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

A 6-minute Limb Function Assessment for Therapeutic Testing in Experimental Peripheral Artery Disease Models

Background

The translation of promising therapies from pre-clinical models of hindlimb ischemia (HLI) to patients with peripheral artery disease (PAD) has been inadequate. While this failure is multifactorial, primary outcome measures in preclinical HLI models and clinical trials involving patients with PAD are not aligned well. For example, laser Doppler perfusion recovery measured under resting conditions is the most used outcome in HLI studies, whereas clinical trials involving patients with PAD primarily assess walking performance. Here, we sought to develop a 6-min limb function test for preclinical HLI models that assess muscular performance and hemodynamics congruently.

Methods

We developed an in situ 6-min limb function test that involves repeated isotonic (shortening) contractions performed against a submaximal load. Continuous measurement of muscle blood flow was performed using laser Doppler flowmetry. Quantification of muscle power, work, and perfusion are obtained across the test. To assess the efficacy of this test, we performed HLI via femoral artery ligation on several mouse strains: C57BL6J, BALBc/J, and MCK-PGC1α (muscle-specific overexpression of PGC1α). Additional experiments were performed using an exercise intervention (voluntary wheel running) following HLI.

Results

The 6-min limb function test was successful at detecting differences in limb function of C57BL6/J and BALBc/J mice subjected to HLI with effect sizes superior to laser Doppler perfusion recovery. C57BL6/J mice randomized to exercise therapy following HLI had smaller decline in muscle power, greater hyperemia, and performed more work across the 6-min limb function test compared to non-exercise controls with HLI. Mice with muscle-specific overexpression of PGC1α had no differences in perfusion recovery in resting conditions, but exhibited greater capillary density, increased muscle mass and absolute force levels, and performed more work across the 6-min limb function test compared to their wildtype littermates without the transgene.

Conclusion

These results demonstrate the efficacy of the 6-min limb function test to detect differences in the response to HLI across several interventions including where traditional perfusion recovery, capillary density, and muscle strength measures were unable to detect therapeutic differences.

Residual force enhancement is affected more by quadriceps muscle length than stretch amplitude

Little is known about how muscle length affects residual force enhancement (rFE) in humans. We therefore investigated rFE at short, long, and very long muscle lengths within the human quadriceps and patellar tendon (PT) using conventional dynamometry with motion capture (rFE_TQ) and a new, non-invasive shear-wave tensiometry technique (rFE_WS). Eleven healthy male participants performed submaximal (50% max.) EMG-matched fixed-end reference and stretch-hold contractions across these muscle lengths while muscle fascicle length changes of the vastus lateralis (VL) were captured using B-mode ultrasound. We found significant rFE_TQ at long (7±5%) and very long (12±8%), but not short (2±5%) muscle lengths, whereas rFE_WS was only significant at the very long (38±27%), but not short (8±12%) or long (6±10%) muscle lengths. We also found significant relationships between VL fascicle length and rFE_TQ (r=0.63, p=0.001) and rFE_WS (r=0.52, p=0.017), but relationships were not significant between VL fascicle stretch amplitude and rFE_TQ (r=0.33, p=0.126) or rFE_WS (r=0.29, p=0.201). Squared PT shear-wave-speed-angle relationships did not agree with estimated PT force-angle relationships, which indicates that estimating PT loads from shear-wave tensiometry might be inaccurate. We conclude that increasing muscle length rather than stretch amplitude contributes more to rFE during submaximal voluntary contractions of the human quadriceps.

The effect of stretch-shortening magnitude and muscle-tendon unit length on performance enhancement in a stretch-shortening cycle

Stretch-induced residual force enhancement (rFE) is associated with increased performance in a stretch–shortening cycle (SSC). Although the influence of different range of motions and muscle–tendon unit lengths has been investigated in pure stretch-hold experiments in vivo, the contribution to a SSC movement in human muscles remains unclear. In two sessions, 25 healthy participants performed isometric reference (ISO), shortening hold (SHO) and SSC contractions on an isokinetic dynamometer. We measured the net knee-joint torque, rotational mechanical work, knee kinematics and fascicle behavior (m. vastus lateralis) of the upper right leg. In session 1 the SHO- and SSC-magnitude was changed respectively (SHO: 50°–20°, 80°–20° and 110°–20°; SSC: 20°–50°–20°, 20°–80°–20° and 20°–110°–20°) and in session 2 the muscle–tendon unit length (SHO: 50°–20°, 80°–50° and 110°–80°; SSC: 20°–50°–20°, 50°–80°–50° and 80°–110°–80°; straight leg = 0°). In both sessions, rotational work was significantly (p < 0.05) increased in the SSC compared to the SHO contractions (in the range of 8.1–17.9%). No significant difference of joint torque was found in the steady-state for all SSC-magnitudes compared to the corresponding SHO contractions in session 1. In session 2, we found only significantly (p < 0.05) less depressed joint torque in the SSC at the longest muscle–tendon unit length compared to the corresponding SHO condition, without any differences in knee kinematics and fascicle behavior. Therefore, the physiological relevance of rFE might be particularly important for movements at greater muscle–tendon unit lengths.

Contribution of Stretch-Induced Force Enhancement to Increased Performance in Maximal Voluntary and Submaximal Artificially Activated Stretch-Shortening Muscle Action

In everyday muscle action or exercises, a stretch-shortening cycle (SSC) is performed under different levels of intensity. Thereby, compared to a pure shortening contraction, the shortening phase in a SSC shows increased force, work, and power. One mechanism to explain this performance enhancement in the SSC shortening phase is, besides others, referred to the phenomenon of stretch-induced increase in muscle force (known as residual force enhancement; rFE). It is unclear to what extent the intensity of muscle action influences the contribution of rFE to the SSC performance enhancement. Therefore, we examined the knee torque, knee kinematics, m. vastus lateralis fascicle length, and pennation angle changes of 30 healthy adults during isometric, shortening (CON) and stretch-shortening (SSC) conditions of the quadriceps femoris. We conducted maximal voluntary contractions (MVC) and submaximal electrically stimulated contractions at 20%, 35%, and 50% of MVC. Isometric trials were performed at 20° knee flexion (straight leg: 0°), and dynamic trials followed dynamometer-driven ramp profiles of 80°–20° (CON) and 20°–80°–20° (SSC), at an angular velocity set to 60°/s. Joint mechanical work during shortening was significantly (p < 0.05) enhanced by up to 21% for all SSC conditions compared to pure CON contractions at the same intensity. Regarding the steady-state torque after the dynamic phase, we found significant torque depression for all submaximal SSCs compared to the isometric reference contractions. There was no difference in the steady-state torque after the shortening phases between CON and SSC conditions at all submaximal intensities, indicating no stretch-induced rFE that persisted throughout the shortening. In contrast, during MVC efforts, the steady-state torque after SSC was significantly less depressed compared to the steady-state torque after the CON condition (p = 0.034), without significant differences in the m. vastus lateralis fascicle length and pennation angle. From these results, we concluded that the contribution of the potential enhancing factors in SSCs of the m. quadriceps femoris is dependent on the contraction intensity and the type of activation.

Force enhancement in the human vastus lateralis is muscle-length-dependent following stretch but not during stretch

Purpose

Force enhancement is the phenomenon of increased forces during (transient force enhancement; tFE) and after (residual force enhancement; rFE) eccentric muscle actions compared with fixed-end contractions. Although tFE and rFE have been observed at short and long muscle lengths, whether both are length-dependent remains unclear in vivo.

Methods

We determined maximal-effort vastus lateralis (VL) force-angle relationships of eleven healthy males and selected one knee joint angle at a short and long muscle lengths where VL produced approximately the same force (85% of maximum). We then examined tFE and rFE at these two lengths during and following the same amount of knee joint rotation.

Results

We found tFE at both short (11.7%, P = 0.017) and long (15.2%, P = 0.001) muscle lengths. rFE was only observed at the long (10.6%, P < 0.001; short: 1.3%, P = 0.439) muscle length. Ultrasound imaging revealed that VL muscle fascicle stretch magnitude was greater at long compared with short muscle lengths (mean difference: (tFE) 1.7 mm, (rFE) 1.9 mm, P ≤ 0.046), despite similar isometric VL forces across lengths (P ≥ 0.923). Greater fascicle stretch magnitude was likely to be due to greater preload forces at the long compared with short muscle length (P ≤ 0.001).

Conclusion

At a similar isometric VL force capacity, tFE was not muscle-length-dependent at the lengths we tested, whereas rFE was greater at longer muscle length. We speculate that the in vivo mechanical factors affecting tFE and rFE are different and that greater stretch of a passive component is likely contributing more to rFE at longer muscle lengths.

Electronic supplementary material

The online version of this article (10.1007/s00421-020-04488-1) contains supplementary material, which is available to authorized users.

Shortening-induced residual force depression in humans

When an isometric muscle contraction is immediately preceded by an active shortening contraction, a reduction in steady-state isometric force is observed relative to an isometric reference contraction at the same muscle length and level of activation. This shortening-induced reduction in isometric force, termed “residual force depression” (rFD), has been under investigation for over a half century. Various experimental models have revealed the positive relationship between rFD and the force and displacement performed during shortening, with rFD values ranging from 5 to 39% across various muscle groups, which appears to be due to a stress-induced inhibition of cross-bridge attachments. The current review will discuss the findings of rFD in humans during maximal and submaximal contractions.

Interdependence of torque, joint angle, angular velocity and muscle action during human multi-joint leg extension

PURPOSE

Force and torque production of human muscles depends upon their lengths and contraction velocity. However, these factors are widely assumed to be independent of each other and the few studies that dealt with interactions of torque, angle and angular velocity are based on isolated single-joint movements. Thus, the purpose of this study was to determine force/torque-angle and force/torque-angular velocity properties for multi-joint leg extensions.

METHODS

Human leg extension was investigated (n = 18) on a motor-driven leg press dynamometer while measuring external reaction forces at the feet. Extensor torque in the knee joint was calculated using inverse dynamics. Isometric contractions were performed at eight joint angle configurations of the lower limb corresponding to increments of 10° at the knee from 30 to 100° of knee flexion. Concentric and eccentric contractions were performed over the same range of motion at mean angular velocities of the knee from 30 to 240° s(-1).

RESULTS

For contractions of increasing velocity, optimum knee angle shifted from 52 ± 7 to 64 ± 4° knee flexion. Furthermore, the curvature of the concentric force/torque-angular velocity relations varied with joint angles and maximum angular velocities increased from 866 ± 79 to 1,238 ± 132° s(-1) for 90-50° knee flexion. Normalised eccentric forces/torques ranged from 0.85 ± 0.12 to 1.32 ± 0.16 of their isometric reference, only showing significant increases above isometric and an effect of angular velocity for joint angles greater than optimum knee angle.

CONCLUSIONS

The findings reveal that force/torque production during multi-joint leg extension depends on the combined effects of angle and angular velocity. This finding should be accounted for in modelling and optimisation of human movement.

The interdependence of Ca2+ activation, sarcomere length, and power output in the heart

Myocardium generates power to perform external work on the circulation; yet, many questions regarding intermolecular mechanisms regulating power output remain unresolved. Power output equals force × shortening velocity, and some interesting new observations regarding control of these two factors have arisen. While it is well established that sarcomere length tightly controls myocyte force, sarcomere length-tension relationships also appear to be markedly modulated by PKA-mediated phosphorylation of myofibrillar proteins. Concerning loaded shortening, historical models predict independent cross-bridge mechanics; however, it seems that the mechanical state of one population of cross-bridges affects the activity of other cross-bridges by, for example, recruitment of cross-bridges from the non-cycling pool to the cycling force-generating pool during submaximal Ca(2+) activation. This is supported by the findings that Ca(2+) activation levels, myofilament phosphorylation, and sarcomere length are all modulators of loaded shortening and power output independent of their effects on force. This fine tuning of power output probably helps optimize myocardial energetics and to match ventricular supply with peripheral demand; yet, the discernment of the chemo-mechanical signals that modulate loaded shortening needs further clarification since power output may be a key convergent point and feedback regulator of cytoskeleton and cellular signals that control myocyte growth and survival.

A multisegmental cross-bridge kinetics model of the myofibril

Striated muscle is a mechanical system that develops force and generates power in serving vital activities in the body. Striated muscle is a complex biological system; a single mammalian muscle fibre contains up to hundred or even more myofibrils in parallel connected via an inter-myofibril filament network. In one single myofibril thousands of sarcomeres are lined up as a series of linear motors. We recently demonstrated that half-sarcomeres (hS) in a single myofibril operate non-uniformly. We outline a mathematical framework based on cross-bridge kinetics for the simulation of the force response and length change of individual hS in a myofibril. The model describes the muscle myofibril in contraction experiments under various conditions. The myofibril is modeled as a multisegmental mechanical system of hS models, which have active and viscoelastic properties. In the first approach, a two-state cross-bridge formalism relates the hS force to the chemical kinetics of ATP hydrolysis, as first described by Huxley [1957. Muscle structure and theories of contraction. Prog. Biophys. Mol. Biol. 7, 255-318]. Two possible types of biological variability are introduced and modeled. Numerical simulations of a myofibril composed of four to eight hS show a non-uniform hS length distribution and complex internal dynamics upon activation. We demonstrate that the steady-state approximation holds only in restricted time zones during activation. Simulations of myofibril contraction experiments that reproduce the classic steady-state force-length and force-velocity relationships, strictly constrained or "clamped" in either end-held isometric or isotonic contraction conditions, reveal a small but conspicuous effect of hS dynamics on force.

Effects of velocity on maximal torque production in poststroke hemiparesis

Impaired torque production is a major physical impairment following stroke, and has been studied extensively in isometric conditions. However, functional use of a limb requires torque production during movement, and the effects of velocity on maximal torque production may be abnormally enhanced in the paretic limb. The purpose of this study was to quantify the effects of movement velocity on maximal torque production during isokinetic, concentric flexion and extension of the elbow in poststroke subjects. Three speeds were tested (30, 75, 120 deg/s) over a 100-deg range of motion. To control for strength variations between subjects and limbs, isokinetic torques were normalized by peak isometric torque. As flexion velocity increased, paretic limb torque decreased at a greater rate than in the unaffected limb. During extension, paretic limb torque was much lower than torque in the unaffected limb at all speeds. In both flexion and extension, the disparity between limbs in the constant-velocity torque-angle curves became more pronounced as velocity increased. Torque decreased 44% +/- 7% in flexion and 63% +/- 9% in extension as velocity increased from 30 to 120 deg/s, whereas the corresponding decreases in the unaffected limb were only 9% +/- 5% in flexion and 16% +/- 4% in extension. No electromyographic (EMG) abnormalities were observed during flexion. During extension, EMG data provided evidence for abnormally increased antagonist coactivation in brachioradialis and markedly reduced activation in triceps as potential contributors to the decreased extension torques. The finding that movement velocity produces large deficits in maximal torque might explain why functional use of the paretic limb is often impaired even though isometric strength appears adequate.

Haemodynamics and mechanics following partial left ventriculectomy: a computer modeling analysis

Mechanics following partial left ventriculectomy is still poorly understood. A computational cylindrical model of the left ventricle was developed, based on the myocardial fibre behaviour for the evaluation of the mechanical and haemodynamical effects of the operation. A healthy left ventricle with physiological geometry and function and a dilated hypokinetic heart were investigated. Haemodynamic and mechanical data were obtained at baseline and compared with those obtained at different degrees of volume reduction. Data included: ejection fraction (EF); stroke volume (SV); end-systolic and end-diastolic pressure-volume relationships (ESPVR and EDPVR), and efficiency. EF increases following volume reduction in both simulation but, concurrently, SV shows modest improvement (dilated ventricle) or reduction (healthy ventricle) at progressive degrees of resection. The ESPVR and EDPVR slope increases and shifts leftward with the resection extent, but the increase of the ESPVR slope is more pronounced in dilated ventricle. Efficiency is improved in the dilated heart after resections, while does not improve when the healthy-heart volume is reduced. The simulation of partial left ventriculectomy suggests an improvement of systolic performance, counterbalanced by increased diastolic stiffness following inverse remodelling. Efficiency of simulated dilated ventricles is enhanced by volume reduction, suggesting a favourable effect of reduction of the metabolic demand of the failing heart.

Rat medial gastrocnemius muscles produce maximal power at a length lower than the isometric optimum length

The interaction of relative muscle length and force-velocity characteristics was investigated in the fully activated rat medial gastrocnemius muscle in situ. Average maximal isometric force (as a percentage of the of the maximal isometric force at L(o,iso)) at relative lengths measured below isometric optimum (L(o,iso)) was 96% at L(o,iso)-2 mm, 88% at L(o,iso)-4 mm and 58% at L(o,iso)-6 mm. Force-velocity curves were obtained at the four relative muscle lengths. There were no significant differences in maximal shortening velocity (approximately 280 mm x s(-1)) between the different muscle lengths. The highest power output (P<0.05) was found at L(o,iso)-2 mm (mean+/-SEM 435+/-19 mW). Peak power values at L(o,iso) (390+/-10 mW) and L(o,iso)-4 mm (395+/-12 mW) were not significantly different, whereas peak power was lowest (P<0.05) at L(o,iso)-6 mm. There was a significant (P<0.01) shift of approximately 1.5 mm in optimum muscle length for force generation during shortening contractions compared with isometric contractions. Shortening velocity had only a minor influence on optimum muscle length for force generation. It is concluded that fully activated muscles produce their maximal power at a length lower than L(o,iso). The difference in optimum length between isometric and dynamic contractions may be related to length-dependent variations in sarcomere length in series during shortening.

Ca2+ dependence of loaded shortening in rat skinned cardiac myocytes and skeletal muscle fibres

This study examined the effects of activator Ca2+ on loaded shortening and power output in skinned rat cardiac myocyte preparations, and fast- and slow-twitch skeletal muscle fibres at 12 degrees C. Shortening velocities were slowed at nearly all relative loads when Ca2+ activation levels were reduced to approximately 70% maximal isometric force (P4.5) in cardiac myocyte preparations, as well as in fast-twitch and slow-twitch skeletal muscle fibres. Peak absolute power outputs declined significantly as Ca2+ activation levels were progressively reduced from maximal to 30% P4.5 in all three striated muscle types, with the greatest change in fast-twitch fibres. In cardiac myocyte preparations, even peak relative power output progressively fell when Ca2+ activation levels were lowered to approximately 70, 50 and 30% P4.5. Peak relative power output also progressively fell in fast-twitch fibres as Ca2+ activation levels were lowered from maximal down to 50% P4.5. However, in slow-twitch fibres, peak relative power output decreased only at 70% P4.5 and then remained unchanged with further reductions in Ca2+ activation levels. The greater Ca2+ dependence of peak relative power output in cardiac myocytes and fast-twitch fibres may arise from a shared mechanism such as cooperative inactivation of the thin filament, which is likely to be slowest in less cooperative slow-twitch fibres. During submaximal Ca2+ activations, the time course of shortening became markedly curvilinear during isotonic shortening in all three muscle types. The progressive slowdown in shortening velocity during isotonic contractions was greatest in fast-twitch fibres, consistent with the higher degree of cooperativity of Ca2+ activation in fast-twitch fibres. Additionally, fast-twitch and slow-twitch fibre stiffness decreased in concert with the curvature of length traces during loaded shortening. These results are consistent with the idea that cooperative inactivation of the thin filament occurs during loaded shortening and such a mechanism may contribute to the progressive slowing and overall Ca2+ dependence of loaded shortening velocity.

The filament lattice of striated muscle

The filament lattice of striated muscle is an overlapping hexagonal array of thick and thin filaments within which muscle contraction takes place. Its structure can be studied by electron microscopy or X-ray diffraction. With the latter technique, structural changes can be monitored during contraction and other physiological conditions. The lattice of intact muscle fibers can change size through osmotic swelling or shrinking or by changing the sarcomere length of the muscle. Similarly, muscle fibers that have been chemically or mechanically skinned can be compressed with bathing solutions containing very large inert polymeric molecules. The effects of lattice change on muscle contraction in vertebrate skeletal and cardiac muscle and in invertebrate striated muscle are reviewed. The force developed, the speed of shortening, and stiffness are compared with structural changes occurring within the lattice. Radial forces between the filaments in the lattice, which can include electrostatic, Van der Waals, entropic, structural, and cross bridge, are assessed for their contributions to lattice stability and to the contraction process.

Changes in the maximum speed of shortening of frog muscle fibres early in a tetanic contraction and during relaxation

1. Isotonic shortening velocities at very light loads were examined in single fibres of the anterior tibialis muscle of the frog, Rana temporaria, using load-clamp recording and slack tests (temperature, 1-3 degrees C; initial sarcomere length, 2.25 microns). 2. Shortening velocities at very light loads (force-clamp recording) were found to be higher early in the rise of a tetanic contraction than during the plateau of the contraction. The upper limit of the load at which there was elevated shortening velocity early in the contraction was 1.5-5.4% of the maximum tetanic tension (Fo) depending on the particular fibre. 3. The maximum shortening velocity determined using the slack test method (Vo) was as much as 30% greater early in a contraction than at the tetanic plateau. Vo was elevated above the plateau level up to about 30 ms after the end of the latent period, which is equivalent to the time required for the force in an isometric contraction to rise to about 30% of Fo. Vo is depressed below the plateau value during relaxation at the cessation of stimulation. 4. Stimulation studies show that the cross-bridge model of Huxley (1957) predicts the maximum shortening velocity to be greater early in a contraction, when new actin binding sites are becoming activated and new cross-bridge connections are being formed rapidly, than during steady-state contraction. The elevated shortening velocity in the model is a consequence of new cross-bridges being formed in the pulling configuration, and there being a delay before the newly added bridges are dragged beyond their equilibrium position so they begin to retard shortening. The model also predicts that maximum shortening velocity should be depressed below the plateau level during early relaxation as cross-bridge binding sites are rapidly removed from the active population.

Negative developed tension in rapidly shortening whole frog muscles

High speed isovelocity shortening using a servo-controlled lever was performed on isolated whole frog sartorius muscles at long lengths to ensure substantial passive tension. The tension records of unstimulated control experiments were subtracted from the tension records of fully-tetanized experiments on the same muscles to yield the developed tension exerted by the contractile proteins alone. There are several main results: (1) the positive developed tension had the same relation with shortening speed observed by other researchers in single fibres with no passive tension present; (2) negative developed tension was always measured at velocities of shortening above Vmax, where Vmax (typically 1.5 muscle-lengths s-1 at 2 degrees C) is defined as the velocity of shortening observed to yield zero developed tension; (3) negative developed tension was roughly asymptotic to -0.05 T(o), where T(o) is the developed isometric tetanic tension for the muscle length at which the developed tension was measured during steady shortening; (4) negative developed tension diminished in magnitude at velocities of shortening above approximately 2.5 Vmax; (5) a 10 degrees C increase in temperature from 2 degrees C to 12 degrees C had no significant effect on the shape of the normalized force-velocity curve (%T(o) versus %Vmax), but did increase Vmax by a factor of 2.6 in agreement with the results of previous studies measuring Vmax in the absence of passive tension; (6) addition of curare in the saline bath did not affect the results.

Dynamic changes in human diaphragm length: maximal inspiratory and expulsive efforts studied with sequential radiography

1. The maximal voluntary pressure generated by the diaphragm (transdiaphragmatic pressure, Pdi) is about 50% greater during maximal expulsive efforts than during maximal inspiratory efforts against a closed airway. However, these pressures cannot be increased by interpolated phrenic stimuli in trained subjects. This suggests that variable neural drive is not responsible for the difference in voluntary pressure. To investigate whether dynamic changes in diaphragm length during inspiratory and expulsive efforts could account for this difference, we used digital sequential radiography at 6 frames per second. 2. During the development of peak Pdi in inspiratory efforts, total diaphragm length decreased by about 20% in the antero-posterior and lateral projections. During maximal expulsive efforts (with glottis open), the diaphragm shortened slightly in the early stage of pressure development but then lengthened due to contraction of abdominal muscles before peak pressure was achieved. 3. Given that force increases when a contracting muscle is lengthened (expulsive effort) and decreases during shortening (inspiratory effort), this study provides a definitive explanation for the difference in maximal voluntary pressure between pure inspiratory and expulsive efforts.

Tension as a function of sarcomere length and velocity of shortening in single skeletal muscle fibres of the frog

1. Simple measurements of muscle tension at fixed fibre or segment length produce a range of length-tension relationships, depending primarily on the duration of the interval between stimulation onset and tension measurement, in contradiction with the simple predictions of current models. This has been explained by non-uniformity in sarcomere lengths, leading to internal motion and, in turn, to increasing tension because the force-velocity relationship has a much greater slope for slow lengthening than for slow shortening. 2. Previous attempts to reduce the effect of internal motion have been focused on decreasing the initial extent of non-uniformity and measuring tension early in a contraction, when non-uniformities are at a minimum. An alternative approach that has not been attempted previously is to reduce the non-linearity of the force-velocity relationship by avoiding the discontinuity in slope at zero velocity. This is accomplished by imposing overall fibre shortening at velocities sufficient to ensure that all sarcomeres are shortening. 3. When the tension maintained during shortening was measured and plotted against sarcomere length for each release velocity used, linear length-tension relationships resulted that extrapolated to a common sarcomere length intercept. This was true whether the release was applied early in the tetanus or near the end of the 'creep phase' of tension rise. These observations were duplicated by computer simulation using a multisarcomere model of a muscle fibre. 4. These results provide strong support for the view that cross-bridges function as independent force generators and for the explanation of the creep phase of fibre or segment isometric tension as being due to internal motion. The results also imply that the force-velocity relationship scales with sarcomere length without changing shape. 5. Using this novel method for obtaining length-tension relationships, the sarcomere length at which active tension fell to zero was found, by extrapolation, to be 3.65 microns in semitendinosus fibres and 3.53 microns in tibialis anterior fibres from the frog (Rana temporaria).

Behaviour of the crossbridges in stretched or compressed muscle fibres

The maximum chord of the myosin heads is comparable to the closest surface-to-surface spacing between the myofilaments in a muscle at the slack length. Therefore, when the sarcomere length increases or when the fibre is compressed, the surface-to-surface myofilament spacing becomes lower than the head long axis. We conclude that, in stretched or compressed fibres, some crossbridges cannot attach, owing to steric hindrance. When the amount of compression is limited, this hindrance may be overcome by a tilting of the heads in the plane perpendicular to the filament axes; in this case, there is no consequence as concerns the crossbridge properties. In highly compressed fibres, the crossbridges become progressively hindered and all the crossbridges are hindered for an axis-to-axis spacing representing about 60% of the spacing observed under zero external osmotic pressure. In this case, both the isometric tension and the ATPase activity of the fibre are zero. In fibres stretched up to 3.77 microns (sarcomere length corresponding to the disappearance of the overlap between the thick and the thin filaments), the ratio of hindered crossbridges over the functional crossbridges may be estimated at about 55%. In stretched fibres, a noticeable proportion of crossbridges are sterically hindered and the crossbridges performance (e.g. constants of attachment and detachment) depends on filament spacing, i.e. on sarcomere length. Therefore, we think it is probably impossible to consider the crossbridges as independent force converters, since this idea requires that the crossbridge properties are independent of sarcomere length. In this connection, all the experiments performed on osmotically compressed fibres are of major importance for the understanding of the true mechanisms of muscle contraction.

Effect of active pre-shortening on isometric and isotonic performance of single frog muscle fibres

1. We studied the effects of shortening history on isometric force and isotonic velocity in single intact frog fibres. Fibres were isometrically tetanized. When force reached a plateau, shortening was imposed, after which the fibre was held isometric again. Isometric force after shortening could then be compared with controls in which no shortening had taken place. 2. Sarcomere length was measured simultaneously with two independent methods: a laser-diffraction method and a segment-length method that detects the distance between two markers attached to the surface of the fibre, about 800 microns apart. 3. The fibre was mounted between two servomotors. One was used to impose the load clamp while the other cancelled the translation that occurred during this load clamp. Thus, translation of the segment under investigation could be minimized. 4. Initial experiments were performed at the fibre level. We found that active preshortening reduced isometric force considerably, thereby confirming earlier work of others. Force reductions as large as 70% were observed. 5. Under conditions in which there were large effects of shortening at the fibre level, we measured sarcomere length changes in the central region of the fibre. These sarcomeres shortened much less than the fibre's average. In fact, when the load was high, these sarcomeres lengthened while the fibre as a whole shortened. Thus, while the fibre-length signal implied that sarcomeres might have shortened to some intermediate length, in reality some sarcomeres were much longer, others much shorter. 6. Experiments performed at the sarcomere level revealed that isometric force was unaffected by previous sarcomere shortening provided the shortening occurred against either a low load or over a short distance. However, if the work done during shortening was high, force after previous shortening was less than if sarcomeres had remained at the final length throughout contraction. The correlation between the force deficit and the work done during shortening was statistically significant (P = 0.0001). 7. Interrupting the tetanus for 0.5-3.0 s did not reverse the effects of shortening on isometric force; at least 5-10 min of rest were required before force recovered completely. 8. Sarcomeres accelerated during the period of shortening under constant load, indicating that the sarcomeres became progressively stronger. However, the acceleration was less than that predicted from the force-velocity relation applicable at each of the sarcomere lengths transversed during shortening. 9. Velocity of shortening appeared to be much more sensitive to previous shortening than isometric force. 10. Results obtained with the diffraction method were the same as those obtained with the segment method.(ABSTRACT TRUNCATED AT 400 WORDS)