Sarcomeric visco-elasticity of chemically skinned skeletal muscle fibres of the rabbit at rest

Spasticity in adults with cerebral palsy and multiple sclerosis measured by objective clinically applicable technique

OBJECTIVE

The present study evaluated ankle stiffness in adults with and without neurological disorders and investigated the accuracy and reproducibility of a clinically applicable method using a dynamometer.

METHODS

Measurements were obtained from 8 healthy subjects (age 39.3), 9 subjects with spastic cerebral palsy (CP) (age 39.8) and 8 subjects with multiple sclerosis (MS) (age 49.9). Slow and fast dorsiflexion stretches of the ankle joint were performed to evaluate passive muscle-tendon-joint stiffness, reflex mediated stiffness and range of movement (ROM), respectively. Intra/inter-rater reliability for passive and reflex mediated ankle muscle stiffness was assessed for all groups.

RESULTS

Subjects with CP and MS showed significantly larger values of passive stiffness in the triceps surae muscle tendon complex and smaller ROM compared to healthy individuals, while no significant difference in reflex mediated stiffness. Measurements of passive muscle-tendon-joint stiffness and reflex mediated stiffness showed good to excellent inter- and intra-rater reliability (ICC: 0.62-0.91) in all groups.

CONCLUSION

Increased stiffness was found in subjects with CP and MS with a clinically applicable method that provides valid and reproducible measurement of passive ankle muscle-tendon-joint stiffness and reflex mediated stiffness.

SIGNIFICANCE

The present technique may provide important supplementary information for the clinician.

The force-generation process in active muscle is strain sensitive and endothermic: a temperature-perturbation study

In experiments on active muscle, we examined the tension decline and its temperature sensitivity at the onset of ramp shortening and at a range of velocities. A segment (∼1.5 mm long) of a skinned muscle fibre isolated from rabbit psoas muscle was held isometrically (sarcomere length ∼2.5 µm) at 8-9°C, maximally Ca²⁺-activated and a ramp shortening applied. The tension decline with a ramp shortening showed an early decrease of slope (the P₁ transition) followed by a slower decrease in slope (the P₂ transition) to the steady (isotonic) force. The tension level at the initial P₁ transition and the time to that transition decreased as the velocity was increased; the length change to this transition increased with shortening velocity to a steady value of ∼8 nm half-sarcomere^-1 A small, rapid, temperature jump (T-jump) (3-4°C, <0.2 ms) applied coincident with the onset of ramp shortening showed force enhancement by T-jump and changed the tension decline markedly. Analyses showed that the rate of T-jump-induced force rise increased linearly with increase of shortening velocity. These results provide crucial evidence that the strain-sensitive cross-bridge force generation, or a step closely coupled to it, is endothermic.

Neural and nonneural contributions to wrist rigidity in Parkinson's disease: an explorative study using the NeuroFlexor

Objective. The NeuroFlexor is a novel method incorporating a biomechanical model for the measurement of neural and nonneural contributions to resistance induced by passive stretch. In this study, we used the NeuroFlexor method to explore components of passive movement resistance in the wrist and finger muscles in subjects with Parkinson's disease (PD). Methods. A cross-sectional comparison was performed in twenty-five subjects with PD with clinically identified rigidity and 14 controls. Neural (NC), elastic (EC), and viscous (VC) components of the resistance to passive extension of the wrist were calculated using the NeuroFlexor. Measurements were repeated during a contralateral activation maneuver. Results. PD subjects showed greater total resistance (P < 0.001) and NC (P = 0.002) compared to controls. EC and VC did not differ significantly between groups. Contralateral activation maneuver resulted in increased NC in the PD group but this increase was due to increased resting tension. Total resistance and NC correlated with clinical ratings of rigidity and with bradykinesia. Conclusions. The findings suggest that stretch induced reflex activity, but not nonneural resistance, is the major contributor to rigidity in wrist muscles in PD. The NeuroFlexor is a potentially valuable clinical and research tool for quantification of rigidity.

Biophysically defined and cytocompatible covalently adaptable networks as viscoelastic 3D cell culture systems

Presented here is a cytocompatible covalently adaptable hydrogel uniquely capable of mimicking the complex biophysical properties of native tissue and enabling natural cell functions without matrix degradation. Demonstrated is both the ability to control elastic modulus and stress relaxation time constants by more than an order of magnitude while predicting these values based on fundamental theoretical understanding and the simulation of muscle tissue and the encapsulation of myoblasts.

Mechanical properties of respiratory muscles

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

Lack of effect of moderate-duration static stretching on plantar flexor force production and series compliance

BACKGROUND

The effects of an acute bout of moderate-duration static stretching on plantar flexor force production, series compliance of the muscle-tendon unit, and levels of neuromuscular activation were examined.

METHODS

Eighteen active individuals (9 men and 9 women) performed four 45-s static plantar flexor stretches and a time-matched control of no stretch (where subjects remained seated in the dynamometer for 4 min with no stretch being performed). Measures of peak isometric moment, rate of force development, neuromuscular activation (interpolated twitch technique and electromyography), twitch force characteristics, passive moment during stretch, and tendon elongation during maximal voluntary contractions were taken before and after the stretching.

FINDINGS

Despite a significant stress-relaxation response during stretch (9.3%, P<0.01) there were no significant differences in peak isometric moment (P=0.35; effect size 0.13), rate of force development (P=0.93; effect size 0.01), neuromuscular activation (interpolated twitch: P=0.86; electromyography: P=0.09; effect size 0.02), or tendon elongation (P=0.61; effect size 0.07) after stretching. Twitch characteristics were also unchanged after stretching, although there was a reduction in the rate of twitch torque relaxation (RR(t); P<0.01).

INTERPRETATION

The acute bout of moderate-duration static stretching did not impair the force generating capacity of the plantar flexors or negatively affect muscle-tendon mechanical properties. Static stretching may not always have detrimental consequences for force production. Thus, clinicians may be able to apply moderate-duration stretches to patients without risk of reducing muscular performance.

A nonlinear model of passive muscle viscosity

The material properties of passive skeletal muscle are critical to proper function and are frequently a target for therapeutic and interventional strategies. Investigations into the passive viscoelasticity of muscle have primarily focused on characterizing the elastic behavior, largely neglecting the viscous component. However, viscosity is a sizeable contributor to muscle stress and extensibility during passive stretch and thus there is a need for characterization of the viscous as well as the elastic components of muscle viscoelasticity. Single mouse muscle fibers were subjected to incremental stress relaxation tests to characterize the dependence of passive muscle stress on time, strain and strain rate. A model was then developed to describe fiber viscoelasticity incorporating the observed nonlinearities. The results of this model were compared with two commonly used linear viscoelastic models in their ability to represent fiber stress relaxation and strain rate sensitivity. The viscous component of mouse muscle fiber stress was not linear as is typically assumed, but rather a more complex function of time, strain and strain rate. The model developed here, which incorporates these nonlinearities, was better able to represent the stress relaxation behavior of fibers under the conditions tested than commonly used models with linear viscosity. It presents a new tool to investigate the changes in muscle viscous stresses with age, injury and disuse.

Validation of a New Biomechanical Model to Measure Muscle Tone in Spastic Muscles

Background. There is no easy and reliable method to measure spasticity, although it is a common and important symptom after a brain injury. Objective. The aim of this study was to develop and validate a new method to measure spasticity that can be easily used in clinical practice. Methods. A biomechanical model was created to estimate the components of the force resisting passive hand extension, namely ( a) inertia (IC), ( b) elasticity (EC), ( c) viscosity (VC), and ( d) neural components (NC). The model was validated in chronic stroke patients with varying degree of hand spasticity. Electromyography (EMG) was recorded to measure the muscle activity induced by the passive stretch. Results. The model was validated in 3 ways: ( a) NC was reduced after an ischemic nerve block, ( b) NC correlated with the integrated EMG across subjects and in the same subject during the ischemic nerve block, and ( c) NC was velocity dependent. In addition, the total resisting force and NC correlated with the modified Ashworth score. According to the model, the neural and nonneural components varied between patients. In most of the patients, but not in all, the NC dominated. Conclusions. The results suggest that the model allows valid measurement of spasticity in the upper extremity of chronic stroke patients and that it can be used to separate the neural component induced by the stretch reflex from resistance caused by altered muscle properties.

Crossbridge and non-crossbridge contributions to force in shortening and lengthening muscle

Analysis of tension responses to ramp length changes in muscle can provide important information about the crossbridge cycle. During a ramp length change, the force response of an active muscle shows an early change in slope (the P₁ transition) followed by a later, gradual change in slope (the P₂ transition). Modeling shows that the first transition reflects the tension change associated with the crossbridge power stroke in shortening and with its reversal in lengthening; the reduction in slope at the second transition occurs when most of the crossbridges (myosin heads) that were attached at the start of the ramp become detached; the steady tension during shortening is borne mainly by post-stroke heads whereas tension during lengthening is borne mostly by pre-stroke heads. After the P₂ transition, the tension reaches a steady level in the model whereas in the experiments the tension continues to increase during lengthening or to decrease during shortening; this tension change is seen at a wide range of sarcomere lengths and even when active force is reduced by a myosin inhibitor. It appears that some non-crossbridge components in muscle fibers stiffen upon activation and contribute to the continued tension rise during lengthening; release of such tension leads to tension decline during shortening. Thus, non-crossbridge visco-elasticity in sarcomeres may also contribute to energy storage and release during in situ muscle function.

Temperature jump induced force generation in rabbit muscle fibres gets faster with shortening and shows a biphasic dependence on velocity

We examined the tension responses to ramp shortening and rapid temperature jump (<0.2 ms, 3-4 degrees C T-jump) in maximally Ca(2+)-activated rabbit psoas muscle fibres at 8-9 degrees C (the fibre length (L(0)) was approximately 1.5 mm and sarcomere length 2.5 microm). The aim was to investigate the strain sensitivity of crossbridge force generation in muscle. The T-jump induced tension rise was examined during steady shortening over a wide range of velocities (V) approaching the V(max) (V range approximately 0.01 to approximately 1.5 L(0) s(1)). In the isometric state, a T-jump induced a biphasic tension rise consisting of a fast (approximately 50 s(1), phase 2b) and a slow (approximately 10 s(1), phase 3) component, but if treated as monophasic the rate was approximately 20 s(1). During steady shortening the T-jump tension rise was monophasic; the rate of tension rise increased linearly with shortening velocity, and near V(max) it was approximately 200 s(1), approximately 10x faster than in the isometric state. Relative to the tension reached after the T-jump, the amplitude increased with shortening velocity, and near V(max) it was 4x larger than in the isometric state. Thus, the temperature sensitivity of muscle force is markedly increased with velocity during steady shortening, as found in steady state experiments. The rate of tension decline during ramp shortening also increased markedly with increase of velocity. The absolute amplitude of T-jump tension rise was larger than that in the isometric state at the low velocities (<0.5 L(0) s(1)) but decreased to below that of the isometric state at the higher velocities. Such a biphasic velocity dependence of the absolute amplitude of T-jump tension rise implies interplay between, at least, two processes that have opposing effects on the tension output as the shortening velocity is increased, probably enhancement of crossbridge force generation and faster (post-stroke) crossbridge detachment by negative strain. Overall, our results show that T-jump force generation is strain sensitive and becomes considerably faster when exposed to negative strain. Thus the crossbridge force generation step in muscle is both temperature sensitive (endothermic) and strain sensitive.

The viscoelastic properties of passive eye muscle in primates. II: testing the quasi-linear theory

We have extensively investigated the mechanical properties of passive eye muscles, in vivo, in anesthetized and paralyzed monkeys. The complexity inherent in rheological measurements makes it desirable to present the results in terms of a mathematical model. Because Fung's quasi-linear viscoelastic (QLV) model has been particularly successful in capturing the viscoelastic properties of passive biological tissues, here we analyze this dataset within the framework of Fung's theory.We found that the basic properties assumed under the QLV theory (separability and superposition) are not typical of passive eye muscles. We show that some recent extensions of Fung's model can deal successfully with the lack of separability, but fail to reproduce the deviation from superposition.While appealing for their elegance, the QLV model and its descendants are not able to capture the complex mechanical properties of passive eye muscles. In particular, our measurements suggest that in a passive extraocular muscle the force does not depend on the entire length history, but to a great extent is only a function of the last elongation to which it has been subjected. It is currently unknown whether other passive biological tissues behave similarly.

Crossbridge and non-crossbridge contributions to tension in lengthening rat muscle: force-induced reversal of the power stroke

Lengthening of active muscle is an essential feature of animal locomotion, but the molecular processes occurring are incompletely understood. We therefore examined and modelled tension responses to ramp stretches (5% fibre length, L0) over a wide range of velocities (0.1-10 L(0) s(-1)) of tetanized intact rat muscle fibre bundles (L0 approximately 2 mm) with a resting sarcomere length of 2.5 microm at 20 degrees C. Tension rose to a peak during stretch and decayed afterwards to a level which was higher than the prestretch tetanic tension. This residual force enhancement was insensitive to velocity. The tension rise during stretch showed an early transition (often appearing as an inflection) at approximately 1 ms. Both the stretch (L1) and the tension rise at this transition increased in proportion to velocity. A second transition, marked by a reduction in slope, occurred at a stretch of approximately 18 nm per half-sarcomere; the rise in tension at this transition increased with velocity towards a plateau. Based on analyses of the velocity dependence of the tension and modelling, we propose that the initial steep increase in tension arises from increasing strain of all attached crossbridges and that the first transition reflects the tension loss due to the original post-stroke heads executing a reverse power stroke. Modelling indicates that the reduction in slope at the second transition occurs when the last of the heads that were attached at the start of the ramp become detached. Thereafter, the crossbridge cycle is largely truncated, with prepower stroke crossbridges rapidly detaching at high strain and attaching at low strain, the tension being borne mainly by the prestroke heads. Analysis of the tension decay after the ramp and the velocity dependence of the peak tension suggest that a non-crossbridge component increasingly develops tension throughout the stretch; this decays only slowly, reaching at 500 ms after the ramp approximately 20% of its peak value. This is supported by the finding that, in the presence of 10 microm N-benzyl-p-toluene sulphonamide (a myosin inhibitor), while isometric tension is reduced to approximately 15%, and the crossbridge contribution to stretch-induced tension rise is reduced to 30-40%, the peak non-crossbridge contribution and the residual force enhancement remain high. We propose that the residual force enhancement is due to changes upon activation in parallel elastic elements, specifically that titin stiffens and C-protein-actin interactions may be recruited.

Dynamic behaviour of half-sarcomeres during and after stretch in activated rabbit psoas myofibrils: sarcomere asymmetry but no 'sarcomere popping'

We examined length changes of individual half-sarcomeres during and after stretch in actively contracting, single rabbit psoas myofibrils containing 10-30 sarcomeres. The myofibrils were fluorescently immunostained so that both Z-lines and M-bands of sarcomeres could be monitored by video microscopy simultaneously with the force measurement. Half-sarcomere lengths were determined by processing of video images and tracking the fluorescent Z-line and M-band signals. Upon Ca2+ activation, during the rise in force, active half-sarcomeres predominantly shorten but to different extents so that an active myofibril consists of half-sarcomeres of different lengths and thus asymmetric sarcomeres, i.e. shifted A-bands, indicating different amounts of filament overlap in the two halves. When force reached a plateau, the myofibril was stretched by 15-20% resting length (L0) at a velocity of approximately 0.2 L0 s(-1). The myofibril force response to a ramp stretch is similar to that reported from muscle fibres. Despite the approximately 2.5-fold increase in force due to the stretch, the variability in half-sarcomere length remained almost constant during the stretch and A-band shifts did not progress further, independent of whether half-sarcomeres shortened or lengthened during the initial Ca2+ activation. Moreover, albeit half-sarcomeres lengthened to different extents during a stretch, rapid elongation of individual sarcomeres beyond filament overlap ('popping') was not observed. Thus, in contrast to predictions of the 'popping sarcomere' hypothesis, a stretch rather stabilizes the uniformity of half-sarcomere lengths and sarcomere symmetry. In general, the half-sarcomere length changes (dynamics) before and after stretch were slow and the dynamics after stretch were not readily predictable on the basis of the steady-state force-sarcomere length relation.

Damped elastic recoil of the titin spring in myofibrils of human myocardium

The giant protein titin functions as a molecular spring in muscle and is responsible for most of the passive tension of myocardium. Because the titin spring is extended during diastolic stretch, it will recoil elastically during systole and potentially may influence the overall shortening behavior of cardiac muscle. Here, titin elastic recoil was quantified in single human heart myofibrils by using a high-speed charge-coupled device-line camera and a nanonewtonrange force sensor. Application of a slack-test protocol revealed that the passive shortening velocity (Vp) of nonactivated cardiomyofibrils depends on: (i) initial sarcomere length, (ii) release-step amplitude, and (iii) temperature. Selective digestion of titin, with low doses of trypsin, decelerated myofibrillar passive recoil and eventually stopped it. Selective extraction of actin filaments with a Ca2+-independent gelsolin fragment greatly reduced the dependency of Vp on release-step size and temperature. These results are explained by the presence of viscous forces opposing myofibrillar passive recoil that are caused mainly by weak actin-titin interactions. Thus, Vp is determined by two distinct factors: titin elastic recoil and internal viscous drag forces. The recoil could be modeled as that of a damped entropic spring consisting of independent worm-like chains. The functional importance of myofibrillar elastic recoil was addressed by comparing instantaneous Vp to unloaded shortening velocity, which was measured in demembranated, fully Ca2+-activated, human cardiac fibers. Titin-driven passive recoil was much faster than active unloaded shortening velocity in early phases of isotonic contraction. Damped myofibrillar elastic recoil could help accelerate active contraction speed of human myocardium during early systolic shortening.

Titin: properties and family relationships

In striated muscles, the rapid production of macroscopic levels of force and displacement stems directly from highly ordered and hierarchical protein organization, with the sarcomere as the elemental contractile unit. There is now a wealth of evidence indicating that the giant elastic protein titin has important roles in controlling the structure and extensibility of vertebrate muscle sarcomeres.

Structural evidence for the interaction of C-protein (MyBP-C) with actin and sequence identification of a possible actin-binding domain

C-protein (MyBP-C) is a myosin-binding protein that is usually seen in two sets of seven to nine positions in the C-zones in each half of the vertebrate striated muscle A-band. Skeletal muscle C-protein is a modular structure containing ten sub-domains (C1 to C10) of which seven are immunoglobulin-type domains and three (C6, C7 and C9) are fibronectin-like domains. Cardiac muscle C-protein has an extra N-terminal domain (C0) and also some sequence insertions, one of which provides phosphorylation sites. It is conceivable that C-protein has both a structural and regulatory role within the sarcomere. The precise mode of binding of C-protein to the myosin filament has not been determined. However, detailed ultrastructural studies have suggested that C-protein, which binds to myosin, can give rise to a longer periodicity (about 435A) than the intrinsic myosin filament repeat of 429A. The reason for this has remained a puzzle for over 25 years. Here we show by modelling and computation that the presence of this longer periodicity could be explained if the myosin-binding part of C-protein binds to myosin with the expected 429A repeat, but if there are systematic interactions of the N-terminal end of C-protein with the neighbouring actin filaments in the hexagonal lattice of filaments in the A-band. We also show that if they occur these interactions would probably only arise in defined muscle states. Further analysis of the MyBP-C sequence identifies a possible actin-binding domain in the Pro-Ala-rich sequence found at the N terminus of skeletal MyBP-C and between domains C0 and C1 in the cardiac sequence.

Cardiac titin: molecular basis of elasticity and cellular contribution to elastic and viscous stiffness components in myocardium

Myocardium resists the inflow of blood during diastole through stretch-dependent generation of passive tension. Earlier we proposed that this tension is mainly due to collagen stiffness at degrees of stretch corresponding to sarcomere lengths (SLS) > or = 2.2 microns, but at shorter lengths, is principally determined by the giant sarcomere protein titin. Myocardial passive force consists of stretch-velocity-sensitive (viscous/viscoelastic) and velocity-insensitive (elastic) components; these force components are seen also in isolated cardiac myofibrils or skinned cells devoid of collagen. Here we examine the cellular/myofibrillar origins of passive force and describe the contribution of titin, or interactions involving titin, to individual passive-force components. We construct force-extension relationships for the four distinct elastic regions of cardiac titin, using results of in situ titin segment-extension studies and force measurements on isolated cardiac myofibrils. Then, we compare these relationships with those calculated for each region with the wormlike-chain (WLC) model of entropic polymer elasticity. Parameters used in the WLC calculations were determined experimentally by single-molecule atomic force-microscopy measurements on engineered titin domains. The WLC modelling faithfully predicts the steady-state-force vs. extension behavior of all cardiac-titin segments over much of the physiological SL range. Thus, the elastic-force component of cardiac myofibrils can be described in terms of the entropic-spring properties of titin segments. In contrast, entropic elasticity cannot account for the passive-force decay of cardiac myofibrils following quick stretch (stress relaxation). Instead, slower (viscoelastic) components of stress relaxation could be simulated by using a Monte-Carlo approach, in which unfolding of a few immunoglobulin domains per titin molecule explains the force decay. Fast components of stress relaxation (viscous drag) result mainly from interaction between actin and titin filaments; actin extraction of cardiac sarcomeres by gelsolin immediately suppressed the quickly decaying force transients. The combined results reveal the sources of velocity sensitive and insensitive force components of cardiomyofibrils stretched in diastole.

Inter-sarcomere dynamics in muscle fibres. A neglected subject?

The sarcomere is the functional unit of muscle, and all sarcomeres are connected in series in myofibrils within a muscle fibre. From this point of view of the structure a single model consisting of a contractile, a series and a parallel element can not account for the description of a real muscle fibre. Additionally, the titin protein filament needs to be considered as a passive visco-elastic element in parallel with the contractile apparatus. Therefore, the structure of a single muscle fibre is complex due mechanical elements ("motors") operating in series and in parallel. Moreover, variability does exist in the mechanical properties along a fibre and hence a multi-segmental model is more realistic and would give rise to many new insights. By attributing a segment model to each half-sarcomere, a fibre can be constructed through rigorous coupling of these units in series and parallel. The dynamics of such a multi-segmental model is much more complex, but it can explain a variety of effects reported in standard classical mechanics experiments. With a relatively simple mechanistic description we can show that the dynamics of such multi-sarcomere systems exhibit a variety of effects (relaxation phenomena, permanent extra-tension, biphasic force-velocity relation) and should therefore not be neglected in muscle fibre modelling. We have observed in single skinned fibre experiments that non-uniformities in sarcomere length changes are prominent during activation and relaxation.

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).