SEARCH · PubMed Health
Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
In experiments on the hyperpermeable papillary muscles (PM), we studied the antimembranous antibodies effects on their contractile responses. It has been found that antibodies enhance the amplitude of isometric contraction. Specific antibodies were suggested to induce the release of Ca2+ from the sarcoplasmatic reticulum in the PM of rats.
The time course of force generation and the time course of muscle stiffness were measured in rabbit soleus muscles during eccentric contraction to understand the underlying basis for the force loss in these muscles. Muscles were activated for 600 msec every 10 sec for 30 min. Soleus muscles contracting isometrically maintained constant tension throughout the treatment period, while muscles subjected to eccentric contraction rapidly dropped tension generation by 75% within the first few minutes and then an additional 10% by the end of 30 min. This indicated a dramatic loss in force-generating ability throughout the 30 min treatment period. To estimate the relative number of cross-bridges attached during the isometric force generation phase immediately preceding each eccentric contraction, stiffness was measured during a small stretch of a magnitude equal to 1.5% of the fiber length. Initially, muscle stiffness exceeded 1300 g/mm and, as eccentric treatment progressed, stiffness decreased to about 900 g/mm. Thus, while muscle stiffness decreased by only 30% over the 30 min treatment period, isometric force decreased by 85%. In isometrically activated muscles, stiffness remained constant throughout the treatment period. These data indicate that, while soleus muscles decreased their force generating capability significantly, there were a number of cross-bridges still attached that were not generating force. In summary, the loss of force generating capacity in the rabbit soleus muscle appears to be related to a fundamental change in myosin cross-bridge properties without the more dramatic morphological changes observed in other eccentric contraction models. These results are compared and contrasted with the observations made on muscles composed primarily of fast fibers.
We examined whether the decline in isometric force and peak stretch force during repeated stretches of activated plantar flexor muscles was larger in old (24 months, N=5) compared with young (4 months, N=5) female Sprague Dawley rats. Thirty stretches were imposed on isometric contractions (stimulation time 1.9 s, rest periods 180 s) by ankle rotation from 90 degrees to 40 degrees at 50 degrees x s(-1). Even though muscle weights were similar [2094 +/- 54 (young) vs 2033 +/- 73 mg (old) (mean +/- SE)], the isometric force (IF) at 90 degrees before stretch 1 and peak stretch force (PSF) at 40 degrees for stretch 1 were lower in old rats [18.9 +/- 0.5 vs 15.5 +/- 1.4 N (p=0.024) and 25.8 +/- 1.3 vs 20.5 +/- 0.9 N (p=0.017)]. After the stretches, the IF deficit was similar [38.5 +/- 3.0% (young) vs 39.8 +/- 3.4% (old)] and did not recover after 1 hour of rest [35.1+/- 4.1% (young) vs 36.9 +/- 4.3% (old)]. The decline in PSF was/also similar [36.0 +/- 5.2% (young) vs 26.6 +/- 1.8% (old)]. Skeletal muscles of 24-month-old female Sprague Dawley rats were weaker than 4-month-old rats but had similar susceptibility to develop IF deficits after stretches of activated muscles.
We have compared the responsiveness of rabbit mesenteric resistance arteries with agonists under isometric and isobaric conditions. When pressurized (60 mmHg), arteries spontaneously reduced their diameter by 18.1%. An equivalent isometric stress did not generate force in a "wire" myograph. Subsequently, much higher concentrations of norepinephrine (NE) and histamine were required to cause isometric contractions than were needed to reduce vascular diameter of pressurized vessels, whereas angiotensin II produced a maintained response only in pressurized arteries. Reducing transmural pressure to 20 mmHg abolished pressure-induced myogenic tone and decreased arterial sensitivity to NE. Under isometric conditions, partial depolarization with KCl increased sensitivity to NE and histamine to within the concentration range effective in pressurized vessels and also "revealed" responses to angiotensin II. The membrane potential of the vascular smooth muscle cells under partially depolarized conditions was similar to that found in vivo and in vessels studied isobarically. These observations demonstrate a fundamental interaction between pressure-induced myogenic tone and the sensitivity of resistance arteries to vasoactive stimuli. This influence was mimicked in isometrically mounted vessels by partial depolarization, indicating a possible pivotal role for membrane potential in determining the reactivity of the resistance vasculature.
This paper is concerned with the development and analysis of a nonlinear approach to modeling of the contraction of electrically stimulated skeletal muscle. The model structure is based on a network of locally valid linear models which are blended together by a scheduler. Data are from experiments with rabbit tibialis anterior muscles in which the muscles contracted isometrically while being stimulated by supramaximal impulses with randomly varying inter-pulse intervals. The model accounts for nonlinear effects due to variations of the stimulation frequency, such as the "catch-like" effect. It is shown that this modeling technique is suitable for modeling the contraction of muscles with very different characteristics, such as muscle with a majority of fast motor units and muscle with mainly slow motor units. The approach is also suitable as a basis for the design of muscle stimulation controllers. Index Terms-Functional electrical stimulation, local model network, muscle modeling, nonlinear system identification.
Hypothyroidism was induced in Wistar-Kyoto rats by adding propylthiouracil to the drinking water (0.8 mg/ml). Initial heat, total activity-related heat, and resting heat rate were measured in left ventricular papillary muscle preparations of propylthiouracil-treated and control rats contracting isometrically at 12 beats/min (21 degrees C), using Hill type, planar vacuum-deposited bismuth and antimony thermopiles. In the propylthiouracil preparations, relative to control, time-to-peak tension increased from 288 +/- 27 (mean +/- SD) to 411 +/- 25 msec (P less than 0.001), dp/dtmax decreased from 38.3 +/- 9.5 to 20.4 +/- 3.5 g X mm-2/sec (P less than 0.001), and peak developed tension decreased from 6.11 +/- 1.75 to 4.64 +/- 0.89 g X mm-2 (P less than 0.05). In the propylthiouracil preparations, initial heat was significantly (P less than 0.001) reduced by 27 or 43% when normalized to peak twitch tension or tension-time integral, respectively. In experiments where the papillary muscles were tetanized, the slope of the linear function of total activity-related heat versus tension-time integral was decreased by 43% (P less than 0.001) in the propylthiouracil preparations, indicating an improved economy of isometric tension maintenance. The predominant myosin isoenzyme of the left ventricular wall, as well as the papillary muscle myocardium, was the V3 variety in the propylthiouracil animals, in contrast to V1 in the controls. Myofibrillar actomyosin calcium-magnesium-stimulated adenosine triphosphatase activity was significantly (P less than 0.02) decreased from 55 +/- 18 (control) to 31 +/- 8 nmol inorganic phosphate ion/mg X min (propylthiouracil).(ABSTRACT TRUNCATED AT 250 WORDS)
BACKGROUND: Exposure to cold causes cutaneous vasoconstriction to reduce body heat loss, while the nasal cavity warms up the inspired cold air. This suggests cooling might evoke a different response in nasal mucosa blood vessels than in cutaneous blood vessels. Thus, the aim of this study was to evaluate the effect of cooling (to 24 degrees C) on the vascular response of isolated nasal mucosa. METHODS: An in vitro isometric contraction of nasal mucosa excised from patients suffered from chronic nasal congestion was continuously recorded. Either electrical field stimulation (EFS) or exogenous norepinephrine (NE) was applied to the turbinate mucosa strip at 37 and 24 degrees C, and the influence of cooling (to 24 degrees C) was evaluated. RESULTS: EFS (1, 2, 4, and 8 Hz) produced frequency-dependent contractions at 37 and 24 degrees C. Cooling did not alter the magnitude but significantly prolonged the duration of EFS-induced contraction. Exogenous NE (10(-8) to 10(-4) M) produced concentration-dependent contractions at 37 and 24 degrees C. Cooling significantly enhanced the contractile responses evoked by NE between 3 x 10(-6) and 1 x 10(-4) M. CONCLUSION: Cooling (to 24 degrees C) prolonged the EFS-induced and enhanced NE (3 x 10(-6) to 1 x 10(-4) M)-induced contraction of isolated human nasal mucosa.
The purpose of this study was to compare the force exerted by the rat medial gastrocnemius (GM) muscle with either fast oxidative or fast glycolytic parts active during concentric and eccentric contractions at different velocities. The proximal end of the GM contains mainly fast oxidative fibres and the distal end predominantly fast glycolytic fibres. Different parts of GM were activated by selective stimulation of nerve branches. Fast oxidative or fast glycolytic muscle parts of anaesthetised male Wistar rats were activated maximally. After assessment of concentric force/velocity (F/v) relations (n=11), some of the muscles were subjected to a fatiguing series of isometric contractions (n=5). Fast oxidative muscle parts showed a significantly lower mean (+/-SD) maximal power output (P(max) 0.12+/-0.06 W) and fatigability than fast glycolytic muscle parts (P(max) 0.20+/-0.06 W). The remaining muscles performed eccentric contractions. The eccentric F/v curves were not significantly different for fast oxidative and fast glycolytic muscle parts (n=6). Maximum eccentric force relative to the maximum isometric force (157+/-3% and 153+/-6% respectively,P=0.99) was reached at a velocity of 60 mm s(-1). It is concluded that eccentric F/v relations of rat GM with either fast oxidative or fast glycolytic parts active are very similar despite the differences in the concentric F/v relations.
Force and isotonic shortening velocities were studied (0.6-4.0 degrees C) in isolated single muscle spindles from the anterior tibialis muscle of Rana temporaria using techniques that enabled measurements both from the spindle as a whole and from marked segments of the preparation. The force-velocity relationship during tetanic stimulation exhibited the same biphasic shape as previously described for extrafusal muscle fibres. However, the maximum speed of shortening of the spindle fibres was merely 0.95 +/- 0.006 lengths s(-1) (mean +/- S.E.M., n = 11), which is approximately half the value recorded in extrafusal fibres of the same muscle. The maximum tetanic force, 91 +/- 10 kN m(-2), n = 14, was likewise only approximately half that produced by extrafusal fibres. The force generated by the capsule segment was lower than that produced by the whole spindle resulting in elongation of the capsule region during a fixed-end tetanus. The intracellular calcium ion concentration reached during the plateau of the tetanus, 1.7 +/- 0.1 microM (n = 8), was substantially lower than the value attained in extrafusal fibres under equivalent conditions. In accordance, the spindle fibres did not become fully activated during supramaximal electrical stimulation as indicated by the finding that the tetanic force could be further increased by 16.6 +/- 0.04 % (n = 5) on addition of 0.5 mM caffeine. Inadequate activation may thus, to a certain extent, account for the relatively low force per cross-sectional area of the spindle fibres. The contractile properties of the intrafusal fibres should make the spindle organ suited to provide feedback control during eccentric (forced lengthening) and static (isometric) contractions and, with reduced effectiveness, during slow muscle shortening.
This study was designed to determine the effects of ketamine on contractions induced by norepinephrine (NE), K+ or histamine (Hist) and on agonist-induced calcium mobilization, in rabbit thoracic aorta with or without endothelium. Contractile responses to NE, K+ or Hist were markedly attenuated by prior exposure to ketamine. Subsequent addition of ketamine to the rabbit aorta undergoing an isometric contraction induced by NE, K+ or Hist also decreased the contractile responses in a calcium ion concentration-dependent manner. Preincubation with ketamine produced a concentration-dependent inhibition of contractile responses elicited by the addition of calcium ion (1.6 mM) to a Ca(++)-free depolarizing solution. However, the phasic contraction produced by NE with 2mM lanthanum pretreatment, which is release of intracellular calcium, was also inhibited by ketamine. Moreover, the tonic contraction produced by NE after depletion of the agonist-releasable pool of intracellular calcium, which is thought to be due to calcium influx, was depressed by ketamine. These data suggest that ketamine relaxes NE-contracted rings of rabbit thoracic aorta by decreasing calcium entry and by producing an extracellular calcium-independent relaxant effect.
Strength measurement of the hand grip is at the core of most protocols of functional assessment of the upper limb and in rehabilitation plays a major role in the analysis of treatment efficacy and patients' occupational ability. The aims of this study were to: a) verify the repeatability of strength measurements made during performance of the hand grip and three types of pinch, carried out under isometric and isokinetic conditions; b) compare maximal isometric strength with the corresponding isokinetic value for each of the manoeuvres studied; c) investigate the correlations between the strength expressed in the different manoeuvres, under both isometric and isokinetic conditions. We studied 14 voluntary subjects over three sessions conducted at 48-hr intervals, employing a computerized isokinetic dynamometer Lido WorkSet equipped with device N(o) 21 for the study of pinch (lateral pinch, pulp pinch, chuck pinch) and device N(o) 52 for the grip study. Isometric contractions resulted stronger than isokinetic ones, and the hand grip was found to be the manoeuvre able to produce most strength. The repeatability of each strength measurement test over the three days was high (Intraclass Correlation Coefficients: 0.89-0.93). Correlations between the isometric and isokinetic performance for each of the manoeuvres examined were always high (Pearson's r coefficients: 0.89-0.95) as were those between the different manoeuvres, whether performed in isometric or isokinetic modality (r: 0.60-0.94).
The effects of indapamide on ionic currents and isometric contractions in both rat myometrium and rat portal vein preparations were studied. Indapamide (3 x 10(-4) M) depressed both inward calcium and outward potassium currents. The decrease in potassium current was dependent on the reduction of the inward calcium current. Indapamide reduced the transient contraction induced in calcium-free, ethylene glycol-bis(B-aminoethylether)-N,N'tetra-acetic acid-containing solutions by either acetylcholine (10(-4) M) or norepinephrine (10(-5) M). The results indicate that indapamide acts primarily on the plasma membrane of spontaneously active smooth muscles by reducing the calcium current; it may also depress contractions supported by release of calcium from the sarcoplasmic reticulum.
1. Simultaneous recordings of cortical activity, recorded as the magnetoencephalogram (MEG), and the electromyogram (EMG) of the ipsilateral and contralateral first dorsal interosseous muscles (1DI) were made during maintained voluntary contractions. 2. The MEG recorded from a localized region of the sensorimotor cortex of the dominant hemisphere was coherent with the EMG from the contralateral 1DI muscle over a limited band of frequencies. The peak coherence was confined largely within the beta range of cortical activity (13-35 Hz). Significant cortical activity at 10 Hz and 40-50 Hz was not correlated with motor output. The MEG and EMG from the ipsilateral 1DI muscle were uncorrelated at all frequencies. 3. Significant coherence between the MEG and the EMG was associated with synchronous behaviour between the MEG and EMG in the time domain. 4. The results demonstrate that synchronized cortical activity contributing to MEG activity within the beta range of frequencies during maintained voluntary contractions is coupled to motor output at frequencies of motor-unit activity associated with motor-unit synchronization. This observation provides further evidence for the involvement of cortical neurones in the generation of motor-unit synchronization. 5. We suggest that the coherence between MEG and contralateral EMG observed during maintained isometric contractions may provide an example of binding within the motor system.
The purpose of this investigation was to compare the effects of halothane on malignant hyperthermia (MH) and normal isolated muscle bundle performance during isometric contraction and relaxation phases. Mechanical parameters were measured: peak tension (PT), time to peak tension (TPT) and positive peak of isometric tension derivative (+dP/dtmax) characterized the contraction phase. Half-relaxation time (RT1/2) and negative peak of isometric tension derivative (-dP/dtmax) characterized the relaxation phase. The ratio R = (+dP/dtmax)/(-dP/dtmax) was used to study the coupling between contraction and relaxation under isometric condition. In normal muscle, halothane increased PT by nearly 40% without altering TPT. The +dP/dtmax value increased concomitantly with the -dP/dtmax values, thus no changes in R was observed. In MH muscle, PT was first potentiated (0.5-1.0 vol% halothane) and then depressed (2.0-3.0 vol% halothane). TPT and +dP/dtmax were not altered whereas RT1/2 increased progressively with concomitant decrease in -dP/dtmax, thus R increased by nearly 40%. The amplitude of MH muscle contracture with stepwise concentrations of halothane was correlated with the increase of RT1/2 and R, and the decrease of -dP/dtmax. These results suggest that halothane alters the relaxation phase more than the contraction phase in MH human skeletal muscle compared to normal muscle.
It has been assumed that force enhancement in single fibres of skeletal muscles only occurs on the descending, and not the ascending or plateau region, of the force-length relationship. This assumption has been based, however, on theoretical considerations or isolated experiments, in which neither stretch conditions nor fibre lengths were optimized for force enhancement. Therefore, the purpose of this study was to investigate the residual, steady-state force enhancement following active stretch in single muscle fibres of frog on the ascending limb of the force-length relationship. Twenty-nine stretch experiments on ten single fibres from the lumbrical muscle of the frog Rana pipiens were carried out on the ascending limb of the force-length relationship. Force enhancement was observed in 28 out of the 29 tests. Moreover, the force produced for stretch experiments finishing at optimal fibre length always exceeded the force obtained for an isometric contraction at optimal length. We conclude from these results that steady-state force enhancement occurs systematically on the ascending limb of the force-length relationship, and that the steady-state force in the enhanced state can easily exceed the maximal isometric force of the fibre.
PURPOSE: The aim of the present study was to test the applicability and sensitivity of new electromyography (EMG) spectral indices in assessing peripheral muscle fatigue during dynamic knee-extension exercise. METHODS: Seven subjects completed 10 sets of 15 repetitions of right knee-extension exercise lifting 50% of their one-repetition maximum. Torque (T), knee-joint angle, and the interference EMG of rectus femoris muscle were recorded simultaneously. Maximal voluntary isometric contraction (MVC) was tested before and after exercise. Median spectral frequency (Fmed) and new spectral indices of muscle fatigue (FInsmk) were calculated for each repetition. RESULTS: The rate and range of FInsmk- and Fmed-relative changes against the first repetition of the corresponding set increased gradually across successive repetitions within the set, reflecting accumulation of peripheral muscle fatigue. The maximal change of FInsmk observed in the present experiment was approximately eightfold, whereas that of Fmed was only 32%. Significant between-subject variability in the range of FInsmk changes (P < 0.0001) was found, so a hierarchical cluster analysis of muscle fatigue indices was conducted. Three distinct subgroups of subjects were identified: high (N = 1, FInsmk change > 400%), medium (N = 4, 200% < FInsmk change < 400%), and low (N = 2, FInsmk change < 200%) muscle fatigability. The changes in muscle performance during (last vs first repetition peak T, P = 0.03) and after (post- vs preexercise MVC, P = 0.012) exercise were significantly different between clusters (one-way ANOVA). The rate of fatigue development was also significantly different between clusters (linear regression analysis of Fmed and FInsmk changes). CONCLUSIONS: The new spectral indices are a valid and reliable tool for assessment of muscle fatigability irrespective of EMG signal variability caused by dynamic muscle contractions, and these indices are more sensitive than those traditionally used.
When Lombardi and colleagues reported the phenomenon of rapid regeneration of the power stroke after a quick release of muscle fibre during a tetanus, they gave an explanation in terms of detachment of cross-bridges and re-attachment further along the thin filament. We show here that the phenomenon can also be explained on assumptions that lead to a majority of myosin molecules being attached by only one head during steady isometric contraction; the other head may then become attached after a quick release and can add its contribution to the early tension recovery after a second release.
Three well-trained male subjects served as volunteers in these experiments to examine the physiological capacity for extended work during lifting tasks. The maximal oxygen uptake (VO2max) during lifting was always lower than work on the bicycle ergometer. However, the work load during lifting which could be maintained for 1--4 h was 50% of the VO2max for lifting each specific weight of box; the limit for lifting light boxes without fatigue was at an oxygen uptake of about 25% of the VO2max obtained from bicycle ergometry. Significant fatigue in the forearm muscles was found during prolonged lifting as assessed from the endurance of isometric contractions and from the surface electromyogram (EMG), and was more pronounced as the weight of the box increased.