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PubMed1990

Effect of imipramine treatment on changes in heart rate and blood pressure during postural and isometric handgrip tests.

Postural and isometric handgrip tests were performed on nine patients with panic disorder and 13 patients with major depression before and after treatment with imipramine for a mean duration of 10 weeks. There were significant increases in resting supine heart rate (HR), systolic, diastolic and mean blood pressures (SBP, DBP, MBP) after treatment. There were significant increases in HR and DBP after standing from supine posture before treatment, and after treatment, there was a significant enhancement of this tachycardia. The decrease of heart rate during the change from standing to supine posture before treatment significantly decreased further after imipramine treatment. Decreases in SBP and DBP during the change from standing to supine posture were significantly smaller during the post treatment condition. Increases in SBP during the standing isometric exercise test were smaller after treatment. These findings suggest abnormal orthostatic reflexes secondary to an altered adrenergic function and/or cholinergic supersensitivity after treatment with imipramine.

Analysis of Variance↗
PubMed1979

Influence of temperature on isometric contraction and passive muscular tension in paramyotonia congenita (Eulenburg).

Four patients without symptoms of episodic hyperkalemic weakness from two families with paramyotonia congenita (Eulenburg) are described. 1. Maximum voluntary muscle contraction of the upper and lower arm was studied under isometric conditions at different temperatures. If the temperature was lowered stepwise, distinct paresis occured at 32--31 degrees C which increased with the amount of muscular effort. The upper arm muscles, however, developed weakness gradually after cooling. 2. During cooling of the resting muscle, the EMG showed dense spontaneous activity of the fibrillary type, which decreased again at about 30 degrees C. It can be assumed that in paramyotonia congenita cooling produces muscle cell membrane depolarization which at a critical level causes the firing of action potentials and finally muscular paresis. 3. Increasing muscular stiffness can be interpreted as abnormally slow muscular relaxation after isometric contraction. In the forearm muscles the time to 3/4 relaxation after cooling was about six times normal, in the upper arm muscles only two times normal. As an additional parameter the mechanical resistance to passive stretching of a muscle has been studied. This passive muscular tension increased simultaneously with the onset of weakness. 4. The close relation between weakness and stiffness suggest that both symptoms are caused by the same basic defect which is probably located in the sarcolemma. It is suggested that a defect of the sodium channel causes a cooling-dependent increase in sodium conductance. Raised intracellular sodium causes in the first place membrane depolarization, and in the second place depression of calcium reuptake through competition by sodium for calcium binding sites. This would explain muscle stiffness and delayed relaxation as well.

Arm↗
PubMed1981

Glycolytic intermediates in human muscle after isometric contraction.

Isometric contraction of the quadriceps muscle sustained to fatigue with a force of 66% of the maximum voluntary contraction force resulted in a mean glycogen utilization of 80.4 (S.D. 58.4) mmol glucosyl units/kg dry muscle (d.m.) and an accumulation of glycolytic intermediates and glucose corresponding to 82.9 (S.D. 17.5) mmol glucosyl units/kg d.m. Accumulation of hexose phosphates (principally glucose 6-phosphate) accounted for 35.4% (S.D. 4.1) of the total increase and lactate for 59.3% (S.D. 2.8). During a 4 min recovery period glucose 6-phosphate content showed a linear decrease with a half time of 2.0 min and lactate decreased exponentially with a half time of 2.5 min. The rate of lactate disappearance from the muscle was approximately 4 times as fast as that observed previously after maximal bicycle exercise. This was probably due to a lower lactate concentration in blood after isometric contraction resulting in a larger muscle-blood gradient for lactate. Muscle content of free glucose was increased after contraction and increased further during recovery. It is concluded that the glucose increase is confined to the intracellular pool and is an effect of hexokinase inhibition by accumulated glucose 6-phosphate. Occlusion of the local circulation after the contraction inhibited the recovery processes for lactate and glucose 6-phosphate.

Energy Metabolism↗
PubMed1976

The relationship between the rate of rise of isometric tension and motor unit recruitment in a human forearm muscle.

The recruitment properties of single motor units from the human extensor indicis muscle were investigated during voluntary isometric contractions of different rate of rise but equal amplitude. Both the electrical and the contractile events associated with recruitment were analyzed. The threshold force of recruitment (measured as the total muscle force at firing onset) decreased with increasing rate of rise of isometric tension. This was consistently found for all units. Differences between low and high threshold units indicating a preferential tonic or phasic mode of activation were not observed. The contractile events associated with recruitment were analyzed analoguously to the electrical events. For this purpose, muscle force was measured at the time of the first twitch as it was measured at the time of the first spike. This separate measurement of the electrical and mechanical recruitment of a unit is necessary, because during a change of muscle force, force is different at firing onset and during the subsequent twitch contraction. Muscle tension at the time of the peak of the first twitch contraction was calculated from measurements of the twitch concentration time of the single motor units. In contrast to firing onset, the peak of the first twitch of a motor unit occurs at approximately the same muscle tension no matter how fast the contraction is performed. This is the consequence of the result that the average decrease of the threshold force of recruitment at successively faster contraction has the same value as the corresponding increase of total muscle force during the mean contraction time of the motor units. On the basis of this precise matching between these two changes, the mechanical recruitment of motor units occurs at approximately the same force level irrespective of the rate of rise of tension.

Electric Stimulation↗
PubMed1988

Effect of sustained-release verapamil therapy on the blood pressure at rest and on the pressor response to isometric exertion in hypertensive patients.

The effect of a new formulation of verapamil sustained release (SR) 240 mg tablets on resting blood pressure (BP) and on the pressor response to isometric exertion have been examined in a single-blind, placebo-controlled, cross-over study in 12 hypertensive patients (mean age 45 years). SR verapamil and placebo were administered every 12 h for 6 consecutive weeks. At the end of each period of treatment BP and heart rate (HR) were measured at rest and during isometric exercise, performed as a handgrip (HG) test for 3 min at 30% of the maximum voluntary contraction. There was a significant reduction in resting systolic and diastolic BP, with no change in HR. BP and HR at peak exercise were lower after verapamil than after placebo, but the maximal absolute increase did not change during verapamil therapy. The results are compatible with a role of SR verapamil b.d. in reducing resting BP in hypertension, and in lowering very high pressure at the peak of a HG test, without modifying the physiological reactivity of cardiovascular system.

Adult↗
PubMed1990

Twitch contractile adaptations are not dependent on the intensity of isometric exercise in the human triceps surae.

Ultrastructural and twitch contractile characteristics of the human triceps surae were determined in six healthy but very sedentary subjects before and after 16 weeks of isometric training at 30% maximal voluntary contraction (MVC). Following training, twitch contraction time was approximately 16% shorter, although no differences were observed in one-half relaxation time or peak twitch torque. Percent fibre type was not changed by training. The mean area of type I and type II fibres in the soleus increased by approximately 30% but only type II fibres showed an increase in area in the lateral gastrocnemius (30%). Despite such changes in fibre area the volume density of the sarcoplasmic reticulum-transverse tubular network averaged 3.2 +/- 0.6% and 5.9 +/- 0.9% in type I and type II fibres respectively, before and after training in the two heads of the gastrocnemius. The results indicate that contractile adaptations to isometric training at 30% MVC were limited to twitch contraction time and were not directly related to changes in percent fibre distribution or the volume of sarcoplasmic reticulum and transverse tubules in either type I or type II fibres. The data further demonstrate that substantial fibre hypertrophy is achieved by training with low-intensity contractions.

Adaptation, Physiological↗
PubMed1996

Changes in ventilation related to changes in electromyograph activity during repetitive bouts of isometric exercise in simulated sailing.

This study examined the control of ventilation during repetitive bouts of isometric exercise in simulated sailing. Eight male sailors completed four successive 3-min bouts of similar isometric effort on a dinghy simulator; bouts were separated by 15-s rest intervals. Quadriceps muscle integrated electromyograph activity (iEMG) was recorded during each bout and expressed as a percentage of activity during maximal voluntary contraction (%iEMGmax). From the first to the fourth bout, the 3-min mean averages for ventilation and for %iEMGmax increased from 19.8 (SEM 1.1) to 37.5 (SEM 3.0) l.min-1 and from 31 (SEM 4) to 39 (SEM 4)% respectively; also, ventilation and %iEMGmax over each minute throughout the four bouts were significantly correlated (r = 0.85; P < 0.05). Progressive hyperventilation reduced the mean end-tidal partial pressure of carbon dioxide from 5.0 (SEM 0.3) kPa during bout 1 to 4.3 (SEM 0.4) kPa during bout 4 [37.7 (SEM 2.0) to 32.4 (SEM 3.0) mmHg]. From the first to the fourth bout the end-of-bout blood lactate concentration did not increase significantly although the concentration from the third bout onwards was significantly greater than at rest. The results suggested that the development of muscle fatigue, which was enhanced by the insufficiency of recovery during the 15-s intervals and mirrored in the progressive increase in iEMG, was linked with stimuli causing progressive hyperventilation. Though these changes in ventilation and iEMG could not be associated with changes in blood lactate concentration, they could both have been related to accumulating metabolites within the muscles themselves.

Adult↗
PubMed1992

Transient and steady-state cardiopulmonary responses to combined rhythmic and isometric exercise.

The transient and steady-state cardiopulmonary responses to combined rhythmic (R) and isometric (I) exercise were examined in nine subjects. Isometric exercise at 30% maximal voluntary contraction (MVC) was started 1.5 min prior to either a 50% or 75% maximal oxygen uptake (VO2max) cycle ride and continued for 1.5 min into the 10-min R. Systolic (Pas) and diastolic (P(ad)) blood pressure, heart rate (fc), inspired ventilation volumes (VI), and oxygen uptake (VO2) were recorded every 30 s throughout each experiment. Responses to I effort alone were recorded for comparison with experiments in which the combined exercises were performed during the first 1.5 min when R had not yet begun. Pas responses in the first 1.5 min of I (no R) showed the typical rapid linear increase. Addition of the R effort further increased Pas to levels which remained nearly constant (steady state) throughout R. R alone produced a slower Pas increase to approximately the same steady-state levels as those of the combined R and I exercise. For P(ad), the linear increase which occurred during the first 1.5 min of I was attenuated with the superimposition of R. Following cessation of I, P(ad) fell rapidly during continued R to levels not different from experiments with R alone. The fc during I alone increased slightly. As I continued, the onset of the R induced a further rapid increase in fc to levels not different from R alone. The VI showed a similar response to fc. VO2 during I alone did not change significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1993

Characterization of a prolate-headed bacteriophage of Lactobacillus delbrueckii subsp. lactis, and its DNA homology with isometric-headed phages.

A new Lactobacillus delbrueckii subsp. lactis bacteriophage, JCL 1032, was characterized. JCL 1032 had a small, elongated prolate head, and a long non-contractile tail with cross-bars. The restriction map of JCL 1032 genome was constructed with five endonucleases. The genome was 45.8 kb in size, and it had cohesive ends (cos). Molecular masses of the phage structural proteins were also determined. JCL 1032 showed DNA homology with morphologically dissimilar, isometric-headed phages of Lb. delbrueckii (subsp. lactis and subsp. bulgaricus) when analyzed by Southern hybridization. Although in general JCL 1032 was only distantly related to isometric-headed phages, there were also a few short highly homologous (minimal homology 84%) DNA regions.

Bacteriophages↗
PubMed1992

Differential effects of isometric exercise on the cutaneous circulation of different regions.

Studies have been made in healthy human subjects of the changes evoked by isometric hand grip in arterial pressure, heart rate and cutaneous red cell flux, the latter being recorded using a laser Doppler meter. Hand grip for 2 min evoked increases in arterial pressure and heart rate, the magnitude of which were graded with the force of contraction (75, 50, or 25% maximum voluntary contraction). Cutaneous red cell flux in the contralateral forearm decreased significantly during 25 and 50% maximum voluntary contraction, while the cutaneous vascular resistance (arterial pressure/cutaneous red cell flux) increased to extents that were graded with the maximum voluntary contraction, indicating graded vasoconstriction. By contrast, cutaneous red cell flux in the face tended to increase, this reaching significance at 75% maximum voluntary contraction. Cutaneous vascular resistance in the face increased in some subjects, but decreased in others, vasodilator responses being most common during 75% maximum voluntary contraction when sweating commonly appeared on the face. In the dorsum of the foot, red cell flux did not change during 75% maximum voluntary contraction, although foot cutaneous vascular resistance increased significantly by the end of the first minute of contraction. At 50 and 25% maximum voluntary contraction most subjects showed an increase in foot cutaneous vascular resistance, but the remainder showed a decrease. We propose that isometric hand grip causes vasoconstriction in the cutaneous circulation of the contralateral forearm, the face and foot, that this response is strongest in the forearm and weakest in the face, and that in the face and foot, the vasoconstriction may be overcome by vasodilatation secondary to sweating.

Blood Pressure↗
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↗
PubMedDate not supplied

Isometric relaxation of hypertrophied right ventricle in rats.

In rats with experimental hypertrophy of the right ventricle due to chronic increase in pressure load -dP/dTmax, a parameter of isometric relaxation, is significantly reduced at a stage of impairment of myocardial function at which the parameter of isometric contractility still remains unaffected.

Animals↗
PubMed1985

Sympathetic control of the forearm blood flow in man during brief isometric contractions.

Experiments were performed to assess the possible neurally mediated constriction in active skeletal muscle during isometric hand-grip contractions. Forearm blood flow was measured by venous occlusion plethysmography on 5 volunteers who exerted a series of repeated contractions of 4 s duration every 12 s at 60% of their maximum strength of fatigue. The blood flows increased initially, but then remained constant at 20-24 ml X min(-1) X 100 ml(-1) throughout the exercise even though mean arterial blood pressure reached 21-23 kPa (160-170 mm Hg). When the same exercise was performed after arterial infusion of phentolamine, forearm blood flow increased steadily to near maximal levels of 38.7 +/- 1.4 ml X min(-1) X 100 ml(-1). Venous catecholamines, principally norepinephrine, increased throughout exercise, reaching peak values of 983 +/- 258 pg X ml(-1) at fatigue. Of the vasoactive substances measured, the concentration of K+ and osmolarity in venous plasma also increased initially and reached a steady-state during the exercise but ATP increased steadily throughout the exercise. These data indicate a continually increasing alpha-adrenergic constriction to the vascular beds in active muscles in the human forearm during isometric exercise, that is only partially counteracted by vasoactive metabolites.

Adult↗
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↗
PubMed1998

Errors associated with the use of adaptive differential pulse code modulation in the compression of isometric and dynamic myo-electric signals.

Muscle activity produces an electrical signal termed the myo-electric signal (MES). The MES is a useful clinical tool, used in diagnostics and rehabilitation. This signal is typically stored in 2 bytes as 12-bit data, sampled at 3 kHz, resulting in a 6 kbyte s-1 storage requirement. Processing MES data requires large bit manipulations and heavy memory storage requirements. Adaptive differential pulse code modulation (ADPCM) is a popular and successful compression technique for speech. Its application to MES would reduce 12-bit data to a 4-bit representation, providing a 3:1 compression. As, in most practical applications, memory is organised in bytes, the realisable compression is 4:1, as pairs of data can be stored in a single byte. The performance of the ADPCM compression technique, using a real-time system at 1 kHz, 2 kHz and 4 kHz sampling rates, is evaluated. The data used include MES from both isometric and dynamic contractions. The percent residual difference (PRD) between an unprocessed and processed MES is used as a performance measure. Errors in computed parameters, such as median frequency and variance, which are used in clinical diagnostics, and waveform features employed in prosthetic control are also used to evaluate the system. The results of the study demonstrate that the ADPCM compression technique is an excellent solution for relieving the data storage requirements of MES both in isometric and dynamic situations.

Electromyography↗
PubMed1991

Influence of nebivolol on the cardiovascular hemodynamics during postural changes and isometric exercise.

Nebivolol (R67555), a drug with beta 1 receptor antagonizing properties, was administered once daily (5 mg) for 7 days in 10 healthy volunteers. The hemodynamic parameters were measured noninvasively during postural changes (supine, sitting, standing) and during isometric handgrip at 50% maximal voluntary contraction, before and 3, 8, and 23 hours after the first nebivolol intake of 5 mg; the same measurements were done 23 hours after the last intake. Nebivolol lowered arterial blood pressure acutely and chronically due to a decrease in heart rate and cardiac output. The stroke volume seemed to be preserved, while the total peripheral vascular resistance did not change. Nebivolol did not change the orthostatic responses, except that the absolute value was lowered. Nebivolol was unable to prevent the blood pressure increase during isometric handgrip. However, this blood pressure increase was obtained by an increase in the total peripheral vascular resistance and not by an increase in the cardiac output, as observed during control measurements before nebivolol intake.

Adrenergic beta-Antagonists↗