. Doppler methods were used to derive cardiac output, total peripheral resistance, and stroke volume before, during, and after a defined protocol of lower extremity isometric exercise. Hemodynamics and blood pressure were evaluated and compared. RESULTS: Mean arterial blood pressure and total peripheral resistance increased during the performance of isometric effort (mean blood pressure +/- SD was 78.9 +/- 7.3 to 97.5 +/- 8.6 mm Hg; total peripheral resistance +/- SD was 924 +/- 148 to 1153 +/- 18.3 dyne.sec.cm-5; p < 0.002 and p < 0.001, respectively). Cardiac output remained unchanged throughout the study period. CONCLUSION: In advanced normal pregnancy isometric exercise increases the mean arterial blood pressure by raising the total peripheral resistance.

Blood Pressure↗

PubMed2006

Skeletal muscle IGF-I isoform expression in healthy women after isometric exercise.

CONTEXT: Little is known about the expression of skeletal muscle-specific isoforms of the insulin-like growth factor-I (IGF-I) gene in response to exercise in humans. Data for women are sparse and the influences of age and physical training status are unknown. Here we aimed to describe the expression of mRNA for the IGF-I isoforms in the quadriceps muscles of women at rest and after isometric exercise. OBJECTIVE: To categorize mRNA expression of the IGF-I splice variants IGF-IEa and IGF-IEc (MGF) in healthy women, determine whether isometric exercise stimulates expression, and to determine the duration of the response. DESIGN: Observational study. SETTING: Clinical research facility. PARTICIPANTS: Six healthy women (median age 38 years, range 25-43 years). INTERVENTION: Single bout of maximal isometric knee extension undertaken as 20 sets of 6 contractions. Muscle samples from the lateral mass of the quadriceps were obtained by needle biopsy at baseline, 2.5h and 50h after exercise. MAIN OUTCOME MEASURES: Quantification of mRNAs for IGF-IEa and MGF. RESULTS: mRNA for the IGF-IEa transcript was significantly elevated by 50h post-exercise compared to baseline (p=0.005) and mRNA for MGF was significantly elevated by 2.5h (p=0.026). There were no statistically significant differences between measurements at 2.5h and 50h for either transcript. CONCLUSIONS: We have shown that the upregulation of the muscle specific IGF-IEa and MGF isoforms occurs within 2.5h of a single bout of isometric exercise in women. The upregulation persists for at least 2 days after exercise. We have also demonstrated a potentially safe and effective way of studying the responsiveness of these isoforms to resistance exercise in future studies of older and/or frail individuals.

Adult↗

PubMed1996

Differential responses in left ventricular diastolic filling dynamics with isometric handgrip versus isotonic treadmill exertion.

Although the hemodynamic responses to isotonic and isometric exercise are different, few data exist comparing the response to left ventricular (LV) diastolic filling dynamics with these two forms of exertion. Therefore we performed Doppler examination before and at the end of isotonic and isometric exercise in 20 normal volunteers to define the differential responses of LV filling to these two forms of exertion. Transmitral inflow velocity signals from the apical view and phonocardiography were recorded before and at the termination of treadmill exercise (TRD) to 11 METs and handgrip (HG) 50% maximal for 2 minutes). Mean blood pressure (mBP), heart rate (HR), early diastolic (E) and late atrial (A) inflow velocities, mean acceleration rate (ACC) of E wave, time velocity integral of inflow (Ti), and isovolumic relaxation time (IRT) from second heart sound to onset mitral inflow were measured. Absolute changes from baseline were significantly different for the two forms of exertion: TRD versus HG: BP = 11 +/- 9 versus 36 +/- 10 mm Hg, HR = 37 +/- 16 versus 16 +/- 9 beats/min, E = 11.6 +/- 11.3 versus -7.0 +/- 9.4 cm/sec, A = 29.9 +/- 14.5 versus 14 +/- 12 cm/sec, ACC = 164 +/- 151 versus -56 +/- 135 cm/sec2, Ti = 1.9 +/- 3.0 versus -1.7 +/- 1.7 cm, and IRT = -12 +/- 9 versus 9 +/- 10 msec, all p < 0.0001 except for A, p < 0.001). Isotonic treadmill exercise resulted in enhanced early diastolic filling manifested by increases in E and ACC and a decreased in IRT. Conversely, isometric handgrip exercise produced evidence of reduced early filling including decreased E and ACC and slightly increased IRT. Thus the response of LV filling dynamics recorded by Doppler differs for isotonic and isometric exertion and likely reflects the variable pressure and flow alterations induced by these two forms of exertion.

Adult↗