Tension Decay Research Articles

Theoretical Analysis of Tension and Pressure Decay of a Tubular Elastic Fabric

This paper attempts to theoretically analyze the tension and pressure decay of a single layer tubular elastic fabric used for pressure garments. In the theoretical analysis, we assume that the exponential decay model of the fabric tube on pressure and tension is used and supported by the experiment. We also reveal that in the experiment, the relative rates of pressure change and tension change of a fabric tube are the same no matter which reduction percent is used for making up the pressure garment. The model in this study could assist medical practitioners in providing effective clinical treatment with pressure garments for patients.

An asymmetry in the phosphate dependence of tension transients induced by length perturbation in mammalian (rabbit psoas) muscle fibres.

The effects of inorganic phosphate (P(i), a product released during ATP hydrolysis in active muscle) on tension transients induced by length perturbation (approximately 0.3 ms) were examined in chemically skinned (0.5 % Brij), maximally Ca(2+)-activated rabbit psoas muscle fibres at 10 degrees C (ionic strength 200 mM, pH 7.1). In one type of experiment, the tension transients induced by length release and stretch of a standard amplitude (0.4-0.5 % of L(o), muscle fibre length) were examined at a range of added [P(i)] (range 3-25 mM). The steady active tension was depressed approximately 45 % with 25 mM added P(i). The initial tension recovery (from T(1), extreme tension reached after length step, to T(2), tension after quick recovery) was analysed by half-time measurement and also by exponential curve fitting - extracting a fast (phase 2a) and a slow (phase 2b) component. The tension decay after a stretch became faster with increased [P(i)], whereas the quick tension rise induced by a length release was insensitive to added P(i). Consequently, the asymmetry in the speed of tension recovery from stretch and release was reduced at high [P(i)]. A plot of the phase 2b rate (or 1/half-time) of tension decay after stretch versus [P(i)] was approximately hyperbolic and showed saturation at higher [P(i)] levels. In a second type of experiment, the tension transients induced by length steps of different amplitudes were examined in control (no added P(i)) and in the presence of 25 mM added P(i). Over a range of length step amplitudes (up to 1 % L(0)), the tension decay after stretch was consistently faster in the presence of P(i) than in the control; this was particularly pronounced in phase 2b. The rate of tension rise after length release remained high but similar in the presence and absence of added P(i). These observations indicate that a stretch and release perturb different molecular steps in the crossbridge cycle. The P(i) sensitivity of tension decay (phase 2b) after stretch is similar to that seen using other perturbations (e.g. [P(i)] jumps, hydrostatic pressure jumps and temperature jumps and sinusoidal length oscillations). The results indicate that the P(i)-sensitive force generation identified in previous studies is strain sensitive (as expected), but it is seen only with respect to positive strain (stretches).

Relaxin Depresses Small Bowel Motility Through a Nitric Oxide-Mediated Mechanism. Studies in Mice1

Gastrointestinal motility is reduced and the incidence of functional gastrointestinal disorders is increased in pregnancy, possibly due to hormonal influences. This study aims to clarify whether the hormone relaxin, which attains high circulating levels during pregnancy and has a nitric oxide-mediated relaxant action on vascular and uterine smooth muscle, also reduces bowel motility and, if it does, whether nitric oxide is involved. Female mice in proestrous or estrous were treated for 18 h with relaxin (1 microg s.c.) or vehicle (controls). Isolated ileal preparations from both groups were used to record contractile activity, either basal or after acute administration of relaxin (5 x 10(-8) M). Drugs inhibiting nitric oxide biosynthesis or neurotransmission were used in combination with relaxin. Expression of nitric oxide synthase isoforms by the ileum was assessed by immunocytochemistry and Western blot analysis. Relaxin caused a clear-cut decay of muscle tension and a reduction in amplitude of spontaneous contractions upon either chronic administration to mice or acute addition to isolated ileal preparations. These effects were significantly blunted by N(G)-nitro-L-arginine, but not by the neural blockers we used. Moreover, relaxin increased the expression of nitric oxide synthases II and III, but not synthase I. Relaxin markedly inhibits ileal motility in mice by exerting a direct action on smooth muscle through the activation of intrinsic nitric oxide biosynthesis.

A New Technique for the Measurement of the Dynamic Evolution of Surface Tension

We develop a novel technique which deduces the surface tension in air of a fluid as a function of surface age, beginning at age zero. The technique utilizes pointwise measurements of perpendicular free surface profiles of a steady oscillating jet corresponding to a discretization interval on the order of 0.1 ms. We implement the technique on constant-surface-tension test fluids (100% ethanol and 15% ethanol/85% water by volume) to demonstrate the extent to which the technique can qualitatively capture that the surface tensions of these fluids are constant in time, and quantitatively produce values of these constants consistent with static measurements. We then implement the technique on jets of two agricultural surfactant mixtures, Triton X-405 and Triton X-100, and quantitatively deduce the decay of surface tension as a function of surfactant concentration.

Energy transfer during stress relaxation of contracting frog muscle fibres.

1. A contracting muscle resists stretching with a force greater than the force it can exert at a constant length, T(o). If the muscle is kept active at the stretched length, the excess tension disappears, at first rapidly and then more slowly (stress relaxation). The present study is concerned with the first, fast tension decay. In particular, it is still debated if and to what extent the fast tension decay after a ramp stretch involves a conservation of the elastic energy stored during stretching into cross-bridge states of higher chemical energy. 2. Single muscle fibres of Rana temporaria and Rana esculenta were subjected to a short ramp stretch (approximately 15 nm per half-sarcomere at either 1.4 or 0.04 sarcomere lengths s(-1)) on the plateau of the force-length relation at temperatures of 4 and 14 degrees C. Immediately after the end of the stretch, or after discrete time intervals of fixed-end contraction and stress relaxation at the stretched length (Delta t(isom) = 0.5-300 ms), the fibre was released against a force ~T(o). Fibre and sarcomere stiffness during the elastic recoil to T(o) (phase 1) and the subsequent transient shortening against T(o) (phase 2), which is expression of the work enhancement by stretch, were measured after different Delta t(isom) and compared with the corresponding fast tension decay during Delta t(isom). 3. The amplitude of fast tension decay is large after the fast stretch, and small or nil after the slow stretch. Two exponential terms are necessary to fit the fast tension decay after the fast stretch at 4 degrees C, whereas one is sufficient in the other cases. The rate constant of the dominant exponential term (0.1-0.2 ms(-1) at 4 degrees C) increases with temperature with a temperature coefficient (Q(10)) of approximately 3. 4. After fast stretch, the fast tension decay during Delta t(isom) is accompanied in both species and at both temperatures by a corresponding increase in the amplitude of phase 2 shortening against T(o) after Delta t(isom): a maximum of approximately 5 nm per half-sarcomere is attained when the fast tension decay is almost complete, i.e. 30 ms after the stretch at 4 degrees C and 10 ms after the stretch at 14 degrees C. After slow stretch, when fast tension decay is small or nil, the increase in phase 2 shortening is negligible. 5. The increase in phase 2 work during fast tension decay (Delta W(out)) is a constant fraction of the elastic energy simultaneously set free by the recoil of the undamped elastic elements. 6. Delta W(out) is accompanied by a decrease in stiffness, indicating that it is not due to a greater number of cross-bridges. 7. It is concluded that, during the fast tension decay following a fast ramp stretch, a transfer of energy occurs from the undamped elastic elements to damped elements within the sarcomeres by a temperature-dependent mechanism with a dominant rate constant consistent with the theory proposed by A. F. Huxley and R. M. Simmons in 1971.

Calcium handling and sarcoplasmic-reticular protein functions during heart-failure transition in ventricular myocardium from rats with hypertension

The objective of this study was to determine the primary event that occurs in Ca 2+-regulatory sarcoplasmic-reticular (SR) proteins during subacute transition from concentric/mechanically-compensated left ventricular (LV) hypertrophy to eccentric/decompensated hypertrophy. Using Dahl salt-sensitive rats with hypertension, changes of myocardial contraction, intracellular Ca 2+ transients, SR Ca 2+ uptake, protein levels of SR Ca 2+ ATPase (SERCA2), phospholamban, and calsequestrin (CSQ), and mRNA levels of SERCA2 and CSQ were serially determined and compared between the established stage of LV hypertrophy (LVH) and the subsequent stage of overt LV dysfunction (CHF). In LVH, isolated LV papillary muscle preparations showed an equal peak-tension level and a mild prolongation of the isometric tension decay compared to those of age-matched controls. The Ca 2+ transients as measured by aequorin were unchanged. The Ca 2+ uptake of isolated SR vesicles and the protein/mRNA levels of SR proteins were also equivalent to those of the controls. In contrast, in CHF, the failing myocardium showed a further prolongation of the contraction time course and a 39% reduction of the peak-tension development. The Ca 2+ transients showed changes consisting of a decrease in the peak level and a prolongation of the time course. In addition, the SR Ca 2+ uptake was decreased by 41%. Despite these functional changes, the protein and mRNA levels of the SR components remained equivalent to those of the age-matched controls. Thus, in this hypertensive animal, 1) at the LVH stage, myocardial contractility and intracellular capability to regulate Ca 2+ remained normal; 2) at the CHF stage, impaired SR Ca 2+ handling and the subsequent reduction of myocardial contraction were in progress; and 3) impairments of SR function occurred at the post-translational protein level rather than at the transcriptional/translational levels. Our findings support the role of SR proteins as the primary determinant of the contractile dysfunction that occurs during the heart-failure transition; however, post-translational modulators of these SR elements may also be critical.

Percutaneous Embolization for Erectile Dysfunction due to Venous Leakage: Prognostic Factors for a Good Therapeutic Result

Objectives: To identify prognostic parameters for patient outcome after embolization for erectile dysfunction (ED) due to venous leakage (VL). Methods: 23 patients presenting with ED due to pure venous leakage were selected. Intracavernous pharmacological testing, Doppler ultrasound, dynamic cavernosography–cavernosometry and cavernous oxygen tension were done. All patients underwent embolization of the leakage areas visualized. The results were assessed in terms of the response: good, partial or absent. Mean values were calculated for diastolic flow, oxygen tension, maintenance flow and pressure decay, and were compared between the group of patients who responded and the group which did not respond using Student’s t test. Results: 26% had a good response, with a mean follow–up of 22 months, if we include partial response and adjuvant therapy this rate rises to 44%. A statistically significant relationship was found between the intracavernous oxygen tension and maintenance flow with a good response to treatment; patients with cavernosography types I and II showed a better response to treatment than those with cavernosography type III. Conclusions: Maintenance flow, intracavernous oxygen tension, and cavernosography types can be prognostic factors of success in the treatment of ED due to pure VL with embolization.

Changes in interfacial behaviour, emulsifying and foaming properties of faba bean legumin after modification with dimethylsuberimidate

The effect of a rising rigidity and surface hydrophobicity of the 11S storage protein from faba beans--legumin--induced by chemical modification with dimethylsuberimidate (DMS) on some surface functional properties was studied. Short-time adsorption kinetics using a droplet-volume tensiometer, pressure transformation and desorption behaviour of monolayer using a film balance, and emulsifying and foaming properties were determined to characterize surface activity and interfacial film forming behaviour. Tensio-active properties at the air-water interface, i.e. decay in surface tension and pressure transformation in monolayer, were improved by modification. However, a decrease in emulsifying activity, foam capacity and foam expansion after modification of the legumin points to an overall deterioration of energy-induced film forming behaviour. The results support the view that surface activity is generally governed more by molecular flexibility than by surface hydrophobicity.

Counter Effects of Higenamine and Coryneine, Components of Aconite Root, on Acetylcholine Release from Motor Nerve Terminal in Mice

The counter effects of higenamine and coryneine, components of aconite root, on acetylcholine (ACh) release from motor nerve terminals in the mouse phrenic nerve–diaphragm muscle preparation were studied by a radioisotope method. Both nerve-evoked release and spontaneous release of [3H]-ACh from the preparation preloaded with [3H]-choline were measured. The change in the tetanic tension of muscle was simultaneously recorded in the same preparation. Higenamine (10 μM) augmented both the nerve-evoked and spontaneous ACh releases, and the muscle tension. The effects were inhibited by pretreatment with propranolol (10 μM), a β-adrenoceptor antagonist. Coryneine reduced the nerve-evoked release of ACh, accelerated the decay of tetanic tension (tetanic fade) at 30 μM, and it depressed the peak amplitude of tetanic tension at a higher concentration of 100 μM. These results suggest that of the two components contained in aconite root, higenamine increases ACh release via activation of β-adrenoceptor, and conversely coryneine depresses ACh release by preferentially acting at motor nerve terminal.

Effect of Loading Path on Mechanical Response of a Glass Fabric Composite at Low Cyclic Fatigue under Tension/Torsion Biaxial Loading

The present paper shows how loading path affects the low cyclic fatigue failure of a glass fabric/vinylester resin composite. The focus is placed on the stress-strain response both in tension and shear as well as on the fatigue life under tension/torsion biaxial loading at low cycle fatigue up to 10,000 cycles. Three types of loading paths were applied: Type I-Biaxial loads were applied proportionally to the specimens. Namely the resultant stresses, tensile and shear stresses increased simultaneously. Type II-A tensile load was first applied. Then, a torsion moment was applied to attain a (biaxial stress ratio) = 3/1 while the tensile load was kept constant. The applied loads were removed in the reverse order. Type Il I-Contrary to Type II loading, first a torsion moment was applied. Then, a tensile load was applied to attain a L = 3/1 while the torsional moment was kept constant. The maximum stress states in the laminate during each loading period are the same for all loading cases. The experimental results revealed that: (1) No significant effect of loading path on the fatigue life exists. (2) The modulus decay in shear with respect to the number of loading cycles is affected by loading path while the modulus decay in tension was not significantly affected. (3) There exist peculiar responses in stress-strain relations both in tension and shear when loading path is changed although the final strain state is almost the same in all loading cases as long as the internal damage is not much accumulated. A model for explaining the peculiar stress-strain behaviors was proposed based on the Continuum Damage Mechanics (CDM) in conjunction with the lamination theory. The calculated stress-strain responses reasonably agree with the experimental stress-strain behaviors under biaxial loadings having different loading paths.

Spreading Dynamics of Surfactant Solutions

The spreading behavior of microdrops of surfactant solutions at solid surfaces has been studied. The influence on the spreading of different factors, such as the drop lifetime prior to surface contact, surface tension dynamics, surface energy, and surfactant properties, was systematically investigated. The results obtained suggest the existence of two spreading regimes exhibiting different spreading characteristics: In the first, nondiffusive regime, the spreading is very rapid and controlled to different extents by inertia, gravity, and capillarity, depending on the drop size, impact energy, and interfacial tension balance. It is shown in this study that the initial drop surface tension, which is set by the surface tension decay rate and the drop lifetime prior to the surface impact, strongly influences the maximum spreading distance in the nondiffusive spreading regime. The second, diffusion-controlled regime, is characterized by slower concentration-dependent spreading rates. The spreading rate is, here, mainly controlled by the diffusive transport of surfactant to the expanding liquid−vapor interface. In this regime, the drop base radius exhibits an approximate rb2 ∝ t dependence on time. The spreading kinetics at hydrophobic surfaces has been discussed in the framework of a simplified theory. Depending on the assumptions regarding the drop shape, rb5/2 ∝ t to rb2 ∝ t spreading laws are obtained. This agrees reasonably with the experimentally observed relationship.

The Effect of Dynamic Surface Tension on the Oscillation of Slender Elliptical Newtonian Jets

We investigate free-surface oscillating jets with elliptical cross section, focusing on behavior associated with decaying surface tension. Previous one-dimensional equations for an oscillating jet are extended to allow variable surface tension on short space and time scales relevant for surfactant mixtures. We presume the decay of surface tension as a function of surface age, and derive the resulting jet behavior. Three plausible forms of decay are studied: an exponential decay, a diffusion model derived in Brazee et al. (1994), and an algebraic form due to Hua and Rosen (1991). Our simulations suggest both experimental regimes, and measurable jet features in these regimes, which may be exploited in an inverse formulation to deduce the unknown rapid surface tension decay of a given surfactant mixture. In particular, we establish numerical relationships between the amplitude and the wavelength of either a sustained far-field oscillation or oscillation at a fixed downstream location and the entire history of surface tension decay. These numerical relationships are ideal for the inverse formulation, in that the complete surface tension evolution may be deduced solely from far-field or downstream jet measurements, away from the confined part of the jet where the surface tension is rapidly changing.

Surface activity properties of cysteine-substituted C-terminal melittin analogues

In order to extend our knowledge of factors important in the surface activity of melittin, cysteine was substituted for lysine-21 and lysine-21/glutamine-25 in a pair of synthetic peptide analogues. The first of these changes resulted in only modest effects on secondary structure (determined in 50% trifluoroethanol), emulsification and surface tension properties. Introduction of a second cysteine greatly reduced both the rate of surface tension decay and the equilibrium surface tension attained, although secondary structure (determined in 50% trifluoroethanol) was only slightly affected by this modification. This latter peptide completely lacked emulsification and haemolytic properties and was found to oligomerise readily due to the formation of intermolecular, disalphide bridges. These results indicate that oligomerisation abolishes surface activity in melittin.

Effect of inotropic interventions on contraction and Ca2+ transients in the human heart.

The present study investigated the influences of inotropic intervention on the intracellular Ca2+ transient (intracellular Ca2+ concentration ([Ca2+]i)) and contractile twitch. Isometric twitch and [Ca2+]i (fura 2 ratio method) were measured simultaneously (1 Hz, 37 degrees C) after stimulation with Ca2+ (0.9-3.2 mM), the cardiac glycoside ouabain (Oua; 0.1 microM), the beta1- and beta2-adrenoceptor-agonist isoprenaline (Iso; 1-10 nM), and the Ca2+ sensitizer EMD-57033 (30 microM) by using isolated human nonfailing right auricular trabeculae (n = 19). Inotropic interventions increased force of contraction and peak rate of tension rise (+T) significantly. Only Iso stimulated peak rate of tension decay (-T) higher than +T (P < 0.05), thereby reducing time of contraction (Ttwitch). EMD-57033 increased +T more effectively than -T and prolonged Ttwitch (P < 0.05). Ca2+, Oua, and Iso, but not EMD-57033, increased systolic Ca2+. Diastolic Ca2+ increased after stimulation with Oua or Ca2+, but not in the presence of EMD-57033. Iso shortened the Ca2+ transient and did not influence diastolic Ca2+. In conclusion, positive inotropic agents differently affect force and [Ca2+]i depending on their mode of action. Inotropic interventions influence diastolic Ca2+ and thus may be less advantageous in a situation with altered intracellular Ca2+ homeostasis (e.g., heart failure due to dilated cardiomyopathy).

Relaxation is impaired in the diaphragm muscle of the cardiomyopathic Syrian hamster.

Relaxation was examined in diaphragm muscle strips of cardiomyopathic Syrian hamsters and control hamsters. Isotonic lengthening velocity and isometric tension decay were analyzed over the load continuum in response to twitch. For each load level (P), we measured the maximum extent of shortening (deltaL), the peak lengthening velocity (VL), and the peak rate of tension decline (-dP/dtmax). The kinetics of sarcomere length (SL) were simultaneously measured by laser diffraction. In an attempt to account for the influence of shortening and/or load on relaxation, we calculated the slopes of the VL - deltaL and -dP/dtmax - P relationships. In both groups, there was a direct relationship between (1) VL and deltaL and (2) -dP/dtmax and P. In myopathic hamsters, we observed a decrease in the slope of the VL - deltaL relationship (p < 0.05), a decrease in VL at any common value of both muscle and sarcomere extent of shortening, and an increase in the duration of overall lengthening. Isometric tension decay was significantly prolonged in myopathic muscle strips, while the -dP/dtmax - P relationship was not significantly different than in controls. At low-to-medium loads, SLs at the onset of tension decline were greater in myopathy. These data indicate that relaxation of the diaphragm was both slowed and prolonged in myopathic Syrian hamsters.

The relationship between tension and slowly varying intracellular calcium concentration in intact frog skeletal muscle.

1. The relationship between intracellular calcium concentration, [Ca2+]i, and fixed-end tension was investigated in intact single muscle fibres from frogs. A slow decline of tension was produced by cyclopiazonic acid (CPA), a sarcoplasmic reticulum Ca2+ uptake pump inhibitor. The fluorescent dyes fura-2 and furaptra (mag-fura-2) were used to estimate [Ca2+]i. 2. Neither the steepness nor the position of the curve changed consistently over a wide range of tension decay times from a few seconds to over 100 s. For these near-steady-state curves, the 10-90% tension change occurred, on average, in 0.07 pCa units, corresponding to a Hill coefficient > 25, much steeper than previously reported. Possible artifacts could reduce that to 15. 3. Methoxyverapamil (D600) reduces the calcium released in response to an action potential. Contractions with D600 and CPA had a slow rise composed of many small steps, and a slow fall. Comparing rise and fall showed little or no hysteresis in the tension-[Ca2+]i relationship. 4. A model involving co-operativity between the binding of Ca2+ and myosin to thin filaments is shown to produce a tension-pCa relationship that is substantially altered by the mean rate constant for detachment of myosin cross-bridges, which in turn is likely to be affected by sarcomere movements. 5. Such a model is shown to be capable of reproducing the small rise in [Ca2+]i previously reported during the late phase of fixed-end relaxation of intact fibres.

Influence of sex, diabetes and ethanol on intrinsic contractile performance of isolated rat myocardium.

The influence of sex on intrinsic contractile performance, diabetes-induced myocardial mechanical dysfunction and the inotropic response to clinically relevant concentrations of ethanol (ETOH) was studied using weight-matched streptozotocin-induced diabetic rats. After 8 weeks, isolated left-ventricular papillary muscles stimulated at 0.5 Hz, were studied under isometric conditions at Lmax. Peak developed tension, time-to-peak tension (TPT), time-to-90% relaxation (RT90), and the maximum velocities of tension development and rate of tension decay were assessed at baseline and in response to changes in stimulation frequency and varying extracellular calcium concentrations. In male but not in female rats, body weight and heart size were significantly correlated with glycemic status. In both sexes, diabetes was associated with prolongation of baseline TPT and RT90 values. However, diabetes-induced prolongations of contraction and relaxation duration were greater in papillary muscles obtained from male than in those from female animals. The negative staircase effect of increasing the frequency of stimulation was not influenced by sex or the diabetic state. Similarly, neither the positive, inotropic effect of increasing extracellular calcium nor the negative inotropic effect of ethanol was modified by sex or experimental diabetes. Our results suggest that: 1) myocardium from female rats is resistant to diabetes-induced myocardial dysfunction and 2) neither baseline developed tension, calcium-mediated nor ethanol-induced inotropic responsiveness are influenced by sex or experimental diabetes.

Sarcomere relaxation in hamster diaphragm muscle.

We characterized instantaneous sarcomere relaxation over the load continuum in isolated hamster diaphragm muscles by means of laser diffraction. In afterloaded twitches, sarcomere relaxation displayed two consecutive phases. The bulk of sarcomere lengthening occurred during the first phase and corresponded in time to muscle lengthening. The second phase of sarcomere relaxation was slower and corresponded in time to tension decay. At initial muscle length, the peak velocity of sarcomere lengthening (SVL) was linearly related to both the maximum extent of sarcomere shortening (delta SL) and sarcomere length at peak shortening (SLmin; each P < 0.01). Varying preload modified the SVL vs. SLmin relationship but not the SVL vs. delta SL relationship. At a given preload, muscle tension decay began at a similar sarcomere length, regardless of the afterload level. In conclusion, our results support the role played by sarcomere length in regulating the diaphragm muscle-lengthening rate but not the rate of tension decline.

Acute and chronic effects of ethanol on papillary muscles from spontaneously hypertensive rats

The effects of chronic ethanol ingestion (12 weeks) on the mechanical properties of hypertrophied papillary muscle and the in vitro effects of ethanol (80–640 mg/dl) was studied. Papillary muscles from spontaneously hypertensive rats (SHRs) and their normotensive controls, the Wistar-Kyoto rat (WKY), were used in this study. Peak- developed tension was significantly less in muscles obtained from SHR compared with WKY even when normalized for muscle cross-sectional area. Chronic ethanol ingestion resulted in a significant shortening of both contraction and relaxation duration in muscles from SHR and WKY. In muscles from SHR and WKY, acute in vitro ethanol exposure produced concentration-dependent negative inotropic effects that were associated with a reduction in the duration of contraction and relaxation and marked slowing in the maximum velocities of tension development and decay. These findings suggest that the contractile response to ethanol exposure, in vitro, is not modified by either chronic ethanol ingestion or hypertension.

Tension relaxation after stretch in resting mammalian muscle fibers: stretch activation at physiological temperatures

Tension responses to ramp stretches of 1-3% Lo (fiber length) in amplitude were examined in resting muscle fibers of the rat at temperatures ranging from 10 degrees C to 36 degrees C. Experiments were done using bundles of approximately 10 intact fibers isolated from the extensor digitorum longus (a fast muscle) and the soleus (a slow muscle). At low temperatures (below approximately 20 degrees C), the tension response consisted of an initial rise to a peak during the ramp followed by a complex tension decay to a plateau level; the tension decay occurred at approximately constant sarcomere length. The tension decay after a standard stretch at approximately 3-4.Lo/s contained a fast, an intermediate, and a (small amplitude) slow component, which at 10 degrees C (sarcomere length approximately 2.5 microns) were approximately 2000.s-1, approximately 150.s-1, and approximately 25.s-1 for fast fibers and approximately 2000.s-1, approximately 70.s-1 and approximately 8.s-1 for slow fibers, respectively. The fast component may represent the decay of interfilamentary viscous resistance, and the intermediate component may be due to viscoelasticity in the gap (titin, connectin) filament. The two- to threefold fast-slow muscle difference in the rate of passive tension relaxation (in the intermediate and the slow components) compares with previously reported differences in the speed of their active contractions; this suggests that "passive viscoelasticity" is appropriately matched to contraction speed in different muscle fiber types. At approximately 35 degrees C, the fast and intermediate components of tension relaxation were followed by a delayed tension rise at approximately 10.s-1 (fast fibers) and 2.5.s-1 (slow fibers); the delayed tension rise was accompanied by sarcomere shortening. BDM (5-10 mM) reduced the active twitch and tetanic tension responses and the delayed tension rise at 35 degrees C; the results indicate stretch sensitive activation in mammalian sarcomeres at physiological temperatures.