Lengthening Phase Research Articles

Tissue response during monofocal and bifocal leg lengthening in patients

The purpose of this investigation was to compare the tissue response during mono- and bifocal limb lengthening. The study includes four patients undergoing leg lengthening. All patients started out bifocally with a total diurnal distraction of 1.75 mm, but proceeded monofocally with a rate of 1 mm a day when the distal distraction was terminated due to contractures or pain. The tissue response was monitored by registration of axial force in the distraction rods. The force increased linearly during bifocal lengthening, but culminated or decreased in the period of monofocal lengthening. Average tissue stiffness, defined as the immediate force increase due to each 0.25 mm distraction increment, was significantly higher in the bifocal lengthening phase. The force decay between each distraction was significantly lower during bifocal lengthening, thus indicating decreased tissue accommodation. Details in the force registrations indicated that the soft tissue, not the regenerate, was the main contributor to the tensile force. Conclusively, the tissues at the two osteotomy sites do not lengthen independently. Bifocal lengthening exposes the entire soft tissue to large loads, resulting in increased tissue stiffness and reduced ability to adapt to the increased length. Accordingly, bifocal leg lengthening requires special attention to soft tissue adaptation.

Bone consolidation is enhanced by rhBMP-2 in a rabbit model of distraction osteogenesis

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is a differentiation factor which has been shown to induce bone formation and heal bony defects in a variety of animal models. A possible application of rhBMP-2 is to accelerate bone regeneration during distraction osteogenesis, which clinically is a long procedure, often involving significant complications. In this study we tested the ability of rhBMP-2 to accelerate the consolidation phase of distraction osteogenesis in a rabbit model of leg lengthening. Tibiae were lengthened 2 cm over a period of ten days. rhBMP-2 was administered at the end of the lengthening phase. Two modes of rhBMP-2 application were tested: surgical implantation of rhBMP-2/ACS (absorbable collagen sponge) into the regenerate (50 μl of 1.5 mg/ml rhBMP-2, total dose=75 μg rhBMP-2), and percutaneous injection of rhBMP-2/buffer (0.1 ml of 0.75 mg/ml rhBMP-2, total dose=75 μ g rhBMP-2) into three sites within the regenerate. Also, there were three groups of control animals: (1) no surgical intervention, (2) surgical implantation of buffer/ACS and (3) percutaneous injection of buffer. Rabbits were sacrificed at 5, 14 and 28 days after the interventions. Radiographic evaluation indicated a significant increase in bony union of the distraction regenerate in the rhBMP-2 treated groups compared with the untreated groups at 5 and 14 days. At 28 days, formation of a cortex and reestablishment of the medullary canal was evident only in the rhBMP-2 treated groups. The bone mineral content (BMC) of the regenerate was significantly higher in the rhBMP-2 treated groups at 5 and 14 days. However, at 28 days, BMC of the regenerate was similar in all groups. The average volumetric density of the regenerate was significantly higher in the rhBMP-2 injection group at day 14. In summary, both injection of rhBMP-2/buffer and implantation of rhBMP-2/ACS enhanced the consolidation stage of distraction osteogenesis in this rabbit model.

A thixotropic effect in contracting rabbit psoas muscle: prior movement reduces the initial tension response to stretch.

Paired ramp stretches and releases ('triangular length changes', typically 0.04 +/- 0.09L0 s-1; mean +/- s.e.m.) were imposed on permeabilised rabbit psoas fibre segments under sarcomere length control. In actively contracting fibres, the tension response to stretch was biphasic; tension rose more rapidly during the first 0. 005L0 of the imposed stretch than thereafter. Tension also dropped in a biphasic manner during shortening, and at the end of the length change was reduced below the steady state. If a second triangular length change was imposed shortly after the first, tension rose less sharply during the initial phase of lengthening, i.e. the stiffness of the muscle during the initial phase of the response was reduced in the second stretch. This is a thixotropic effect. If a third triangular length change was imposed on the muscle, the response was the same as that to the second. The time required to recover the original tension response was measured by varying the interval between triangular length changes. Recovery to steady state occurred at a rate of approximately 1 s-1. The stiffness of the muscle during the initial phase of the response scaled with the developed tension in pCa (= -log10[Ca2+]) solutions ranging from 6.3 (minimal activation) to 4.5 (saturating effect). The relative thixotropic reduction in stiffness measured using paired length changes was independent of the pCa of the activating solution. The thixotropic behaviour of contracting skeletal muscle can be explained by a cross-bridge model of muscle contraction in which the number of attached cross-bridges is temporarily reduced following an imposed movement.

Reflex facilitation during the stretch-shortening cycle

Maximal torque during the concentric phase of a movement has been shown to be enhanced by prior eccentric muscle actions, a movement strategy referred to as the stretch-shortening cycle. Although the mechanical basis for this enhancement is well established, the neural component is not. We hypothesized that brief high-frequency bursts of spindle afferent discharge during the eccentric phase of the stretch-shortening cycle could be one mechanism for facilitating the volitional drive. To test this hypothesis, three sets of experiments were done. In the first ( N=15), we demonstrated that both the peak and mean EMG of the soleus (S) and lateral gastrocnemius (LG) muscles were considerably greater during a reciprocal hopping (RHOP) task than for maximum isometric contractions (MIVCs). In the second experiment, we tested whether the dynamic nature of the RHOP or the eccentric phase of the RHOP contributed to the EMG potentiation. Peak and mean EMG produced with a concentric hop (CHOP), in which the lengthening phase of the hop was eliminated, were compared with that produced with the RHOP and MIVCs conditions ( N=7). The RHOP produced greater peak EMG than either the CHOP or the MIVCs while the mean EMG for both hopping conditions was considerably more than the MIVCs. In the final experiment, we attempted to mimic the brief high-frequency burst of spindle afferent activity during the lengthening phase of the stretch-shortening cycle in the absence of muscle length changes. High-frequency (100 Hz) afferent stimulation (HFS) was delivered during MIVCs. At rest, the HFS produced negligible EMG activity but when superimposed over MIVCs produced a marked potentiation of the S EMG over values obtained during MIVCs alone. Evidence that HFS synchronizes the EMG associated with volitional activation is also provided. We conclude that a substantial but brief facilitation and possible synchronization of the neural drive is provided by the spindle afferents during the eccentric phase of the stretch-shortening cycle.

Power output of sound-producing muscles in the tree frogs Hyla versicolor and Hyla chrysoscelis.

Sound-producing muscles provide the opportunity of studying the limits of power production at high contractile frequencies. We used the work loop technique to determine the power available from the external oblique muscles in two related species of North American gray tree frog, Hyla chrysoscelis and Hyla versicolor. These trunk muscles contract cyclically, powering high-intensity sound production in anuran amphibians. The external oblique muscles in H. chrysoscelis have an in vivo operating frequency of 40-55 Hz at 20-25 degrees C, whereas in H. versicolor these muscles contract with a frequency of 20-25 Hz at these temperatures. In vivo investigations have shown that these muscles use an asymmetrical sawtooth length trajectory (with a longer shortening phase compared with the lengthening phase) during natural cycles. To study the influence of this particular length trajectory on power output, we subjected the muscles to both sinusoidal and sawtooth length trajectories. In both species, the sawtooth trajectory yielded a significantly higher power output than the sinusoidal length pattern. The maximum power output during sawtooth cycles was similar in both species (54 W kg(-)(1) in H. chrysoscelis and 58 W kg(-)(1) in H. versicolor). These values are impressive, particularly at the operating frequencies and temperatures of the muscle. The sinusoidal length trajectory yielded only 60 % of the total power output compared with the sawtooth trajectory (34 W kg(-)(1) for H. chrysoscelis and 36 W kg(-)(1) for H. versicolor). The optimum cycle frequencies maximizing the power output using a sawtooth length pattern were approximately 44 Hz for H. chrysoscelis and 21 Hz for H. versicolor. These frequencies are close to those used by the two species during calling. Operating at higher frequencies, H. chrysoscelis maximized power at a strain amplitude of only 8 % compared with a value of 12 % in H. versicolor. These strains match those used in vivo during calling. The stimulus timing observed in vivo during calling was also similar to that yielding maximum power at optimal frequency in both species (6 ms and 8 ms before the start of shortening in H. chrysoscelis and H. versicolor, respectively). As expected, twitch duration in H. chrysoscelis is much shorter than that in H. versicolor (23 ms and 37 ms, respectively). There was a less remarkable difference between their maximum shortening velocities (V(max)) of 13.6 L(0 )s(-)(1) in H. chrysoscelis and 11.1 L(0 )s(-)(1) in H. versicolor, where L(0) is muscle length. The force-velocity curves are very flat, which increases power output. At the myofibrillar level, the flat force-velocity curves more than compensate for the lower peak isometric force found in these muscles. The data presented here emphasize the importance of incorporating in vivo variables in designing in vitro studies.

Energy absorption during running by leg muscles in a cockroach

Biologists have traditionally focused on a muscle's ability to generate power. By determining muscle length, strain and activation pattern in the cockroach Blaberus discoidalis, we discovered leg extensor muscles that operate as active dampers that only absorb energy during running. Data from running animals were compared with measurements of force and power production of isolated muscles studied over a range of stimulus conditions and muscle length changes.We studied the trochanter-femoral extensor muscles 137 and 179, homologous leg muscles of the mesothoracic and metathoracic legs, respectively. Because each of these muscles is innervated by a single excitatory motor axon, the activation pattern of the muscle could be defined precisely. Work loop studies using sinusoidal strains at 8 Hz showed these trochanter-femoral extensor muscles to be quite capable actuators, able to generate a maximum of 19-25 W kg-1 (at 25 degreesC). The optimal conditions for power output were four stimuli per cycle (interstimulus interval 11 ms), a strain of approximately 4 %, and a stimulation phase such that the onset of the stimulus burst came approximately half-way through the lengthening phase of the cycle. High-speed video analysis indicated that the actual muscle strain during running was 12 % in the mesothoracic muscles and 16 % in the metathoracic ones. Myographic recordings during running showed on average 3-4 muscle action potentials per cycle, with the timing of the action potentials such that the burst usually began shortly after the onset of shortening. Imposing upon the muscle in vitro the strain, stimulus number and stimulus phase characteristic of running generated work loops in which energy was absorbed (-25 W kg-1) rather than produced. Simulations exploring a wide parameter space revealed that the dominant parameter that determines function during running is the magnitude of strain. Strains required for the maximum power output by the trochanter-femoral extensor muscles simply do not occur during constant, average-speed running. Joint angle ranges of the coxa-trochanter-femur joint during running were 3-4 times greater than the changes necessary to produce maximum power output. None of the simulated patterns of stimulation or phase resulted in power production when strain magnitude was greater than 5 %. The trochanter-femoral extensor muscles 137/179 of a cockroach running at its preferred speed of 20 cm s-1 do not operate under conditions which maximize either power output or efficiency. In vitro measurements, however, demonstrate that these muscles absorb energy, probably to provide control of leg flexion and to aid in its reversal.

In vivo performance of trunk muscles in tree frogs during calling.

We used high-speed video and electromyography (EMG) to measure in vivo performance of the trunk muscles (external obliques) in two related species of North American gray tree frogs, Hyla versicolor and Hyla chrysoscelis. Both species produce trilled calls with high sound intensity, but the sound pulse frequency within calls in H. chrysoscelis is twice that in H. versicolor. In both species, sound pulse frequency is directly correlated with the active contractions of the trunk muscles. The length trajectory during contraction and relaxation displays a saw-tooth pattern with a longer shortening phase compared with the lengthening phase. The longer time spent shortening may enhance power production, because the shortening phase is the active part of the cycle during which the muscle produces positive work. A similar total strain (approximately 21 % and approximately 19 % in H. versicolor and H. chrysoscelis respectively) is achieved in the first few pulses, and during subsequent pulses the muscle cycles with a reduced pulse strain (approximately 12 % and approximately 7.3 % in H. versicolor and H. chrysoscelis respectively). The higher pulse frequencies of H. chrysoscelis are thus associated with lower pulse strains. The EMG pattern is different in the two species. A single EMG stimulus occurs for each cycle in H. chrysoscelis, but two stimuli per cycle are found in H. versicolor. Indirect evidence suggests that the initial phase of shortening during a pulse is partly due to elastic recoil of the trunk.

Rational management of gait pathology in Duchenne muscular dystrophy

Because of extensor weakness, children with Duchenne muscular dystrophy (DMD) maintain internal flexion moments at the joints of the lower extremities when they walk. We believe that at the ankle, the plantar flexion moments caused by contractures may contribute significantly to the production of the net ankle flexion moment during the gait in these children. The goal of the present study is to quantify ankle plantar flexion passive moments that may be associated with the presence of flexion contractures and to estimate their contribution to the net moment during the gait of children with DMD. Kinematic and kinetic parameters were collected during gait of eleven subjects with DMD. Ankle plantar flexion passive moments were also measured experimentally during the same session. Fourteen control children participated in the study in order to have normal reference values. The presence of ankle plantar flexion contractures in children with DMD was reflected by a rigidity coefficient obtained at a common moment of −7 Nm that was higher for these children (0.75 Nm/° vs. 0.48 Nm/°; p < 0.05). The relative passive moment contribution to the net plantar flexion moments was higher for the children with DMD at the end of the lengthening phase of the plantar flexors (25% vs. 18%; p < 0.05). We believe that the passive moments can compensate for the presence of progressive muscle weakness in the children with DMD and help these children with gait.

Postnatal growth of lung parenchyma in the piglet: Morphometry correlated with mechanics

A previous study of piglet lung growth (Mansell et. al. 1989. J. Appl. Physiol., 67:1422-1427) showed transient stiffness to changes in shape and volume immediately after birth. Later, elastic recoil was found to increase as the lung grew in weight and volume. The present study uses morphometry to test possible structural correlates of these two mechanical changes. Piglet lungs were fixed near full inflation via the airways during the immediate newborn period (6-12 hours, n = 3), at 3-5 days (n = 6), 25-30 days (n = 5), and 80-85 days (n = 3). Morphometry comprised arithmetic and harmonic mean thicknesses of alveolar septae and average mean surface curvature. Measurements of curvature and airspace volume were combined to differentiate alveolar expansion from septal proliferation as mechanisms for volumetric growth. The unique mechanical behavior of the newborn lungs was associated with relatively thick alveolar septae. Marked thinning of the septae and resolution of the stiffness to shape and volume change had occurred by 3-5 days. An increase in elastic recoil during the first postnatal month was found to be associated with simple airspace expansion. The second and third months were characterized by septal proliferation and increase in arithmetic mean septal thickness but elastic recoil did not increase further. Harmonic mean septal thickness and airspace volume per gram of lung tissue did not change over the course of the study. 1) A relative stiffness to shape and volume change in freshly newborn piglet lungs is associated with relatively thick alveolar septal walls; 2) postnatal development of piglet lung parenchyma involves septal lengthening and thinning followed by septal proliferation; 3) the initial phase of septal lengthening, rather than the later phase of septal proliferation, is associated with increase in parenchymal recoil.

Imaging in Bone Lengthening

Limb lengthening using callotasis needs careful preoperative planning and great care during the lengthening period. The most common method used to monitor limb lengthening is plain radiography, but the time of frame removal is still left to the judgment of the surgeon. This paper reviews some of the methods used experimentally and clinically to monitor the process, outlining their possible advantages and disadvantages for application in routine clinical practice. Considering availability, cost, and safety, an imaging protocol could involve preoperative assessment with computed tomography scan; immediate postoperative anteroposterior (AP) and lateral radiographs; weekly ultrasound scanning for the 1st 8 weeks; and monthly AP and lateral radiographs during the lengthening phase and if there is any clinical suspicion of complication during the consolidation phase. If available, dual energy bone densitometry can be performed every 2 to 4 weeks from 8 weeks, until removal of the fixator.

Recruitment Patterns and Contractile Properties of Fast Muscle Fibres Isolated From Rostral and Caudal Myotomes of the Short-Horned Sculpin

ABSTRACT Muscle action during swimming and the contractile properties of isolated muscle fibres were studied in the short-horned sculpin Myoxocephalus scorpius at 5 ° C. Semi-steady swimming, startle responses and prey-capture events were filmed with a high-speed video at 200frames s−1, using fish 22–26cm in total length (L). Electromyographical (EMG) recordings, synchronised with the video, were made from fast muscle in rostral and caudal myotomes at points 0.40L and 0.80L along the body. Fast muscle fibres were first recruited at tail-beat frequencies of 3.7–4.2Hz, corresponding to a swimming speed of 1.7 L s−1. Electrical activity in the muscles occurred during 16–38% of each tail-beat cycle regardless of frequency. Muscle fibres were activated during the lengthening phase of the cycle. In caudal myotomes, the onset of the muscle activity occurred at a phase of 75–105 ° at 3.7Hz, decreasing to approximately 50 ° at frequencies greater than 4.5Hz (0 ° phase was defined as the point at which muscle fibres passed through their resting lengths in the stretch phase of the cycle; a full cycle is 360 °). Prey capture was a stereotyped behaviour consisting of a preparatory movement, a powerstroke at 7–9Hz and a glide of variable duration. The delay between the activation of muscle fibres in rostral and caudal myotomes during prey capture and startle responses was approximately 10ms. Fast muscle fibres isolated from rostral and caudal myotomes were found to have similar isometric contractile properties. Maximum tetanic stress was 220kN m−2, and half-times for force development and relaxation were approximately 50ms and 135ms respectively. Power output was measured by the ‘work loop’ technique in muscle fibres subjected to sinusoidal length changes at the range of frequencies found during swimming. Under optimal conditions of strain and stimulation, muscle fibres from rostral and caudal myotomes produced similar levels of work (3.5 J kg−1) and generated their maximum power output of 25–30 W kg−1 at the tail-beat frequencies used in swimming (4–8Hz). Progressively delaying the onset of stimulation relative to the start of the strain cycle resulted in an initial modest increase, followed by a decline, in the work per cycle. Maximum positive work and net negative work were done at stimulus phase values of 20–50° and 120–140° respectively. The EMG and swimming studies suggest that fast muscle fibres in both rostral and caudal myotomes do net positive work under most conditions.

Spontaneous sarcomeric oscillations at intermediate activation levels in single isolated cardiac myofibrils.

Spontaneous oscillations observed in various heart muscle preparations are widely thought to be triggered by spontaneous release of Ca2+ from the sarcoplasmic reticulum (SR). Here, we report undamped propagated oscillations that occur in the absence of SR. In single cardiac myofibrils treated with Triton X-100 to remove SR and held isometrically, partial activation initiated periodic fluctuations of sarcomere length persisting up to 1 hour. Oscillation characteristics could be readily quantitated by virtue of the small size of the preparation. In an individual sarcomere, the oscillation cycle generally consisted of a slow shortening phase, followed by a phase of rapid lengthening. Oscillations usually propagated along the myofibril--frequently along the entire specimen--in a wavelike fashion (average velocity, 12.3 microns/s at 10 degrees C; Q10, approximately 1.3). The oscillation period was 2.30 and 1.72 seconds at 10 degrees and 20 degrees C, respectively, and was insensitive to stretch. The average oscillation amplitude, which was temperature independent, decreased with stretch from more than 20% of the mean sarcomere length at lengths below 2 microns to zero beyond a sarcomere length of 3 microns. Stiffening of the Z line by labeling with anti-alpha-actinin resulted in a dose-dependent decrease of oscillation amplitude, while the period was not affected. Tension oscillations could not be detected in single myofibrils but were frequently detectable in myofibril doublets, where the oscillation magnitude (approximately 1 microgram) was above the noise floor. Addition of 10 mumol/L ryanodine to the activating solution did not alter oscillation characteristics, as expected, since the oscillations are unrelated to SR calcium release. On the basis of our results, we consider a mechanism for the oscillations in which a length dependence of myofibrillar Ca2+ sensitivity and a dynamic Z-line structure are essential.

Mechanical properties of skinned rabbit psoas and soleus muscle fibres during lengthening: effects of phosphate and Ca2+.

1. Mechanical properties of permeabilized single fibres from rabbit psoas and soleus muscle were determined by measuring the length responses due to abrupt changes in load and the force responses due to isovelocity length changes at different phosphate and Ca2+ concentrations. 2. The length responses due to abrupt increases in load from psoas fibres showed a rapid lengthening during the change in load followed by a phase of lengthening during which the velocity gradually decreased. In soleus fibres an abrupt lengthening during the change in load was followed by a phase of lengthening during which the velocity remained constant or decreased slightly for increases in load to less than 1.45 of the isometric force (F0). For larger increases in load the velocity during this later phase first increased and thereafter decreased. 3. The initial force-velocity curve, derived from the early part of the isotonic responses after the change in load, as well as the late force-velocity curve derived from the force level attained during isovelocity length changes, were sensitive to phosphate. Phosphate caused a shift of the absolute force-velocity curves of both psoas and soleus fibres towards lower values of force. In psoas fibres, the relative force-velocity curves derived by normalization of the force level to the force developed isometrically was shifted by phosphate to smaller velocities. In soleus fibres, the initial velocity at low and intermediate relative loads (less than 1.75 F0) was increased by phosphate but at higher loads it decreased, while the late force-velocity curve showed an overall decrease in velocity. 4. The force responses during isovelocity lengthening of psoas fibres showed an early rapid increase in force followed by a slow rise in force. The position of this break point in force was sensitive to the phosphate concentration. In soleus fibres, the force responses without phosphate showed an overshoot followed by a slow rise in force. The overshoot diminished with increasing phosphate concentration. 5. Phosphate and Ca2+ affected the force responses in psoas and soleus fibres in different ways. When the isometric starting levels were the same, force during and after the length change at submaximal activation was always less than at maximal activation in the presence of 15 mM-phosphate. 6. The changes in the mechanical performance during lengthening caused by phosphate in psoas as well as in soleus fibres, are in agreement with a decrease in the average force per attached crossbridge.(ABSTRACT TRUNCATED AT 400 WORDS)

Behaviour of triceps surae muscle-tendon complex in different jump conditions.

The force-length relationship of the human muscle-tendon complex (MTC) of the triceps surae and the achilles tendon was investigated in various stretch load conditions. Six male subjects performed various vertical jumps with maximal effort: squat jumps (SJ), counter movement jumps (CMJ) and drop jumps (DJ) from a height of 24 cm, 40 cm and 56 cm. The force-length relationship was calculated from the signals of the components of the ground reaction forces and the kinematic data obtained from the high-speed film records. Surface electromyograms (EMG) of the soleus, gastrocnemius and tibialis anterior muscles were also recorded. The force-length diagrams showed individually high sensitivity to the imposed stretch load. In conditions with relatively low stretch load requirements there was a counter-clockwise direction observable, indicating that the energy absorbed during the eccentric, or lengthening phase was lower than the energy delivered during the concentric, or shortening phase. In high load conditions this relationship was reversed indicating a negative energy balance. The EMG-length diagrams of SJ and CMJ consisted of an initial isometric loading of the muscle, followed by a shortening phase with only slightly reduced EMG amplitudes. In DJ, however, the diagrams showed an initial lengthening of the MTC with fairly constant activation amplitudes. After 40 ms an isometric loading of the muscle, lasting for approximately 80 ms, was followed by a shortening phase. It was concluded that segmental stretch reflex activation represented the predominant activation process during the isometric loading phase, to meet the adequate stiffness properties of the MTC.

Injury to skeletal muscles of mice by forced lengthening during contractions.

Our purpose was to determine the amount of injury to extensor digitorum longus (EDL) and anterior tibial (ATB) muscles of mice from 1 h to 30 days following passive shortening and lengthening, shortening contractions and lengthening contractions. The shortening-lengthening cycle was of 600 ms duration and was repeated every 5 s for 30 min. Contractions were produced during either the shortening or lengthening phase with stimulation at 100 Hz for 300 ms. The amount of injury was evaluated by the decrease in the number of fibres in a cross-section of the muscle and in the maximum isometric tetanic force (Po). Passively shortened and lengthened and actively shortened EDL and ATB muscles showed no evidence of injury. At 3-5 days after lengthening contractions, EDL and ATB muscles showed an inflammatory response. At 3-5 days, the number of fibres in the cross-section of the injured EDL muscles was 65% of that of the contralateral control EDL muscle, whereas no change was observed in the number of fibres in the injured ATB muscles. By 3 days, the presence of myoblasts, myotubes and central nuclei indicated regeneration of fibres. One hour after shortening contractions, the Po of EDL and ATB muscles decreased to about 75% of the contralateral control value, but recovered completely by 3 days. In contrast, 1 h after lengthening contractions the Po of EDL and ATB muscles decreased to 52 and 40% respectively of the contralateral control value and then both recovered to about 65% within 3 h. For the ATB muscle, the Po showed a secondary decrease to 49% of the contralateral control value at 1 day and for the EDL muscle, to 47% at 3 days. Each muscle recovered gradually and by 30 days the number of fibres and the Po of the injured muscles were not significantly different from values for contralateral control muscles.

Spontaneous oscillatory contraction of sarcomeres in skeletal myofibrils.

We found that the lengths of all sarcomeres spontaneously oscillated in an isolated skeletal myofibril, when both ends were fixed, submillimolar to millimolar concentrations of ATP, ADP and inorganic phosphate (Pi) were present, and Ca2+ was removed. Narrowing and widening of an H-zone and an I-band were observed corresponding to the shortening and lengthening of a sarcomere, suggesting that thick and thin filaments slide past each other. The oscillation of each sarcomere was asymmetrical, consisting of a rapid lengthening phase and a slow shortening phase. The period of oscillation was about 3s; the peak-to-peak amplitude of oscillation reached as much as 30% of the average sarcomere length. The propagation of the sarcomere oscillation along the long axis of the myofibril was observed occasionally in single myofibrils and frequently in bundles of myofibrils. The 'state'-diagram showing the concentration range of ADP and Pi in which contraction, oscillation or relaxation of myofibrils occurs in the presence of ATP and the absence of Ca2+ suggested that the oscillation is a third state of skeletal muscle located in between the contracting and relaxing states.

Isotonic length transient of cat heart muscle in Ba2+-induced contracture.

To characterize mechanical properties of activated heart muscle, the length response to step (4 ms) decrease in tension in Ba2+-induced contracture in kitten papillary muscle was analyzed. The amplitudes of tension steps were varied at the maximum contracture level (Tc), different initial muscle lengths [0.90 Lmax to Lmax], and different temperatures (20-35 degrees C). When the tension decrease was less than 0.7 Tc, the length response comprised four different phases. The amplitude of shortening in the second phase, after the initial rapid shortening in the first phase, increased up to approximately 3.5% of the initial muscle length quite linearly with increasing amplitude of tension reduction. The amount of lengthening in the third phase, however, increased to a maximum (up to approximately 1.4% initial length) at the tension reduction of approximately 0.4 Tc. The third phase of lengthening was larger at shorter initial muscle length. Increasing temperature markedly decreased the amplitude and shortened the duration of the length response in the second and third phase independently of initial muscle length. Unlike that of the muscle in contracture, the length response of the resting and of the rigor muscle changed in a nearly stepwise fashion. One explanation of these phenomena is that the transient length responses following a rapid tension reduction are mostly determined by the kinetics of the attachment and detachment of cross bridges between actin and myosin filaments.

Regional circumferential lengthening patterns in canine left ventricle.

We employed sonomicrometers in open-chest dogs to study lengthening of short segments of circumferentially oriented myocardium located at the base, midportion, and apex of the anterior left ventricular free wall. Left ventricular pressure was varied by inferior vena caval occlusion and volume expansion. Diastole was divided into rapid and slow lengthening phases. Rapid lengthening was completed first at the basal site at each of three successive levels of left ventricular diastolic pressure (LVDP). At the base, significant further lengthening occurred during the slow lengthening phase while at the apex virtually all lengthening was completed during the rapid phase. At low LVDPs, peak lengthening rates (dl/dt) were greatest at the apex. As LVDP was increased, regional differences in dl/dt diminished. These results indicate that there is regional variation in the timing of the phases of diastole and in lengthening patterns of the left ventricle. The volume-dependent variation in lengthening rates that we observed is consistent with the concept of regional differences in elastic recoil, which may contribute to active ventricular filling.

Activities of neurons in area 3a of the cerebral cortex during voluntary movements in the monkey

Neurons in the area 3a of the sensory cortex are known to receive inputs from group I afferent fibers arising from the muscle. The discharge pattern of these neurons during trained wrist movements was studied in the monkey and the following results were obtained. (1) About half of the neurons which received inputs from group I afferent fibers arising from wrist flexors increased their discharges during the shortening phase of the muscle from which the impulses originated. (2) About half of the neurons which received inputs from wrist flexors behaved in the opposite way, i.e. they increased discharges during the lengthening phase of the muscle. (3) A small group of neurons increased discharges during both phases of movements. (4) Possible functional significance of the afferent inputs was discussed in relation to the responsible receptors for these inputs.

Jaw muscle spindle activity in the cat during normal movements of eating and drinking

A method is described for recording from single unit afferents from spindles in jaw closing muscles of the normally behaving cat. The spindle discharge is partly or completely suppressed during active muscle shortening in chewing and lapping. In the lengthening phase, it can reach 120 imp./sec in some units (believed secondaries) and 220–400 imp./sec in others (believed primaries). There is clear evidence of fluctuating fusimotor drive accompanying muscle contraction, in some cases timed to maintain almost constant afferent frequency. There is considerable variability in the relation of fusimotor to α motor drive and this is discussed in relation to control theory with reference to optimal systems.