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At least 289 records · Page 16Linked to original sources
PubMed1988

Intensity changes of actin-based layer lines from frog skeletal muscles during an isometric contraction.

X-ray diffraction studies of actin-containing thin filaments from frog skeletal muscles were done using a new type of integrating X-ray area detector (an imaging plate) and synchrotron radiation. The high sensitivity and wide dynamic range of the imaging plate made it possible to clearly record the weak thin filament pattern from a muscle during isometric contraction in less than 20 sec exposure time. The intensity distributions of the actin-based layer lines were measured. During contraction, most of the actin-based layer lines increased in intensity without noticeable changes of their layer-line profile and axial spacing in the resting state. The difference cylindrically symmetrical Patterson function was calculated using the intensity data of 15 actin-based layer lines. During contraction, the Patterson map was quite different from the rigor map and no significant indication of binding of the myosin heads was detected in the contracting map. This revealed that the labeling of myosin heads was not in accordance with the actin symmetry. Assuming that the intensity change during contraction is due to the structural change occurring within the thin filaments by the interaction with the myosin heads, model-calculating studies were done to interpret the intensity change. The observed intensity increase could be explained by changing the structure of the actin subunit and the position of tropomyosin molecules in the thin filament. With reference to the three-dimensional reconstructions of the thin filament electron micrographs, some models of the thin filament in the resting and contracting states were presented.

Actin Cytoskeleton↗
PubMed1983

Skeletal muscle tension, flow, pressure, and EMG during sustained isometric contractions in humans.

In five healthy males sustained isometric torques during elbow flexion, knee extension, and plantar flexion correlated positively with intramuscular tissue pressure (MTP) in the range 0-80% of the maximal voluntary contraction (MVC). During passive compression of the muscle at rest 133-Xenon muscle clearance stopped when MTP reached diastolic arterial pressure (DAP) indicating that the muscle vascular bed was occluded. However, during sustained contraction this relation between DAP, flow and MTP was not seen. In two cases 133-Xenon clearance from M. soleus did not stop in spite of an 80% maximal contraction and MTP stayed below DAP. In other cases MTP would reach as high as 240 mm Hg before clearance was zero. In the deeper parts of the muscles MTP during contraction was increased in relation to the more superficial parts. The means values for the % MVC that would stop MBF varied between 50 and 64% MVC for the investigated muscles. Mean rectified EMG (MEMG) showed a high correlation to MTP during sustained exhaustive contractions: When MEMG was kept constant MTP also remained constant while the exerted force decreased; when force was kept constant both MEMG and MTP increased in parallel. This demonstrated that muscle tissue compliance is decreasing during fatigue. Muscle ischemia occurring during sustained isometric contractions is partly due to the developed MTP, where especially the MTP around the veins in the deeper parts of the muscle can be considered of importance. However, ischemia is also affected by muscle fiber texture and anatomical distorsion of tissues.

Animals↗
PubMedDate not supplied

Assessing hand grip endurance with repetitive maximal isometric contractions.

The purpose of this study was to identify a method for measuring hand grip endurance in patients with hand injury as well as in healthy subjects. Twenty hand-injured patients and 60 healthy subjects participated in the study. Each subject performed two sets of maximal repetitive isometric contractions on the Grippit instrument for 180 seconds, with a contraction time of 2.5 seconds and a rest time of 1 second. Reliability data were high for initial grip force and for absolute and relative endurance values at 90 seconds (r range, 0.73 to 0.97, p < 0.001). Relative endurance at 180 seconds, though, showed weaker reproducibility (r = 0.54, p < 0.01). A measuring period of 90 seconds instead of 180 seconds is suggested, because of the results of test-retest reliability. The method allows endurance to be described in relative force values (as a percentage) as well as in absolute force values (Newtons).

Adult↗
PubMed1997

Circadian rhythm in the torque developed by elbow flexors during isometric contraction. Effect of sampling schedules.

Time-dependent changes in elbow flexion torque have been documented according to two different sampling schedules. Seven physical education students took part in the first series of experiments, and 7 other similar subjects in the second. In both sets of experiments, the subjects performed isometric contractions: maximal and submaximal at 90 degrees in the first experiments and maximal at different angular positions in the second. After a 30-minute rest period, the torque developed was measured at 00:00, 06:00, 09:00, 12:00, 15:00, 18:00, and 21:00 h on the day of the experiment. These subjects remained in the laboratory for 24 h. In the second series of experiments, the torque developed was measured at 01:00, 05:00, 09:00, 13:00, 17:00, and 21:00 h over the subsequent 6 days with only one test session per day. In this case, there was an interval of 20 h between two successive test sessions. In the first experiment, a significant time-of-day effect was observed for the torque of the elbow flexors under isometric conditions with an acrophase at 17:58 h. The 24 h normalized mean score was 92.85% with an amplitude of 7.63% of the daily mean. In the second series of experiments, there was evidence of a circadian rhythm in the torque developed by the elbow flexors at every angle position, especially at 90 degrees, the angle investigated in the first set of experiments. The peak torque was calculated to have occurred at 17:55 h. The amplitude of the rhythm was equal to 6.99% of the daily mean. There were no statistically significant differences in the characteristics of the circadian rhythm observed between the two experimental designs. We concluded that an experiment extending over several days could be employed to evaluate circadian rhythms in muscular activity reliably.

Adult↗
PubMed2004

Normalization procedures using maximum voluntary isometric contractions for the serratus anterior and trapezius muscles during surface EMG analysis.

The serratus anterior and trapezius muscles are considered to be the only upward rotators of the scapula and are very important for normal shoulder function. A variety of methods have been used to produce a maximum voluntary isometric contraction (MVIC) of these muscles for normalization of EMG data. The purpose of this study was to quantify the surface EMG activity of the serratus anterior muscle and the upper, middle, and lower parts of the trapezius during 9 manual muscle tests performed with maximum effort in 30 subjects. It was found that no one muscle test produced a MVIC for all individuals. Therefore, to perform normalization within each subject, it is suggested that the 2 or 3 tests identified in this study that produce high levels of EMG activity for each muscle be performed. The scapular protraction muscle test that is often used to normalize data for the serratus anterior muscle produced relatively low levels of EMG activity and was not found to be an optimal test. Muscle tests in which an attempt was made to de-rotate the scapula from an upwardly rotated position produced much higher levels of EMG activity in the serratus anterior muscle.

Adult↗
PubMed1986

Muscle economy of isometric contractions as a function of stimulation time and relative muscle length.

For rat medial gastrocnemius muscle economy (i.e. the ratio of time integral of force and total energy-rich phosphate consumption) was calculated. Muscles in situ at 35 degrees C were stimulated to perform either one continuous or several repetitive isometric contractions at one muscle length in the range from 70% to 130% of optimum muscle length for force generation. Whereas during one continuous contraction economy increased, no differences in economy were found between 6, 12 or 18 successive contractions. Economy during intermittent exercise was always lower than during continuous exercise. The difference in economy is a result of different rates of metabolism, whereas no difference was found for force generation. Economy was highest at optimum muscle length for force generation and decreased at muscle lengths smaller as well as greater than optimum muscle length. Force-dependent energy consumption was calculated by subtracting the force-independent part (obtained by extrapolation) from total energy consumption. The calculated force produced per mumol force-dependent energy-rich phosphate consumption was similar in muscles stretched beyond optimum length. In contrast, a decreasing amount of force per mumol force-dependent energy-rich phosphate consumption was observed at lengths smaller than optimum length.

Animals↗
PubMed1989

Distribution of cross-bridges in isometrically contracting muscle: its deduction from the Huxley-Simmons theory and the effect of filament elasticity.

The author attempted to deduce a possible cross-bridge (XB) distribution as a function of x, the amount of stretch of the XB, by applying the Huxley-Simmons theory with their recent experimental data to a steady isometric contraction. I found no XB distribution which satisfied at once the Huxley-Simmons theory, their data and observations in muscle energetics and biochemistry. Further, a computation with a simple model suggested that the XB distribution could be broadened by the elasticity of the thick and/or the thin filament if it existed.

Elasticity↗
PubMed1982

Force sensation in isometric contractions: a relative force effect.

A contralateral limb matching paradigm was employed to investigate the perception of isometric forces. Both force and brachial biceps and triceps EMG were recorded from each arm as subjects matched isometric contractions on the basis of equal sensation. It was found that smaller forces were consistently overestimated in magnitude and that the most accurate estimation of force occurred around the middle of the force domain. It is proposed that these results reflect the properties of the biceps muscle which is predominantly involved in gross motor activity.

Arm↗
PubMed1974

Isometric contractions of motor units in self-reinnervated fast and slow twitch muscles of the cat.

1. Isometric contractions of motor units have been studied in self-reinnervated soleus and flexor digitorum longus muscles, the motor nerves of which had been transected 6 months earlier.2. The distributions of motor unit tetanic tensions in both muscles were skewed towards large values and showed greater variance than normal: there were motor units which produced more tension and others which developed less tension than in control muscles. Mean tensions of motor units are compared with those of the controls.3. The variance of times to peak of motor units in both muscles was significantly smaller than normal.4. Conduction velocities of axons were slower than normal above the point of nerve section. A number of axons did not functionally cross this point and those that did conducted slowly below the neuroma.5. Many of the correlations seen in normal muscle between axon conduction velocity, motor unit tetanic tension, twitch time to peak and ratio of twitch to tetanic tension were demonstrated in reinnervated muscle.6. We suggest that the pattern of motor unit organization in self-reinnervated muscle can in part be due to random regrowth of axons but is largely produced by specific processes related to axon size which are not identical in fast and slow muscle.

Animals↗
PubMed1980

Measurement of heat production during the summation of isometric contraction in frog skeletal muscle with an infrared radiometer.

The heat production of frog skeletal muscle during isometric contraction was measured with an infrared radiometer at room temperature (23--26 degrees C), attention being focused on the heat produced during the summation of twitches by two successive stimuli. When the amount of heat produced by the second stimulus was plotted against the corresponding tension increment, the data points fell on two regression lines of approximately the same slope except for the intermediate stimulus intervals. One regression line started from the origin, while the other intersected the ordinate at about 1 mcal/g. If a twitch was produced at various times after a 0.5 sec tetanus, all the data points on the heat versus tension increment plot distributed around a single regression line starting from the origin. These results are explained by assuming that the relation between the tension-dependent heat and the tension increment in a twitch produced after a preceding mechanical response remains nearly constant irrespective of the stimulus interval, while the amount of tension-independent heat production varies depending on the time after the preceding stimulation.

Animals↗
PubMed1991

Crossbridge states in isometrically contracting fish muscle: evidence for swinging of myosin heads on actin.

We have previously shown that time-resolved X-ray diffraction studies of the 2-D pattern from isometrically contracting flatfish (turbot) fin muscle have considerable advantages over similar studies of other vertebrate muscles due to the simple lattice and the better long-range order in these muscles (5, 24). Here we show not only that two structurally different myosin head to actin attached states must exist in the crossbridge cycle but that we are also able to define the likely crossbridge configurations in these states. A non-force producing "weak-binding" state is evident from the lead of the (11) equatorial reflections (and actin mass) time-course relative to that of tension (and the (10) equatorial reflection decrease) by about 30 msec. The first myosin layer line at 429 A has a weakened but altered intensity distribution, with no change in axial spacing, in patterns from active muscle. We show this to be consistent with myosin heads binding in the non-specific manner envisaged for the "weak-binding" state. Evidence for the second force-producing attached state, or series of states, comes from the observation of only a small increase in the intensity of the 360 A actin layer line between resting and active muscle patterns. It might be thought that a substantial increase in this layer line would be expected if myosin heads were even transiently attached to the thin filaments in a force-producing state. However, this is not so because internal changes in the structure of the thin filaments in active muscle have the opposite effect of causing this layer line to decrease in intensity. Observation of a small net intensity increase is therefore evidence for myosin head attachment with the symmetry of the actin helix. From the equatorial diffraction pattern, Fourier synthesis maps were computed at 10 msec intervals throughout the isometric tetanus, enabling changes in the mass distribution to be visualised between the force- and non-force producing populations of crossbridges. This difference map shows that in the force-producing state myosin heads have their centres of mass on average at a smaller radius from the thin filament axis compared to the case for non-force producing myosin heads. Since there is good evidence that there is no substantial change in myosin head shape during contraction (30) these observations are consistent with myosin heads swinging on actin as fairly rigid structures.

Actins↗
PubMed1992

Short-term synchrony of motor unit discharge during weak isometric contraction in Parkinson's disease.

Short-term synchrony between the discharges of motor units has been assessed in Parkinson's disease (PD) and normal man. The discharges of single motor units were recorded in the extensor digitorum communis (EDC) muscle of the forearm or the tibialis anterior (TA) muscle of the leg during weak, voluntary isometric contraction. Short-term synchrony was defined as a narrow peak (total width less than 25 ms) in cross-correlograms constructed from the discharges of pairs of motor units. There was no difference in the incidence of short-term synchrony between PD and normal age-matched subjects for either the EDC or TA muscle. On average, 60% of pairs of motor units exhibited synchrony, but this varied between 0% and 100% for both groups. The amount of short-term synchrony was assessed as the probability (above chance) of discharge of one motor unit with respect to the other. In TA, but not EDC, this index was greater for PD than for normal subjects. The high indices of synchrony in TA in PD were not related to lower discharge rates of motor units. Parkinson's disease subjects, but not normal subjects, also showed broad correlations that were invariably associated with periodic discharges in the range 4-6 Hz. In some instances, a peak of short-term synchrony was observed superimposed on the broad correlation. The periodic correlograms were often associated with overt tremor which accompanied the contraction. Motor units occasionally discharged paired impulses (doublet discharges) with short interspike intervals of 5-15 ms (normal and PD) or, as a more persistent feature in PD, longer interspike doublets (20-60 ms) associated with periodic synchrony (4-6 Hz). The abnormal discharge characteristics of motor units are discussed in relation to the bulbospinal control of presynaptic drive to motor neurons in PD.

Adult↗
PubMed2005

Repeatability of surface EMG parameters at various isometric contraction levels and during fatigue using bipolar and Laplacian electrode configurations.

The objective of this work was to assess the repeatability of two surface electromyographic (sEMG) recording techniques, the classical bipolar configuration and a Laplacian configuration to document their ability to provide reliable information during follow-up studies. The signals were recorded on 10 healthy subjects during voluntary isometric contractions of the biceps brachii muscle at different constant contraction levels. Slopes, area ratios (at 60% of the maximal voluntary contraction (MVC)) and initial values (at 20%, 40%, 60%, 80% and 100% MVC) of the root mean square (RMS), the mean power frequency (MPF) and the muscle fibre conduction velocity (CV) were estimated. Experimental sessions were repeated on three different days with both electrode sets to evaluate the repeatability of sEMG parameter estimates. Classical results were observed, such as an increase in the RMS and the CV with the contraction level. Only initial values of RMS and MPF were shown to be dependent on electrode type. These two parameters presented intra-class correlation coefficient values higher than .80 for high contraction levels. On the whole, the repeatability of the measures was good; however it was better for all sEMG parameter estimates with bipolar electrodes than Laplacian electrodes. Because a bipolar configuration is less selective than a Laplacian one, it provides a global view of muscular activity, which is more repeatable, hence more suitable for follow-up studies.

Adult↗
PubMed1978

Tension and heat production during isometric contractions and shortening in the anterior byssus retractor muscle of Mytilus edulis.

1. Tension and heat production were measured during phasic isometric contractions and isovelocity shortening in the anterior byssus retractor muscle (ABRM) of Mytilus edulis at 20 degrees C. 2. Isometric tension at lo was 550 +/- 40 mN/mm2 (S.D. for 173 observations in nine muscles), while the isometric maintenance heat rate was 1.0 +/- 0.2 mW/g wet wt. (S.D. for seventy-eight observations in eight muscles). 3. Isometric tension and heat production were measured as functions of muscle length over a range of 0.79--1.14 lo and were found to bear a linear relation to each other. 4. The force-velocity relation was determined in isovelocity releases imposed during tetanic stimulation and was found to fit the Hill equation with parameters alpha/Po = 0.07 +/- 0.01 and b/lo = 0.016 +/- 0.0007 sec-1 (S.E. from non-linear least-squares regression of the pooled data from seven experiments). 5. Heat production measured in the same experiments showed that shortening heat is produced with a shortening heat coefficient alpha/Po of 0.15. Shortening heat does not appear to be force-dependent, and separate experiments confirmed that it is a linear function of the amount of shortening.

Animals↗
PubMed1993

Correlation of myosin light chain phosphorylation with isometric contraction of fibroblasts.

In vitro studies have indicated that the enzymatic activity of myosin II from non-muscle cells is controlled by phosphorylation of its regulatory light chain (LC20). We have studied one likely functional consequence of phosphorylating LC20 in living chick embryo fibroblasts (CEF) by measuring contractile force developed by these cells. Using a recently developed method, we recorded quantitative changes in isometric force generated by a population of cells following mitogenic stimulation. Fetal bovine serum, thrombin, and lysophosphatidic acid stimulate rapid isometric contraction of CEF. Cells stimulated with thrombin develop maximal force within 5-10 min. Force development correlates temporally with a 3-5-fold increase in the overall fraction of LC20 phosphorylated and with the fractions of LC20 in both the monophosphorylated and diphosphorylated states. Unloaded shortening velocity also increases after thrombin stimulation. Although both force and phosphorylation begin to decline 10 min after stimulation, the level of phosphorylation declined more rapidly than the force. These results suggest that the role of LC20 phosphorylation in regulating fibroblast contractility is analogous to its well established role in regulating smooth muscle contraction and that quantitative measurements of the force developed by populations of fibroblasts (or other cultured cells) can be used to study the regulation of non-sarcomeric myosin at the molecular level in vivo.

Animals↗
PubMedDate not supplied

Distribution of muscle fiber conduction velocity of m. masseter during voluntary isometric contraction.

Muscle fiber conduction velocity (MFCV) is the velocity of an interference wave due to muscle fiber action potentials. In general, it has been reported that the value of MFCV in m. masseter is larger than that in limb and trunk muscles. But the values of MFCV in the reports were measured in part of the muscle, and the distribution of MFCV in the whole muscle including the end-plate and the tendon has not been measured. In this study, surface myoelectric signals are recorded in m. masseter during voluntary isometric contractions of 20, 30, and 40% MVC (maximum voluntary contraction) in eleven healthy male subjects with the use of array electrodes. The value of MFCV is found directly using an averaging method. The end-plate zone is concentrated in the lower quarter of the muscle. The distribution of MFCV depends on the location of the measured electrode. The largest value of MFCV of more than 20.0 m/s is obtained in the locations of the end-plate and the tendon. The minimum value of MFCV is obtained at the location of 10 mm measured from the end-plate along the direction of the muscle fiber to the tendon of the upper side. The mean values with the standard deviations during 20, 30, and 40% MVC are 10.3 +/- 0.7, 11.6 +/- 0.7, and 12.2 +/- 0.8 m/s, respectively. The MFCVs between the different neighboring locations are compared and found to be significant by a level of 1% for each contraction level. The MFCVs increase depending on the contraction levels for various electrode locations on the muscle.

Adult↗