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PubMedDate not supplied

Surface EMG and motor unit activity of partially denervated human muscle during fatiguing submaximal isometric contraction.

The extent to which the normal fatigue compensatory mechanisms are disturbed in partially denervated muscles was investigated in human patients. Surface EMG, as well as motor unit electrical and mechanical activity, were analyzed from the partially denervated first interosseous muscle, during fatiguing isometric submaximal contraction. The EMG power and frequency changes which reveal the local fatigue process of healthy muscle have not been systematically found. Motor unit firing rate changes were rather normal and twitch contraction time did not increase during the fatiguing exercise. Differences between normal and partially denervated muscles could be explain by the occurrence of a central fatigue process more or less important in neurogenic lesions.

Adult↗
PubMed2005

Firing rates of motor units in human vastus lateralis muscle during fatiguing isometric contractions.

We investigated the firing rate of motor units in the vastus lateralis muscle in five healthy young men (mean = 21.4 yr, SD = 0.9) during a sequence of isometric constant-torque contractions repeated to exhaustion. The contractions were sustained at 20% of the maximal voluntary level, measured at the beginning of the test sequence. Electromyographic (EMG) signals were recorded via quadrifilar fine-wire electrodes and subsequently decomposed into their constituent motor unit action potentials to obtain the motor unit firing times. In addition, we measured the whole muscle mechanical properties during the fatigue task using electrical stimulation. The firing rate of motor units first decreased within the first 10-20% of the endurance time of the contractions and then increased. The firing rate increase was accompanied by recruitment of additional motor units as the force output remained constant. The elicited twitch and tetanic torque responses first increased and then decreased. The two processes modulated in a complementary fashion at the same time. Our data suggest that, when the vastus lateralis muscle is activated to maintain a constant torque output, its motoneuron pool receives a net excitatory drive that first decreases to compensate for the short-lived potentiation of the muscle force twitch and then increases to compensate for the diminution of the force twitch. The underlying inverse relationship between the firing rate and the recruitment threshold that has been reported for nonfatigued contractions is maintained. We, therefore, conclude that the central nervous system control of vastus lateralis motor units remains invariant during fatigue in submaximal isometric isotonic contractions.

Action Potentials↗
PubMed1996

Comparison of force and peak EMG during a maximal voluntary isometric contraction at selected angles in the range of motion for knee extension.

For 5 adults maximal isometric strength at 110 degrees, 90 degrees, 70 degrees, 50 degrees, and 30 degrees of knee flexion for the vastus medialis, rectus femoris, and vastus lateralis varied across angles but EMG responses were similar. That motor recruitment patterns were similar throughout the range of joint motion requires replication on a larger sample.

Adult↗
PubMed2006

Isometric contraction of microvascular pericytes from mouse brain parenchyma.

Pericytes were isolated and cultured from mouse cerebroparenchymal microvessels. A single pericyte clone was three-dimensionally cultured in a collagen gel by adding tensile stress, resulting in the reconstruction of narrow stringy fibers. When the contractility of these fibers was evaluated isometrically, they contracted in response to acetylcholine (ACh)1 or noradrenaline; this was accompanied by an increase in intracellular calcium concentration ([Ca(2+)]i). The fibers that were pre-contracted by ACh were completely relaxed by papaverine, which is a smooth-muscle relaxant. Moreover, the muscarinic ACh receptor-antagonist atropine depressed the [Ca(2+)]i response that was induced by ACh. This study demonstrates for the first time the quantitative measurement of the contractions produced by cultured microvascular pericytes from mouse brain parenchyma.

Acetylcholine↗
PubMedDate not supplied

[Isometric contractions of the myocardium: mathematic modeling].

A mathematical model describing a single contraction of a cardiac muscle strip under isometric conditions is proposed. The adequacy of the model was checked in experiments on cardiac strips from patients with chronic coronary insufficiency. It was shown that the contraction-relaxation cycle is rather completely characterized by the parameters characterizing the association-dissociation kinetics of actomyosin bridges.

Humans↗
PubMed1991

The precision of motor control in human jaw and limb muscles during isometric contraction in the presence of visual feedback.

Experiments showed that the human capacity to maintain a particular isometric force in the presence of visual feedback during a force-level, pursuit-tracking experiment is less developed for the jaw-closing muscles than for the limb muscles. This finding may indicate that the projection on the trigeminal motoneurone pool from visual inputs is poor, or that the trigeminal effector system itself is less finely tuned.

Adult↗
PubMed1990

Increased fatigue of isovelocity vs. isometric contractions of canine diaphragm.

A comparison of fatigue as a loss of force with repeated contractions over time was performed in canine respiratory muscle by isometric (nonshortening) and isovelocity (shortening) contractions. In situ diaphragm muscle strips were attached to a linear ergometer and electrically stimulated (30 or 40 Hz) via the left phrenic nerve to produce either isometric (n = 12) or isovelocity (n = 12) contractions (1.5 s) from optimal muscle length (Lo = 8.8 cm). Similar velocities of shortening between isovelocity experiments [0.19 +/- 0.02 (SD) Lo/S] were produced by maximizing the mean power output (Wmax = 210 +/- 27 mW/cm2) that could be developed over 1.5 s when displacement was approximately 0.30 Lo. Initial peak isometric tension was 1.98 kg/cm2, whereas initial peak isovelocity tension was 1.84 kg/mc2 (P less than 0.01) or 93% of initial isometric tension. Fatigue trials of 5 min were conducted on muscles contracting at a constant duty cycle (0.43). At the end of the trials, peak isovelocity tension had fallen to 50% of initial isometric tension (P less than 0.01), whereas peak isometric tension had only fallen by 27%. These results indicate that muscle shortening during force production has a significant influence on diaphragm muscle fatigue. We conclude that the effects of shortening on fatigue must be considered in models of respiratory muscle function, because these muscles typically shorten during breathing.

Animals↗
PubMed1988

Blood flow to different rat skeletal muscle fiber type sections during isometric contractions in situ.

In whole skeletal muscle, peak blood flow is known to correlate with the capacity for oxidative metabolism. Since most mammalian skeletal muscle is comprised of different fiber types whose oxidative potential varies widely, the possibility of heterogeneous blood flow distribution within a given muscle was investigated. An in situ preparation of the rat gastrocnemius-soleus-plantaris muscle group was used and blood flow measured by the radiolabeled microsphere technique. Blood flow at rest averaged 5-6 ml.min-1.100 g-1 in fast-twitch white (FTW) fiber sections, while flows to fast-twitch red (FTR) and slow-twitch red (STR) muscle were 10-12 ml.min-1.100 g-1. Isometric twitch contractions generated large (12-15 times above rest) increases in blood flow to all fiber type sections that tended to decrease at higher frequencies. Tetanic contractions result in greater tension development and higher blood flows in the red fiber sections. The highest blood flow to the FTR section was 300 ml.min-1.100 g-1, a value 3-4-fold greater than the maximum for the FTW fiber section. Peak blood flow to the STR (soleus) was intermediate between the two fast fiber types. Differences in blood flow response between fiber sections could not be dismissed due to measurement artifact. Thus, the capacity for blood flow is fairly proportional to the differences in oxidative capacity among fiber types. Blood flow to the skeletal muscle fiber types of the rat also differed qualitively in response to contractions.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1975

A one-dimensional viscoelastic model of cat heart muscle studied by small length perturbations during isometric contraction.

To develop a model of heart muscle, we studied cat papillary muscle contracting in a quasi-isometric condition under a fixed inotropic state. The properties of resting muscle were determined by using a step stretch of less than 1.2% of Lmax for initial lengths from 85 to 100% Lmax. The passive force response suggested the model of the passive branch (Fig. 1). All five parameters were small at muscle lengths below 95% of Lmax but increased markedly at longer lengths. The properties of contracting muscle were studied with a sinusoidal length change (amplitude less than 0.15% of Lmax, frequency 0.1-35.0 Hz). The frequency response of active (total minus passive) stiffness suggested the model of the active branch (Fig. 1). We determined the dependency of the elastic elements (K, Ks) and the viscous element (C) on length and time by recording the frequency response at various combinations of length and time Ks varied linearly with active force (FA). K and C exhibited time courses that paralleled FA up to 0.6tmax, and they maintained their values until 1.4tmax. K then fell toward zero, whereas C exhibited a secondary rise before it fell toward zero. K was dependent of length up to 95% of Lmax and then began to decline, but C varied in proportion to muscle length.

Animals↗
PubMed1994

EMG changes in respiratory and skeletal muscles during isometric contraction under normoxic, hypoxemic, or ischemic conditions.

The consequences of general hypoxemia (PaO2 = 51 mmHg) on two muscle groups (adductor pollicis and diaphragm) sustaining 80% maximal isometric voluntary contraction were studied in healthy individuals. For adductor pollicis, contractions were also executed after 10-s or 3-min rest ischemia. Compared to control, i.e., normoxic, sustained isometric workloads, significant shortening of endurance time occurred only when adductor pollicis contracted under hypoxemic conditions. In both muscle groups, a 3-min ischemia test as well as hypoxemia reduced the rate of changes in integrated surface EMG in a low frequency band and lowered, or did not modify, the rate of change in the high above low frequency ratio. Recovery of normal patterns of EMG changes was prolonged only after the adductor pollicis contracted under hypoxemic conditions. The present data show that both hypoxemia and prolonged rest ischemia reduced the rate of changes in quantitative EMG activity, with the more significant effects being measured under hypoxemia.

Adult↗
PubMed2006

Cross bridges account for only 20% of total ATP consumption during submaximal isometric contraction in mouse fast-twitch skeletal muscle.

It is generally believed that cross bridges account for >50% of the total ATP consumed by skeletal muscle during contraction. We investigated the effect of N-benzyl-p-toluene sulfonamide (BTS), an inhibitor of myosin ATPase, on muscle force production and energy metabolism under near-physiological conditions (50-Hz stimulation frequency at 30 degrees C results in 35% of maximal force). Extensor digitorum longus muscles from mice were isolated and stimulated to perform continuous isometric tetanic contractions. Metabolites of energy metabolism were analyzed with fluorometric techniques. ATP turnover was estimated from the changes in phosphocreatine (PCr), ATP, and lactate (-2DeltaATP - DeltaPCr + [1.5Deltalactate]). During contractions (2-10 s), BTS decreased force production to approximately 5% of control. Under these conditions, BTS inhibited ATP turnover by only 18-25%. ATP turnover decreased markedly and similarly with and without BTS as the duration of contraction progressed. In conclusion, cross bridges (i.e., actomyosin ATPase) account for only a small fraction (approximately 20%) of the ATP consumption during contraction in mouse fast-twitch skeletal muscle under near-physiological conditions, suggesting that ion pumping is the major energy-consuming process.

Adenosine Triphosphate↗