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PubMed1986

Influence of partial activation on force-velocity properties of frog skinned muscle fibers in millimolar magnesium ion.

Segments of briefly glycerinated muscle fibers from Rana pipiens were activated rapidly by a brief exposure to 2.5 mM free calcium followed by a solution containing calcium buffered with EGTA to produce the desired level of force. Steps to isotonic loads were made using a servomotor, usually 3-5 s after the onset of activation. The relative isotonic forces (P/P0) and velocities from contractions obtained under similar circumstances were grouped together and fitted with hyperbolic functions. Under the condition of 6 mM MgCl2 and 5 mM ATP, there was no significant difference in the relative force-velocity relations obtained at full activation compared with those obtained at partial activation when developed force was approximately 40% of its full value. Control experiments showed that a variety of factors did not alter either the relative force-velocity relations or the finding that partial activation did not change these properties. The factors investigated included the decline in force that occurs with each successive contraction of skinned fibers, the segment length (over a range of 1-3 mm), the sarcomere length (over a range of 1.9-2.2 microns), the magnesium ion concentration (26 microM and 1.4 mM were tested), the ATP concentration, the presence of free calcium, and the age of the preparation (up to 30 h). Attempts to repeat earlier experiments by others showing a dependence of shortening velocity on activation were unsuccessful because the low ionic strength used in those experiments caused the fibers to break after a few contractions. The main conclusion, that the shortening velocity is independent of the level of activation, is consistent with the hypothesis that the cross-bridges act independently and that activating calcium acts only as an all-or-none switch for individual cross-bridge attachment sites, and does not otherwise influence the kinetics of cross-bridge movement.

Adenosine Triphosphate↗

PubMed1990

In vitro effects of etomidate on intrinsic myocardial contractility in the rat.

Etomidate is available in two different solvents: propylene glycol for induction of anesthesia and ethanol for maintenance of anesthesia. The direct effect of etomidate (1 and 5 micrograms/ml) and of its solvents on cardiac muscle was studied using rat left ventricular papillary muscle. Etomidate induced a slight positive inotropic effect in both solvents, as shown by an increase in maximum unloaded shortening velocity (Vmax) but not in force. At 0.5 mM Ca++ 5 micrograms/ml etomidate increased Vmax (128 +/- 18%, P less than 0.05) but not force (103 +/- 16%, NS). Using various afterloaded twitches, the peak power output (Emax) was calculated: 1 and 5 micrograms/ml etomidate increased Emax (107 +/- 8%, P less than 0.05, and 108 +/- 10%, P less than 0.05, respectively). This increase was related to the increase in Vmax and not in isometric force. Etomidate did not modify the elastic components of papillary muscle, isometric relaxation, and contraction-relaxation coupling under high load. Several findings suggest that etomidate in propylene glycol impaired the sarcoplasmic reticulum (SR) function: 1) it impaired the isotonic relaxation, the contraction-relaxation coupling under low load, and the load sensitivity of relaxation; and 2) it decreased postrest potentiated contraction, which is highly dependent on the SR. Nevertheless, alteration of SR function was only significant at high [Ca++]o and the beat-to-beat postrest recovery was not modified, indicating that the deleterious effects on SR function were moderate. The isotonic relaxation (max Vr) was more impaired by etomidate in propylene glycol (78 +/- 9%, P less than 0.001) and by propylene glycol alone (69 +/- 9%, P less than 0.001) than by etomidate in ethanol (97 +/- 12%, NS) and by ethanol alone (92 +/- 8%, P less than 0.05). This suggests that propylene glycol was responsible for the decrease in SR function. Etomidate in propylene glycol thus has a dual action on rat myocardium: 1) a slight positive inotropic effect due to etomidate per se, and 2) a slight decrease in SR function probably related to propylene glycol. However, because etomidate in propylene glycol induced a slight decrease in isometric force under certain experimental conditions (i.e., after isometric stabilization), etomidate in propylene glycol may induce a slight negative inotropic effect in some clinical conditions as a result of its dual action on the myocardium.

Animals↗

PubMed1990

Mechanical, energetic, and biochemical changes in long-term pressure overload of rabbit heart.

The mechanical and energetic consequences of long-term pressure-overload (POL) hypertrophy have been investigated in rabbits and compared with sham-operated controls (SOC). Hypertrophy was induced by banding the pulmonary artery of young rabbits and examining the mechanical, biochemical, and energetic properties of the compensated heart 10-16 wk later. Experiments were undertaken on papillary muscles from the hypertrophic hearts. At 27 degrees C and a stimulus frequency of 1 Hz there was a modest depression of peak stress development but no significant changes in isometric rise times and one-half widths or in isotonic maximum velocity of shortening and power output. The inverse relationship between peak stress and cross-sectional area (CSA) was practically identical in the POL and SOC groups. Both polarographic and myothermic investigations were made on papillary muscles. Hypertrophy nearly halved basal metabolism, and in isometric contractions there was increased isometric economy due to a combination of a lower stress cost and a reduced activation heat. Hypertrophy did significantly depress the extent of shortening leading to a reduced work output per beat. In isotonic contractions the reduced work output was offset by a reduced energy output such that there was no significant change in suprabasal mechanical efficiency. Biochemical studies showed that the transition of myosin isoenzymes to the V3 form was essentially complete in the POL group, but that the SOC group was also predominantly V3 when the animals were killed. There was a significant 30% decline in the Ca2(+)-stimulated adenosinetriphosphatase activity of the sarcoplasmic reticulum. It is concluded that in long-term compensated hypertrophy of rabbit hearts there are only a few mechanical and energetic differences between control and hypertrophic muscles. The changes that can be detected appear to predominantly reflect disturbances in cellular Ca2+ regulation.

Animals↗

PubMed1995

Repetitive training of isolated movements improves the outcome of motor rehabilitation of the centrally paretic hand.

The effect of a standardized training on movements of the affected hand has been studied in 27 hemiparetic patients using a multiple baseline approach across individuals. The training consisted of repetitive hand and finger flexions and extensions against various loads and was carried out twice daily during 15-min periods. Grip strength (p < 0.006), peak force of isometric hand extensions (p < 0.05), peak acceleration (p < 0.05) of isotonic hand extensions as well as contraction velocities as indicators of motor performance significantly improved during the training period. In contrast to the standardized training of hand and finger movements, therapeutic strategies following the Bobath concept aim at reducing enhanced muscle tone without reinforcing the activity in centrally paretic distal muscle groups directly. Patients undergoing this treatment approach alone did not experience a significant improvement in the motor capacity of the hand. Therefore, the results of the present study emphasize the importance of frequent movement repetition for the motor rehabilitation of the centrally paretic hand and challenge conventional physiotherapeutic strategies that focus on spasticity reduction instead of early initiation of active movements.

Adult↗

PubMed2002

Effects of halothane and isoflurane on the contraction, relaxation and energetics of rat diaphragmatic muscle.

BACKGROUND: The inotropic effects of halogenated anaesthetics on diaphragmatic muscle remain a matter of debate. Their effects on its relaxation are poorly understood, although diaphragmatic relaxation is recognized as an important physiological process that may interfere with diaphragmatic performance, fatigue and arterial blood flow. METHODS: The effects of halothane and isoflurane (1 or 2 x minimum alveolar concentration [1 or 2 MAC]) on contraction and relaxation of rat diaphragm muscle strips (n = 40) were studied in vitro from force-velocity curves obtained at various loads from isotonic to isometric conditions. From these curves we determined the peak power output and the curvature. Data are mean (SD) percentage of baseline values. RESULTS: At I MAC, isoflurane and halothane induced no significant inotropic and lusitropic effects. At 2 MAC, isoflurane induced a negative inotropic effect (active force, 93(5)% of baseline). Halothane and isoflurane induced a significant decrease in the peak power output at 2 MAC (88(8) and 86(9)% of baseline; P < 0.05), without significant changes in the curvature of the force-velocity curve. At 2 MAC isoflurane under high loads and halothane under low loads induced moderate negative lusitropic effects. CONCLUSION: Halothane and isoflurane induced very moderate inotropic and lusitropic effects, suggesting that the decrease in diaphragm function observed in vivo is not related to a direct effect on diaphragmatic contractility.

Anesthetics, Inhalation↗

PubMed1991

Coincident thresholds in intracellular phosphorylation potential and pH during progressive exercise.

Dynamic changes in intracellular phosphocreatine (PCr), inorganic phosphate (Pi), and pH in human forearm muscle were studied from rest through heavy exercise by means of a ramp exercise protocol and 31P nuclear magnetic resonance spectroscopy. Eighteen healthy volunteers performed an isotonic wrist flexion exercise of repeated contractions at a frequency of 0.5 Hz. Work rate was increased continuously (ramped) at approximately 0.13 W each minute from 0.34 to 1.5 W or until fatigue. Pi/PCr was used as an estimate of the cellular phosphorylation potential of the muscle. Exercise caused a progressive increase in Pi/PCr with an initial slow and later fast component. The transition between these components was distinct and corresponded to the onset of pH decline in all subjects. These changes in Pi/PCr and pH were best fit (P less than 0.05) by a piecewise linear regression model with a break point or threshold. Repeated ramp testing of six subjects showed that the threshold was reproducible (r = 0.98). The results of this study demonstrate the existence of an intracellular metabolic threshold and suggest that indirect threshold measures (lactate and ventilatory thresholds) may reflect events at the cellular level.

Adult↗

PubMed1985

[Dynamics of efferent regulation of muscle contraction. Determination of transition processes: external load--muscle length].

Transition processes of the length changes were studied in the ankle extensors of anesthetized cats in case of abrupt transition from isometric to isotonic condition of muscle contraction which was evoked by distributed stimulation of efferents supplied these muscles. Dynamic properties of muscles were analyzed in the framework of the two-components (inertialess and aperiodic) approximation of the transition processes. These properties were nonlinear because of the complex dependence of time constants and weight of an aperiodic component on the amplitudes of input signals. Both of these parameters were significantly lower in the shortening processes comparing with the lengthening ones. When amplitudes of length changes were small, the average limit values of the time constants were 1.38s for slow soleus muscle shortening and 0.39s for its lengthening. The same parameters for fast muscles (gastrocnemius and plantaris) were 0.55 and 0.25s, respectively. The observed dynamical properties of muscle contraction are discussed for their functional significance.

Animals↗

PubMedDate not supplied

Why does the cardiac force-velocity relationship not follow a Hill hyperbola? Possible implications of feedback loops involved in cardiac excitation-contraction coupling.

1. If cardiac force-velocity (PV) relationships are determined in a rhythmically beating isolated preparation by progressively changing afterloads, the PV curves obtained exhibit a characteristic nonhyperbolic shape with an upward convexity in the force domain close to P0. It was suggested by others that this might be due to contractile element (CE) shortening against series elastic component (SEC) and/or the early decay of the active state at isometric peak tension. 2. The present investigation performed in isolated cat papillary muscles demonstrates that neither CE shortening against SEC nor the variation of the intensity of the active state can fully account for the nonhyperbolic shape of cardiac PV curves. 3. On the other hand large parts of the remaining deviation could be eliminated if the "progressive loading sequence" (PLS) was replaced for an "interpolated loading sequence" (ILS). Here, the muscle contracts under steady state conditions either isometrically or isotonically (zero load). Single afterloaded test contractions are interpolated after every 10th beat. 4. It is concluded that the nonhyperbolic shape of cardiac PV curves, determined by progressively changing afterloads, is the expression of at least four factors: (i) the existence of displacement dependent variation of excitation-contraction coupling (including the contraction-excitation-contraction recoupling loop), (ii) CE shortening against non CE bound SEC, (iii) a variable active state and (iiii) a fourth unknown mechanism.

Animals↗

PubMed2002

Load-dependent induction of apoptosis in multicellular myocardial preparations.

Increased mechanical load has been proposed as an inductor of apoptosis, but it is unknown whether this can occur in the range of pre- and afterloads that prevail in the beating heart. We investigated apoptosis in cultured rabbit multicellular myocardial preparations over several days. Muscles contracted in absence of pre- and afterload (unloaded isotonic), in absence of preload but in presence of afterload (unloaded isometric), or in presence of both (loaded isometric). After up to 48 h of continuous contractions, apoptosis was assessed by TdT-mediated nick-end labeling (TUNEL) assay and DNA ladder analysis. In muscles that contracted loaded isometric, apoptosis was detected after 6-24 h. After 48 h, apoptosis was most prominent in this group, reflected by a high level of DNA ladder intensity (DLI; 27.8 +/- 11.5), whereas Bcl-xL (on RNA level) was significantly downregulated, and Fas remained unchanged. In unloaded isometric preparations, apoptosis was significantly less (6.9 +/- 5.9 DLI) and very similar to those contracting unloaded isotonic (6.1 +/- 5.1 DLI). We conclude that load-dependent apoptosis can occur at sarcomere lengths achievable in vivo and may mainly result from increased preload.

Animals↗

PubMed1998

The isometric force that induces maximal surface muscle deoxygenation.

To determine the external force that induces maximal deoxygenation of brachioradialis muscle 32 trained male subjects maintained isometric contractions using the elbow flexor muscles up to the limit time (isotonic part of the isometric contraction, IIC) and beyond that time for 120 s (anisotonic part of the isometric contraction). During IIC each subject maintained relative forces of either 25% and 70% maximal voluntary contraction (MVC), 50% and 100% MVC, or 40% and 60% MVC. Muscle oxygenation was assessed using a near infrared spectroscope, and expressed as a percentage of the reference value (deltaO2rest) which was the difference between the minimal oxygenation obtained after 6 min of ischaemia at rest and the maximal reoxygenation following the release of the tourniquet. During IIC at 25% MVC, muscle oxygenation decreased to 17 (SEM 3)% deltaO2rest, then it levelled off [25 (SEM 1)% deltaO2rest]. After the point at which target force could not be maintained, reoxygenation was very weak. During IIC at 40%, 50%, 60%, and 70% MVC, the lowest muscle oxygenation values were obtained after 15-20 s of contraction and corresponded to -18 (SEM 6), -59 (SEM 12) -31 (SEM 6), and -29 (SEM 6)% deltaO2rest, respectively. For the contraction at 100% MVC, the lowest oxygenation [-19 (SEM 9)% deltaO2rest] was obtained while force was decreasing (69% MVC). During the anisotonic part of the isometric contractions, the greatest reoxygenation rate was obtained after 50% MVC IIC (P < 0.001). Our results showed that during isometric elbow flexions between 25% and 100% MVC, there was no linear relationship between external force and muscle oxygenation, and that the maximal deoxygenation of the brachioradialis muscle was obtained at 50% MVC.

Adult↗

PubMedDate not supplied

Quantitative end qualitative analysis of the electrical activity of rectus abdominis muscle portions.

The purpose of this study was to investigate the electrical behavior pattern of the Rectus abdominis muscle by qualitative and quantitative analysis of the electromyographic signal obtained from its superior, medium and inferior portions during dynamic and static activities. Ten voluntaries (aged X = 17.8 years, SD = 1.6) athletic males were studied without history of muscle skeletal disfunction. For the quantitative analysis the RMS (Root Mean Square) values obtained in the electromyographic signal during the isometric exercises were normalized and expressed in maximum voluntary isometric contraction percentages. For the qualitative analysis of the dynamic activity the electromyographic signal was processed by full-wave rectification, linear envelope and normalization (amplitude and time), so that the resulting curve of the processed signal was submitted to descriptive graphic analysis. The results of the quantitative study show that there is not a statistically significant difference among the portions of the muscle. Qualitative analysis demonstrated two aspects: the presence of a common activation electric pattern in the portions of Rectus abdominis muscle and the absence of significant difference in the inclination angles in the electrical activity curve during the isotonic exercises.

Adolescent↗

PubMed1980

Inhibitory action of lanthanum(La) on PGF2 alpha - induced contraction of guinea-pig stomach muscle.

The effects of lanthanum(La) on contractions induced by prostaglandin F2 alpha(PGF2 alpha) or isotonic K+ were investigated in the isolated stomach muscle of guinea-pig. Low concentrations of La( 0.1-1 microM) inhibited the contraction to PGF2 alpha 1 microM in a dose-dependent manner, without affecting the tonic contraction to isotonic K+. 0.1 and 1 microM La shifted the dose-response curve for PGF2 alpha( 0.001 - 1 microM) to the right and reduced the maximum response. The IC50 of La against PGF2 alpha and K+ were 0.6 microM and 30 microM, respectively. These results support the suggestion that PGF2 alpha -induced contraction in the stomach muscle depends mainly on the intracellular release of sequestered Ca, which would be depleted or immobilized by La.

Animals↗

PubMed1959

Paramyosin and contraction of catch muscles.

The isotonic shortening of glycerol-extracted preparations of molluscan catch muscles is inhibited at pH's and ionic strengths at which extracted paramyosin crystallizes. The isometric tension development is hardly altered under the same conditions. Consequently, the "catch mechanism" is explained on the basis of the crystallization of paramyosin.

Animals↗

PubMed1980

[Dynamic characteristics of motor reactions induced by motor cortex stimulation of unanesthetized cats].

Temporal characteristics of muscle contraction evoked by bipolar macrostimulation of cat motor cortex were studied in chronic experiments. Cortically induced flexion of the contralateral hindlimb was recorded isometrically and in some cases isotonically. Latency of muscle contraction exceeded 100 ms when low frequency (100-150/s) stimulation was applied. An increase in the stimulation frequency and intensity led to the shortening of the latency: under "optimal" parameters of the stimulation the minimal values were 30-35 ms. Time constant of the contraction increment was 50-80 ms whereas that of the contraction decrement after stimulation cessation reached 150-300 ms. The results obtained are considered from the view point of automatic regulation theory.

Animals↗

PubMed1975

Isokinetic vs isotonic strength training in adult men.

The differences in changes in strength, body composition, anthropometric measurements and selected motor performance tasks, between groups, trained isotonically and isokinetically were compared. Thirty-six male volunteers were randomly assigned to one of four groups: isotonic; isokinetic low speed contraction; isokinetic high speed contraction; and control. Strength training was conducted three days per week, 40 minutes per day for eight weeks. The results demonstrated a clear superiority of the isokinetic training procedures over the isotonic procedures relative to strength, antrhopometric measures and motor performance tasks. The three training groups exhibited similar changes in body composition. The isokinetic high speed group demonstrated the greatest gains overall. The significance of these results is discussed.

Adult↗

PubMed1985

Myosin phosphorylation and contraction of feline esophageal smooth muscle.

We tested the hypothesis that phosphorylation of the 20,000-Da light chain of myosin (LC 20) is related to mechanical activation of esophageal smooth muscle. Circular muscle layer strips of cat esophagus were taken from the lower esophageal sphincter (LES) and the distal esophageal body (EB). The LES strips developed tone spontaneously, and the EB strips were tonically contracted with carbachol. Both tissues relaxed in response to electrical-field stimulation. Phosphorylation of the LC 20 was determined in tissues quick-frozen during relaxation and during stress redevelopment after cessation of field stimulation. Stress and phosphorylation levels were low after 30 s of field stimulation, and a rapid contraction followed field stimulation. Phosphorylation in the LES increased from 0.043 +/- 0.029 to 0.328 +/- 0.043 mol Pi/mol LC 20 within 10 s after stimulation of the inhibitory nerves was terminated, while stress was still rising rapidly. Phosphorylation in the LES then declined to a steady-state value of 0.162 +/- 0.034 mol Pi/mol LC 20 after 10 min. Isotonic shortening velocities at a constant afterload following a quick release showed changes with time that were proportional to the level of phosphorylation. This was also true for values of maximal shortening velocity estimated for zero external load and for the rate of stress redevelopment after a step shortening. Comparable measurements were made in the carbachol-contracted EB. These results indicate that visceral smooth muscles, which normally function tonically (LES) or phasically (EB), exhibit an initial rapid mechanical activation associated with myosin phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Tetanic contraction in vocal fold muscle.

Active properties of canine vocalis muscle tissue were investigated through a series of experiments conducted in vitro. Samples of the vocalis muscle were dissected from dog larynges excised a few minutes before death and kept in Krebs solution at a temperature of 37 +/- 1 degree C and a pH of 7.4 +/- 0.05. Isometric and isotonic tetanic responses of the vocalis muscle were obtained electronically with a Dual Servo System (ergometer). Isometric tension was recorded at various levels of elongation and stimulation rate. Isotonic shortening was recorded at various levels of force, and shortening velocity was obtained by numerical analysis of recorded data. It was found that fused tetanus occurred at stimulation rates of about 90 Hz, where the isometric tetanic force saturates. Repeated stimulation of the muscle in vitro not only caused nonrecoverable fatigue in the tissue, but also decreased its passive tension. The combined active and passive isometric tension increased with elongation of the muscle. Results of isometric active responses were normalized with respect to average passive response. This normalization allowed for better comparison between tetanic contraction and twitch contraction. It was found that maximum tetanic contraction was 6.4 times greater than maximum twitch contraction obtained in a previous study. A tetanic contraction period was defined and investigated for eight samples of vocalis muscle tissue from different dogs. The tetanic contraction period showed a linear increasing trend with strain.

Animals↗

PubMed2026

Comparing the Effects of Push and Hold Isometric Training on Strength and Musculotendinous Adaptations: A Within-Subject Randomized Controlled Trial.

Lum, D, Oranchuk, DJ, Chen, SE, and Kong, PW. Comparing the effects of push and hold isometric training on strength and musculotendinous adaptations: A within-subject randomized controlled trial. J Strength Cond Res 40(9): 1050-1058, 2026-Despite recent interest in delineating pushing (PIMA) and holding (HIMA) isometric muscle actions, training-induced adaptations have not been examined. As such, we compared the strength and morphological adaptations between PIMA and HIMA training. Twenty limbs across 10 adults (5 men, 5 women, age: 31.4 ± 6.5 years) were randomly assigned to PIMA or HIMA conditions and underwent 12 training sessions over 6 weeks. PIMA required subjects to exert force against a fixed lever while the HIMA limb maintained a set joint angle while resisting an isotonic lever. During each contraction, subjects had to exert force at 70% of maximal voluntary contraction for 20 seconds at an 80° knee angle, for 4-6 repetitions. Subjects completed isometric, concentric, and eccentric knee extension assessments for both limbs before and after the intervention. Pre- and postintervention ultrasound scans were performed to determine quadriceps muscle architecture and patellar tendon thickness. Increased isometric torque was found following both conditions (p < 0.05, g = 0.30-0.36), while concentric (p < 0.05, g = 0.31) and eccentric (p < 0.01, g = 0.50) torque only increased following PIMA. Both conditions increased muscle thickness (p < 0.05, g = 0.27-0.88) and vastus lateralis fascicle length (p < 0.05, g = 0.20-0.31). Only HIMA increased patellar tendon thickness (p < 0.05, g = 0.12). The increase in total quadriceps (p < 0.05, g = 0.46) and especially rectus femoris (p < 0.05, g = 1.44) thickness was greater in HIMA. The results suggest that PIMA may be more effective at improving strength, while HIMA may be superior for inducing morphological adaptations. Larger sample sizes and more ecologically valid training programs are warranted to further elucidate potential differences between different isometric types.

Humans↗