The visco-elasticity of resting intact mammalian (rat) fast muscle fibres

Impact of lengthening velocity on the generation of eccentric force by slow-twitch muscle fibers in long stretches

After an initial increase, isovelocity elongation of a muscle fiber can lead to diminishing (referred to as Give in the literature) and subsequently increasing force. How the stretch velocity affects this behavior in slow-twitch fibers remains largely unexplored. Here, we stretched fully activated individual rat soleus muscle fibers from 0.85 to 1.3 optimal fiber length at stretch velocities of 0.01, 0.1, and 1 maximum shortening velocity, vmax, and compared the results with those of rat EDL fast-twitch fibers obtained in similar experimental conditions. In soleus muscle fibers, Give was 7%, 18%, and 44% of maximum isometric force for 0.01, 0.1, and 1 vmax, respectively. As in EDL fibers, the force increased nearly linearly in the second half of the stretch, although the number of crossbridges decreased, and its slope increased with stretch velocity. Our findings are consistent with the concept of a forceful detachment and subsequent crossbridge reattachment in the stretch's first phase and a strong viscoelastic titin contribution to fiber force in the second phase of the stretch. Interestingly, we found interaction effects of stretch velocity and fiber type on force parameters in both stretch phases, hinting at fiber type-specific differences in crossbridge and titin contributions to eccentric force. Whether fiber type-specific combined XB and non-XB models can explain these effects or if they hint at some not fully understood properties of muscle contraction remains to be shown. These results may stimulate new optimization perspectives in sports training and provide a better understanding of structure-function relations of muscle proteins.

Investigating the passive mechanical behaviour of skeletal muscle fibres: Micromechanical experiments and Bayesian hierarchical modelling

Characterisation of the skeletal muscle's passive properties is a challenging task since its structure is dominated by a highly complex and hierarchical arrangement of fibrous components at different scales. The present work focuses on the micromechanical characterisation of skeletal muscle fibres, which consist of myofibrils, by realising three different deformation states, namely, axial tension, axial compression, and transversal compression. To the best of the authors' knowledge, the present study provides a novel comprehensive data set representing of different deformation states. These data allow for a better understanding of muscle fibre load transfer mechanisms and can be used for meaningful modelling approaches. As the present study shows, axial tension and compression experiments reveal a strong tension-compression asymmetry at fibre level. In comparison to the tissue level, this asymmetric behaviour is more pronounced at the fibre scale, elucidating the load transfer mechanism in muscle tissue and aiding in the development of future modelling strategies. Further, a Bayesian hierarchical modelling approach was used to consider the experimental fluctuations in a parameter identification scheme, leading to more comprehensive parameter distributions that reflect the entire observed experimental uncertainty. STATEMENT OF SIGNIFICANCE: This article examines for the first time the mechanical properties of skeletal muscle fibres under axial tension, axial compression, and transversal compression, leading to a highly comprehensive data set. Moreover, a Bayesian hierarchical modelling concept is presented to identify model parameters in a broad way. The results of the deformation states allow a new and comprehensive understanding of muscle fibres' load transfer mechanisms; one example is the effect of tension-compression asymmetry. On the one hand, the results of this study contribute to the understanding of muscle mechanics and thus to the muscle's functional understanding during daily activity. On the other hand, they are relevant in the fields of skeletal muscle multiscale, constitutive modelling.

Effects of myostatin on the mechanical properties of muscles during repeated active lengthening in the mouse

The aim of the present study was to investigate how myostatin dysfunction affects fast and slow muscle stiffness and viscosity during severe repeated loading. Isolated extensor digitorum longus (EDL) and soleus muscles of young adult female mice of the BEH (dysfunctional myostatin) and BEH+/+ (functional myostatin) strains were subjected to 100 contraction-stretching loading cycles during which contractile and mechanical properties were assessed. BEH mice exhibited greater exercise-induced muscle damage, although the effect was muscle- and age-dependent and limited to the early phases of simulated exercise. The relative reduction of the EDL muscle isometric force recorded during the initial 10-30 loading cycles was greater in BEH mice than in BEH+/+ mice and exceeded that of the soleus muscle of either strain. The induced damage was associated with lower muscle stiffness. The effects of myostatin on the mechanical properties of muscles depend on muscle type and maturity.

The MyoRobot: A novel automated biomechatronics system to assess voltage/Ca2+ biosensors and active/passive biomechanics in muscle and biomaterials

We engineered an automated biomechatronics system, MyoRobot, for robust objective and versatile assessment of muscle or polymer materials (bio-)mechanics. It covers multiple levels of muscle biosensor assessment, e.g. membrane voltage or contractile apparatus Ca²⁺ ion responses (force resolution 1µN, 0-10mN for the given sensor; [Ca²⁺] range ~ 100nM-25µM). It replaces previously tedious manual protocols to obtain exhaustive information on active/passive biomechanical properties across various morphological tissue levels. Deciphering mechanisms of muscle weakness requires sophisticated force protocols, dissecting contributions from altered Ca²⁺ homeostasis, electro-chemical, chemico-mechanical biosensors or visco-elastic components. From whole organ to single fibre levels, experimental demands and hardware requirements increase, limiting biomechanics research potential, as reflected by only few commercial biomechatronics systems that can address resolution, experimental versatility and mostly, automation of force recordings. Our MyoRobot combines optical force transducer technology with high precision 3D actuation (e.g. voice coil, 1µm encoder resolution; stepper motors, 4µm feed motion), and customized control software, enabling modular experimentation packages and automated data pre-analysis. In small bundles and single muscle fibres, we demonstrate automated recordings of (i) caffeine-induced-, (ii) electrical field stimulation (EFS)-induced force, (iii) pCa-force, (iv) slack-tests and (v) passive length-tension curves. The system easily reproduces results from manual systems (two times larger stiffness in slow over fast muscle) and provides novel insights into unloaded shortening velocities (declining with increasing slack lengths). The MyoRobot enables automated complex biomechanics assessment in muscle research. Applications also extend to material sciences, exemplarily shown here for spider silk and collagen biopolymers.

Non-uniform distribution of strain during stretch of relaxed skeletal muscle fibers from rat soleus muscle

Tension and regional average sarcomere length (L(s)) behavior were examined during repeated stretches of single, permeabilized, relaxed muscle fibers isolated from the soleus muscles of rats. We tested the hypothesis that during stretches of single permeabilized fibers, the global fiber strain is distributed non-uniformly along the length of a relaxed fiber in a repeatable pattern. Each fiber was subjected to eight constant-velocity stretch and release cycles with a strain of 32% and strain rate of 54% s(-1). Stretch-release cycles were separated by a 4.5 min interval. Throughout each stretch-release cycle, sarcomere lengths were measured using a laser diffraction technique in which 20 contiguous sectors along the entire length of a fiber segment were scanned within 2 ms. The results revealed that: (1) the imposed length change was not distributed uniformly along the fiber, (2) the first stretch-release cycle differed from subsequent cycles in passive tension and in the distribution of global fiber strain, and (3) a characteristic "signature" for the L(s) response emerged after cycle 3. The findings support the conclusions that longitudinal heterogeneity exists in the passive stiffness of individual muscle fibers and that preconditioning of fibers with stretch-release cycles produces a stable pattern of sarcomere strains.

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.

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.

The viscoelastic properties of passive eye muscle in primates. I: static forces and step responses

The viscoelastic properties of passive eye muscles are prime determinants of the deficits observed following eye muscle paralysis, the root cause of several types of strabismus. Our limited knowledge about such properties is hindering the ability of eye plant models to assist in formulating a patient's diagnosis and prognosis. To investigate these properties we conducted an extensive in vivo study of the mechanics of passive eye muscles in deeply anesthetized and paralyzed monkeys. We describe here the static length-tension relationship and the transient forces elicited by small step-like elongations. We found that the static force increases nonlinearly with length, as previously shown. As expected, an elongation step induces a fast rise in force, followed by a prolonged decay. The time course of the decay is however considerably more complex than previously thought, indicating the presence of several relaxation processes, with time constants ranging from 1 ms to at least 40 s. The mechanical properties of passive eye muscles are thus similar to those of many other biological passive tissues. Eye plant models, which for lack of data had to rely on (erroneous) assumptions, will have to be updated to incorporate these properties.

Influence of muscle cooling on the passive mechanical properties of the human gastrocnemius muscle

The purpose of the present study was to examine the influence of muscle cooling on the passive mechanical properties of the human gastrocnemius muscle (GAS) in vivo. In a thermoneutral (a room temperature of 18-23°C) and a local cooling (placing the right lower leg into cold water with a temperature of 5-8°C for 60 min) conditions, the change in passive plantarflexion force (F), which is produced only by the GAS length change, was taken in five subjects during passive knee extension from 90° to 0° with a constant ankle joint angle of 10° dorsiflexion. To evaluate an elastic component of the passive plantarflexion force of GAS, the subject held full knee extended position for 1 min (i.e. relaxation period). Skin and muscle temperature of GAS were also measured using a core temperature thermistor. The peak value of F (Fve) that was measured at the end of the knee extension phase, the decrease of F (ΔF) during the relaxation period, and the F at the end of the relaxation period (Fe) were measured in the two conditions. Muscle cooling decreased the skin and muscle temperature by 6.7 ± 1.1°C and 8.1 ± 2.5°C, respectively. Fve increased by 24% ± 22% by muscle cooling. ΔF in the thermoneutral and local cooling conditions were 11.5 ± 4.9 N and 12.5 ± 2.9 N, respectively. Fe increased by 28% ± 21% by muscle cooling.

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.

Resting tension characteristics in differentiating intact rat fast- and slow-twitch muscle fibers

The postnatal changes in resting muscle tension were investigated at 20 degrees C by using small muscle fiber bundles isolated from either the extensor digitorum longus or the soleus of both neonatal (7-21 days old) and adult rats. The results show that the tension-extension characteristics of the bundles depended on the age of the rats. For example, both the extensor digitorum longus and soleus bundles of rats older than 14 days showed characteristic differences that were absent in bundles from younger rats. Furthermore, the tension-extension relation of the adult slow muscle fiber bundles were similar to those of the two neonatal muscles and were shifted to longer sarcomere lengths relative to those of the adult fast-fiber bundles. Thus, at the extended sarcomere length of 2.9 microm, the adult fast muscle fiber bundles developed higher resting tensions (5.6 +/- 0.5 kN/m2) than either the two neonatal ( approximately 3 kN/m2) or the adult slow (3.1 +/- 0.4 kN/m2) muscle fiber bundles. At all ages examined, the resting tension responses to a ramp stretch were qualitatively similar and consisted of three components: a viscous, a viscoelastic, and an elastic tension. However, in rats older than 14 days, all three tension components showed clear fast- and slow-fiber type differences that were absent in younger rats. Bundles from 7-day-old rats also developed significantly lower resting tensions than the corresponding adult ones. Additionally, the resting tension characteristics of the adult muscles were not affected by chemical skinning. From these results, we conclude that in rats resting muscle tension, like active tension, differentiates within the first 3 wk after birth.

A mechanical stretch induces contractile activation in unstimulated developing rat skeletal muscle in vitro

The effects of a stretch-release cycle (approximately 25% of the resting muscle fibre length, Lo) on both tension and [Ca2+]i in small, unstimulated, intact muscle fibre bundles isolated from adult and neonatal rats were investigated at 20 degrees C. The results show that the effects of the length change depended on the age of the rats. Thus, the length change produced three effects in the neonatal rat muscle fibre bundles, but only a single effect in the adult ones. In the neonatal fibre bundles, the length change led to an increase in resting muscle tension and to a transient increase in [Ca2+]i. The stretch-release cycle was then followed by a twitch-like tension response. In the adult fibre bundles, only the increase in resting tension was seen and both the transient increase in [Ca2+]i and the stretch-induced twitch-like tension response were absent. The amplitude of the twitch-like tension response was affected by both 2,3-butanedione monoxime and sarcomere length in the same manner as active twitch tension, suggesting that it arose from actively cycling crossbridges. It was also reversibly abolished by 25 mM K+, 1 microM tetrodotoxin and 1.5 mM lidocaine (lignocaine), and was significantly depressed (P < 0.001) by lowering [Ca2+]o. These findings suggest that a rapid stretch in neonatal rats induces a propagated impulse that leads to an increase in [Ca2+]i, and that abolishing the action potential abolishes the stretch-induced twitch-like tension response. In 5- to 7-day-old rats, the twitch-like tension response was approximately 50 % of the isometric twitch. It then decreased progressively with age and was virtually absent by the time the rats were 21 days old. Interestingly, this is the same period over which rat muscles differentiate from their neonatal to their adult types.

The effects of ramp stretches on active contractions in intact mammalian fast and slow muscle fibres

The effects of a ramp stretch (amplitude <6% muscle fibre length (L0), speed < 13L0 s(-1)) on twitch tension and twitch tension re-development were examined in intact mammalian (rat) fast and slow muscle fibre bundles. The experiments were done in vitro at 20 degrees C and at an initial sarcomere length of 2.68 microm. In both fibre types, a stretch applied during the rising phase of the twitch response (including the time of stimulation) increased the re-developed twitch tension (15-35%). A stretch applied before the stimulus had little or no effect on the twitch myogram in fast muscle fibres, but it increased the twitch tension (approximately 5%) in slow muscle fibres. A similar stretch had little or no effect on tetanic tension in either muscle fibre type. In general, the results indicate that the contractile-activation mechanism may be stretch sensitive and this is particularly pronounced in slow muscle fibres. Recorded at a high sampling rate and examined at an appropriate time scale, the transitory tension response to a stretch rose in at least two phases; an initial rapid tension rise to a break (break point tension, P1a) followed by a slower tension rise (apparent P2a) to a peak reached at the end of the stretch. Plotted against stretch velocity, P1a tension increased in direct proportion to stretch velocity (viscous-like) whereas, P2a tension (calculated as peak tension minus P1a tension) increased with stretch velocity to a plateau (visco-elastic). Examined at the peak of a twitch, P1a tension had a slope (viscosity coefficient) of 1.8 kN m(-2) per L0 s(-1) in fast fibres and 4.7 kN m(-2) per L0 s(-1) in slow muscle fibres. In the same preparations, P2a tension had a relaxation time of 8 ms in the fast muscle fibres and 25 ms in the slow muscle fibres. The amplitudes of both tension components scaled with the instantaneous twitch tension in qualitatively the same way as the instantaneous fibre stiffness. These fast/slow fibre type differences probably reflect differences in their cross-bridge kinetics.

Effects of sprint and endurance training on passive stress-strain relation of fast- and slow-twitch skeletal muscle in Wistar rat

We investigated the effects of endurance and sprint training on the passive mechanical properties of fast-twitch (FT) and slow-twitch (ST) skeletal muscles. Eight-week-old male Wistar rats (n=18) were divided into three groups: control (C), sprint-trained (S) and endurance-trained (E). The trained animals exercised for 10 weeks on a treadmill. Under anaesthesia, Plantaris and Soleus muscles were deformed cyclically in vivo at 0.33 mm x s(-1) with length increments of 1 mm in successive cycles until rupture. The rupture of muscle occurs at belly. Stress-strain relation were constructed using the maximum stress and maximum strain in each cycle. The data were fitted to an S-shaped curve. The curve-fitting parameters for trained and untrained muscles showed significant statistical differences. Stress and strain at rupture and maximum deformation energy were statistically greater for trained ST muscles (both groups) than for the controls. The changes induced by the present training protocols were not significant in Plantaris. The above results suggest the plasticity of passive structure caused by activity-demands.

Sarcomere length changes during end-held (isometric) contractions in intact mammalian (rat) fast and slow muscle fibres

The sarcomere length change, within a 2 mm region, during end-held isometric contractions in intact rat fast and slow muscle fibre bundles was investigated at 20 degrees C and an initial sarcomere length of 2.68 microm using He-Ne laser diffraction. In some experiments, the fibre segment displacement was monitored with markers (pieces of human hair) placed at regular intervals on the surface of the muscle fibre bundles. The sarcomere length changes, monitored near the proximal end of the bundle (transducer end), during tetanic contractions were similar to those previously reported in frog muscle fibres. Thus, throughout the tension plateau, sarcomere length remained constant (and shortened) but showed evidence of non-uniform sarcomere behaviour (further shortening) during the rapid tension relaxation phase. Such non-uniform behaviour was not seen during twitch contractions. During a twitch contraction, sarcomeres at the proximal end shortened rapidly at first and continued to shorten--or remained shortened--until the tension had relaxed to between 20-23% of its peak value before lengthening back to the original length. The maximum twitch sarcomere shortening (mean +/- SEM) was 5.9 +/- 0.2% (n = 16) in fast and 5.4 +/- 0.3% (n = 14) in slow fibre bundles at 20 degrees C; sarcomere shortening near body temperature (approximately 35 degrees C) was greater, 8.8 +/- 0.2% (n = 7) in fast and 8.1 +/- 0.2% (n = 5) in slow fibre bundles. Increasing the initial sarcomere length of a preparation decreased the extent of sarcomere shortening and reducing the amount of sarcomere shortening, by sarcomere length clamping, markedly increased the peak twitch tension without significantly altering the twitch time course. When examined at different positions along muscle fibres, a sarcomere shortening was observed along much of the fibre length in most preparations. However, in about a third of the preparations some sarcomere lengthening was recorded in the distal end, but its amplitude was too small to accommodate the fibre shortening elsewhere. Complementary data were obtained using the surface marker technique. The displacement was largest and in opposite--but fibre shortening--direction in the markers placed approximately 0.5-1.0 mm away from the two tendon attachments; the markers placed at or near the centre of the fibre bundle showed the least amount of displacement. The findings suggest that the compliant region, where lengthening occurs, is at fibre ends, i.e. near myotendinous junction.

Do cross-bridges contribute to the tension during stretch of passive muscle?

The tension rise during stretch of passive skeletal muscle is biphasic, with an initial steep rise, followed by a subsequent more gradual change. The initial rise has been interpreted as being due to the presence of numbers of long-term, stable cross-bridges in resting muscle fibres. A point of weakness with the cross-bridge interpretation is that the initial stiffness reaches its peak value at muscle lengths beyond the optimum for myofilament overlap. To explain this result it has been suggested that despite the reduced overlap at longer lengths, the closer interfilament spacing and a higher sensitivity of the myofilaments to Ca2+ allows more stable cross-bridges to form. Recently the stretch responses of passive muscle have been re-examined and it has been suggested that it is not necessary to invoke cross-bridge mechanisms at all. Explanations based on a viscous resistance to interfilament sliding and mechanical properties of the elastic filaments, the gap filaments, were thought to adequately account for the observed tension changes. However, an important property of passive muscle, the dependence of stretch responses on the immediate history of contraction and length changes, thixotropy, cannot be explained simply in terms of viscous and viscoelastic properties. The review discusses the cross-bridge interpretation of muscle thixotropy and the relationship of passive stiffness to filament resting tension and latency relaxation. It is proposed that cross-bridges can exist in three states; one, responsible for the resting stiffness, requires resting levels of calcium. When, during activation, calcium levels rise, cross-bridges enter a low-force, high-stiffness state, signalled by latency relaxation, before they move to the third, force-generating state. It is concluded that, compared with viscoelastic models, a cross-bridge-based explanation of passive muscle properties is better able to accommodate the currently known facts although, as new information becomes available, this view may need to be revised.

Force generation upon hydrostatic pressure release in tetanized intact frog muscle fibres

Single intact muscle fibres isolated from the tibialis anterior muscle of the frog were exposed to hydrostatic pressures of 1-10 MPa, at 2-4 degrees C and sarcomere length of 2.1-2.2 microm. The pressure was rapidly released (ca. 1 ms) to atmospheric level (0.1 MPa) during the plateau of a tetanic contraction (Po) and the resultant tension (= force) transient examined. The pressure release induced tension transient consisted of an initial tension drop coincident with pressure release (ca. 4% Po per MPa, Phase 1), followed by a rapid recovery (Phase 2a) and a slower rise of tension (Phase 2b). Phase 1 was partly due to a length release at fibre ends (ca. 0.1 nm per half-sarcomere per MPa) induced by pressure-release effects on the steel chamber and fibre attachments, and partly due to 'expansion' upon pressure release within muscle fibre (ca. 0.2 nm per half-sarcomere per MPa), probably in the myofilaments and cross-bridges. The rate of tension recovery during phase 2a (ca. 600/s) was comparable to that of the quick tension recovery (T1-T2 transition) reported from moderately fast small length releases; the time course of Phase 2b (rate ca. 40/s) was similar to the late phase of tension rise in a tetanus, and hence compared with Phase 4 (T4) of a length release tension transient. Results are compared with the previously reported findings from analogous experiments on Ca2+ -activated skinned (rabbit) muscle fibres.

A cross-bridge mechanism can explain the thixotropic short-range elastic component of relaxed frog skeletal muscle

1. The passive tension and sarcomere length of relaxed frog skeletal muscle fibres were measured in response to imposed length stretches. The tension response to a constant-velocity stretch exhibited a clear discontinuity. Tension rose more rapidly during the initial approximately 0.4 % L0 of the stretch than during the latter stages (where L0 is the resting length of the fibre). This initial tension response is attributed to the short-range elastic component (SREC). 2. The use of paired triangular stretches revealed that the maximum tension produced during the SREC response of the second stretch was significantly reduced by the first stretch. This history-dependent behaviour of the SREC reflects thixotropic stiffness changes that have been previously described in relaxed muscle. 3. The biphasic nature of the SREC tension response to movement was most apparent during the first imposed length change after a period at a fixed length, irrespective of the direction of movement. 4. If a relaxed muscle was subjected to an imposed triangular length change so that the muscle was initially stretched and subsequently shortened back to its original fibre length, the resting tension at the end of the stretch was reduced relative to its initial pre-stretch value. Following the end of the stretch, tension slowly increased towards its initial value but the tension recovery was not accompanied by a contemporaneous increase in sarcomere length. This finding suggests that the resting tension of a relaxed muscle fibre is not entirely due to passive elasticity. The results are compatible with the suggestion that a portion of the resting tension - the filamentary resting tension (FRT) - is produced by a low level of active force generation. 5. If a second identical stretch was imposed on the muscle at a time when the resting tension was reduced by the previous stretch, the maximal tension produced during the second stretch was the same as that produced during the first, despite the second stretch commencing from a lower initial resting tension. 6. In experiments using paired triangular length changes, an inter-stretch interval of zero did not produce a substantially greater thixotropic reduction in the second stretch elastic limit force than an inter-stretch interval in the range 0.5-1 s. 7. A theoretical model was developed in which the SREC and FRT arise as manifestations of a small number of slowly cycling cross-bridges linking the actin and myosin filaments of a relaxed skeletal muscle. The predictions of the model are compatible with many of the experimental observations. If the SREC and FRT are indeed due to cross-bridge activity, the model suggests that the cross-bridge attachment rate must increase during interfilamentary movement. A mechanism (based on misregistration between the actin binding sites and the myosin cross-bridges) by which this might arise is presented.

Temperature-dependent changes in the viscoelasticity of intact resting mammalian (rat) fast- and slow-twitch muscle fibres

1. The tension and sarcomere length responses induced by ramp stretches (at amplitudes of 1-3 % fibre length (Lo) and speeds of 0.01-12 Lo s-1) were examined at different temperatures (range, 10-35 degrees C) in resting intact muscle fibre bundles isolated from the soleus (a slow-twitch muscle) and extensor digitorum longus (a fast-twitch muscle) of the rat. Some observations are also presented on the effects of chemical skinning on passive viscoelasticity at 10 degrees C. 2. As previously reported, the tension response to a ramp stretch, in different preparations and under various conditions, could be resolved into a viscous (P1), a viscoelastic (P2) and an elastic (P3) component and showed characteristic differences between slow and fast muscle fibres. 3. Chemical skinning of the muscle fibres led to a decrease in the amplitude of all three tension components. However, the fast-slow fibre differences remained after skinning. For example, the viscosity coefficient derived from P1 tension data decreased from 0.84 +/- 0.06 before skinning to 0.44 +/- 0.06 kN s m-2 after skinning in fast fibres; the corresponding values in slow fibres were 2.1 +/- 0.08 and 0.87 +/- 0.09 kN s m-2, respectively. 4. Increasing the experimental temperature from 10 to 35 degrees C led to a decrease in all the tension components in both fast and slow muscle fibre bundles. The decrease of P1 (viscous) tension was such that the viscosity coefficient calculated using P1 data was reduced from 0.84 +/- 0.1 to 0.43 +/- 0.05 kN s m-2 in fast fibres and from 2.0 +/- 0.1 to 1.0 +/- 0.1 kN s m-2 in slow fibres (Q10 of approximately 1.3 in both). 5. In both fast and slow muscle fibre preparations, the plateau tension of the viscoelastic component (P2) decreased by 60-80 % as the temperature was increased from 10 to 35 degrees C giving P2 tension a Q10 of approximately 1.4 in slow fibres and approximately 1.7 in the fast fibres. Additionally, the relaxation time of the viscoelasticity decreased from 11.9 +/- 1 ms (fast) and 43.1 +/- 1 ms (slow) at 10 degrees C to 3 +/- 0.5 ms (fast) at 25 C degrees and 8. 7 +/- 0.6 ms (slow) at 35 degrees C (Q10 of approximately 2.0 in slow and approximately 2.5 in fast fibres). 6. The fast-slow fibre differences in passive viscoelasticity remained at the high physiological temperatures. The physiological significance of such fibre-type differences and their possible underlying mechanisms are discussed.

The viscous, viscoelastic and elastic characteristics of resting fast and slow mammalian (rat) muscle fibres

1. The tension and sarcomere length responses induced by ramp stretches (amplitude 1-3% of initial fibre length (Lzero) and speeds of 0.01-12 Lzero s-1) were examined, at 10 degrees C and sarcomere lengths of approximately 2.7 microns, in resting intact muscle fibre bundles isolated from the soleus (a slow muscle) and extensor digitorum longus (a fast muscle) of the rat. 2. In both fibre types, the tension response to moderately fast ramp stretches consists of a viscous, a viscoelastic and an elastic component. At low stretch velocities, where the viscous component is very small, the tension response consists of only the viscoelastic and elastic components. 3. The viscosity coefficient (mean +/- S.E.M., 2 +/- 0.01 kN s m-2, n = 12) and the relaxation time of the viscoelasticity (44 +/- 2 ms, n = 12) of the slow muscle fibres were significantly larger than those of the fast muscle fibres (0.8 +/- 0.1 kN s m-2 and 11 +/- 1 ms, respectively, n = 20). 4. The relaxation time, in either fibre type, is too long for the viscoelasticity to be due to rapidly cycling, weakly attached cross-bridges. Moreover, the tension components increased with sarcomere length and were insensitive to 5-10 mM 2,3-butanedione 2-monoxime (BDM), which inhibited active contractions. 5. The possibility that the fast-slow fibre differences may reflect differences in myoplasmic viscosity and connectin (titin) isoforms (in their gap filaments) is discussed.