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PubMed1976

Lengthening contraction and interpretations of active state tension in the isometric twitch response of skeletal muscle.

The plateau of tetanic tension in a lengthening contraction of muscle at any given length is always greater than that in an isometric contraction at the same given length. To account for this excess of tension, it is suggested that the contractile machinery is "locked" in a strained condition that might make the muscle capable of bearing a greater tension in a lengthening contraction than it can develop in an isometric contraction. It is proposed that this strained condition also occurs in a lengthening contraction of the twitch response. If this proposal is valid it seems certain that the plateu of tension in the lengthening contraction of the twitch response cannot be equated with the plateau of tension in the isometric contraction of tetanus. The design of the original concept, that the plateau of active state tension in the lengthening contraction of the twitch is equal to the plateau of tension (Po) in the isometric contraction of tetanus, is based upon the assumption that the contractile component is neither lengthening nor shortening during both of these plateaus. This assumption is made without considering the possibility that the plateau in the lengthening contraction might be due to concurrent lengthening of series elastic elements and shortening of the contractile component. To account for the plateau of tension observed after quick lengthening in the early phase of twitch contraction indirect evidence is presented favoring the view that the quick lengthening during stretch is followed by slow lengthening and stress relaxation (decrease of tension) in series elastic elements and simultaneous shortening and increase of tension in the contractile component. When the original concept of active state tension in the twitch response is interpreted in the light of lengthening contraction, it is concluded that the labeled or implied Po for the plateau of the active state tension is unwarranted and confusing. It seems that the primary source of confusion is the assumption that the tension a muscle is capable of bearing in a lengthening contraction is equatable with the tension it can develop in an isometric contraction.

Animals↗

PubMed1984

Superiority of supine bicycle over isometric handgrip exercise in the assessment of ischemic heart disease: an evaluation of left ventricular ejection fraction response using radionuclide angiography.

Left ventricular ejection fraction (LVEF) response to supine bicycle and isometric handgrip exercise was evaluated in 15 patients with documented coronary artery disease (CAD) and stress-induced ischemia using radionuclide angiography. For purposes of analysis, the patients were divided into two groups: group I (n = 7) with single-vessel disease and group II (n = 8) with multiple-vessel disease including 3 with left main artery disease. The studies were repeated 18 days later at similar external workloads to assess reproducibility of both tests. LVEF response to bicycle exercise was different for the two groups. The change in LVEF from rest to peak exercise was +0.04 +/- 0.02 for group I and -0.07 +/- 0.04 for group II (p less than .001). LVEP response to isometric handgrip exercise was not different between the two groups. The change from rest to end of handgrip exercise was -0.02 + 0.02 for group I and -0.05 +/- 0.02 for group II. The reproducibility of LVEF response to bicycle exercise at similar workloads on day 1 and day 19 was good (r = 0.85) while it was poor for isometric handgrip testing (r = 0.67). Our data demonstrate that radionuclide angiographic measurement of LVEF response to supine bicycle exercise testing is superior to LVEF response to isometric handgrip testing in the evaluation of patients with CAD.

Adult↗

PubMed2006

Acute effect of isometric exercise on desire to smoke and tobacco withdrawal symptoms.

A brief bout of aerobic exercise (e.g. stationary bicycle) has been shown to result in an acute reduction in tobacco withdrawal symptoms and cravings in abstinent smokers. However, aerobic exercise is often not practical and it is of interest to examine whether non-aerobic exercise has a similar effect. We investigated whether isometric exercise (involving muscular contractions against resistance without movement, e.g. placing the palms of the hands together and pushing) reduces desire to smoke and tobacco withdrawal symptoms. Following overnight abstinence smokers were randomized to 5-min of: isometric exercises (n = 20), body scanning (focusing attention on sensations in different areas of the body, n = 20, control), or sitting passively (n = 20, control). Desire to smoke and tobacco withdrawal symptoms ('irritable', 'depressed', 'stressed', 'tense', 'restless' and 'poor concentration') were rated at baseline, immediately post-intervention, and 5-, 10-, 15- and 20-min post-intervention. Isometric exercise produced a significantly greater reduction in desire to smoke versus passive control at immediate post-intervention and 5-min post-intervention, relative to baseline (p < 0.05). Most withdrawal symptoms were significantly moderated by exercise versus controls at some point between 5- to 20-min post-intervention, relative to baseline (p < 0.05). Brief isometric exercise has potential for offering immediate relief from a desire to smoke.

Adolescent↗

PubMed1983

Relationship of isokinetic torque to isometric torque.

Isokinetic muscle testing has become an increasingly popular method for measuring muscle performance. However, the relationship of isokinetic strength measurements to static strength measurements remains unclear. The relationship between maximal isokinetic and maximal isometric torque was investigated at angular velocities of 24, 48, 96, and 192 degrees/s for both flexion and extension of the elbow joint. Twenty-four subjects were tested bilaterally, and discrete torque values at the positions of 60 and 90 degrees of flexion were obtained from the continuous isokinetic records for comparison with isometric torques recorded at the same two positions. Data were analyzed using an analysis of covariance with isokinetic torque, the dependent variable, expressed as a function of isometric torque and as a function of the design variables side and position. The results demonstrated significant effects of side and position and also demonstrated the dependence of isokinetic strength on velocity. Confidence intervals were determined for the prediction of isokinetic torque from observed isometric torque for normal individuals, providing a potentially useful tool to aid in the diagnosis of muscular disorders.

Adult↗

PubMed1996

ATPase activity and force production in skinned rat cardiac muscle under isometric and dynamic conditions.

ATPase activity and force were determined in maximally activated skinned cardiac trabeculae from rat at 20 degrees C, 2.2 microns sarcomere length and pH 7.0. Measurements were performed during isometric contraction, and when square-wave shaped length changes with an amplitude of 0.5-5% of the resting length (Lo) and a repetition frequency of 2-167 Hz were imposed. Isometric ATPase activity amounted to 0.48 +/- 0.04 mM/s, and isometric force was 53 +/- 3 kN/m2. During length changes, an increase in ATP consumption was observed, reminiscent of the Fenn effect, to a maximal value of 1.7 times the isometric value, while the average force decreased. The frequency dependent increase in ATPase activity during length changes is compatible with a mono exponential recovery process, with a rate constant of 32 +/- 7/s. In a two-state scheme of crossbridge cycling, an apparent attachment rate of 28.6/s and an apparent detachment rate of 3.4/s were determined. To relate these observations with crossbridge kinetics during isotonic shortening, ramp-shaped length changes (2.5% Lo, 23 Hz) were imposed. ATPase activity and average force during ramp shortenings were similar to the values obtained during square-wave shaped length changes with the same amplitude and frequency. During lengthening ramps, the increase in ATPase activity and the decrease in average force were significantly less than during the square-wave protocol. These results indicate that the energetic effect of stretches is relatively small.

Adenosine Diphosphate↗

PubMed1984

1 kgf/cm2--the isometric tension of muscle contraction: implications to cross-bridge and hydraulic mechanisms.

Attention is drawn to experimental results from many laboratories which indicate that the isometric force (F) in the contraction of striated muscle fibers is linearly proportional to their variable cross-section area (A). Reversible swelling of intact, skinned, or glycinerated fibers can be induced by changes in tonicity, ionic strength or pH. In all cases where careful measurements of F and A are reported, the maximal isometric tension namely, T = F/A, is found around 1 kgf /cm2, even though F and A may change more than threefold for a given fiber at a certain length. These results seem to be independent of the fiber length or temperature. Thus, the isometric tension T in striated muscle does not depend on the number or the rate of the interacting cross-bridges. This result of constant isometric tension, which has so far received little attention, is however, a simple prediction of the hydraulic mechanism which is proposed for muscle contraction. Therefore, the hydraulic model, which is based on the hypothesis of vectorial flux of energetic protons deserves serious consideration.

Actins↗

PubMed1993

Isometric tension exerted by a myofibril of the frog at 0 degree C: geometrical considerations.

For a number of years, the isometric force per myosin head has been taken as 1 pN by the authors who studied in vitro movement. This value was deduced from the value of the isometric tension exerted by a living or a skinned fibre at approximately 0 degree C and for the frog (1-3 kg cm-2). Starting from an isometric tension exerted by a single unit cell of approximately 10 kg cm-2, it has been shown that the isometric force per myosin head is (8 +/- 1)pN. The value of approximately 10 kg cm-2 was deduced from theoretical and semi-empirical reasonings independent of the mechanical roles of the crossbridges. Here, we discuss this latest value by using simple geometrical considerations.

Animals↗

PubMed1992

High energy phosphates and related compounds, glycogen levels and histology in the rat tibialis anterior muscle after forced lengthening and isometric exercise.

Eccentric exercise may elicit damage to the contractile elements. This primary damage is followed by secondary changes, consisting of histological changes and changes in glycogen and energy metabolism. The mechanism underlying changes in glycogen homeostasis and energy metabolism is not well established. The aim of this study was to investigate the possible relationship between changes in adenine and guanine nucleotides, inosine monophosphate (IMP), creatine phosphate, glycogen content and histology in the rat tibialis anterior (TA) muscle after forced lengthening or isometric exercise. The right muscles were either forcibly lengthened or isometrically exercised, while the contralateral muscles served as non-exercised controls. The exercised muscles were dissected 0, 6 and 24 h post-exercise and the contents of adenine and guanine nucleotides, IMP, creatine phosphate, and glycogen determined. In addition, histological changes were assessed. Immediately after both types of exercise increases in tissue IMP levels were found. Irrespective of the type of exercise, glycogen content was decreased immediately post-exercise, but restored 6 h post-exercise. Twenty-four hours later a second decline in glycogen content was found after both types of exercise. In forcibly lengthened muscles ATP content was decreased 24 h post-exercise. In isometrically exercised muscles ATP was not decreased at any time. Gross structural changes were found in all forcibly lengthened muscles (9-12% of TA muscle volume). In isometrically exercised muscles structural changes were minor (up to 0.1% of muscle volume), were found only immediately post-exercise and in only 4 out of 18 muscles. It is concluded that forced lengthening results in decreased ATP levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenine Nucleotides↗

PubMed1988

Changes in the surface electromyogram during increasing isometric shoulder forward flexions.

When using electromyographic techniques in the evaluation of muscular load it is necessary to determine the mathematical relationship between the torque and the amplitude of the electromyographic signal. Isometric gradually increasing contractions up to 100% MVC can then be used. Often more than linear increases for the amplitude (RMS)--force regression have been reported. The present study was designed to test whether changes in power spectral density function take place during a gradually increasing isometric contraction (duration 10 s). Twenty-two clinically healthy females performed an increasing isometric shoulder forward flexion for 10 s using an isokinetic dynamometer. Electromyographic activity was measured in trapezius, deltoid, infraspinatus and biceps brachii using surface electrodes. Mean torque values were determined together with mean power frequency (MPF) and root mean square values (RMS) from the EMG signals for each 256 ms period. The RMS-torque regressions showed higher regression coefficients during the 6th to 9th sec than during the first 5 s. No significant correlation existed between MPF for the four muscles and the torque. A gradual decrease in MPF was generally found from the 6th s. It is concluded that this decrease in power spectral density function might have contributed to the significantly higher regression coefficient for the RMS torque regression at the high output part of the gradually increasing isometric contraction.

Adult↗

PubMed1980

The influence of temperature on the amplitude and frequency components of the EMG during brief and sustained isometric contractions.

The influence of temperature on the amplitude and frequency components of the EMG power spectra of the surface EMG recorded over the forearm muscles was examined in five male and five female subjects during brief and fatiguing isometric contractions of their handgrip muscles. Brief (3 s) isometric contractions were exerted at tensions ranging between 10 and 100% of each subject's maximum strength while fatiguing contractions were exerted at tensions of 25, 40, and 70% of their maximum strength. The temperature of the muscles during those contractions was varied by placing the forearms of the subjects in a controlled temperature water bath at temperatures of 10, 20, 30, and 40 degrees C. The results of these experiments showed that the center frequency of the power spectra of the surface EMG was directly related to the temperature of the exercising muscles during brief isometric contractions. During fatiguing isometric contractions, the amplitude of the EMG increased while the center frequency of the EMG power spectra decreased for all tensions examined.

Adult↗

PubMed1980

The blood pressure response during isometric exercise in fast and slow twitch skeletal muscle in the cat.

The blood pressure response during fatiguing isometric contractions was examined in a slow twitch muscle (the soleus) and a mixed muscle (the medial gastrocnemius) of the cat. The results of these experiments showed that electrical stimulation of the ventral roots of the spinal cord which carried the efferent innervation to the soleus muscle failed to result in a blood pressure response during isometric exercise. Further, although stimulation of the fast twitch motor units in the medial gastrocnemius muscle was associated with a potent pressor response to isometric exercise, stimulation of the slow twitch motor units was associated with a markedly reduced response throughout the duration of the exercise. These findings infer that the pressor response to isometric exercise may be a function of the fast twitch motor units in the muscle.

Animals↗

PubMed1985

Serum creatine kinase activity following forearm flexion isometric exercise.

The purpose of this study was to compare serum creatine kinase (CK) activity following two forearm flexion isometric exercise regimens differing in work to rest ratio, and examine the CK response to a repeated bout of isometric exercise. Eleven males were tested on two sessions (bouts) spaced 1 week apart. For bout 1, five subjects (group A) performed a forearm flexion isometric exercise consisting of 40 10-s maximal contractions with 20-s inter-trial rests (10:20), while six (group B) performed 40 maximal 10-s contractions with 5-s inter-trial rests (10:5). The increase in serum CK activity following the 10:20 exercise (143%) was significantly greater than that following the 10:5 exercise (52%). The 10:20 exercise was also associated with greater tension generation over trials. One week later, both groups performed a bout of 10:20 exercise. A substantial reduction in the serum CK response was found following this second bout. The data suggest that for bout 1 the isometric exercise associated with the greater overall tension levels resulted in the greater CK response. However, when the 10:20 exercise was repeated 1 week later, a substantial reduction in the CK response was found which was unrelated to the tension generated.

Adult↗

PubMed1979

Measurement of forearm blood flow by venous occlusion plethysmography: influence of hand blood flow during sustained and intermittent isometric exercise.

The requirement for using an arterial occlusion cuff at the wrist when measuring forearm blood flows by plethysmography was tested on a total of 8 subjects at rest and during and after sustained and intermittent isometric exercise. The contribution of the venous effluent from the hand to the forearm flow during exercise was challenged by immersing the arm in water at 20, 34, and 40 degrees C. Occlusion of the circulation to the hand reduced the blood flow through the resting forearm at all water temperatures. There was an inverse relationship between the temperature of the water and the proportion in the reduction of forearm blood flow upon inflation of the wrist-cuff, ranging from 45 to 19% at 20 degrees to 40 degrees C, respectively. However, during sustained isometric exercise at 10% of the subjects maximum voluntary contraction (MVC) there was no reduction in the measured forearm flow when an arterial occlusion cuff was inflated aroung the wrist. Similarly, there was no alteration in the blood flow measured 2 s after each of a series of intermittent isometric contractions exerted at 20% or 60% MVC for 2 s whether or not circulation to the hand was occluded nor of the post-exercise hyperemia following 1 min of sustained contraction at 40% MVC. These results indicate that a wrist-cuff is not required for accurate measurement of forearm blood flows during or after isometric exercise.

Forearm↗

PubMed1994

Reproducibility of computer measurement of maximal isometric strength and electromyography in sedentary middle-aged women.

The objectives of this study were (i) to determine the reproducibility of computer measurements of isometric strength and related electromyography in several muscle groups in sedentary middle-aged women, (ii) to evaluate the effects of different digital signal averaging methods on the reproducibility, (iii) to determine the final test score to be preferred in terms of improved reproducibility of isometric strength measurements, and (iv) to evaluate potential advantages provided by the computer measurement. Fifteen subjects were measured three times within a 2-week period. The measurements consisted of recordings of maximal isometric strength and rate of force production during trunk extension and flexion, leg extension and dominant forearm flexion with simultaneous recordings of surface electromyography, except in the trunk flexors. The following four final test scores were determined for each trial: the maximum of the three scores, the mean of the two highest scores, the median of the three scores and the mean of the three scores. The scores for the strength measurement were generally more reproducible (coefficient of variation, CV, approximately 6% and intraclass correlation coefficient ICCC, approximately 0.90) than those of the other measurements (CV > 10%, ICCC 0.13-0.97). There was no obvious preference for any type of final test score or for the width of the averaging window in the computer analysis. For isometric strength the reproducibility of the computer measurements was comparable to that of the voltmeter assessments. Computer analysis seems to be a versatile method for determining parameters of neuromuscular performance with reasonable reproducibility.

Computers↗

PubMed1985

In vitro comparison of isometric and stop-test contractility parameters for the urinary bladder.

Contractility parameters in the urinary bladder can be calculated from isometric contractions (no extra patient load as compared to routine cystometry) or from stop-tests (more accurate, simpler analysis). A stop-test involves a voluntarily interrupted micturition with pressure and flow measurement. In a series of measurements in vitro on pig urinary bladder strips, parameters of the first type, obtained either by analyzing isometric contractions in terms of the Hill model, or by making phase plots, were compared to parameters of the second type. A good correlation was found. Th parameter correlating best with the maximal contraction velocity of the bladder, normalized for differences in initial muscle length, as obtained from stop-test, is the isometric contraction force, which can be obtained from an isometric contraction by either of the two analysis techniques. Clinically, making phase plots seems more promising than analyzing contractions in terms of the Hill model.

Animals↗

PubMed1987

Autonomic nervous control of the heart rate during isometric exercise in normal man.

The relative contribution of the efferent components of the autonomic nervous system to the regulation of tachycardia induced by isometric exercise was assessed in 23 normal males. The isometric exercise (handgrip) was performed at the maximum intensity tolerated by the individual over a period of 10 s (maximal voluntary contraction-MVC) and at levels equivalent to 75, 50 and 25% of MVC for 20, 40 and 10 s, respectively. The study was performed both under control conditions and after pharmacological blockade with atropine (12 individuals) or propranolol (11 individuals). Under control conditions, the heart rate (HR) responses to isometric effort were dependent on the intensity and duration of the exercise, showing a tendency towards progressive elevation with the maintenance of muscular contraction at the levels studied. The tachycardia evoked by this effort was of considerable magnitude and of rapid onset, especially at the more intense levels of activity. Parasympathetic blockade markedly decreased tachycardia, which manifested itself during the first 10 s of exercise at all levels of intensity, whereas sympathetic blockade markedly modified the HR response after 10 s of effort at the 75 and 50% MVC levels. A slight depression of the tachycardiac response could be observed already after 10 s of maximum effort after propranolol. The present results suggest that the autonomic regulation of these responses is based on a biphasic mechanism, with the initial phase depending on the rapid withdrawal of the parasympathetic influence, followed by a marked sympathetic contribution to the induction of tachycardia after 10 s of isometric contraction or even a little before at maximum exertion.

Adult↗

PubMed1985

The effects of two forms of isometric training on the mechanical properties of the triceps surae in man.

The training effects of rhythmic and sustained isometric contractions on the contractile characteristics of the triceps surae have been investigated in four healthy subjects over a period of 8 weeks. One leg (ST) of each subject was trained by performing repeated daily sustained (1 min) isometric contractions at 30% of maximal voluntary contraction force (MVC), and the other using rhythmic isometric contractions at 100% of MVC. The protocol was so arranged that the total area under the training force/time curves was the same for each limb. Electrically evoked maximal twitch (Pt) and tetanic (Po) tensions of the triceps surae were measured weekly on both legs using a standard procedure. The results showed that the rhythmic regime increased the MVC at the rate of 5.5% per week and the sustained training increased MVC at the rate of 3.3% per week. Twitch and tetanic tensions were not altered by either regime. However, only training using sustained contractions produced a progressive increase in endurance as measured by performance in a fatigue test. The results suggest that increases in voluntary strength resulting from short term isometric training are not necessarily associated with a rise in the intrinsic involuntary capacity of muscle fibres to generate force.

Adult↗

PubMed1986

Physiological modifications of local haemodynamic conditions during bilateral isometric contractions.

Nine subjects (five women and four men) simultaneously performed two isometric contractions sustained until exhaustion at different relative forces: 40% of maximum voluntary contraction (MVC) for the right elbow flexors; 50% MVC for the left elbow flexors. Contraction of the left elbow flexors commenced at 50% of the limit time (maximum maintenance time) of isometric contraction of the right elbow flexors. Increase in heart rate during concomitant contraction of the left elbow flexors led to an increase in blood flow to the right elbow flexors. Under these conditions, the limit time of isometric contraction of the right elbow flexors was prolonged with respect to the limit time obtained for an isolated isometric contraction at the same relative tension. The difference was more significant in the female (+40%, P less than 0.05) than in the male subjects (+20%, P greater than 0.05).

Adult↗