A quantitative model of intersarcomere dynamics during fixed-end contractions of single frog muscle fibers.

A numerical model of a muscle fiber as 400 sarcomeres, identical except for their initial lengths, was used to simulate fixed-end tetanic contractions of frog single fibers at sarcomere lengths above the optimum. The sarcomeres were represented by a lumped model, constructed from the passive and active sarcomere length-tension curves, the force-velocity curve, and the observed active elasticity of a single frog muscle fiber. An intersarcomere force was included to prevent large disparities in lengths of neighboring sarcomeres. The model duplicated the fast rise, slow creep rise, peak, and slow decline of tension seen in tetanic contractions of stretched living fibers. Decreasing the initial non-uniformity of sarcomere length reduced the rate of rise of tension during the creep phase, but did not decrease the peak tension reached. Limitations of the model, and other processes that might contribute to the shape of the fixed end tetanic tension record are discussed. Taking account of model and experimental results, it is concluded that the distinctive features of the tension records of fixed end tetanic contraction at lengths beyond optimum can be explained by internal motion within the fiber.     

ULTRASTRUCTURE OF THE RESTING AND CONTRACTED STRIATED MUSCLE FIBER AT DIFFERENT DEGREES OF STRETCH

Passive stretch, isometric contraction, and shortening were studied in electron micrographs of striated, non-glycerinated frog muscle fibers. The artifacts due to the different steps of preparation were evaluated by comparing sarcomere length and fiber diameter before, during, and after fixation and after sectioning. Tension and length were recorded in the resting and contracted fiber before and during fixation. The I filaments could be traced to enter the A band between the A filaments on both sides of the I band, creating a zone of overlap which decreased linearly with stretch and increased with shortening. This is consistent with a sliding filament model. The decrease in the length of the A and I filaments during isometric contraction and the finding that fibers stretched to a sarcomere length of 3.7 µ still developed 30 per cent of the maximum tetanic tension could not be explained in terms of the sliding filament model. Shortening of the sarcomeres near the myotendinous junctions which still have overlap could account for only one-sixth of this tension, indicating that even those sarcomeres stretched to such a degree that there is a gap between A and I filaments are activated during isometric contraction (increase in stiffness). Shortening, too, was associated with changes in filament length. The diameter of A filaments remained unaltered with stretch and with isometric contraction. Shortening of 50 per cent was associated with a 13 per cent increase in A filament diameter. The area occupied by the fibrils and by the interfibrillar space increased with shortening, indicating a 20 per cent reduction in the volume of the fibrils when shortening amounted to 40 per cent.     

Model of muscle-tendon interaction during frog semitendinosis fixed-end contractions.

A structural model was developed to explain sarcomere shortening at the expense of tendon lengthening in the frog semitendinosis (ST) muscle-tendon system. The model was based on the data of Lieber et al. [Am. J. Physiol. 261, C86-C92 (1991)], who determined the relationship between the sarcomere length, tendon load (as a fraction of maximum isometric tension) and tendon, bone-tendon junction (BTJ), and aponeurosis strain. The model was generated assuming a finite time-course of cross-bridge attachment [Huxley, Prog. Biophys. 7,255-318 (1957)], an ideal sarcomere length-tension relationship [Gordon et al., J. Physiol. 184, 170-192 (1966)] and an ideal force-velocity relationship [Katz, J. Physiol. 96, 45-64 (1939); Edman, J. Physiol. 291, 143-159 (1979)]. Functionally, sarcomeres operated on three distinct regions of the length-tension curve: (1) regions where the muscle force decreased as sarcomeres shortened (the shallow and steep ascending limbs); (2) regions where the muscle force increased as sarcomeres shortened and there was little passive tension (descending limb, where sarcomere length greater than or equal to 3.0 microns); and (3) regions where the muscle force increased as sarcomeres shortened and there was a significant passive tension (descending limb where sarcomere length greater than 3.0 microns). Using such a physiological model, it was found that the effect of tendon compliance was to 'skew' the sarcomere length-tension curve to the right and to increase the operating range of the muscle-tendon unit.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sarcomere overextension reduces stretch-induced tension loss in myofibrils of rabbit psoas

Stretch-induced damage to skeletal muscles results in loss of isometric tension. Although there is no direct evidence, loss of tension has been implicitly assumed to be the consequence of permanent loss of myofilament overlap in some sarcomeres ('sarcomere overextension'). Using isolated myofibrils of rabbit psoas muscle (n=38; 6 control and 32 test specimens) at 12-15°C, we directly tested the idea that loss of tension following stretch is caused by sarcomere overextension. Experimental myofibrils were maximally activated at the edge of the descending limb (sarcomere length ～ 2.9 μm) of the sarcomere length-tension relationship and then stretched by 1 μm sarcomere(-1) at a constant speed of 0.1 μms(-1)sarcomere(-1) to result in an average strain of 33.6 ± 0.9% (mean ± 1 SE). Myofibrils were immediately returned to the original lengths and relaxed. Isometric tension measured in a subsequent re-activation 3-5 min later was reduced by 24.6 ± 1.5% from its original value. In 22 out of the 32 test specimens, all sarcomeres maintained myofilament overlap, while in 10 myofibrils one or two sarcomeres were stretched permanently beyond myofilament overlap (>4.0 μm), and thus exhibited overextended sarcomeres. Loss of tension following stretch was significantly smaller in myofibrils with overextended sarcomeres compared to myofibrils with no overextended sarcomeres (19.5 ± 2.3% and 27.1 ± 1.8%, respectively; p=0.017). Combined, these results suggest that the loss of tension associated with stretch-induced damage can occur in the absence of sarcomere overextension and that sarcomere overextension limits rather than causes stretch-induced tension loss.     

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.     

CONTRACTILITY AND ULTRASTRUCTURE IN GLYCEROL-EXTRACTED MUSCLE FIBERS : I. The Relationship of Contractility to Sarcomere Length

This study was undertaken to determine whether glycerol-extracted rabbit psoas muscle fibers can develop tension and shorten after being stretched to such a length that the primary and secondary filaments no longer overlap. A method was devised to measure the initial sarcomere length and the ATP-induced isotonic shortening in prestretched isolated fibers subjected to a small preload (0.02 to 0.15 P₀). At all degrees of stretch, the fiber was able to shorten (60 to 75 per cent): to a sarcomere length of 0.7 µ when the initial length was 3.7 µ or less, and to an increasing length of 0.9 to 1.8 µ with increasing initial sarcomere length (3.8 to 4.4 µ). At sarcomere lengths of 3.8 to 4.5 µ, overlap of filaments was lost, as verified by electron microscopy. The variation in sarcomere length within individual fibers has been assessed by both light and electron microscopic measurements. In fibers up to 10 mm in length the stretch was evenly distributed along the fiber, and with sarcomere spacings greater than 4 µ there was only a slight chance of finding sarcomeres with filament overlap. These observations are in apparent contradiction to the assumption that an overlap of A and I filaments is necessary for tension generation and shortening.     

A non-cross-bridge stiffness in activated frog muscle fibers

Force responses to fast ramp stretches of various amplitude and velocity, applied during tetanic contractions, were measured in single intact fibers from frog tibialis anterior muscle. Experiments were performed at 14 degrees C at approximately 2.1 microm sarcomere length on fibers bathed in Ringer's solution containing various concentrations of 2,3-butanedione monoxime (BDM) to greatly reduce the isometric tension. The fast tension transient produced by the stretch was followed by a period, lasting until relaxation, during which the tension remained constant to a value that greatly exceeded the isometric tension. The excess of tension was termed "static tension," and the ratio between the force and the accompanying sarcomere length change was termed "static stiffness." The static stiffness was independent of the active tension developed by the fiber, and independent of stretch amplitude and stretching velocity in the whole range tested; it increased with sarcomere length in the range 2.1-2.8 microm, to decrease again at longer lengths. Static stiffness increased well ahead of tension during the tetanus rise, and fell ahead of tension during relaxation. These results suggest that activation increased the stiffness of some sarcomeric structure(s) outside the cross-bridges.     

OBLIQUELY STRIATED MUSCLE : III. Contraction Mechanism of Ascaris Body Muscle

Segments of the obliquely striated body muscle of Ascaris were fixed at minimum body length after treatment with acetylcholine and at maximum body length after treatment with piperazine citrate and then studied by light and electron microscopy. Evidence was found for two mechanisms of length change: sliding of thin filaments with respect to thick filaments such as occurs in cross-striated muscle, and shearing of thick filaments with respect to each other such that the degree of their stagger increases with extension and decreases with shortening. The shearing mechanism could account for great extensibility in this muscle and in nonstriated muscles in general and could underlie other manifestations of "plasticity" as well. In addition, it is suggested that the contractile apparatus is attached to the endomysium in such a way that the sarcomeres can act either in series, as in cross-striated muscle, or individually. Since the sarcomeres are virtually longitudinal in orientation and are almost coextensive with the muscle fiber, it would, therefore, be possible for a single sarcomere contracting independently to develop tension effectively between widely separated points on the fiber surface, thus permitting very efficient maintenance of isometric tension.     

Sarcomere length dependence of the force-velocity relation in single frog muscle fibers.

The force-velocity relation of single frog fibers was measured at sarcomere lengths of 2.15, 2.65, and 3.15 microns. Sarcomere length was obtained on-line with a system that measures the distance between two markers attached to the surface of the fiber, approximately 800 microns apart. Maximal shortening velocity, determined by extrapolating the Hill equation, was similar at the three sarcomere lengths: 6.5, 6.0, and 5.7 microns/s at sarcomere lengths of 2.15, 2.65, and 3.15 microns, respectively. For loads not close to zero the shortening velocity decreased with increasing sarcomere length. This was the case when force was expressed as a percentage of the maximal force at optimal fiber length or as a percentage of the sarcomere-isometric force at the respective sarcomere lengths. The force-velocity relation was discontinuous around zero velocity: load clamps above the level that kept sarcomeres isometric resulted in stretch that was much slower than when the load was decreased below isometric by a similar amount. We fitted the force-velocity relation for slow shortening (less than 600 nm/s) and for slow stretch (less than 200 nm/s) with linear regression lines. At a sarcomere length of 2.15 microns the slopes of these lines was 8.6 times higher for shortening than for stretch. At 2.65 and 3.15 microns the values were 21.8 and 14.1, respectively. At a sarcomere length of 2.15 microm, the velocity of stretch abruptly increased at loads that were 160-170% of the sarcomere isometric load, i.e., the muscle yielded. However, at a sarcomere length of 2.65 and 3.15 microm yield was absent at such loads. Even the highest loads tested (260%) resulted in only slow stretch.(ABSTRACT TRUNCATED AT 250 WORDS)     

Overextended sarcomeres regain filament overlap following stretch

Sarcomere overextension has been widely implicated in stretch-induced muscle injury. Yet, sarcomere overextensions are typically inferred based on indirect evidence obtained in muscle and fibre preparations, where individual sarcomeres cannot be observed during dynamic contractions. Therefore, it remains unclear whether sarcomere overextensions are permanent following injury-inducing stretch-shortening cycles, and thus, if they can explain stretch-induced force loss. We tested the hypothesis that overextended sarcomeres can regain filament overlap in isolated myofibrils from rabbit psoas muscles. Maximally activated myofibrils (n=13) were stretched from an average sarcomere length of 2.6±0.04μm by 0.9μm sarcomere(-1) at a speed of 0.1μm sarcomere(-1)s(-1) and immediately returned to the starting lengths at the same speed (sarcomere strain=34.1±2.3%). Myofibrils were then allowed to contract isometrically at the starting lengths (2.6μm) for ～30s before relaxing. Force and individual sarcomere lengths were measured continuously. Out of the 182 sarcomeres, 35 sarcomeres were overextended at the peak of stretch, out of which 26 regained filament overlap in the shortening phase while 9 (～5%) remained overextended. About 35% of the sarcomeres with initial lengths on the descending limb of the force-length relationship and ～2% of the sarcomeres with shorter initial lengths were overextended. These findings provide first ever direct evidence that overextended sarcomeres can regain filament overlap in the shortening phase following stretch, and that the likelihood of overextension is higher for sarcomeres residing initially on the descending limb.     

Sarcomere strain and heterogeneity correlate with injury to frog skeletal muscle fiber bundles.

Sarcomere length and first-order diffraction line width were measured by laser diffraction during elongation of activated frog tibialis anterior muscle fiber bundles (i.e., eccentric contraction) at nominal fiber strains of 10, 25, or 35% (n = 18) for 10 successive contractions. Tetanic tension, measured just before each eccentric contraction, differed significantly among strain groups and changed dramatically during the 10-contraction treatment (P < 0.01). Average maximum tetanic tension for the three groups measured before any treatment was 203.7 +/- 6.8 kN/m2, but after the 10-eccentric contraction sequence decreased to 180.3 +/- 3.8, 125.1 +/- 7.8, and 78.3 +/- 5.1 kN/m2 for the 10, 25, and 35% strain groups, respectively (P < 0.0001). Addition of 10 mM caffeine to the bathing medium decreased the loss of tetanic tension in the 10% strain group but had only a minimal effect on either the 25 or 35% strain groups. Diffraction pattern line width, a measure of sarcomere length heterogeneity, increased significantly with muscle activation and then continued to increase with successive stretches of the activated muscle. Line width increase after each stretch was significantly correlated with the lower yield tension of the successive contractile record. These data demonstrate a direct association and, perhaps, a causal relationship between sarcomere strain and fiber bundle injury. They also demonstrate that muscle injury is accompanied by a progressive increase in sarcomere length heterogeneity, yielding lower yield tension as injury progresses.     

Optical Diffraction Studies of Muscle Fibers

A new technique to monitor light diffraction patterns electrically is applied to frog semitendinosus muscle fibers at various levels of stretch. The intensity of the diffraction lines, sarcomere length change, and the length-dispersion (line width) were calculated by fast analogue circuits and displayed in real time. A heliumneon laser (wavelength 6328 Å) was used as a light source. It was found that the intensity of the first-order diffraction line drops significantly (30-50%) at an optimal sarcomere length of 2.8 μm on isometric tetanic stimulation. Such stimulation produced contraction of half-sarcomeres by about 22 nm presumably by stretching inactive elements such as tendons. The dispersion of the sarcomere lengths is extremely small, and it is proportional to the sarcomere length (less than 4%). The dispersion increases on stimulation. These changes on isometric tetanic stimulation were dependent on sarcomere length. No vibration or oscillation in the averaged length of the sarcomeres was found during isometric tetanus within a resolution of 3 nm; however, our observation of increased length dispersion of the sarcomeres together with detection of the averaged shortening of the sarcomere lengths suggests the presence of asynchronous cyclic motions between thick and thin filaments. An alternative explanation is simply an increase of the length dispersion of sarcomeres without cyclic motions.     

Length-tension relation in Limulus striated muscle

Laser diffraction techniques coupled with simultaneous tension measurements were used to determine the length-tension relation in intact, small (0.5-mm thick, 10-mm wide, 20-25-mm long) bundles of a Limulus (horseshoe crab) striated muscle, the telson levator muscle. This muscle differs from the model vertebrate systems in that the thick filaments are not of a constant length, but shorten from 4.9 to approximately 2.0 micrometers as the sarcomeres shorten from 7 to 3 micrometers. In the Limulus muscle, the length-tension relation plateaued to an average maximum tension of 0.34 N/mm2 at a sarcomere length of 6.5 micrometers (Lo) to 8.0 micrometers. In the sarcomere length range from 3.8 to 12.5 micrometers, the muscle developed 50% or more of the maximum tension. When the sarcomere lengths are normalized (expressed as L/Lo) and the Limulus data are compared to those from frog muscle, it is apparent that Limulus muscle develops tension over a relatively greater range of sarcomere lengths.     

Optimal design of vertebrate and insect sarcomeres

This paper offers a model for the normalized length-tension relation of a muscle fiber based upon sarcomere design. Comparison with measurements published by Gordon et al. ('66) shows an accurate fit as long as the inhomogeneity of sarcomere length in a single muscle fiber is taken into account. Sequential change of filament length and the length of the cross-bridge-free zone leads the model to suggest that most vertebrate sarcomeres tested match the condition of optimal construction for the output of mechanical energy over a full sarcomere contraction movement. Joint optimization of all three morphometric parameters suggests that a slightly better (0.3%) design is theoretically possible. However, this theoretical sarcomere, optimally designed for the conversion of energy, has a low normalized contraction velocity; it provides a poorer match to the combined functional demands of high energy output and high contraction velocity than the real sarcomeres of vertebrates. The sarcomeres in fish myotomes appear to be built suboptimally for isometric contraction, but built optimally for that shortening velocity generating maximum power. During swimming, these muscles do indeed contract concentrically only. The sarcomeres of insect asynchronous flight muscles contract only slightly. They are not built optimally for maximum output of energy across the full range of contraction encountered in vertebrate sarcomeres, but are built almost optimally for the contraction range that they do in fact employ.     

The sarcomere length-tension relation in skeletal muscle

Tension development during isometric tetani in single fibers of frog semitendinosus muscle occurs in three phases: (a) in initial fast-rise phase; (b) a slow-rise phase; and (c) a plateau, which lasts greater than 10 s. The slow-rise phase has previously been assumed to rise out of a progressive increase of sarcomere length dispersion along the fiber (Gordon et al. 1966. J. Physiol. [Lond.]. 184:143--169;184:170-- 192). Consequently, the "true" tetanic tension has been considered to be the one existing before the onset of the slow-rise phase; this is obtained by extrapolating the slowly rising tension back to the start of the tetanus. In the study by Gordon et al. (1966. J. Physiol. [Lond.] 184:170--192), as well as in the present study, the relation between this extrapolated tension and sarcomere length gave the familiar linear descending limb of the length-tension relation. We tested the assumption that the slow rise of tension was due to a progressive increase in sarcomere length dispersion. During the fast rise, the slow rise, and the plateau of tension, the sarcomere length dispersion at any area along the muscle was less than 4% of the average sarcomere length. Therefore, a progressive increase of sarcomere length dispersion during contraction appears unable to account for the slow rise of tetanic tension. A sarcomere length-tension relation was constructed from the levels of tension and sarcomere length measured during the plateau. Tension was independent of sarcomere length between 1.9 and 2.6 microgram, and declined to 50% maximal at 3.4 microgram. This result is difficult to reconcile with the cross-bridge model of force generation.     

Residual force enhancement after stretch of contracting frog single muscle fibers

Single fibers from the tibialis anterior muscle of Rana temporaria at 0.8-3.8 degrees C were subjected to long tetani lasting up to 8 s. Stretch of the fiber early in the tetanus caused an enhancement of force above the isometric control level which decayed only slowly and stayed higher throughout the contraction. This residual enhancement was uninfluenced by velocity of stretch and occurred only on the descending limb of the length-tension curve. The absolute magnitude of the effect increased with sarcomere length to a maximum at approximately 2.9 micrometers and then declined. The phenomenon was further characterized by its dependence on the amplitude of stretch. The final force level reached after stretch was usually higher than the isometric force level corresponding to the starting length of the stretch. The possibility that the phenomenon was caused by nonuniformity of sarcomere length along the fiber was examined by (a) laser diffraction studies that showed sarcomere stretch at all locations and (b) studies of 9-10 segments of approximately 0.6-0.7 mm along the entire fiber, which all elongated during stretch. Length-clamped segments showed residual force enhancement after stretch when compared with the tetanus produced by the same segment held at the short length as well as at the long length. It is concluded that residual force enhancement after stretch is a property shown by all individual segments along the fiber.     

Critical sarcomere extension required to recruit a decaying component of extra force during stretch in tetanic contractions of frog skeletal muscle fibers

29 single frog skeletal muscle fibers were stretched during fused tetanic contractions. The force increase during stretch exhibited a breakpoint at a critical length change (average: 16.6 nm per one-half sarcomere) that was independent of velocity of stretch and of sarcomere length between 1.8 and 2.8 microns. After stretch there was an early decaying force component with a force-extension curve similar to that during stretch, which disappeared over approximately 2 s. This component was removed by a small, quick release, leaving a longer- lasting component. The critical amplitude of release required to produce this result was found by clamping the fiber to a load at which there was zero velocity of shortening. This amplitude increased with time up to the angle in the force record during stretch, was constant for the remainder of the stretch, and decreased with time after the end of stretch; it was consistently less than the critical amplitude of stretch required to reach the breakpoint of force enhancement during stretch but was also independent of sarcomere length. The force drop accompanying the critical release showed a small increase up to an optimum magnitude at 2.4--2.7 microns sarcomere length, with a decrease at longer lengths.     

Load-dependent regulation of neuromuscular system

Roles of gravitational loading, sarcomere length, and/or tension development on the electromyogram (EMG) of soleus and afferent neurogram recorded at the L5 segmental level of spinal cord were investigated during parabolic flight of a jet airplane or hindlimb suspension in conscious rats. Both EMG and neurogram levels were increased when the gravity levels were elevated from 1-G to 2-G during the parabolic flight. They were decreased when the hindlimbs were unloaded by exposure to actual microgravity or by suspension. These phenomena were related to passive shortening of muscle fibers and/or sarcomeres. Unloading-related decrease in sarcomere length was greater at the central rather than the proximal and distal regions of fibers. These activities and tension development were not detected when the mean sarcomere length was less than 2.03 micrometers. It is suggested that load-dependent regulation of neuromuscular system is related to the tension development which is influenced by sarcomere length.     

Interplay between passive tension and strong and weak binding cross- bridges in insect indirect flight muscle. A functional dissection by gelsolin-mediated thin filament removal

The interplay between passive and active mechanical properties of indirect flight muscle of the waterbug (Lethocerus) was investigated. A functional dissection of the relative contribution of cross-bridges, actin filaments, and C filaments to tension and stiffness of passive, activated, and rigor fibers was carried out by comparing mechanical properties at different ionic strengths of sarcomeres with and without thin filaments. Selective thin filament removal was accomplished by treatment with the actin-severving protein gelsolin. Thin filament, removal had no effect on passive tension, indicating that the C filament and the actin filament are mechanically independent and that passive tension is developed by the C filament in response to sarcomere stretch. Passive tension increased steeply with sarcomere length until an elastic limit was reached at only 6-7% sarcomere extension, which corresponds to an extension of 350% of the C filament. The passive tension-length relation of insect flight muscle was analyzed using a segmental extension model of passive tension development (Wang, K, R. McCarter, J. Wright, B. Jennate, and R Ramirez-Mitchell. 1991. Proc. Natl. Acad. Sci. USA. 88:7101-7109). Thin filament removal greatly depressed high frequency passive stiffness (2.2 kHz) and eliminated the ionic strength sensitivity of passive stiffness. It is likely that the passive stiffness component that is removed by gelsolin is derived from weak-binding cross-bridges, while the component that remains is derived from the C filament. Our results indicate that a significant number of weak-binding cross-bridges exist in passive insect muscle at room temperature and at an ionic strength of 195 mM. Analysis of rigor muscle indicated that while rigor tension is entirely actin based, rigor stiffness contains a component that resists gelsolin treatment and is therefore likely to be C filament based. Active tension and active stiffness of unextracted fibers were directly proportional to passive tension before activation. Similarly, passive stiffness due to weak bridges also increased linearly with passive tension, up to a limit. These correlations lead us to propose a stress-activation model for insect flight muscle in which passive tension is a prerequisite for the formation of both weak-binding and strong-binding cross-bridges.