The mechanics of mouse skeletal muscle when shortening during relaxation

The dynamic properties of relaxing skeletal muscle have not been well characterised but are important for understanding muscle function during terrestrial locomotion, during which a considerable fraction of muscle work output can be produced during relaxation. The purpose of this study was to characterise the force-velocity properties of mouse skeletal muscle during relaxation. Experiments were performed in vitro (21 degrees C) using bundles of fibres from mouse soleus and EDL muscles. Isovelocity shortening was applied to muscles during relaxation following short tetanic contractions. Using data from different contractions with different shortening velocities, curves relating force output to shortening velocity were constructed at intervals during relaxation. The velocity component included contributions from shortening of both series elastic component (SEC) and contractile component (CC) because force output was not constant. Early in relaxation force-velocity relationships were linear but became progressively more curved as relaxation progressed. Force-velocity curves late in relaxation had the same curvature as those for the CC in fully activated muscles but V(max) was reduced to approximately 50% of the value in fully activated muscles. These results were the same for slow- and fast-twitch muscles and for relaxation following maximal tetani and brief, sub-maximal tetani. The measured series elastic compliance was used to partition shortening velocity between SEC and CC. The curvature of the CC force-velocity relationship was constant during relaxation. The SEC accounted for most of the shortening and work output during relaxation and its power output during relaxation exceeded the maximum CC power output. It is proposed that unloading the CC, without any change in its overall length, accelerated cross-bridge detachment when shortening was applied during relaxation.     

Frequency-dependent power output and skeletal muscle design

Cyclically contracting muscles provide power for a variety of processes including locomotion, pumping blood, respiration, and sound production. In the current study, we apply a computational model derived from force–velocity relationships to explore how sustained power output is systematically affected by shortening velocity, operational frequency, and strain amplitude. Our results demonstrate that patterns of frequency dependent power output are based on a precise balance between a muscle's intrinsic shortening velocity and strain amplitude. We discuss the implications of this constraint for skeletal muscle design, and then explore implications for physiological processes based on cyclical muscle contraction. One such process is animal locomotion, where musculoskeletal systems make use of resonant properties to reduce the amount of metabolic energy used for running, swimming, or flying. We propose that skeletal muscle phenotype is tuned to this operational frequency, since each muscle has a limited range of frequencies at which power can be produced efficiently. This principle also has important implications for our understanding muscle plasticity, because skeletal muscles are capable of altering their active contractile properties in response to a number of different stimuli. We discuss the possibility that muscles are dynamically tuned to match the resonant properties of the entire musculoskeletal system.     

Shortening velocity and power output of skinned muscle fibers from mammals having a 25,000-fold range of body mass

The shortening velocities of single, skinned, fast and slow skeletal muscle fibers were measured at 5-6 degrees C in five animal species having a 25,000-fold range of body size (mouse, rat, rabbit, sheep, and cow). While fiber diameter and isometric force showed no dependence on animal body size, maximum shortening velocity in both fast and slow fibers and maximum power output in fast fibers were found to vary with the -1/8 power of body size. Maximum power output in slow fibers showed a slightly greater (-1/5 power) dependence on body size. The isometric force produced by the fibers was correlated (r = 0.74) inversely with fiber diameter. For all sizes of animal the average maximum velocity was 1.7 times faster in fast fibers than in slow fibers. The large difference in mechanical properties found between fibers from large and small animals suggests that properties of the contractile proteins vary in a systematic manner with the body size. These size-dependent changes can be used to study the correlations of structure and function of these proteins. Experimental results also suggest that the different metabolic rates observed in different sizes of animals could be accounted for, at least in part, by the difference in the properties of the contractile proteins.     

Relative shortening velocity in locomotor muscles: turkey ankle extensors operate at low V/V(max)

The force-velocity properties of skeletal muscle have an important influence on locomotor performance. All skeletal muscles produce less force the faster they shorten and typically develop maximal power at velocities of approximately 30% of maximum shortening velocity (V(max)). We used direct measurements of muscle mechanical function in two ankle extensor muscles of wild turkeys to test the hypothesis that during level running muscles operate at velocities that favor force rather than power. Sonomicrometer measurements of muscle length, tendon strain-gauge measurements of muscle force, and bipolar electromyographs were taken as animals ran over a range of speeds and inclines. These measurements were integrated with previously measured values of muscle V(max) for these muscles to calculate relative shortening velocity (V/V(max)). At all speeds for level running the V/V(max) values of the lateral gastrocnemius and the peroneus longus were low (<0.05), corresponding to the region of the force-velocity relationship where the muscles were capable of producing 90% of peak isometric force but only 35% of peak isotonic power. V/V(max) increased in response to the demand for mechanical power with increases in running incline and decreased to negative values to absorb energy during downhill running. Measurements of integrated electromyograph activity indicated that the volume of muscle required to produce a given force increased from level to uphill running. This observation is consistent with the idea that V/V(max) is an important determinant of locomotor cost because it affects the volume of muscle that must be recruited to support body weight.     

Mechanism and significance of early, rapid shortening in sensitized airway smooth muscle

It has been reported that sensitization of animals to allergens increases both early shortening velocity and myosin light-chain kinase of their airway smooth muscle without increasing force generated by these muscles. Since early shortening sets muscle length for the duration of a contraction, these responses might be expected to produce greater airway obstruction. Here, it is explained how the more rapid early shortening without increased force production is predicted by the 2-stage process of activation followed by contraction posited by the crossbridge theory of contraction when the rate, but not the extent, of activation is increased. The experimental results are reproduced by a simple model in which activation rate is increased 1.6-fold without any other changes in contractile parameters. These results reinforce suggestions that sensitized animals are a model for reactive airway disease.     

Relationship among fibre type, myosin ATPase activity and contractile properties

Summary At least two types of skeletal muscle myosin have been described which differ in ATPase activity and stability in alkaline or acidic media. Differences in ATPase characteristics distinguish Type I and Type II fibres histochemically. In this study, ATPase activity of myosin from muscles of several species with known histochemical and contractile properties has been determined to test the hypothesis that (1) myosin ATPase activity, (2) histochemical determination of fibre types and (3) maximum shortening velocity, all provide equivalent estimates of contractile properties in muscles of mixed fibre types. Maximum shortening velocity appears to be proportional to ATPase activity as expected from previous reports by Barany. However, both myosin ATPase and the maximum shortening velocity exhibit curvilinear relationships to the fraction of cross-sectional area occupied by Type II fibres. Therefore, we reject the hypothesis and conclude that histochemically determined myofibrillar ATPase does not accurately reflect the intrinsic ATPase activity or shortening velocity in muscles of mixed fibre types. Our data are consistent with the presence of more than two myosin isozymes or with a mixture of isozymes within single muscle fibres.     

Force-velocity properties of two avian hindlimb muscles

Recent work has provided measurements of power output in avian skeletal muscles during running and flying, but little is known about the contractile properties of avian skeletal muscle. We used an in situ preparation to characterize the force-velocity properties of two hind limb muscles, the lateral gastrocnemius (LG) and peroneus longus (PL), in Wild Turkeys (Meleagris gallopavo). A servomotor measured shortening velocity for at least six different loads over the plateau region of the length-tension curve. The Hill equation was fit to the data to determine maximum shortening velocity and peak instantaneous power. Maximum unloaded shortening velocity was 13.0+/-1.6 L s(-1) for the LG muscle and 14.8+/-1.0 L s(-1) for the PL muscle (mean+/-S.E.M.). These velocities are within the range of values published for reptilian and mammalian muscles. Values recorded for maximum isometric force per cross-sectional area, 271+/-28 kPa for the LG and 257+/-30.5 kPa for the PL, and peak instantaneous power output, 341.7+/-36.4 W kg(-1) for the LG and 319.4+/-42.5 W kg(-1) for the PL, were also within the range of published values for vertebrate muscle. The force-velocity properties of turkey LG and PL muscle do not reveal any extreme differences in the mechanical potential between avian and other vertebrate muscle.     

Season and testosterone affect contractile properties of fast calling muscles in the gray tree frog Hyla chrysoscelis

In anurans, circulating levels of androgens influence certain secondary sexual characteristics that are expressed only during the breeding season. We studied the contractile properties of external oblique muscles (used to power sound production) in a species of North American gray tree frog, Hyla chrysoscelis, during the breeding season and also in testosterone-treated captive males and females after the breeding season. Compared with the muscles of breeding-season males, the trunk muscles of postbreeding-season males have 50% less mass, 60% longer twitches, and 40% slower shortening velocities. Testosterone levels similar to those found in breeding-season male hylid frogs restore the contractile speed and mass of male trunk muscles and also convert the small slow trunk muscles of females into larger fast-contracting muscles. We conclude that androgens likely play a key role in altering the contractile properties of these muscles in males during the annual cycle, allowing them to operate in the breeding season at the frequencies required to produce the characteristic rapidly pulsed calls of this species. Females as well as nonbreeding-season males do not produce advertising calls, and therefore the slower muscles found in these animals may allow more economic operation of these muscles. The effects of testosterone on female trunk muscles indicate the potential of this hormone in contributing to the sexual dimorphism in size and contractile properties of these muscles, but this dimorphism is likely due to the interaction of more than one hormone.     

Comparison of isometric contractile properties in hindlimb extensor muscles of the frogs Rana pipiens and Bufo marinus: functional correlations with differences in hopping performance

The leopard frog (Rana pipiens) is an excellent jumper that can reach high take-off velocities and accelerations. It is diurnal, using long, explosive jumps to capture prey and escape predators. The marine toad (Bufo marinus) is a cryptic, nocturnal toad, typically using short, slow hops, or sometimes walking, to patrol its feeding area. Typical of frogs with these different locomotor styles, Rana has relatively long hindlimbs and large (by mass) hindlimb extensor muscles compared to Bufo. We studied the isometric contractile properties of their extensor muscles and found differences that correlate with their different hopping performances. At the hip (semimembranosus, SM), knee (peroneus, Per) and ankle (plantaris longus, PL), we found that Rana's muscles tended to produce greater maximum isometric force relative to body mass, although the difference was significant only for PL. This suggests that differences in force capability at the ankle may be more important than at other joints to produce divergent hopping performances. Maximum isometric force scaled with body mass so that the smaller Rana has relatively larger muscles and force differences between species may reflect size differences only. In addition, Rana's muscles exhibited greater passive resistance to elongation, implying more elastic tissue is present, which may amplify force at take-off due to elastic recoil. Rana's muscles also achieved a higher percentage of maximum force at lower stimulus inputs (frequencies and durations) than in Bufo, perhaps amplifying the differences in force available for limb extension during natural stimulation. Twitch contraction and relaxation times tended to be faster in Rana, although variation was great, so that differences were significant only for Per. Fatigability also tended to be greater in Rana muscles, although, again, values reached significance in only one muscle (PL). Thus, in addition to biomechanical effects, differences in hopping performance may also be determined by diverse physiological properties of the muscles.     

Muscle function in avian flight: achieving power and control

Flapping flight places strenuous requirements on the physiological performance of an animal. Bird flight muscles, particularly at smaller body sizes, generally contract at high frequencies and do substantial work in order to produce the aerodynamic power needed to support the animal's weight in the air and to overcome drag. This is in contrast to terrestrial locomotion, which offers mechanisms for minimizing energy losses associated with body movement combined with elastic energy savings to reduce the skeletal muscles' work requirements. Muscles also produce substantial power during swimming, but this is mainly to overcome body drag rather than to support the animal's weight. Here, I review the function and architecture of key flight muscles related to how these muscles contribute to producing the power required for flapping flight, how the muscles are recruited to control wing motion and how they are used in manoeuvring. An emergent property of the primary flight muscles, consistent with their need to produce considerable work by moving the wings through large excursions during each wing stroke, is that the pectoralis and supracoracoideus muscles shorten over a large fraction of their resting fibre length (33-42%). Both muscles are activated while being lengthened or undergoing nearly isometric force development, enhancing the work they perform during subsequent shortening. Two smaller muscles, the triceps and biceps, operate over a smaller range of contractile strains (12-23%), reflecting their role in controlling wing shape through elbow flexion and extension. Remarkably, pigeons adjust their wing stroke plane mainly via changes in whole-body pitch during take-off and landing, relative to level flight, allowing their wing muscles to operate with little change in activation timing, strain magnitude and pattern.     

Skeletal muscle contractile protein function is preserved in human heart failure

Skeletal muscle weakness is a common finding in patients with chronic heart failure (CHF). This functional deficit cannot be accounted for by muscle atrophy alone, suggesting that the syndrome of heart failure induces a myopathy in the skeletal musculature. To determine whether decrements in muscle performance are related to alterations in contractile protein function, biopsies were obtained from the vastus lateralis muscle of four CHF patients and four control patients. CHF patients exhibited reduced peak aerobic capacity and knee extensor muscle strength. Decrements in whole muscle strength persisted after statistical control for muscle size. Thin filaments and myosin were isolated from biopsies and mechanically assessed using the in vitro motility assay. Isolated skeletal muscle thin-filament function, however, did not differ between CHF patients and controls with respect to unloaded shortening velocity, calcium sensitivity, or maximal force. Similarly, no difference in maximal force or unloaded shortening velocity of isolated myosin was observed between CHF patients and controls. From these results, we conclude that skeletal contractile protein function is unaltered in CHF patients. Other factors, such as a decrease in total muscle myosin content, are likely contributors to the skeletal muscle strength deficit of heart failure.     

Velocity transients and viscoelastic resistance to active shortening in cat papillary muscle.

When isotonic force steps were applied to activated papillary muscles, the velocity was almost never constant. Early rapid shortening associated with the step persisted for 2-7 ms after the step ends. The early rapid shortening is attributed to lightly damped series elastic recoil and velocity transients of the contractile elements. In most steps, the subsequent velocity declines progressively with shortening, and most of the decline in velocity can be accounted for by compression of a viscoelastic element in parallel with the contractile elements. To demonstrate this, the time course of isotonic velocity was compared with a model in which the force-velocity characteristics of the contractile element were assumed to be constant, and the decline in velocity was due to increasing compression of the viscoelastic element. This model predicted the observed results except that the predicted velocities rose progressively above the measured values for steps to light loads applied late in the twitch, and fell below the velocity trace for heavy loads applied early in the twitch. These deviations would occur if rapid shortening caused deactivation late in the twitch, and if activation were rising early in the twitch. A conditioning step applied to the muscle during the rise of force depressed both isometric force and maximum velocity measured at the peak of force; isometric force was more depressed with later conditioning steps than with earlier steps, while maximum velocity was depressed by about the same extent with both early and late steps. This difference between the effects on isometric force and maximum velocity are explained by a combination of deactivation and viscoelastic load.     

In vitro estimates of power output by epaxial muscle during feeding in largemouth bass

Recent work has employed video and sonometric analysis combined with hydrodynamic modeling to estimate power output by the feeding musculature of largemouth bass in feeding trials. The result was an estimate of approximately 69 W kg(-1) of power by the epaxial muscle during maximal feeding strikes. The present study employed in vitro measurements of force, work and power output by fast-twitch epaxial muscle bundles stimulated under activation conditions measured in vivo to evaluate the power output results of the feeding experiments. Isolated muscle bundles from the epaxial muscle, the sternohyoideus and the lateral red or slow-twitch muscle were tied into a muscle mechanics apparatus, and contractile properties during tetanic contractions and maximum shortening velocity (Vmax) were determined. For the epaxial muscles, work and power output during feeding events was determined by employing mean stimulation conditions derived from a select set of maximal feeding trials: 17% muscle shortening at 3.6 muscle lengths/s, with activation occurring 5 ms before the onset of shortening. Epaxial and sternohyoideus muscle displayed similar contractile properties, and both were considerably faster (Vmax approximately 11-13 ML s(-1)) than red muscle (Vmax approximately 5 ML s(-1)). Epaxial muscle stimulated under in vivo activation conditions generated approximately 60 W kg(-1) with a 17% strain and approximately 86 W kg(-1) with a 12% strain. These values are close to those estimated by hydrodynamic modeling. The short lag time (5 ms) between muscle activation and muscle shortening is apparently a limiting parameter during feeding strikes, with maximum power found at an offset of 15-20 ms. Further, feeding strikes employing a faster shortening velocity generated significantly higher power output. Power production during feeding strikes appears to be limited by the need for fast onset of movement and the hydrodynamic resistance to buccal expansion.     

Axial allometry in a neutrally buoyant environment: effects of the terrestrial‐aquatic transition on vertebral scaling

Ecological diversification into new environments presents new mechanical challenges for locomotion. An extreme example of this is the transition from a terrestrial to an aquatic lifestyle. Here, we examine the implications of life in a neutrally buoyant environment on adaptations of the axial skeleton to evolutionary increases in body size. On land, mammals must use their thoracolumbar vertebral column for body support against gravity and thus exhibit increasing stabilization of the trunk as body size increases. Conversely, in water, the role of the axial skeleton in body support is reduced, and, in aquatic mammals, the vertebral column functions primarily in locomotion. Therefore, we hypothesize that the allometric stabilization associated with increasing body size in terrestrial mammals will be minimized in secondarily aquatic mammals. We test this by comparing the scaling exponent (slope) of vertebral measures from 57 terrestrial species (23 felids, 34 bovids) to 23 semi‐aquatic species (pinnipeds), using phylogenetically corrected regressions. Terrestrial taxa meet predictions of allometric stabilization, with posterior vertebral column (lumbar region) shortening, increased vertebral height compared to width, and shorter, more disc‐shaped centra. In contrast, pinniped vertebral proportions (e.g. length, width, height) scale with isometry, and in some cases, centra even become more spool‐shaped with increasing size, suggesting increased flexibility. Our results demonstrate that evolution of a secondarily aquatic lifestyle has modified the mechanical constraints associated with evolutionary increases in body size, relative to terrestrial taxa.     

Resistance to radial expansion limits muscle strain and work

The collagenous extracellular matrix (ECM) of skeletal muscle functions to transmit force, protect sensitive structures, and generate passive tension to resist stretch. The mechanical properties of the ECM change with age, atrophy, and neuromuscular pathologies, resulting in an increase in the relative amount of collagen and an increase in stiffness. Although numerous studies have focused on the effect of muscle fibrosis on passive muscle stiffness, few have examined how these structural changes may compromise contractile performance. Here we combine a mathematical model and experimental manipulations to examine how changes in the mechanical properties of the ECM constrain the ability of muscle fibers and fascicles to radially expand and how such a constraint may limit active muscle shortening. We model the mechanical interaction between a contracting muscle and the ECM using a constant volume, pressurized, fiber-wound cylinder. Our model shows that as the proportion of a muscle cross section made up of ECM increases, the muscle’s ability to expand radially is compromised, which in turn restricts muscle shortening. In our experiments, we use a physical constraint placed around the muscle to restrict radial expansion during a contraction. Our experimental results are consistent with model predictions and show that muscles restricted from radial expansion undergo less shortening and generate less mechanical work under identical loads and stimulation conditions. This work highlights the intimate mechanical interaction between contractile and connective tissue structures within skeletal muscle and shows how a deviation from a healthy, well-tuned relationship can compromise performance.     

Effects of a chronic wasting infection on skeletal muscle size and contractile properties

To evaluate the effects of chronic infection on skeletal muscle dimensions and contractile properties, we used a hamster model of visceral leishmaniasis, a parasitic infection of the reticuloendothelial system produced by Leishmania donovani (LD). To distinguish between effects of reduced caloric intake and infection per se, we also studied healthy control animals and noninfected animals subjected to caloric restriction. Three muscles were tested in vitro: plantaris, soleus, and diaphragm. Both caloric restriction and LD infection caused loss of body weight and reduced muscle cross-sectional areas and wet weights. The interventions had variable effects on in vitro contractile properties, the most pronounced being reduction in peak tension in response to tetanic stimulation. Tension loss was 35-45%, except for a loss of 65% in plantaris of LD-infected animals. We conclude that chronic LD infection affects skeletal muscles in both indirect and direct ways. 1) Reduced caloric intake due to anorexia decreases muscle size and active tension. Disuse probably enhances this effect in limb muscles. 2) Infection produces profound weakness of inactive fast-twitch muscle by unknown mechanisms.     

Mechanical output of the cat soleus during treadmill locomotion: in vivo vs in situ characteristics

To study the mechanical output of skeletal muscle, four adult cats were trained to run on a treadmill and then implanted under sterile conditions and anesthesia with a force transducer on the soleus tendon and EMG electrodes in the muscle belly. After a two-week recovery period, five consecutive step cycles were filmed at treadmill speeds of 0.8, 1.3 and 2.2 m s-1. Locomotion data in vivo included individual muscle force, length and velocity changes and EMG during each step cycle. Data for an average step cycle at each speed were compared to the force-velocity properties obtained on the same muscle under maximal nerve stimulation and isotonic loading conditions in situ. Results indicate that the force and power generated at a given velocity of shortening during late stance in vivo were greater at the higher speeds of locomotion than the force and power generated at the same shortening velocity in situ. Strain energy stored in the muscle-tendon unit during the yield phase in early stance is felt to be a major contributor to the muscle's enhanced mechanical output during muscle shortening in late stance.     

Contribution of muscle series elasticity to maximum performance in drop jumping

The contribution of muscle in-series compliance on maximum performance of the muscle tendon complex was investigated using a forward dynamic computer simulation. The model of the human body contains 8 Hill-type muscles of the lower extremities. Muscle activation is optimized as a function of time, so that maximum drop jump height is achieved by the model. It is shown that the muscle series elastic energy stored in the downward phase provides a considerable contribution (32%) to the total muscle energy in the push-off phase. Furthermore, by the return of stored elastic energy all muscle contractile elements can reduce their shortening velocity up to 63% during push-off to develop a higher force due to their force velocity properties. The additional stretch taken up by the muscle series elastic element allows only m. rectus femoris to work closer to its optimal length, due to its force length properties. Therefore the contribution of the series elastic element to muscle performance in maximum height drop jumping is to store and return energy, and at the same time to increase the force producing ability of the contractile elements during push-off.     

The number of Z-repeats and super-repeats in nebulin greatly varies across vertebrates and scales with animal size

Nebulin is a skeletal muscle protein that associates with the sarcomeric thin filaments and has functions in regulating the length of the thin filament and the structure of the Z-disk. Here we investigated the nebulin gene in 53 species of birds, fish, amphibians, reptiles, and mammals. In all species, nebulin has a similar domain composition that mostly consists of ∼30-residue modules (or simple repeats), each containing an actin-binding site. All species have a large region where simple repeats are organized into seven-module super-repeats, each containing a tropomyosin binding site. The number of super-repeats shows high interspecies variation, ranging from 21 (zebrafish, hummingbird) to 31 (camel, chimpanzee), and, importantly, scales with body size. The higher number of super-repeats in large animals was shown to increase thin filament length, which is expected to increase the sarcomere length for optimal force production, increase the energy efficiency of isometric force production, and lower the shortening velocity of muscle. It has been known since the work of A.V. Hill in 1950 that as species increase in size, the shortening velocity of their muscle is reduced, and the present work shows that nebulin contributes to the mechanistic basis. Finally, we analyzed the differentially spliced simple repeats in nebulin''s C terminus, whose inclusion correlates with the width of the Z-disk. The number of Z-repeats greatly varies (from 5 to 18) and correlates with the number of super-repeats. We propose that the resulting increase in the width of the Z-disk in large animals increases the number of contacts between nebulin and structural Z-disk proteins when the Z-disk is stressed for long durations.     

Serotonin modulates muscle function in the medicinal leech Hirudo verbana

The body wall muscles of sanguivorous leeches power mechanically diverse behaviours: suction feeding, crawling and swimming. These require longitudinal muscle to exert force over an extremely large length range, from 145 to 46 per cent of the mean segmental swimming length. Previous data, however, suggest that leech body wall muscle has limited capacity for force production when elongated. Serotonin (5-HT) alters the passive properties of the body wall and stimulates feeding. We hypothesized that 5-HT may also have a role in allowing force production in elongated muscle by changing the shape of the length-tension relationship (LTR). LTRs were measured from longitudinal muscle strips in vitro in physiological saline with and without the presence of 10 μM 5-HT. The LTR was much broader than previously measured for leech muscle. Rather than shifting the LTR, 5-HT reduced passive muscle tonus and increased active stress at all lengths. In addition to modulating leech behaviour and passive mechanical properties, 5-HT probably enhances muscle force and work production during locomotion and feeding.