Mechanical deactivation induced by active shortening in isolated muscle fibres of the frog.

1. The effect of active shortening on the time course and magnitude of isometric tension development during a single twitch and during an incompletely fused tetanus was studied at 0-2-1-2 degres C in isolated semitendinosus muscle fibres of the frog. 2. Active shortening caused a depression of the contractile force without markedly affecting the total duration of the twitch. The depressant effect increased with increasing amounts of sarcomere shortening. Sarcomere shortenings of 0-05 mum and 0-3 mum reduced the twitch force by approximately 5 and 20 percent of the maximal tetanic tension, respectively. 3. A given sarcomere shortening induced the same absolute amount of depression of the contractile strength when the movement was carried out at different times during the initial 200-250 msec after the stimulus. 4. The influence of load and velocity of shortening during the movement phase was studied. Differences in load ranging between zero and 1/3 of the maximal tetanic tension (with concomitant changes in speed of shortening from Vmax to approximately 1/5 of Vmax) did not affect the degree of depression markedly. Underthe conditions studied, the extent of movement appeared to be the only significant determinant of the depressant effect. 5. The reduction in force induced by active shortening persisted for 800-900 msec during an incompletely fused tetanus. 6. It is suggested that the depressant effect is based on a structural change in the myofilament system that is produced as the A and I filaments slide along each other during muscle activity.     

Shortening induced deactivation of skinned fibres of frog and mouse striated muscle

The depressant effect of active shortening, previously established in intact muscle fibres, was studied during calcium induced contractures of chemically skinned fibres from the semitendinosus muscle of Rana temporaria and the psoas muscle of the mouse. The decrease in contractile activity was determined by comparing the rate of force redevelopment (at a given tension level) after a large (test) and a small (control) release step. Under standard experimental conditions (ionic strength: frog 135 mM, mouse 190 mM; Ca²⁺ 3.0 μM; Mg²⁺: frog 25 μM, mouse 100 μM; MgATP^2-: frog 1.0 mM, mouse 2.0 mM) active shortening of 0.15 μm per sarcomere (in excess of control release) reduced the contractile activity by approximately 50% of the control in both frog and mouse muscle fibres. Full contractile activity was regained within 4 s during isometric activity after the shortening phase. The depressant effect of shortening was steadily reduced, to almost complete disappearance of the effect, by increasing the free calcium concentration within the range 1.5–12.0 μM. Similarly, an increase in ionic strength from 105 to 235 mM reduced the depressant effect by approximately 40%. In contrast, there was a progressive enhancement of the shortening effect as the magnesium ion concentration was increased from 25 to 590 μM. It is proposed that interaction between the myosin cross-bridges and the thin filament during sarcomere shortening leads to a decrease in troponin-calcium binding resulting in a temporary deactivation of the contractile system.     

The influence of dantrolene, diazepam, pentobarbital and 4-aminopyridine on shortening induced depression in isolated muscle fibres of the frog

The influence of dantrolene, diazepam, pentobarbital and 4-aminopyridine on the depressant effect of active shortening during twitch and tetanus was investigated in isolated muscle fibres of the frog. These drugs are all presumed to alter the kinetics of the release and re-uptake of activator calcium during the excitation-contraction process. The shortening induced depression was calculated as the difference in redeveloped force after a small (control) and a larger (test) release step and was expressed in per cent of the control. The depressant effect in response to a given amount of shortening during twitch contraction as well as during the plateau of a tetanus was approximately doubled after addition of 10 microM dantrolene. In contrast, diazepam (150 microM) reduced the shortening effect during the twitch to about one tenth of the value obtained in ordinary Ringer solution, while the force depression by shortening during tetanic stimulation remained essentially unaltered. Pentobarbital (1.0 mM) reduced the shortening induced depression during the twitch by 95-100%. 4-aminopyridine (3.0 mM) completely abolished the depressant effect of shortening obtained during the twitch and approximately halved the shortening effect during tetanic contraction.     

Fatigue vs. shortening-induced deactivation in striated muscle

Fatigue and shortening-induced deactivation, two conditions that both lead to reversible depression of the mechanical performance of striated muscle are briefly reviewed. Fatigue. Isolated fibres from frog skeletal muscle (1–3 °C) that are stimulated to produce a 1 s fused tetanus at 15 s intervals are brought into a state of reduced to 70–75% of the control) that is attributable to reduced performance of the myofibrils with no significant change in activation of the contractile system. A more intense stimulation programme (a single stimulus applied at 1–2 s intervals) reduces the tetanic force below 70% of the rested-state level. Under these conditions, failure of activation becomes increasingly important as a cause of the force decline. Deficient inward spread of activation is likely to account for at least part of the force decline after a period of intense fatiguing stimulation. Shortening-induced deactivation. Striated muscle that is allowed to shorten during activity loses some of its capacity to produce force, full restoration of the contractile strength being attained 1–2 s after the shortening phase. The depressant effect of shortening is demonstrable in skinned preparations as well as in intact muscle fibres and the magnitude of the effect is dependent on the state of activation of the muscle fibre when the movement occurs. The experimental evidence supports the view that sliding of the thick and thin filaments during activity reduces the affinity for calcium at the regulatory sites on the thin filament, leading to a transitory deactivation of the contractile system.     

The effects of 2,3-butanedione monoxime (BDM) on the force-velocity relation in single muscle fibres of the frog

The effects of 2,3-butanedione monoxime (BDM) on the forcevelocity relaion were studied in single fibres from the anterior tibialis muscle of Rana temporaria (2.2 μm sarcomere length, temperature 1.9-2.4 d?C). BDM (1.0 and 1.8 mM) suppressed the maximum tetanic force (P_o) and the maximum speed of shortening (V_max), and increased the main curvature of the force-velocity relation. The biphasic shape of the forcevelocity curve was maintained well in the presence of BDM, but the interrelation between the two portions of the forcevelocity relation was significantly changed. Caffeine (0.5 mM) added in the presence of BDM increased the initial rate of rise of force during twitch and tetanus, increased the twitch amplitude, but did not affect the maximum tetanic force. The latter finding suggests that the contractile system was fully activated during tetanus in the presence of BDM. The results support the view that BDM affects the cross-bridge function by exerting a direct action upon the contractile apparatus. The decrease in tetanic force and the change of the force-velocity relation induced by BDM may be interpreted to show that a larger fraction of the attached cross-bridges is in a state of low force production under the influence of BDM. This view is further supported by the observation that the instantaneous stiffness of the muscle fibre is reduced proportionally less by BDM than the tetanic force.     

Depressant effect of active shortening in the anterior byssus retractor muscle of Mytilus edulis

The effect of shortening during activity, previously characterized in vertebrate striated muscle, was investigated in the anterior byssus retractor muscle (ABRM) of the mollusc Mytilus edulis. This muscle is considered to have an essentially myosin-linked Ca²⁺ - regulatory system. Release steps of different amplitude were performed during isometric phasic contraction, and force redevelopment was recorded at a muscle length L₁, defined as 90% of the muscle length at which a slight resting tension, approximately 1 mN, appeared in the presence of 2.5 × 10^-5 M 5-HT. Active shortening caused a graded depression of the contractile force without affecting the total duration of the mechanical response. Peak redeveloped force after muscle shortening of 0.06 L₁ and 0.18 L₁ was reduced by approximately 1.5% and 7.0%, respectively, of the isometric tension value at L₁. The shortening effect was fully reversible, and had a lifetime of approximately 8 to 9 s. The depressant effect of active shortening was augmented at a reduced degree of activation of the muscle. The presence of caffeine and dantrolene and altered tonicity of the extracellular medium (0.9 T-1.2 T) did not significantly affect the shortening induced depression obtained at maximum phasic activation of the preparation. The nature of the shortening effect is compared to that obtained in vertebrate striated muscle and is discussed on the basis of differences in Ca²⁺ -regulation of the contractile system in these two muscles.     

Effects of amrinone on twitch, tetanus and shortening kinetics in mammalian skeletal muscle

The contractile effects of amrinone were studied on twitch and tetanus responses of mouse lumbrical muscles. Amrinone (1.1 mM) increased the maximum force level during the tetanus, reduced the rate of rise of force during the onset of tetanus and prolonged the time from the last stimulus to half-relaxation. The rate of redevelopment of force after a release during the tetanus plateau was likewise reduced. Amrinone (greater than or equal to 0.5 mM) increased the time to peak twitch force and the time to half-relaxation during the twitch. The peak twitch force was not significantly changed. The force-velocity relation was markedly affected by amrinone: there was a decrease in maximum velocity of shortening, an increase in maximum isometric force and a decrease in curvature of the force-velocity relation. The results suggest that amrinone modulates the kinetic properties of the myosin cross-bridges by exerting a specific effect upon the myofilament system. The latter effects are mainly responsible for the contractile changes produced by amrinone in mouse skeletal muscle. Amrinone appears to have little effect on the metabolism of activator calcium in this particular preparation.     

Relationship between Sarcomere Length and Active Force in Rabbit Papillary Muscle1

Isometric peak twitch force (stimulation frequency 0.5/s; 29.5–30.5°C) was correlated with sarcomere length in isolated papillary muscles of the rabbit. Sarcomere length was measured from photographic recordings (1.5 ms exposure time) performed at rest between contractions and at the time of isometric peak twitch force. The sarcomere length at rest was found to be relatively uniform throughout the preparation and to be linearly related to the overall muscle length within the range L_max- 0.85 L_max. The distribution of sarcomere lengths increased considerably as the muscle went from rest to activity. Studies of surface markers showed different degrees of shortening (or elongation) of individual segments along the length of the preparation. The mean resting sarcomere length at L_max (the optimum muscle length for force production) was 2.44±0.01 μm (grand mean ± S.E., 7 muscles). The mean active sarcomere length at L_max was 2.29 ± 0.04 /μm. Active force declined steeply as the muscle length was reduced below L_max. At a resting sarcomere length of 2.0 μm, active force was approximately 1/3 of the maximum. The observed differences between the length-tension relationships in myocardium (twitch responses) and skeletal muscle (tetanic contractions) are discussed on the basis of a length dependency of the activation process in cardiac muscle.     

The influence of varied tonicity of the extracellular medium on the depressant effect of active shortening in vertebrate striated muscle

The depressant effect of active shortening was studied during isometric twitch contraction in intact single muscle fibres of the frog at varied tonicity of the extracellular medium. The shortening effect was calculated as the difference in peak redeveloped force after a small (control) and a larger (test) release step and was expressed in per cent of the isometric tetanic force. The solutions were made hypertonic by addition of sucrose (relative tonicity 1.22T and 1.44T) and hypotonic by reduction of NaCl (relative tonicity 0.81T and 0.62T). The shortening induced depression decreased from 13.0 ± 1.2% in normal Ringer solution to 7.8 ± 1.3% after immersion of the fibre in 1.22T solution (mean ± SE, n = 7). This reduction of the depressant effect by shortening was less than half the size of that obtained in 1.44T solution. An increased force depression by shortening, from 12.7 ± 1.2% to 17.1 ± 1.5% (mean ± SE, n=7), was obtained when normal Ringer was replaced by 0.81T solution. This enhancement was further augmented in 0.62T solution. Experimental evidence is presented supporting the view that the influence of tonicity on the depressant effect of shortening is not due to tonicity induced changes in fibre width. The effect of varied tonicity on the shortening induced depresison appears to be essentially related to alterations in intracellular ionic strength.     

Moderate fatigue studied at great sarcomere lengths in frog single muscle fibres

Single fibres from the anterior tibialis muscle of Rana temporaria (temperature, 2–3 ^oC) were moderately fatigued at a prestretched sarcomere length (approximately 3.6 μm) by reducing the intervals between 2 s tetani from 240 to 20 s. Changes in length of marked segments along the muscle fibre were monitored during fixed–end tetani using a photoelectric recording system. In contrast to the situation at 2.2 μm sarcomere length, the central region of the fibre was elongated during tetanus at the prestretched fibre length, whereas the segments next to the fibre–tendon junctions (end segments) shortened correspondingly. These length changes were associated with a slow climb of force (tension creep). During development of fatigue, the maximum tetanic tension was reduced to about 78% of the control value and the time course of the tetanus was markedly changed. The tension creep observed in control tetanus was greatly reduced during fatigue, the tetanic tension remaining nearly constant after an initial rising phase. The change in shape of the force myogram during fatigue was associated with a considerable reduction in the amplitude of segment movements along the fibre. The results are explainable by assuming that the end segments (having shorter sarcomeres) become more fatigued than other parts in series along the fibre. At the same time the central segments are likely to have acquired a higher ability to resist stretch by the end segments.     

The time course of the contractile force measured during a twitch under fixed sarcomere length

Summary A sarcomere length-controlled feedback system was constructed utilizing the laser diffraction technique of Haugen & Sten-Knudsen (1976) to detect sarcomere length changes. The system allowed the sarcomere length to be kept constant within 0.02% during an isometric twitch. The contractile force developed approximates closely to the force exerted by the crossbridges when their translatory movements are prevented. Thus, the force developed under this condition should correspond to the intensity of the active state as defined by A. V. Hill (1949). The time course of the twitch under constant sarcomere length differs substantially from that of the active state curves obtained using quick stretches and quick releases. Thus, (1) the force does not rise quickly to its maximum but rather resembles the fixed-end twitch curve by leading it only slightly (5 ms at 5°C). (2) Its peak value does not reach the level of the tetanic plateau, but is only 9% higher than the maximum fixed-end twitch tension. (3) The force remains above the curve of the fixed-end twitch during its entire course. It is shown that the quick-stretch procedure which results in active state curves as those obtained by A. V. Hill (1949) led to a considerable elongation of the sarcomeres.It is concluded that the slow rise of the contractile force under ordinary isometric conditions is due to properties inherent in the contractile machinery other than those resulting from the extension of series elastic components.     

Dynamic properties of denervated fast and slow twitch muscle of the cat

1. Isometric and isotonic contractions were recorded from cat flexor digitorum longus (FDL) and soleus muscles denervated for 28-35 days and compared with the responses of contralateral control muscles.2. Isometric twitch time to peak was longer in the denervated muscles than in the controls. Tetanic tension was reduced whether expressed as absolute units, per gram or per unit area. Absolute twitch tensions were reduced less below the control values with a consequent increase of twitch-tetanus ratio. These results agree with earlier reports; but, in contrast, there was no reduction in the relative rate of rise of tension in the isometric tetanus of denervated compared with control FDL. The maximum rate of rise of tension in the twitch was only a little less than that in the fully fused tetanus in the denervated muscles.3. Whole muscle shortening velocity was reduced in the denervated soleus with no change in the number of sarcomeres per fibre. There was a smaller change in FDL muscles. This was the result of an increase in the number of sarcomeres per fibre which followed denervation in FDL, compensating for a decreased velocity of shortening of the sarcomere. Maximum sarcomere shortening velocity was reduced by 30% relative to control values in both muscles.4. Measurements were made of the isometric contractions resulting from pairs of stimuli in an attempt to assess the period of maximal activation in the twitch. Maximum summation occurred when the second stimulus was given close to the time of the peak of the twitch in the denervated muscles.     

Shortening induced deactivation in the guinea-pig urinary bladder

Shortening induced deactivation, the depressant effect of active muscle shortening on the subsequently measured isometric force, has been shown in smooth muscle strips and rings. The guinea-pig bladder permits the investigation of this phenomenon in a whole organ preparation. Previous work in our laboratory showed that shortening of the in vitro guinea-pig detrusor muscle had a depressant effect on the isovolumetric pressure that could be generated immediately afterwards. To test the hypothesis that this was caused by deactivation, the effects of active and passive detrusor shortening on the subsequently measured isovolumetric pressure were compared. The isovolumetric pressures measured after 5 min periods of recovery were taken as control values. It was found that the isovolumetric pressure after passive shortening was 7% smaller than the isovolumetric pressure without preceding shortening. This difference was ascribed to viscoelastic relaxation during shortening. Active shortening had an additional 8% depressant effect on isovolumetric pressure compared with passive shortening. The effects of active and passive shortening differed significantly. It was concluded that shortening induced deactivation in the guinea-pig urinary bladder smooth muscle in toto can be considered proven. The fact that deactivation is shown both by striated and smooth muscle preparations is in line with the assumption that it is caused by reduced actin-myosin interaction. The hypothesis that (in striated muscle) the latter is effected by a decrease in troponin–calcium binding, however, needs reconsideration.     

2,3‐Butanedione monoxime increases speed of relaxation in single muscle fibres of frog

The effects of 2,3‐butanedione monoxime (BDM) on intracellular Ca²⁺ transient and cross‐bridge function were studied in frog single fibres from the anterior tibialis muscle of Rana temporaria (sarcomere length, 2.2 μm; temperature, 2–4 °C). The fluorescent dye fluo‐3 was used to monitor the intracellular free calcium concentration ([Ca²⁺]_i) during isometric contractions. BDM (1–5 mM ) reduced the amplitude of the Ca²⁺ transient during twitches, but this effect was too small to explain the marked inhibition of BDM on twitch force. [Ca²⁺]_i reached at the end of 1‐s tetanic stimulation was not significantly affected by BDM (1.0 and 1.8 mM ) while the maximum tetanic tension was substantially reduced. The rate of relaxation during isometric tetanus was increased by BDM whereas the rate of decay of the Ca²⁺ transient was reduced in the presence of BDM. The results strongly suggest that BDM, under the experimental conditions used, mainly affects the contractile machinery resulting in altered performance of the cross‐bridges. These effects of BDM were evaluated in terms of the cross‐bridge model of 17 which was fitted to the experimental force–velocity data in the presence and absence of BDM.     

Effects of 2,3-butanedione monoxime on the crossbridge kinetics in frog single muscle fibres

Summary The effects of 2,3-butanedione monoxime (BDM) on contraction characteristics were studied at 5C in single intact fibres isolated from the tibialis anterior muscle of the frog. The force-velocity relation was determined using the controlled-velocity method in either whole fibres or short fibre segments in which sarcomere shortening was measured by a laser light diffraction method. It is shown that 3mm BDM decreases the speed of rise and the amount of tetanus tension, reduces the maximum velocity of shortening and increases the curvature of the force-velocity relation, as well as the value for the stiffness to tension ratio. BDM also slowed down the redevelopment of tetanus tension after a period of unloaded shortening both in fixed-end and in length-clamp conditions. In normal and in BDM-treated fibres length-clamping increased the speed of the initial rise of tetanus tension but not that of the recovery after shortening. The observed force-velocity data points were fitted by the Huxley (1957) equation. It was found that BDM produces a conspicuous decrease of the rate constant for crossbridge attachment. This effect, and also a reduction of the force per crossbridge, are responsible for the depression of the contractile characteristics produced by BDM.     

The effects of temperature on the mechanical performance in fatigued single muscle fibers of the frog induced by twitch and tetanus

Muscle fatigue induced by consecutive twitches or tetani was studied in single skeletal muscle fibers of the frog, Rana japonica. The fatigue by twitch appeared sooner after the start of stimulation at lower temperatures (2-5 degrees C) than at higher ones (15-20 degrees C), while the fatigue by tetanus appeared sooner at higher temperatures. When a twitch-fatigued fiber was bathed in a solution with caffeine (15 mM), the contracture force was much higher than the fatigued force, while in tetanus fatigue, the force by caffeine was not different from the fatigued force. The length-force relation in fatigued fibers was compared with that in pre-fatigue at low and high temperatures. It was noticed that the ascending limb of the length-force curve in fatigued fibers by twitch was lower than that in pre-fatigue at the low temperatures; namely, the fatigue by twitch was more marked in shorter muscle length, while no marked change in the length-force relation was detected in the tetanus fatigue at the low and high temperatures. The maximum shortening velocity, measured by the slack test, decreased in both types of fatigue. These results suggest that the fatigue by twitch may be mainly due to the failure of activation of the contractile system, while in the fatigue by tetanus, the rate of the interaction between actin and myosin may be impaired due to the change in intracellular chemical environment.     

Crossbridge kinetics in single frog muscle fibres in presence of ethylene glycol

Single fibres isolated from frog muscle were tetanically stimulated at 14°C to produce isometric tetani at a sarcomere length of about 2.16 m, using a striation follower device to measure the sarcomere length of a selected segment of fibre. Force–velocity data were obtained by applying ramp releases at pre-set velocity at the tetanus plateau. Sarcomere stiffness was measured at isometric plateau and during isotonic shortening by using sinusoidal length changes at 2 kHz frequency and about 1 nm per half sarcomere (hs) peak to peak amplitude. A correction method was used to compensate for the force truncation due to the quick recovery. After data collection, the bathing solution was substituted with Ringer plus ethylene glycol (EG) at 2 M (11.2% v/v). When the fibre was fully equilibrated with the new solution, the measurements were repeated. Ethylene glycol reduced the speed of the tetanus rise and tetanus relaxation without altering the isometric tension, and reduced the maximum shortening velocity by about 20%. During isotonic contraction tension and stiffness at each given shortening velocity were reduced by about the same amount, so that the stiffness/tension ratio remained almost unaltered. Force–velocity and stiffness data in both standard and EG Ringer were analysed in terms of a two state model (Huxley, 1957). The analysis showed that our results can be accounted for by assuming that EG at 2 M concentration reduces all the rate constants for crossbridges interaction by about the same amount.     

The effects of length and stimulus rate on tension in the isometric cat soleus muscle

1. By subdividing ventral roots and supplying stimulating pulses to different groups of motor units in rotation, smooth contractions of soleus could be obtained with low rates of stimulation.2. Isometric tension was recorded with different rates of stimulation, and at different muscle lengths.3. Longitudinal histological sections were cut from muscles fixed at different lengths, and sarcomeres were measured. Mean sarcomere lengths in soleus could then be related to the angle at the ankle.4. At high rates of stimulation the maximum active tension was obtained at a length corresponding to an angle of about 60 degrees at the ankle, and a mean sarcomere length of about 2.8 mu. The isometric tension fell only slightly on shortening the muscle to a length equivalent to 100 degrees , and a mean sarcomere length about 2.3 mu. Further shortening caused a marked fall in tension.5. There was a reciprocal relationship between stimulus rate and muscle length; when the muscle was long low rates of stimulation gave near maximal tension, whereas at short lengths the maximum tension was reached only when the stimulus rate was very high. It is suggested that stimulating pulses activate the contractile machinery of the muscle more effectively at long than at short muscle lengths.6. When at low rates of stimulation pulses were distributed among the motor units in rotation to give a smooth contraction, the tension rose higher than during the unfused tetanus that accompanied synchronous stimulation of the same motor units at the same rate. It is suggested that in an unfused tetanus internal movement of the muscle reduces the tension below that developed in a truly isometric state.7. The rate of rise of tension in an isometric tetanus varied with both muscle length and rate of stimulation. At each stimulus rate there was a range of lengths in which the isometric tension developed slowly, this was the same length range in which, at that stimulus rate, the length tension curve was steep.     

Fatigue in frog skeletal muscle fibres and effects of methylxanthine derivatives

The influence of theophylline and the related drug caffeine on the mechanical performance of fatigued muscle fibre isolated from semitendinosus muscle of Rana temporaria (2.5–6.7 °C) was investigated. The fibre was stimulated supramaximally to produce 1 s fused tetani and 2 s and 10 s partially fused tetani at intervals of 10 min. Fatigue was produced by shortening the contraction interval to 15 and 30 s. This caused a 15–20% decline in the maximum tension during fused tetanus and a 40–50% decline during partially fused tetanus. Theophylline and caffeine (0.1-0.5 mM) did not change the maximum tension developed by the fatigued fibre during fused tetanization. Both drugs, however, markedly increased the tension output of the fatigued muscle fibre during partially fused tetanus. It was observed that the increase in partially fused tetanic tension by theophylline and caffeine was associated with an increase in the degree of fusion. This later effect was even more pronounced in the presence of diethyl-stilboestrol. It is concluded that these drugs may not reverse the metabolic changes caused by fatigue, since they are unnable to increase fused tetanic force of a fatigued muscle fibre. The increase in partially fused tetanic tension of a fatigued muscle fibre by these drugs is probably due to enhancement of the activator calcium release from the sarcoplasmic reticulum in response to stimulation.     

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°C and an initial sarcomere length of 2.68 m 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°C; sarcomere shortening near body temperature (35°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 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.