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PubMedDate not supplied

Analytical approach to mathematical modelling of the isometric contraction of the hypertrophied myocardium.

It is attempted on the basis of the Hill two-element model to simulate mathematically the rising phase of the isometric mechanogram in the pressure-hypertrophied rat myocardium (Goldblatt II). The evaluation is based on the experimentally determined force-velocity relation as well as on the characteristic of the series-elastic stiffness. The changes in the isometric mechanogram of the hypertrophied heart muscle observed in the experiment can be mathematically predicted solely by means of the alterations in the force-velocity relationships induced by the hypertrophy, whereby the characteristic of the series-elastic stiffness is considered to be unchanged.

Cardiomegaly↗
PubMed1976

Influence of low extracellular pH upon the Ca inward current and isometric contractile force in mammalian ventricular myocardium.

In isolated papillary muscles of cats the changes in Ca inward current and isometric contractile force following a decrease of extracellular pH from 7.4 to 5.5 were studied. The Ca current was analyzed (a) by measuring the upstroke velocity of Ca-mediated action potentials and (b) in voltage clamp experiments using the double sucrose gap technique. 1. At a pH of 5.5 the upstroke velocity of the Ca-mediated action potential decreased to 65% of the control, while overshoot and action potential duration remained almost unchanged. Furthermore, the relative refractory period was prolonged and in some cases, a "Wenckebach-like" phenomenon occurred. In voltage clamp experiments, the slow inward current was found to be diminished to 50-60% of the initial control value and over a broad voltage range the current voltage relationship curve was shifted to weaker currents. Acidosis did not influence the steady state inactivation but altered the kinetics of inactivation of the slow inward current and induced an increase of tauinactivation and taurecovery. This indicates that acidosis exerts a complex effect on the slow membrane channel. 2. The normal response of the Ca current towards variations of the extracellular Ca concentration (0.5-4 mM) or towards the addition of the beta-stimulating compound isoproterenol (2 mg/l) was not altered by the lowered extracellular pH. 3. In the acid medium, isometric contractile force declined to 40% of the control value within 25 min and, thus, reacted stronger than the Ca current. This indicates that those forms of acidosis used in the present experiments caused their negative inotropic effect not exclusively via a depression of the Ca current. Rather an additional intracellular effect has to be assumed which finally leads to a reduced activity of the contractile system. 4. At pH 5.5 excess Ca (4 mM) induced the same quantitative response of the contractile system as obtained at normal pH. In contrast, the positive-inotropic effect of 2 mg/l isoproterenol was more pronounced, whilst the sensitivity of the Ca inward current towards this beta-stimulating compound remained unchanged.

Action Potentials↗
PubMed1979

Effect of estradiol and progesterone on the isometric tension developed by longitudinal and circular isthmus of rabbit fallopian tube.

The effect of estradiol and progesterone on the isometric tension developed by longitudinal and circular muscle of isthmus of rabbit fallopian tube to various agonists was studied. The isometric tension developed was reduced by about 50% under progesterone dominance as compared to estrogen dominance. This inhibition was of non-specific nature and did not involve adrenergic receptors. The non-specific effect of progesterone may have a role in regulating the entry of ova into the isthmus after their initial retention at the ampullary--isthmic junction.

Animals↗
PubMed1979

The time-course of energy balance in an isometric tetanus.

Unpoisoned sartorius muscles of Rana temporaria were stimulated tetanically in isometric contractions lasting up to 20 s at 0 degrees C. The observed enthalpy (heat + work) production and the chemical changes in these contractions were measured, and a comparison was made between the observed enthalpy and the enthalpy that could be explained by the chemical changes. Like earlier workers, we found that the only net known reaction of energetic significance that occurred was dephosphorylation of n-phosphoryl creatine (PC), and we found a significant evolution of unexplained enthalpy (UE), a portion of the observed enthalpy which could not be explained by the extent of PC dephosphorylation. We measured the total quantity and the rate of production of the UE, and we found that its rate of evolution, which was most rapid during the first 750 ms of contraction, fell progressively to zero by the 8th s of contraction: i.e., after 8 s of contraction, all the observed enthalpy is adequately explained by PC dephosphorylation. The time-course of evolution of the UE was slower than that of the labile enthalpy (a component of the enthalpy evolved in isometric contraction whose rate of production declines exponentially at approximately 1 s-1). We conclude that, although the magnitudes of these enthalpy quantities may be similar, they are not derived from the same chemical reaction in muscle.

Animals↗
PubMed1976

The responses of human muscle spindle endings to vibration during isometric contraction.

1. An human subjects, vibration of amplitude 1-5 mm and frequency 20-220 Hz was applied to the tendons of muscles in the leg to examine the effects on the discharge of primary and secondary endings during manoeuvres designed to alter the level of fusimotor drive. 2. In four experiments, the peroneal nerve was completely blocked with lidocaine proximal to the recording site in order to de-efferent spindle endings temporarily. The responses to muscle stretch and vibration, as seen in multi-unit recordings and in single unit recordings, were similar during the block as in the relaxed state prior to the block. Thus, these experiments provided no evidence for a functionally effective resting fusimotor drive. 3. The responses to vibration of nine primary endings and four secondary endings were examined during isometric voluntary contractions of the receptor-bearing muscles. Providing that the endings were responding submaximally in the relaxed state, voluntary contraction enhanced the response to vibration, suggesting co-activation of the fusimotor system sufficient to compensate for mechanical unloading. Unloading effects were observed during contractions of neighbouring synergistic muscles, indicating a close spatial relationship between the co-activated skeletomotor and fusimotor outflows. 4. Recordings were obtained from ten primary endings and seven secondary endings during isometric reflex contractions resulting from the vibratory stimulus (TVR contractions). For twelve endings, the appearance of the tonic vibration reflex in the receptor-bearing muscle resulted in a significant decrease in the response to vibration, suggesting that the endings were unloaded by the extrafusal contraction. On voluntary suppression of the reflex contraction spindle responses reverted to their previous levels. 5. These results suggest that the tonic vibration reflex, like the tendon jerk reflex, operates predominantly or exclusively on alpha motoneurones and that it does not utilize the same cortically originating efferent pathways as are used in the performance of voluntary contractions.

Action Potentials↗
PubMed1978

Recruitment order of human spindle endings in isometric voluntary contractions.

1. The responses of twenty-two spindle endings in the anterior tibial and toe extensor muscles of human subjects were studied during isometric voluntary contractions of the receptor-bearing muscle with the ankle joint fixed in 25 degrees plantar flexion.2. The discharge of eighteen endings accelerated in voluntary contractions when the contraction strength exceeded a threshold level which differed for different endings but was reproducible for the same ending.3. With contractions of slow onset the latency to spindle acceleration varied with the speed of onset of the contraction. Endings with a background discharge were often unloaded by contractions until the contraction strength exceeded the threshold for activation of the ending.4. No correlation was found between the sensitivity of a spindle to external length changes and its ease of activation in a voluntary contraction. For two spindle endings with a background discharge there was no change in either discharge frequency or the regularity of spindle discharge during contractions which were below the threshold for activation of the endings. It is concluded that the threshold for activation of a spindle ending in an isometric voluntary contraction is determined by its fusimotor innervation, and that fusimotor neurones probably have a recruitment order, much as do skeletomotor neurones.5. Once activated, the discharge of spindle endings fluctuated with changes in skeletomotor activity but the relationship for some endings contained non-linearities. Such non-linearities were not as apparent in multi-unit recordings from a number of spindle endings in the contracting muscle. It is concluded that the fusimotor drive to a muscle is proportional to the skeletomotor drive to the muscle, and that skeletomotor and fusimotor neurones are subjected to similar if not identical descending command signals. From the work of Evarts (1968), it seems likely that these command signals are related more to desired muscle force than to desired muscle length.

Action Potentials↗
PubMed1978

Force-velocity relation in normal and nitrate-treated frog single muscle fibres during rise of tension in an isometric tetanus.

1. The force-velocity (P-V) relation for normal or NO-3 treated single fibres isolated from the semitendinosus muscle of the frog was determined at given times during the rise of tension and the plateau of isometric tetani. Experiments were made at about 2.25 micron sarcomere length and at constant temperatures, from 3 to 4.5 degrees C and from 19 to 21 degrees C. The controlled-velocity release method was used. 2. During the rise of tension, at any initial tension higher than about 0.2 P0, the lowest release velocity required to drop the tension to zero was the same as at the tetanic plateau, independent of the temperature and the presence of NO-3 ions in the bathing solution. 3. The degree of activation (measured by the steady force exerted at a given velocity of shortening lower than V0) increased with time, but attained its steady-state level before isometric tension. 4. At about 20 degrees C, frog muscle fibres at about 2.2 micron sarcomere length were only partially activated after a single stimulus. 5. NO-3 ions did not affect the steady-state P-V relation. At about 20 degrees C, NO-3 ions increased the rate of development of activation. Potentiation of the twitch contraction was due at least in part to this mechanism. 6. The 'relative' P-V relation appears to be independent of both the time after start of stimulation and the presence of NO-3 ions in the bathing solution. 7. The results are discussed in terms of the sliding filament model of Huxley (1957), assuming that either the number of actin sites available for cross-bridge formation, or the value of the rate constant for making of cross-bridges, is time dependent.

Animals↗
PubMed1979

The velocity of unloaded shortening and its relation to sarcomere length and isometric force in vertebrate muscle fibres.

1. The velocity of shortening at zero load was studied during fused tetanic contractions and single twitches in isolated skeletal muscle fibres of Rana temporaria. 2. The technique used for determination of the speed of unloaded shortening consisted of a series of quick releases of different amplitudes applied at a given instant during activity. The time, delta t, needed for the fibre to take up the slack was plotted against the amplitude of release, delta L. The slope of the straight line relating delta t-delta L provided a measure of the velocity of shortening at zero load, V0. 3. V0 was compared with force-velocity data obtained at finite loads (load-clamp recordings). The predicted velocity of shortening at zero load, derived by hyperbolic extrapolation from velocities at low and intermediate loads, was not significantly different from V0. 4. The temperature dependence of isometric force and of shortening velocity was investigated between 2 and 12 degrees C in the same fibres. Q10 was 2.67 +/- 0.07 (S.E. of mean, n = 6) for V0 and 1.24 +/- 0.01 for tetanic force. 5. The velocity of unloaded shortening was determined at different sarcomere lengths in the range 1.4--3.1 microns. V0 was constant between 1.65 microns and approximately 2.7 microns. It decreased below 1.65 microns and increased above 2.7 microns. 6. The decrease in velocity at short sarcomere lengths probably reflects an increase of the passive resistance to shortening. The increase in velocity at long sarcomere lengths can be accounted for by the passive compressive force that is produced by the parallel elastic elements of the prestretched fibre. 7. V0 was determined at the peak of the twitch and during the plateau of the fused tetanus in the same fibre. Whereas the peak twitch force varied between 38 and 85% of the tetanic tension in the different fibres (mean: 71 +/- 5%, n = 8), V0 during the twitch was 99 +/- 2% of the value recorded during the tetanus. Depression of the isometric twitch amplitude to 10% of the control value by dantrolene did not cause any significant reduction of V0.

Animals↗
PubMed1979

Tremor in the tension developed isometrically by soleus during the tonic vibration reflex in the decerebrate cat.

1. Irregularities in the development of tension during the tonic vibration reflex of the soleus muscle of the decerebrate cat have been analysed into their frequency components. The reflex was recorded isometrically and elicited by longitudinal vibration, normally at 150 Hz. The amplitude of vibration was set so as to elicit a maximal reflex response, suggesting 1:1 driving of the majority of the Ia afferents at the frequency of vibration. 2. The resulting power spectrum regularly showed a well marked tremor peak separated by a trough from any slow irregularities. The predominant frequency of this tremor varied from 4 to 11 Hz in different preparations, with a mean of 7.4 Hz; on average, frequencies within 1.7 Hz on either side contained over half the power of the predominant frequency. Altering the frequency of vibration did not alter the distribution of tremor frequencies. 3. The root mean square value of the tension irregularities, over the range 4-14 Hz, varied from 12 to 110 mN in different preparations (median value, 23 mN); this was superimposed on mean active reflex tensions varying from 2 to 10 N. 4. The 'tremor' due to a single motor unit was estimated from spectral analysis of tetanic contractions of the whole muscle and decreased with increasing frequency of activation. Comparison of the single unit values with the tremor seen during vibration in the same preparations showed that equivalent amounts of tremor to the latter could typically have been produced by the continued synchronous contraction of about five 'average' motor units firing at the predominant tremor frequency. 5. When a tonic stretch reflex was present its tremor frequencies did not differ consistently from those of the tonic vibration reflex. On average, the tremor was smaller for the stretch reflex than for the tonic vibration reflex; the difference was usually slight and might have been related to the stretch refex tension being smaller. 6. Evidence was obtained that the tremor was not due to any insecurity of 1:1 driving of the Ia afferents by the vibration. First, the tremor did not increase when the amplitude of vibration was decreased sufficiently to ensure that the degree of 1:1 driving must have been reduced. Secondly, the introduction of a comparable 'artificial tremor' by sinusoidally oscillating the muscle at low frequency did not produce the e.m.g. response that would have been expected if the applied 'tremor' had been modulating the firing of the Ia or any other group of afferents. 7. It is concluded that the observed tremor cannot be attributed to 'oscillation in the stretch reflex arc', though without prejudice to the role of this mechanism under other conditions and especially when the recording is not isometric. However, the genesis of the tremor has not been established and much of it might result simply from the chance synchronization of motor units that are firing below their tetanic fusion frequency.

Action Potentials↗
PubMed1975

Constituents of human muscle in isometric fatigue.

Three subjects performed five successive isometric contractions to fatigue; the tension in any one experiment was constant at tensions varying from 20 to 80% of the maximal voluntary contraction (MVC). The interval between contractions was held constant at 11 min. Muscle biopsy specimens were obtained at the start of the experiment, after the first, fourth, and fifth, and before the second and fifth of the successive contractions. The concentrations of ATP, CP, glycogen, and lactate were measured in each sample of muscle. Changes in ATP and glycogen were insufficient to be held accountable for the development of isometric fatigue. Changes in CP and lactate were large after fatigue at intermediate tensions, but those of CP were considered unlikely to be responsible for the fatigue. At tensions of 30-50% MVC the increase in lactate could be responsible for fatigue either directly or by indirect changes in pH; at higher and lower tensions the possibility that lactate is directly implicated in the development of fatigue seems remote.

Adenosine Triphosphate↗
PubMed1979

Relation between integrated electromyographic activity and biting force during voluntary isometric contraction in human masticatory muscles.

The relation between integrated electromyographic activity and computed biting force during voluntary isometric contraction was evaluated in the masticatory muscles of healthy subjects. The slopes of the curves relating integrated electromyographic activity to computed biting force in masseter muscles were steeper on the non-preferred chewing side than on the preferred chewing side, and they progressively became steeper during the course of continuous isometric contraction of a given biting force.

Action Potentials↗
PubMed1976

The effect of SO24-, NO-3, Ca2+ and temperature upon sodium-free contracture, isotonic and isometric contraction of the isolated rat right ventricle.

Sodium-free contracture was directly dependent on the presence of Ca2+ in Tyrode sulphate solution. The first phase of contracture increased in Tyrode nitrate solution more quickly than in sulphate solution, while the second phase was the same. Higher amplitude of isotonic contractions was observed on substituting nitrates for chlorides in the Tyrode solution than in sulphate substitution. In Tyrode sulphate solution, spontaneous contractions appeared in 50% of the experiments. The relationship between the duration and tension of isotonic contraction, given the same pre-loading, showed greater diminution of the contractions, at the lower temperature (22 degrees C) than at the higher temperature (35 degrees C), whereas passive stretching of the muscle was not influenced by the temperature. Isometric contrations lasted longer at the lower temperature. The amphlitude of isometric contractions was directly correlated to the frequency of electrical stimulation at a constant temperature (22 degrees C). The results showed that shifts of the calcium participating in contraction of the rat ventricle are influenced both by the anion and cation content of the medium and by its temperature.

Animals↗
PubMed1977

Enhancement of platelet sensitivity to ADP-aggregation by isometric exercise in arteriosclerosis patients and its prevention.

A new method for assessment of platelet sensitivity to ADP-aggregation was devised. Its reproducibility and the correlations between the values obtained by this method, the optical density (O. D.) method, and the screen filtration pressure (SFP) method were assessed. In summary, this method may be said to have three main points: 1. It can be performed without centrifugation, avoiding mechanical stress to platelets, using only 0.8 ml. of blood and inexpensive equipment. 2. It may reflect different aspects of platelet function from the O. D. method and the SFP method, despite the positve significant correlations between the values obtained by these three methods. 3. It was proved to be highly reproducible and is thought to be useful clinically. By using this method, the effect of sustained isometric exercise by handgripping on platelet aggregability was assessed in coronary sclerotic and cerebral arteriosclerotic patients on placebo and EG-626, a newly synthesized cyclic AMP phosphodiesterase inhibitor. On placebo, an enhancement platelet sensitivity was observed after isometric exercise in coronary and cerebral arteriosclerotic patients but not in healthy control subjects. The enhancement was prevented by pretreatment of EG-626, administered orally 1.5 hours prior to exercise.

3',5'-Cyclic-AMP Phosphodiesterases↗
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↗
PubMed1979

Effect of beta-adrenergic blockade on haemodynamic responses to dynamic and isometric exercise in angina pectoris.

The effect of treatment for 1--4 weeks with metoprolol, a beta 1-selective blocking agent, or alprenolol, on the heart rate and blood pressure response to isometric exercise was studied in two groups of 12 patients with angina. Measurements were made during the peak effect of metoprolol 10, 40 or 50 mg, and alprenolol 200 mg as Aptin Durules. After 1 min of sustained handgrip at 50% of maximal voluntary contraction, systolic (6--15%) and diastolic (8--12%) blood pressure after both drugs was significantly lower than without any beta-blockade; Heart rate was decreased by 19--22% by metroprolol but not by alprenolol. The blood pressure rise during handgrip was not attenuated by either drug. The rise in heart rate was significantly reduced (by 36--50%) by metoprolol 40 and 50 mg and alprenolol 200 mg. No patient experienced angina during handgrip. In contrast, all but one were restricted by angina during bycycle exercise without treatment, at a level that produced the same increase in heart rate as the handgrip test, vis. 3 min at a load of 33 W). The cardiovascular response to sustained handgrip is too small to provide a useful challenge for determinatin of the anti-anginal efficacy of drugs. However, slight ECG changes of ischaemia did occur during handgrip, which were reversed by beta-blockade.

Adrenergic beta-Antagonists↗
PubMed1979

Effects of labetalol and propranolol on blood pressure at rest and during isometric and dynamic exercise.

The influence of intravenous labetalol and propranolol on the blood pressure response to isometric and dynamic exercise was examined in a double blind study in eight, young, normotensive volunteers. Effects were recorded after propranolol 7.5, 15 and 30 mg i. v., and after labetalol 30, 60 and 120 mg i. v. In control experiments saline was administered. Mean blood pressure rose with successive handgrip tests following saline and propranolol, but not after labetalol, and the difference was significant. The total dose of each drug produced the same reduction in heart rate during sub-maximal bicycle exercise. The exercise-induced systolic blood pressure response did not differ between the drugs.

Adult↗
PubMed1979

EMG, bite force, and elongation of the masseter muscle under isometric voluntary contractions and variations of vertical dimension.

The relation EMG activity, bite force, and muscular elongation was studied in eight subjects with complete natural dentition during isometric contractions of the masseter muscle, measured from 7 mm to almost maximum jaw opening. EMG was registered with superficial electrodes and bite force with a gnathodynamometer. In series 1, recordings of EMG activity maintaining bite force constant (10 and 20 kg) show that EMG is high when the bite opening is 7 mm, decreases from 15 to 20 mm, and then increases again as jaw opening approaches maximum opening. In series 2, recordings of bite force maintaining EMG constant show that bite force increases up to a certain range of jaw opening (around 15 to 20 mm) and then decreases as we approach maximum jaw opening. Results show that there is for each experimental subject a physiologically optimum muscular elongation of major efficiency where the masseter develops highest muscular force with least EMG activity.

Adult↗