Contractile behavior of the forelimb digital flexors during steady-state locomotion in horses (Equus caballus): an initial test of muscle architectural hypotheses about in vivo function

The forelimb digital flexors of the horse display remarkable diversity in muscle architecture despite each muscle-tendon unit having a similar mechanical advantage across the fetlock joint. We focus on two distinct muscles of the digital flexor system: short compartment deep digital flexor (DDF(sc)) and the superficial digital flexor (SDF). The objectives were to investigate force-length behavior and work performance of these two muscles in vivo during locomotion, and to determine how muscle architecture contributes to in vivo function in this system. We directly recorded muscle force (via tendon strain gauges) and muscle fascicle length (via sonomicrometry crystals) as horses walked (1.7 m s(-1)), trotted (4.1 m s(-1)) and cantered (7.0 m s(-1)) on a motorized treadmill. Over the range of gaits and speeds, DDF(sc) fascicles shortened while producing relatively low force, generating modest positive net work. In contrast, SDF fascicles initially shortened, then lengthened while producing high force, resulting in substantial negative net work. These findings suggest the long fibered, unipennate DDF(sc) supplements mechanical work during running, whereas the short fibered, multipennate SDF is specialized for economical high force and enhanced elastic energy storage. Apparent in vivo functions match well with the distinct architectural features of each muscle.     

Characterization of the passive mechanical properties of spine muscles across species

Passive mechanical properties differ between muscle groups within a species. Altered functional demands can also shift the passive force-length relationship. The extent that passive mechanical properties differ within a muscle group (e.g. spine extensors) or between homologous muscles of different species is unknown. It was hypothesized that multifidus, believed to specialize in spine stabilization, would generate greater passive tensile stresses under isometric conditions than erector spinae, which have more generalized functions of moving and stabilizing the spine; observing greater multifidus moduli in different species would strengthen this hypothesis. Permeabilized fibre bundles (n = 337) from the multifidus and erector spinae of mice, rats, and rabbits were mechanically tested. A novel logistic function was fit to the experimental data to fully characterize passive stress and modulus. Species had the greatest effect on passive muscle parameters with mice having the largest moduli at all lengths. Rats generated less passive stress than rabbits due to a shift of the passive force-length relationship towards longer muscle lengths. Rat multifidus generated slightly greater stresses than erector spinae, but no differences were observed between mouse muscles. The secondary objective was to determine the parameters required to simulate the passive force-length relationship. Experimental data were compared to the passive muscle model in OpenSim. The default OpenSim model, optimized for hindlimb muscles, did not fit any of the spine muscles tested; however, the model could accurately simulate experimental data after adjusting the input parameters. The optimal parameters for modelling the passive force-length relationships of spine muscles in OpenSim are presented.     

Characterization of passive elastic properties of the human medial gastrocnemius muscle belly using supersonic shear imaging

The passive elastic properties of a muscle-tendon complex are usually estimated from the relationship between the joint angle and the passive resistive torque, although the properties of the different structures crossing the joint cannot be easily assessed. This study aimed to determine the passive mechanical properties of the gastrocnemius medialis muscle (GM) using supersonic shear imaging (SSI) that allows the measurement of localized muscle shear modulus (μ). The SSI of the GM was taken for 7 subjects during passive ankle dorsiflexion at a range of knee positions performed on an isokinetic dynamometer. The relationship between normalized μ and the length of the gastrocnemius muscle-tendon units (GMTU) was very well fitted to an exponential model (0.944相似文献   

Reports of the length dependence of fatigue are greatly exaggerated.

Relative force depression associated with muscle fatigue is reported to be greater when assessed at short vs. long muscle lengths. This appears to be due to a rightward shift in the force-length relationship. This rightward shift may be caused by stretch of in-series structures, making sarcomere lengths shorter at any given muscle length. Submaximal force-length relationships (twitch, double pulse, 50 Hz) were evaluated before and after repetitive contractions (50 Hz, 300 ms, 1/s) in an in situ preparation of the rat medial gastrocnemius muscle. In some experiments, fascicle lengths were measured with sonomicrometry. Before repetitive stimulation, fascicle lengths were 11.3 +/- 0.8, 12.8 +/- 0.9, and 14.4 +/- 1.2 mm at lengths corresponding to -3.6, 0, and 3.6 mm where 0 is a reference length that corresponds with maximal active force for double-pulse stimulation. After repetitive stimulation, there was no change in fascicle lengths; these lengths were 11.4 +/- 0.8, 12.6 +/- 0.9, and 14.2 +/- 1.2 mm. The length dependence of fatigue was, therefore, not due to a stretch of in-series structures. Interestingly, the rightward shift that was evident when active force was calculated in the traditional way (subtraction of the passive force measured before contraction) was not seen when active force was calculated by subtracting the passive force that was associated with the fascicle length reached at the peak of the contraction. This calculation is based on the assumption that passive force decreases as the fascicles shorten during a fixed-end contraction. This alternative calculation revealed similar postfatigue absolute active force depression at all lengths. In relative terms, a length dependence of fatigue was still evident, but this was greatly diminished compared with that observed when active force was calculated with the traditional method.     

Endotoxin administration alters the force vs. pCa relationship of skeletal muscle fibers

Recent work indicates that endotoxemia elicits severe reductions in skeletal muscle force-generating capacity. The subcellular alterations responsible for these decrements have not, however, been fully characterized. One possibility is that the contractile proteins per se are altered in endotoxemia and another is that the mechanism by which these proteins are activated is affected. The purpose of the present study was to assess the effects of endotoxin administration on the contractile proteins by examining the maximum calcium-activated force (F(max)) and calcium sensitivity of single Triton-skinned fibers of diaphragm, soleus, and extensor digitorum longus (EDL) muscles taken from control and endotoxin-treated (8 mg/kg) rats. Fibers were mounted on a force transducer and sequentially activated by serial immersion in solutions of increasing Ca(2+) concentration (i.e., pCa 6.0 to pCa 5.0); force vs. pCa data were fit to the Hill equation. All fibers were typed at the conclusion of studies using gel electrophoresis. F(max), the calcium concentration required for half-maximal activation (Ca(50)), and the Hill coefficient were compared as a function of muscle and fiber type for the control and endotoxin-treated animals. Control group F(max) was similar for diaphragm, soleus, and EDL fibers, i.e., 112.34 +/- 2.64, 111.55 +/- 3.66, and 104.05 +/- 4.33 kPa, respectively. Endotoxin administration reduced the average F(max) for fibers from all three muscles to 80.25 +/- 2.30, 72.47 +/- 2.97, and 78.32 +/- 2.43 kPa, respectively (P < 0.001 for comparison of each to control). All fiber types in diaphragm, soleus, and EDL muscles manifested similar endotoxin-related reductions in F(max). The Ca(50) and the Hill coefficient for all fiber types and all muscles were unaffected by endotoxin administration. We speculate that these alterations in the intrinsic properties of the contractile proteins represent a major mechanism by which endotoxemia reduces muscle force-generating capacity.     

Three-Dimensional Muscle Architecture and Comprehensive Dynamic Properties of Rabbit Gastrocnemius,Plantaris and Soleus: Input for Simulation Studies

The vastly increasing number of neuro-muscular simulation studies (with increasing numbers of muscles used per simulation) is in sharp contrast to a narrow database of necessary muscle parameters. Simulation results depend heavily on rough parameter estimates often obtained by scaling of one muscle parameter set. However, in vivo muscles differ in their individual properties and architecture. Here we provide a comprehensive dataset of dynamic (n = 6 per muscle) and geometric (three-dimensional architecture, n = 3 per muscle) muscle properties of the rabbit calf muscles gastrocnemius, plantaris, and soleus. For completeness we provide the dynamic muscle properties for further important shank muscles (flexor digitorum longus, extensor digitorum longus, and tibialis anterior; n = 1 per muscle). Maximum shortening velocity (normalized to optimal fiber length) of the gastrocnemius is about twice that of soleus, while plantaris showed an intermediate value. The force-velocity relation is similar for gastrocnemius and plantaris but is much more bent for the soleus. Although the muscles vary greatly in their three-dimensional architecture their mean pennation angle and normalized force-length relationships are almost similar. Forces of the muscles were enhanced in the isometric phase following stretching and were depressed following shortening compared to the corresponding isometric forces. While the enhancement was independent of the ramp velocity, the depression was inversely related to the ramp velocity. The lowest effect strength for soleus supports the idea that these effects adapt to muscle function. The careful acquisition of typical dynamical parameters (e.g. force-length and force-velocity relations, force elongation relations of passive components), enhancement and depression effects, and 3D muscle architecture of calf muscles provides valuable comprehensive datasets for e.g. simulations with neuro-muscular models, development of more realistic muscle models, or simulation of muscle packages.     

The role of passive structures in force enhancement of skeletal muscles following active stretch

We recently found that force enhancement following active stretch in skeletal muscles is accompanied by an increase in passive force following deactivation (J. Exp. Biol. 205 (2002) 1275). However, it is not known if this increase in passive force contributes to the force enhancement observed in the active muscle, and if it is observed at all muscle lengths. The purposes of this study were to quantify the amount of passive force increase as a function of muscle lengths, and to determine if this passive force contributes to the force enhancement observed in the active muscle. Experiments were performed on cat soleus (n = 24) using techniques published previously (J. Biomech. 30(9) (1997) 865). Conceptually, tests involved comparisons of force enhancement and passive force increase for a variety of stretch tests in soleus. Furthermore, in one test, activation of the soleus was interrupted for 1s in the force-enhanced state, and soleus was then re-activated. We found that total force enhancement and passive force increase were positively correlated for all test conditions, that passive force increase following stretch of the active soleus only occurred at muscle lengths corresponding to the descending limb of the force-length relationship, that increases in passive force for a given stretch magnitude became greater at long muscle lengths, and that upon reactivation, there was a remnant passive force enhancement. We conclude from these results that the passive force enhancement following stretch of an active muscle contributes to the total force enhancement, that this passive contribution increases with increasing muscle length, and that there must be at least one other factor than passive force increase that contributes to the total force enhancement, as the passive force increase was always smaller than the total force enhancement. A by-product of this investigation was that we observed a shift in the passive force-length relationship that was dependent on muscle activation, stretch magnitude and muscle length. Therefore, the passive force-length relationship is not a constant property of skeletal muscle, but depends critically on the muscle's contractile history.     

Stretch-induced,steady-state force enhancement in single skeletal muscle fibers exceeds the isometric force at optimum fiber length

Stretch-induced force enhancement has been observed in a variety of muscle preparations and on structural levels ranging from single fibers to in vivo human muscles. It is a well-accepted property of skeletal muscle. However, the mechanism causing force enhancement has not been elucidated, although the sarcomere-length non-uniformity theory has received wide support. The purpose of this paper was to re-investigate stretch-induced force enhancement in frog single fibers by testing specific hypotheses arising from the sarcomere-length non-uniformity theory. Single fibers dissected from frog tibialis anterior (TA) and lumbricals (n=12 and 22, respectively) were mounted in an experimental chamber with physiological Ringer's solution (pH=7.5) between a force transducer and a servomotor length controller. The tetantic force-length relationship was determined. Isometric reference forces were determined at optimum length (corresponding to the maximal, active, isometric force), and at the initial and final lengths of the stretch experiments. Stretch experiments were performed on the descending limb of the force-length relationship after maximal tetanic force was reached. Stretches of 2.5-10% (TA) and 5-15% lumbricals of fiber length were performed at 0.1-1.5 fiber lengths/s. The stretch-induced, steady-state, active isometric force was always equal or greater than the purely isometric force at the muscle length from which the stretch was initiated. Moreover, for stretches of 5% fiber length or greater, and initiated near the optimum length of the fiber, the stretch-enhanced active force always exceeded the maximal active isometric force at optimum length. Finally, we observed a stretch-induced enhancement of passive force. We conclude from these results that the sarcomere length non-uniformity theory alone cannot explain the observed force enhancement, and that part of the force enhancement is associated with a passive force that is substantially greater after active compared to passive muscle stretch.     

The effects of muscle stretching and shortening on isometric forces on the descending limb of the force-length relationship

The purpose of this study was to examine the effects of stretching and shortening on the isometric forces at different lengths on the descending limb of the force-length relationship. Cat soleus (N = 10) was stretched and shortened by various amounts on the descending limb of the force-length relationship, and the steady-state forces following these dynamic contractions were compared to the isometric forces at the corresponding muscle lengths. We found a shift of the force-length relationship to greater force values following muscle stretching, and to smaller force values following muscle shortening. Shifts in both directions critically depended on the magnitude of stretching/shortening and the final muscle length. We confirm recent findings that the steady-state isometric force following some stretch conditions clearly exceeded the maximal isometric forces at optimum muscle length, and that force enhancement was associated with an increase in the passive force, i.e., a passive force enhancement. When the passive force enhancement was subtracted from the total force enhancement, forces following stretch were always equal to or smaller than the isometric force at optimum muscle length. Together, these findings led to the conclusions: (a). that force enhancement is composed of an "active and a "passive" component; (b). that the "passive" component of force enhancement allows for forces greater than the maximal isometric forces at the muscle's optimum length; and (c). that force enhancement and force depression are critically affected by muscle length and stretch/shortening amplitude.     

V(O2) max is unaffected by altering the temporal pattern of stimulation frequency in rat hindlimb in situ.

It might be anticipated that fatiguing contractions would impair the aerobic metabolic response in skeletal muscle if significant fatigue developed before full activation of aerobic metabolism. On the basis of this premise, we examined two groups of rats to test the hypothesis that a gradual increase in stimulation frequency would yield a higher maximal O2 uptake (Vo2 max) than beginning immediately with an intense stimulation frequency because of a slower progression of fatigue under the former conditions. In one group of animals, the distal hindlimb muscles were electrically stimulated at a frequency of 60 tetani/min for 4 min (F60; n = 6 rats); in the other group, the muscles were incrementally stimulated for 1 min at each of 7.5, 15, 30, and 60 tetani/min and for 2 min at 90 tetani/min (FInc; n = 5 rats). Despite large differences in rate of fatigue [time to 60% of initial force was 47 +/- 3 (SE) vs. 188 +/- 1 s in F60 and FInc, respectively] and the time at which Vo2 max occurred (120 +/- 15 vs. 264 +/- 6 s), Vo2 max was not different (419 +/- 24 vs. 381 +/- 44 micromol x min-1. 100 g-1). Furthermore, time x tension integral at Vo2 max (3.82 +/- 0.41 vs. 4.07 +/- 0.31 N. s) and peak lactate efflux (910 +/- 45 vs. 800 +/- 98 micromol x min-1. 100 g-1) were not different between groups. Thus our results show that the more rapid progression of fatigue in F60 did not compromise the aerobic metabolic response in electrically stimulated rat hindlimb muscles. However, in both groups, O2 uptake and lactate efflux declined after Vo2 max was attained in similar proportion to a further fall in force, suggesting that ongoing fatigue with intense contractions reduced ATP demand below that requiring maximal aerobic and glycolytic metabolic responses once Vo2 max was reached.     

Sprint training attenuates the deficits of force and dynamic stiffness in rat soleus muscle caused by eccentric contractions

We investigated whether sprint training attenuates the deficits in force and dynamic stiffness caused by eccentric contractions to the soleus muscles of Wistar rats. Two groups of male rats were analyzed: sedentary (C, n=8) and trained (T, n=8). T rats were sprint trained for 10 weeks. Subsequently, the right soleus muscles of rats were freed under anesthesia, leaving the bone insertion and blood supply intact. Eccentric contractions were induced by lengthening muscles during tetanic contractions. Force and dynamic stiffness were tested before and after 20 rounds of eccentric contractions. Tension decline was analyzed using a two-state model (first-order kinetics) in the context of Kramer's theory. Training improved the twitch tension (C, 6.44+/-0.6N/cm(2); T, 10.90+/-0.8N/cm(2)), tetanic force (C, 61.74+/-0.6N/cm(2); T, 85.62+/-0.8N/cm(2)), and increased the dynamic stiffness (C, 41.28+/-1.0N/cm(2); T, 49.56+/-3.2N/cm(2)). Twitch tension after eccentric contractions declined to 73% and 75% in C and T groups, respectively, while tetanic tension decreased to 60% and 36% in C and T groups, respectively. After eccentric contractions, dynamic stiffness decreases were smaller in T rats (from 49.56+/-3.2 to 36.09+/-2.1N/cm(2)) than in C rats (from 41.28+/-1.0 to 20.73+/-1.8N/cm(2)). Sprint training increased the dynamic stiffness and tetanic tension of the soleus muscle and protected against the attenuation induced by eccentric contractions. Finally, the two-state model provided evidence that the number of force-generating cross-bridges increases in trained muscle.     

A new method for measuring passive length-tension properties of human gastrocnemius muscle in vivo

The study of muscle growth and muscle length adaptations requires measurement of passive length-tension properties of individual muscles, but until now such measurements have only been made in animal muscles. We describe a new method for measuring passive length-tension properties of human gastrocnemius muscles in vivo. Passive ankle torque and ankle angle data were obtained as the ankle was rotated through its full range with the knee in a range of positions. To extract gastrocnemius passive length-tension curves from passive torque-angle data it was assumed that passive ankle torque was the sum of torque due to structures which crossed only the ankle joint (this torque was a 6-parameter function of ankle joint angle) and a torque due to the gastrocnemius muscle (a 3-parameter function of knee and ankle angle). Parameter values were estimated with non-linear regression and used to reconstruct passive length-tension curves of the gastrocnemius. The reliability of the method was examined in 11 subjects by comparing three sets of measurements: two on the same day and the other at least a week later. Length-tension curves were reproducible: the average root mean square error was 5.1+/-1.1 N for pairs of measurements taken within a day and 7.3+/-1.2 N for pairs of measurements taken at least a week apart (about 3% and 6% of maximal passive tension, respectively). Length-tension curves were sensitive to mis-specification of moment arms, but changes in length-tension curves were not. The new method enables reliable measurement of passive length-tension properties of human gastrocnemius in vivo, and is likely to be useful for investigation of changes in length-tension curves over time.     

Androgen therapy improves muscle mass and strength but not muscle quality: results from two studies

The relationship of strength to muscle area was used to assess change in muscle quality after anabolic interventions. Study 1: asymptomatic human immunodeficiency virus-positive men (39 +/- 9 yr) were randomized to nandrolone (600 mg/wk) +/- resistance training (RT). Study 2: older healthy men (72 +/- 5 yr) were randomized to oxandrolone (20 mg/day) or placebo. Maximum voluntary strength was determined by the 1-repetition maximum (1-RM) method for leg press, flexion and extension, and cross-sectional area of leg muscles by MRI. From study week 0 to study week 12, muscle quality was unchanged with nandrolone, oxandrolone, or oxandrolone placebo, respectively, for total thigh muscles (1.23 +/- 0.012 vs. 1.27 +/- 0.29 kg/cm2; 9.0 +/- 1.1 vs. 8.9 +/- 1.2 N/cm2; 8.9 +/- 1.2 vs. 8.9 +/- 1.9 N/cm2) and hamstrings (0.41 +/- 0.08 vs. 0.43 +/- 0.07 kg/cm2; 0.90 +/- 0.14 vs. 0.95 +/- 0.016 N/cm2; 0.94 +/- 0.23 vs. 0.93 +/- 0.21 N/cm2). Lower-extremity 1-RM strength increased several times greater with RT+nandrolone (51-63% increases) than with nandrolone alone (4.7-16%), despite similar increases in muscle area; therefore, muscle quality increased from 1.13 +/- 0.17 to 1.51 +/- 0.18 kg/cm2 (+36 +/- 19%; P < 0.001) for total thigh muscle, 0.37 +/- 0.10 to 0.53 +/- 0.08 kg/cm2 (+49 +/- 39%; P < 0.001) for hamstrings, and 0.73 +/- 0.19 to 1.07 +/- 0.16 kg/cm2 (+55 +/- 36%; P < 0.001) for quadriceps. Thus androgen therapy alone did not improve muscle quality, but the addition of RT to nandrolone produced substantive improvements.     

Force-length properties and functional demands of cat gastrocnemius, soleus and plantaris muscles.

The purpose of this study was to measure isometric force-length properties of cat soleus, gastrocnemius and plantaris muscle-tendon units, and to relate these properties to the functional demands of these muscles during everyday locomotor activities. Isometric force-length properties were determined using an in situ preparation, where forces were measured using buckle-type tendon transducers, and muscle-tendon unit lengths were quantified through ankle and knee joint configurations. Functional demands of the muscles were assessed using direct muscle force measurements in freely moving animals. Force-length properties and functional demands were determined for soleus, gastrocnemius and plantaris muscles simultaneously in each animal. The results suggest that isometric force-length properties of cat soleus, gastrocnemius and plantaris muscles, as well as the region of the force-length relation that is used during everyday locomotor tasks, match the functional demands.     

A fuzzy-genetic model for estimating forces from electromyographical activity of antagonistic muscles due to planar lower arm movements: the effect of nonlinear muscle properties

The aim of this paper is to create a model for mapping the surface electromyogram (EMG) signals to the force that generated by human arm muscles. Because the parameters of each person's muscle are individual, the model of the muscle must have two characteristics: (1) The model must be adjustable for each subject. (2) The relationship between the input and output of model must be affected by the force-length and the force-velocity behaviors are proven through Hill's experiments. Hill's model is a kinematic mechanistic model with three elements, i.e. one contractile component and two nonlinear spring elements.In this research, fuzzy systems are applied to improve the muscle model. The advantages of using fuzzy system are as follows: they are robust to noise, they prove an adjustable nonlinear mapping, and are able to model the uncertainties of the muscle.Three fuzzy coefficients have been added to the relationships of force-length (active and passive) and force-velocity existing in Hill's model. Then, a genetic algorithm (GA) has been used as a biological search method that can adjust the parameters of the model in order to achieve the optimal possible fit.Finally, the accuracy of the fuzzy genetic implementation Hill-based muscle model (FGIHM) is invested as following: the FGIHM results have 12.4% RMS error (in worse case) in comparison to the experimental data recorded from three healthy male subjects. Moreover, the FGIHM active force-length relationship which is the key characteristics of muscles has been compared to virtual muscle (VM) and Zajac muscle model. The sensitivity of the FGIHM has been evaluated by adding a white noise with zero mean to the input and FGIHM has proved to have lower sensitivity to input noise than the traditional Hill's muscle model.     

Aerobic power declines with aging in rat skeletal muscles perfused at matched convective O2 delivery.

Although it is well established that maximal O(2) uptake (Vo(2 max)) declines from adulthood to old age, the role played by alterations in skeletal muscle is unclear. Specifically, because during whole body exercise reductions in convective O(2) delivery to the working muscles from adulthood to old age compromise aerobic performance, this obscures the influence of alterations within the skeletal muscles. We sought to overcome this limitation by using an in situ pump-perfused hindlimb preparation to permit matching of muscle convective O(2) delivery in young adult (8 mo; muscle convective O(2) delivery = 569 +/- 42 micromol O(2) x min(-1) x 100 g(-1)) and late middle-aged (28-30 mo; 539 +/- 62 micromol O(2) x min(-1) x 100 g(-1)) Fischer 344 x Brown Norway F1 hybrid rats. The distal hindlimb muscles were electrically stimulated for 4 min (60 tetani/min), and Vo(2 max) was determined. Vo(2 max) normalized to the contracting muscle mass was 22% lower in the 28- to 30-mo-old (344 +/- 17 micromol O(2). min(-1) x 100 g(-1)) than the 8-mo-old (441 +/- 20 micromol O(2) x min(-1) x 100 g(-1); P < 0.05) rats. The flux through the electron transport chain complexes I-III was 45% lower in homogenates prepared from the plantaris muscles of the older animals. Coincident with these alterations, the tension at Vo(2 max) and lactate efflux were reduced in the 28- to 30-mo-old animals, whereas the percent decline in tension was greater in the 28- to 30-mo-old vs. 8-mo-old animals. Collectively, these results demonstrate that alterations within the skeletal muscles, such as a reduced mitochondrial oxidative capacity, contribute to the reduction in Vo(2 max) with aging.     

Mechanical properties of rat middle cerebral arteries with and without myogenic tone

The inner diameter and wall thickness of rat middle cerebral arteries (MCAs) were measured in vitro in both a pressure-induced, myogenically-active state and a drug-induced, passive state to quantify active and passive mechanical behavior. Elasticity parameters from the literature (stiffness derived from an exponential pressure-diameter relationship, beta, and elasticity in response to an increment in pressure, Einc-p) and a novel elasticity parameter in response to smooth muscle cell (SMC) activation, Einc-a, were calculated. beta for all passive MCAs was 9.11 +/- 1.07 but could not be calculated for active vessels. The incremental stiffness increased significantly with pressure in passive vessels; Einc-p (10(6) dynes/cm2) increased from 5.6 +/- 0.5 at 75 mmHg to 14.7 +/- 2.4 at 125 mmHg, (p < 0.05). In active vessels, Einc-p (10(6) dynes/cm2) remained relatively constant (5.5 +/- 2.4 at 75 mmHg and 6.2 +/- 1.0 at 125 mmHg). Einc-a (10(6) dynes/cm2) increased significantly with pressure (from 15.1 +/- 2.3 at 75 mmHg to 49.4 +/- 12.6 at 125 mmHg, p < 0.001), indicating a greater contribution of SMC activity to vessel wall stiffness at higher pressures.     

Relationships between range of motion,Lo, and passive force in five strap-like muscles of the feline hind limb

The relationships between range of motion, optimal length for force production (l_o), and passive force provide useful insights into the structure and function of muscles but are unknown for most individual muscles. We measured these values and examined their relationships in five strap-like muscles of the cat hind limb: caudofemoralis, semitendinosus, sartorius anterior, tenuissimus, and biceps femoris anterior. The range of motion relative to l_o was found to vary significantly between different muscles and even between different specimens of the same muscle. The passive force-length (FL) curve was found to be correlated with both l_o and l_max (maximal in situ muscle length) but was correlated more strongly with l_max. The mean passive force produced by these muscles at l_max was less than 7% of estimated maximal isometric force, suggesting that passive force may not be important in these muscles during normal activation patterns. The variance in passive FL curves between specimens of the same muscle was found to be significantly lower when length was scaled by l_max as opposed to l_o. These results suggest that l_max may provide a more useful scaling factor for generic models of muscle. However, the passive length-tension properties of mammalian muscle appear to reflect a complex mix of structures at both the myofilament and connective tissue levels that may differ depending on muscle-fiber architecture and perhaps on the history of trophic influences on a particular specimen. © 1996 Wiley-Liss, Inc.     

Length dependence of active force production in skeletal muscle.

The sliding filament and cross-bridge theories of muscle contraction provide discrete predictions of the tetanic force-length relationship of skeletal muscle that have been tested experimentally. The active force generated by a maximally activated single fiber (with sarcomere length control) is maximal when the filament overlap is optimized and is proportionally decreased when overlap is diminished. The force-length relationship is a static property of skeletal muscle and, therefore, it does not predict the consequences of dynamic contractions. Changes in sarcomere length during muscle contraction result in modulation of the active force that is not necessarily predicted by the cross-bridge theory. The results of in vivo studies of the force-length relationship suggest that muscles that operate on the ascending limb of the force-length relationship typically function in stretch-shortening cycle contractions, and muscles that operate on the descending limb typically function in shorten-stretch cycle contractions. The joint moments produced by a muscle depend on the moment arm and the sarcomere length of the muscle. Moment arm magnitude also affects the excursion (length change) of a muscle for a given change in joint angle, and the number of sarcomeres arranged in series within a muscle fiber determines the sarcomere length change associated with a given excursion.     

Micromechanical function of myofibrils isolated from skeletal and cardiac muscles of the zebrafish

The zebrafish is a potentially important and cost-effective model for studies of development, motility, regeneration, and inherited human diseases. The object of our work was to show whether myofibrils isolated from zebrafish striated muscle represent a valid subcellular contractile model. These organelles, which determine contractile function in muscle, were used in a fast kinetic mechanical technique based on an atomic force probe and video microscopy. Mechanical variables measured included rate constants of force development (k(ACT)) after Ca(2+) activation and of force decay (τ(REL)(-1)) during relaxation upon Ca(2+) removal, isometric force at maximal (F(max)) or partial Ca(2+) activations, and force response to an external stretch applied to the relaxed myofibril (F(pass)). Myotomal myofibrils from larvae developed greater active and passive forces, and contracted and relaxed faster than skeletal myofibrils from adult zebrafish, indicating developmental changes in the contractile organelles of the myotomal muscles. Compared with murine cardiac myofibrils, measurements of adult zebrafish ventricular myofibrils show that k(ACT), F(max), Ca(2+) sensitivity of the force, and F(pass) were comparable and τ(REL)(-1) was smaller. These results suggest that cardiac myofibrils from zebrafish, like those from mice, are suitable contractile models to study cardiac function at the sarcomeric level. The results prove the practicability and usefulness of mechanical and kinetic investigations on myofibrils isolated from larval and adult zebrafish muscles. This novel approach for investigating myotomal and myocardial function in zebrafish at the subcellular level, combined with the powerful genetic manipulations that are possible in the zebrafish, will allow the investigation of the functional primary consequences of human disease-related mutations in sarcomeric proteins in the zebrafish model.