Mechanical performance of scallop adductor muscle during swimming

What good is a measure of muscle length? The how and why of direct measurements of skeletal muscle motion

Over the past 50 years our understanding of the central role that muscle motion has in powering movement has accelerated significantly. Fundamental to this progress has been the development of methods for measuring the length of muscles and muscle fibers in vivo. A measurement of muscle fiber length might seem a trivial piece of information on its own. Yet when combined with knowledge of the properties of skeletal muscle it has proven a powerful tool for understanding the mechanics and energetics of locomotion and informing models of motor control. In this perspective we showcase the value of direct measurements of muscle fiber length from four different techniques: sonomicrometry, fluoromicrometry, magnetomicrometry, and ultrasound. For each method, we review its history and provide a high-level user's guide for researchers choosing tools for measuring muscle length in vivo. We highlight key insights that these measurements have provided, including the importance of passive elastic mechanisms and how skeletal muscle properties govern locomotor performance. The diversity of locomotor behaviors revealed across comparative studies has provided an important tool for discovering the rules for muscle function that span vertebrate locomotion more broadly, including in humans.

The mechanical properties of the mantle muscle of European cuttlefish (Sepia officinalis)

The circular muscles surrounding the mantle cavity of European cuttlefish (Sepia officinalis) generate the mechanical power to compress the cavity, forcing a jet of water out of the funnel, propelling the animal during jet propulsion swimming. During ontogeny, jetting frequency decreases in adults compared with juveniles, and this is expected to be reflected in the contractile properties of the locomotory muscles. To develop greater insight into how the locomotion of these animals is powered during ontogeny, we determined the mechanical properties of bundles of muscle fascicles during isometric, isotonic and cyclic length changes in vitro, at two life stages: juveniles and adults. The twitch kinetics were faster in juveniles than in adults (twitch rise time 257 ms compared with 371 ms; half-twitch relaxation 257 ms compared with 677 ms in juveniles and adults, respectively); however, twitch and tetanic stress, the maximum velocity of shortening and curvature of the force-velocity relationship did not differ. Under cyclic conditions, net power exhibited an inverted U-shaped relationship with cycle frequency in both juveniles and adults; the frequency at which maximum net power was achieved was shifted to lower cycle frequencies with increased maturity, which is consistent with the slower contraction and relaxation kinetics in adults compared with juveniles. The cycle frequency at which peak power was achieved during cyclical contractions in vitro was found to match that seen in vivo in juveniles, suggesting power is being maximised during jet propulsion swimming.

Cell type diversity in scallop adductor muscles revealed by single-cell RNA-Seq

Studies on cell atlas in marine invertebrates provide a better understanding of cell types, stem cell maintenance, and lineages of cell differentiation. To investigate the molecular features of various cell types in molluscan muscles, we performed single-cell RNA sequencing (scRNA-seq) to map cell types in scallop adductor muscles. We uncovered the cell type-specific features of 20 cell clusters defined by the expression of multiple specific molecular markers. These cell clusters are mainly classified into four broad classes, including mesenchymal stem cells, muscle cells, neurons, and haemolymph cells. In particular, we identified a diverse repertoire of neurons in the striated adductor muscle, but not in the smooth muscle. We further reconstructed the cell differentiation events using all the cell clusters by single-cell pseudotemporal trajectories. By integrating dual BrdU-PCNA immunodetection, neuron-specific staining and electron microscopy observation, we showed the spatial distribution of mesenchymal stem cells and neurons in striated adductor muscle of scallops. The present findings will not only be useful to address the cell type-specific gene expression profiles in scallop muscles, but also provide valuable resources for cross-species comparison of marine organisms.

In vitro virtual reality: an anatomically explicit musculoskeletal simulation powered by in vitro muscle using closed-loop tissue-software interaction

Muscle force-length dynamics are governed by intrinsic contractile properties, motor stimulation and mechanical load. Although intrinsic properties are well characterised, physiologists lack in vitro instrumentation to account for combined effects of limb inertia, musculoskeletal architecture and contractile dynamics. We introduce in vitro virtual reality (in vitro-VR) which enables in vitro muscle tissue to drive a musculoskeletal jumping simulation. In hardware, muscle force from a frog plantaris was transmitted to a software model where joint torques, inertia and ground reaction forces were computed to advance the simulation at 1 kHz. To close the loop, simulated muscle strain was returned to update in vitro length. We manipulated (1) stimulation timing and (2) the virtual muscle's anatomical origin. This influenced interactions among muscular, inertial, gravitational and contact forces dictating limb kinematics and jump performance. We propose that in vitro-VR can be used to illustrate how neuromuscular control and musculoskeletal anatomy influence muscle dynamics and biomechanical performance.

Software for convenient correction of artifacts in sonomicrometry

Sonomicrometry is widely applied in biomechanics and physiology to measure precise distances with high temporal resolution. Although commonly used, its usefulness is often limited by the presence of artifacts that require correction. Unfortunately, procedures reported in the literature for artifact correction are often unclear. Furthermore, currently available tools for artifact correction require significant manual manipulations, the consistency of which takes painstaking effort to verify. To improve the efficiency and consistency of sonomicrometry, we have developed a new software tool for the correction of sonomicrometry artifacts. This tool provides a framework for artifact correction requiring fewer and more limited manipulations from the user. We aimed to make this tool more transparent and easier to use than commercially available tools. To facilitate its application, we describe the relevant properties of sonomicrometry artifacts and detail the software's correction algorithm. The software is available for MacOS and Linux with source code and documentation.

Bluegill sunfish use high power outputs from axial muscles to generate powerful suction-feeding strikes

Suction-feeding fish rapidly expand the mouth cavity to generate high-velocity fluid flows that accelerate food into the mouth. Such fast and forceful suction expansion poses a challenge, as muscle power is limited by muscle mass and the muscles in fish heads are relatively small. The largemouth bass powers expansion with its large body muscles, with negligible power produced by the head muscles (including the sternohyoideus). However, bluegill sunfish - with powerful strikes but different morphology and feeding behavior - may use a different balance of cranial and axial musculature to power feeding and different power outputs from these muscles. We estimated the power required for suction expansion in sunfish from measurements of intraoral pressure and rate of volume change, and measured muscle length and velocity. Unlike largemouth bass, the sternohyoideus did shorten to generate power, but it and other head muscles were too small to contribute more than 5-10% of peak expansion power in sunfish. We found no evidence of catapult-style power amplification. Instead, sunfish powered suction feeding by generating high power outputs (up to 438 W kg^-1) from their axial muscles. These muscles shortened across the cranial half of the body as in bass, but at faster speeds that may be nearer the optimum for power production. Sunfish were able to generate strikes of the same absolute power as bass, but with 30-40% of the axial muscle mass. Thus, species may use the body and head muscles differently to meet the requirements of suction feeding, depending on their morphology and behavior.

Speed of sound in muscle for use in sonomicrometry

Converting ultrasound transit time into a measure of distance when using sonomicrometry requires that the speed of sound be known. A number of different values for the speed of sound in muscle have been assumed in studies of skeletal and cardiac muscle, and in some cases the effect of temperature has been ignored. The speed of ultrasound with frequencies greater than 1MHz in skeletal and cardiac muscle is briefly reviewed, including the effects of temperature and contractile state. A simplified equation for the speed of sound in pure water is presented for the temperature range from 0-50°C. This equation can be used when calibrating sonomicrometer transducers in water. The data available indicate that the speed of sound in both cardiac and skeletal muscle can be approximated by multiplying the speed of sound in pure water at the measurement temperature by 1.045. Differences in the speed of sound in the longitudinal and transverse directions and changes with contractile state appear to be small and in most cases can probably be safely ignored. The normal variation in muscle composition does not greatly affect the speed of ultrasound in muscle, but investigators placing sonomicrometer transducers near tendons should be conscious of the much greater speed of sound in tendon and variation with loading.

Swimming muscles power suction feeding in largemouth bass

Most aquatic vertebrates use suction to capture food, relying on rapid expansion of the mouth cavity to accelerate water and food into the mouth. In ray-finned fishes, mouth expansion is both fast and forceful, and therefore requires considerable power. However, the cranial muscles of these fishes are relatively small and may not be able to produce enough power for suction expansion. The axial swimming muscles of these fishes also attach to the feeding apparatus and have the potential to generate mouth expansion. Because of their large size, these axial muscles could contribute substantial power to suction feeding. To determine whether suction feeding is powered primarily by axial muscles, we measured the power required for suction expansion in largemouth bass and compared it to the power capacities of the axial and cranial muscles. Using X-ray reconstruction of moving morphology (XROMM), we generated 3D animations of the mouth skeleton and created a dynamic digital endocast to measure the rate of mouth volume expansion. This time-resolved expansion rate was combined with intraoral pressure recordings to calculate the instantaneous power required for suction feeding. Peak expansion powers for all but the weakest strikes far exceeded the maximum power capacity of the cranial muscles. The axial muscles did not merely contribute but were the primary source of suction expansion power and generated up to 95% of peak expansion power. The recruitment of axial muscle power may have been crucial for the evolution of high-power suction feeding in ray-finned fishes.

Forces and mechanical energy fluctuations during diagonal stride roller skiing; running on wheels?

Mechanical energy is conserved during terrestrial locomotion in two ways; the inverted pendulum mechanism for walking and the spring-mass mechanism for running. Here, we investigated if diagonal stride cross-country roller skiing (DIA) utilizes similar mechanisms. Based on previous studies, we hypothesized that running and DIA would share similar patterns of kinetic energy (KE), gravitational potential energy (GPE) and elastic energy fluctuations as if roller skiing is “running on wheels”. Experienced skiers (N=9) walked, ran and roller skied with DIA at 1.25m/s and 3 m/s on a level dual-belt treadmill that recorded perpendicular and parallel forces. We calculated the KE and GPE of the COM from the force recordings. As expected, the KE and GPE fluctuated with an out-of-phase pattern during walking and an in-phase pattern during running. Unlike walking, during DIA, the KE and GPE fluctuations were in-phase as they are in running. However, during the glide phase, KE was dissipated as frictional heat and could not be stored elastically in the tendons like in running. Elastic energy storage and return epitomize running and thus we reject our hypothesis. Diagonal stride cross-country skiing is a biomechanically unique movement that only superficially resembles walking or running.

Escape responses by jet propulsion in scallops

The impressive swimming escape response of scallops uses a simple locomotor system that facilitates analysis of the functional relationships between its primary components. One large adductor muscle, two valves, the muscular mantle, and the rubbery hinge ligament are the basic elements allowing swimming by jet propulsion. Although these basic functional elements are shared among scallop species, the exact nature of the escape response varies considerably within and among species. Valve shape and density have opposing influences upon the capacity for swimming and the ease of attack by predators once captured. Patterns of muscle use can partly overcome the constraints imposed by shell characteristics. The depletion of muscle reserves during gametogenesis leads to a trade-off between escape response performance and reproductive investment. However, changes in muscle energetic status influence repeat performance more than initial escape performance. Escape response performance is influenced by habitat temperature and mariculture techniques. During scallop ontogeny, changes in susceptibility to predation and in reproductive investment may influence escape response capacities. These ontogenetic patterns are likely to vary with the longevity and maximal size of each species. Although the basic elements allowing swimming by jet propulsion are common to scallops, their exact use varies considerably among species.

Joint-specific power production during submaximal and maximal cycling

Separate authors have reported that knee extension dominates power production during submaximal cycling (SUB(cyc)) and hip extension is the dominant action during maximal cycling (MAX(cyc)). Changes in joint-specific powers across broad ranges of net cycling powers (P(net)) within one group of cyclists have not been reported.

PURPOSE

Our purpose was to determine the extent to which ankle, knee, and hip joint actions produced power across a range of P(net) . We hypothesized that relative knee extension power would decrease and relative knee flexion and hip extension powers would increase as P(net) increased.

METHODS

Eleven cyclists performed SUB(cyc) (250, 400, 550, 700, and 850 W) and MAX(cyc) trials at 90 rpm. Joint-specific powers were calculated and averaged over complete pedal revolutions and over extension and flexion phases. Portions of the cycle spent in extension (duty cycle) were determined for the whole leg and ankle, knee, and hip joints. Relationships of relative joint-specific powers with P(net) were assessed with linear regression analyses.

RESULTS

Absolute ankle, knee, and hip joint-specific powers increased as P(net) increased. Relative knee extension power decreased (r(2) = 0.88, P = 0.01) and knee flexion power increased (r(2) = 0.98, P < 0.001) as P(net) increased. Relative hip extension power was constant across all P(net) . Whole-leg and ankle, knee, and hip joint duty cycle values were greater for MAX(cyc) than for SUB(cyc).

CONCLUSIONS

Our data demonstrate that 1) absolute ankle, knee, and hip joint-specific powers substantially increase as a function of increased P(net) , 2) hip extension was the dominant power-producing action during SUB(cyc) and MAX(cyc), 3) knee flexion power becomes relatively more important during high-intensity cycling, and 4) increased duty cycle values represent an important strategy to increase maximum power.

The effects of asymmetric length trajectories on the initial mechanical efficiency of mouse soleus muscles

Asymmetric cycles with more than half of the cycle spent shortening enhance the mechanical power output of muscle during flight and vocalisation. However, strategies that enhance muscle mechanical power output often compromise efficiency. In order to establish whether a trade-off necessarily exists between power and efficiency, we investigated the effects of asymmetric muscle length trajectories on the maximal mechanical cycle-average power output and initial mechanical efficiency (Ei). Work and heat were measured in vitro in a mouse soleus muscle undergoing contraction cycles with 25% (Saw25%), 50% (Saw50%) and 75% (Saw75%) of the cycles spent shortening. Cycle-average power output tended to increase with the proportion of the cycle spent shortening at a given frequency. Maximum cycle-average power output was 102.9±7.6 W kg–1 for Saw75% cycles at 5 Hz. Ei was very similar for Saw50% and Saw75% cycles at all frequencies (approximately 0.27 at 5 Hz). Saw25% cycles had Ei values similar to those of Saw50% and Saw75% cycles at 1 Hz (approximately 0.20), but were much less efficient at 5 Hz (0.08±0.03). The lower initial mechanical efficiency of Saw25% cycles at higher frequencies suggests that initial mechanical efficiency is reduced if the time available for force generation and relaxation during shortening is insufficient. The similar initial mechanical efficiency of Saw50% and Saw75% cycles at all frequencies shows that increasing the proportion of the contraction cycle spent shortening is a strategy that allows an animal to increase muscle mechanical power output without compromising initial mechanical efficiency.

Convergent and parallel evolution in life habit of the scallops (Bivalvia: Pectinidae)

Background

We employed a phylogenetic framework to identify patterns of life habit evolution in the marine bivalve family Pectinidae. Specifically, we examined the number of independent origins of each life habit and distinguished between convergent and parallel trajectories of life habit evolution using ancestral state estimation. We also investigated whether ancestral character states influence the frequency or type of evolutionary trajectories.

Results

We determined that temporary attachment to substrata by byssal threads is the most likely ancestral condition for the Pectinidae, with subsequent transitions to the five remaining habit types. Nearly all transitions between life habit classes were repeated in our phylogeny and the majority of these transitions were the result of parallel evolution from byssate ancestors. Convergent evolution also occurred within the Pectinidae and produced two additional gliding clades and two recessing lineages. Furthermore, our analysis indicates that byssal attaching gave rise to significantly more of the transitions than any other life habit and that the cementing and nestling classes are only represented as evolutionary outcomes in our phylogeny, never as progenitor states.

Conclusions

Collectively, our results illustrate that both convergence and parallelism generated repeated life habit states in the scallops. Bias in the types of habit transitions observed may indicate constraints due to physical or ontogenetic limitations of particular phenotypes.

The weak link: do muscle properties determine locomotor performance in frogs?

Muscles power movement, yet the conceptual link between muscle performance and locomotor performance is poorly developed. Frog jumping provides an ideal system to probe the relationship between muscle capacity and locomotor performance, because a jump is a single discrete event and mechanical power output is a critical determinant of jump distance. We tested the hypothesis that interspecific variation in jump performance could be explained by variability in available muscle power. We used force plate ergometry to measure power produced during jumping in Cuban tree frogs (Osteopilus septentrionalis), leopard frogs (Rana pipiens) and cane toads (Bufo marinus). We also measured peak isotonic power output in isolated plantaris muscles for each species. As expected, jump performance varied widely. Osteopilus septentrionalis developed peak power outputs of 1047.0 ± 119.7 W kg(-1) hindlimb muscle mass, about five times that of B. marinus (198.5 ± 54.5 W kg(-1)). Values for R. pipiens were intermediate (543.9 ± 96.2 W kg(-1)). These differences in jump power were not matched by differences in available muscle power, which were 312.7 ± 28.9, 321.8 ± 48.5 and 262.8 ± 23.2 W kg(-1) muscle mass for O. septentrionalis, R. pipiens and B. marinus, respectively. The lack of correlation between available muscle power and jump power suggests that non-muscular mechanisms (e.g. elastic energy storage) can obscure the link between muscle mechanical performance and locomotor performance.

Modulation of in vivo muscle power output during swimming in the African clawed frog (Xenopus laevis)

The goal of this study is to explore how swimming animals produce the wide range of performance that is seen across their natural behaviors. In vivo recordings of plantaris longus muscle length change were obtained by sonomicrometry. Simultaneous with muscle length data, force measurements were obtained using a novel tendon buckle force transducer placed on the Achilles tendon of Xenopus laevis frogs during brief accelerating bursts of swimming. In vivo work loops revealed that the plantaris generates a variable amount of positive muscle work over a range of swimming cycle durations (from 0.23 to 0.76 s), resulting in a large range of cycle power output (from 2.32 to 74.17 W kg(-1) muscle). Cycle duration correlated negatively with cycle power, and cycle work correlated positively (varying as a function of peak cycle stress and, to a much lesser extent, fascicle strain amplitude). However, variation in cycle duration only contributed to 12% of variation in power, with cycle work accounting for the remaining 88%. Peak cycle stress and strain amplitude were also highly variable, yet peak stress was a much stronger predictor of cycle work than strain amplitude. Additionally, EMG intensity correlated positively with peak muscle stress (r(2)=0.53). Although the timing of muscle recruitment (EMG phase and EMG duty cycle) varied considerably within and among frogs, neither parameter correlated strongly with cycle power, cycle work, peak cycle stress or strain amplitude. These results suggest that relatively few parameters (cycle duration, peak cycle stress and strain amplitude) vary to permit a wide range of muscle power output, which allows anurans to swim over a large range of velocities and accelerations.

Jet propulsion in the cold: mechanics of swimming in the Antarctic scallopAdamussium colbecki

Unlike most bivalves, scallops are able to swim, relying on a shell with reduced mass and streamlined proportions, a large fast-twitch adductor muscle and the elastic characteristics of the shell's hinge. Despite these adaptations, swimming in scallops is never far from failure, and it is surprising to find a swimming scallop in Antarctica, where low temperature increases the viscosity of seawater, decreases the power output of the adductor muscle and potentially compromises the energy storage capability of the hinge material (abductin, a protein rubber). How does the Antarctic scallop, Adamussium colbecki, cope with the cold? Its shell mass is substantially reduced relative to that of temperate and tropical scallops, but this potential advantage is more than offset by a drastic reduction in adductor-muscle mass. By contrast, A. colbecki's abductin maintains a higher resilience at low temperatures than does the abductin of a temperate scallop. This resilience may help to compensate for reduced muscle mass,assisting the Antarctic scallop to maintain its marginal swimming ability. However, theory suggests that this assistance should be slight, so the adaptive value of increased resilience remains open to question. The high resilience of A. colbecki abductin at low temperatures may be of interest to materials engineers.

Design and function of superfast muscles: new insights into the physiology of skeletal muscle

Superfast muscles of vertebrates power sound production. The fastest, the swimbladder muscle of toadfish, generates mechanical power at frequencies in excess of 200 Hz. To operate at these frequencies, the speed of relaxation has had to increase approximately 50-fold. This increase is accomplished by modifications of three kinetic traits: (a) a fast calcium transient due to extremely high concentration of sarcoplasmic reticulum (SR)-Ca2+ pumps and parvalbumin, (b) fast off-rate of Ca2+ from troponin C due to an alteration in troponin, and (c) fast cross-bridge detachment rate constant (g, 50 times faster than that in rabbit fast-twitch muscle) due to an alteration in myosin. Although these three modifications permit swimbladder muscle to generate mechanical work at high frequencies (where locomotor muscles cannot), it comes with a cost: The high g causes a large reduction in attached force-generating cross-bridges, making the swimbladder incapable of powering low-frequency locomotory movements. Hence the locomotory and sound-producing muscles have mutually exclusive designs.

Muscle function and power output during suction feeding in largemouth bass, Micropterus salmoides

Muscle power output is thought to limit suction feeding performance, yet muscle power output during suction feeding has never been directly measured. In this study, epaxial activation and strain, hyoid depression, and intra-oral pressure were simultaneously measured during suction feeding in the largemouth bass (Micropterus salmoides). A mechanical model of muscle force transmission between the neurocranium and oral cavity was used to estimate muscle stress, work, and power. The epaxials shortened from rest an average of 9% of their length, with the highest efforts producing greater than 20% strain. Onset of shortening was simultaneous with or shortly after (< 10 ms) onset of activation. Maximal net power for individual fish ranged from 17 to 137 W kg(-1). Muscle power was significantly correlated with rectified EMG area (r = 0.80; p < 0.0001). The power required for cranial expansion was significantly correlated with epaxial power (r = 0.81; p < 0.0001), and the power exponent of this relationship ( approximately 1 for 3 of the 4 fish) implies that epaxial power accounts for most of the power of cranial expansion. The limitations imposed by the kinematic requirements and loading environment of suction feeding (short delay between activation and strain, maximal stress occurring after shortening, operation at lengths shorter than resting length) may prevent maximal muscular power production.

Joint work and power associated with acceleration and deceleration in tammar wallabies (Macropus eugenii)

Measurements of joint work and power were determined using inverse dynamics analysis based on ground reaction force and high-speed video recordings of tammar wallabies as they decelerated and accelerated while hopping over a force platform on level ground. Measurements were obtained over a range of accelerations ranging from -6 m s(-2) to 8 m s(-2). The goal of our study was to determine which joints are used to modulate mechanical power when tammar wallabies change speed. From these measurements, we also sought to determine which hind limb muscle groups are the most important for producing changes in mechanical work. Because our previous in vivo analyses of wallaby distal muscle function indicated that these muscle-tendon units favor elastic energy savings and perform little work during steady level and incline hopping, we hypothesized that proximal muscle groups operating at the hip and knee joint are most important for the modulation of mechanical work and power. Of the four hind limb joints examined, the ankle joint had the greatest influence on the total limb work, accounting for 89% of the variation observed with changing speed. The hip and metatarsophalageal (MP) joints also contributed to modulating whole limb work, but to a lesser degree than the ankle, accounting for 28% (energy production) and -24% (energy absorption) of the change in whole limb work versus acceleration, respectively. In contrast, the work produced at the knee joint was independent of acceleration. Based on the results of our previous in vivo studies and given that the magnitude of power produced at the ankle exceeds that which these muscles alone could produce, we conclude that the majority of power produced at the ankle joint is likely transferred from the hip and knee joints via proximal bi-articular muscles, operating in tandem with bi-articular ankle extensors, to power changes in hopping speed of tammar wallabies. Additionally, over the observed range of performance, peak joint moments at the ankle (and resulting tendon strains) did not increase significantly with acceleration, indicating that having thin tendons favoring elastic energy storage does not necessarily limit a tammar wallaby's ability to accelerate or decelerate.

Submaximal power output from the dorsolongitudinal flight muscles of the hawkmothManduca sexta

To assess the extent to which the power output of a synchronous insect flight muscle is maximized during flight, we compared the maximum potential power output of the mesothoracic dorsolongitudinal (dl1) muscles of Manduca sexta to their power output in vivo. Holding temperature and cycle frequency constant at 36°C and 25 Hz, respectively,we varied the phase of activation, mean length and strain amplitude. Under in vivo conditions measured in tethered flight, the dl1muscles generated only 40–67% of their maximum potential power output. Compared to the in vivo phase of activation, the phase that maximized power output was advanced by 12% of the cycle period, and the length that maximized power output was 10% longer than the in vivo operating length.

Mechanical function of two ankle extensors in wild turkeys: shifts from energy production to energy absorption during incline versus decline running

We investigated the mechanical function of two ankle extensor muscles, the lateral gastrocnemius (LG) and peroneus longus (PL), in wild turkeys Meleagris gallopavo during steady speed running. We hypothesized that mechanical work output of the LG and PL during running parallels the demand for mechanical work on the body. The turkeys ran on level, inclined (+6 degrees, +12 degrees ) and declined (-6 degrees, -12 degrees ) treadmills to change the demand for mechanical work. Simultaneous measurements of muscle length (from sonomicrometry) and muscle force (from tendon strain gauges) were used to calculate mechanical work output. During level running at a speed of 2 m s(-1), the LG and PL were both active in stance but produced peak force at different times, at approximately 21% of stance duration for the LG and 70% for the PL. The LG and PL also had different length patterns in stance during level running. The LG underwent little shortening during force production, resulting in negligible net positive work (2.0+/-0.8 J kg(-1)). By contrast, the PL produced force across a stretch-shorten cycle in stance and did significant net positive work (4.7+/-1.6 J kg(-1)). Work outputs for both the LG and PL were directly proportional to running slope. When we increased the demand for net positive work by running the turkeys on an incline, the LG and PL increased stance net positive work output in direct proportion to slope (P<0.05). Stance net positive work output increased to 7.0+/-1.3 J kg(-1) for the LG and 8.1+/-2.9 J kg(-1) for the PL on the steepest incline. Increases in stance net positive work for the LG and PL were associated with increases in net shortening strain and average shortening velocity, but average force in stance remained constant. The LG and PL muscles were also effective energy absorbers during decline running, when there is demand for net negative work on the body. During decline running at 2 m s(-1) on the steepest slope, the LG absorbed 4.6+/-2.2 J kg(-1) of net work in stance and the PL absorbed 2.4+/-0.9 J kg(-1) of net work. Shifts in muscle mechanical function from energy production during incline running to energy absorption during decline running were observed over a range of running speeds from 1-3 m s(-1) for both the LG and PL. Two fundamentally different mechanisms for changing work output were apparent in the mechanical behavior of the LG and PL. The LG simply altered its length pattern; it actively shortened during incline running to produce mechanical energy and actively lengthened during decline running to absorb mechanical energy. The PL changed mechanical function by altering its length pattern and by shifting the timing of force production across its stretch-shorten cycle. During incline running, the PL produced force during late stance shortening for positive work, but during decline running, the timing of force production shifted into early stance, to align with lengthening for negative work. In addition, during decline running, the PL greatly reduced or eliminated late stance shortening, thus reducing the potential for positive work. Our results show that the changing demands for whole body work during steady speed running are met, at least in part, by an ability of single muscles to shift mechanical function from net energy production to net energy absorption.

Muscle activation and strain during suction feeding in the largemouth bassMicropterus salmoides

Activation and strain in the sternohyoideus (SH) were measured in vivo in five largemouth bass Micropterus salmoides. The SH is thought to actuate lower jaw depression, hyoid depression and suspensorial abduction during suction feeding in teleost fish. Sonomicrometry was used to measure fascicle shortening and lower jaw kinematics, while activity was measured by electromyography (EMG). SH fascicles shortened by an average of 11% during suction feeding. In three fish SH fascicles consistently shortened during fast lower jaw depression, but in two individuals they contracted isometrically or lengthened slightly during fast lower jaw depression. The SH continued shortening after peak gape, presumably actuating hyoid depression and lateral expansion of the buccal cavity. Onset of SH relengthening and onset of lower jaw elevation were simultaneous, as were the return of the SH to resting length and gape closure. Activation followed the onset of shortening by an average of 23 ms, although the muscle was active an average of 15 ms before the onset of rapid shortening. SH fascicles reached sustained shortening velocities averaging –2.5 fascicle lengths per second, and generally increased shortening velocity after peak gape. The shortening velocities measured in this study suggest that the SH actively shortens to generate power during suction feeding. This study is the first direct measurement of in vivo muscle function during suction feeding, the most common mechanism of prey capture among aquatic vertebrates.

High-energy phosphate metabolism during exercise and recovery in temperate and Antarctic scallops: an in vivo 31P-NMR study

In vivo (31)P-nuclear magnetic resonance (NMR) spectroscopy was used to measure the levels of ATP, phospho-l-arginine (PLA), and inorganic phosphate in the adductor muscle of the Antarctic scallop Adamussium colbecki and two temperate species, Aequipecten opercularis and Pecten maximus. Graded exercise regimes from light (one to two contractions) to exhausting (failing to respond to further stimulation) were imposed on animals of each species at its habitat temperature (0 degrees vs. 12 degrees C, respectively). NMR spectroscopy allowed noninvasive measurement of metabolite levels and intracellular pH at high time resolution (30-120-s intervals) during exercise and throughout the recovery period. Significant differences were shown between the magnitude and form of the metabolic response with increasing levels of exercise in each species. After exhaustion, short-term (first 15 min) muscle alkalosis was followed by acidosis of up to 0.2 pH units during the recovery process. Aequipecten opercularis had similar resting muscle PLA levels compared with either P. maximus or A. colbecki but used a fivefold greater proportion of this store per contraction and was able to perform only half as many claps (maximum of 24) as the other species before exhaustion. All species regenerated their PLA store at a similar rate despite different environmental temperatures. These findings argue for some cold compensation of muscular performance and recovery capacities in the Antarctic scallop, albeit at levels of performance similar to scallops with low activity lifestyles from temperate latitudes.

Mechanical power output during running accelerations in wild turkeys

We tested the hypothesis that the hindlimb muscles of wild turkeys(Meleagris gallopavo) can produce maximal power during running accelerations. The mechanical power developed during single running steps was calculated from force-plate and high-speed video measurements as turkeys accelerated over a trackway. Steady-speed running steps and accelerations were compared to determine how turkeys alter their running mechanics from a low-power to a high-power gait. During maximal accelerations, turkeys eliminated two features of running mechanics that are characteristic of steady-speed running: (i) they produced purely propulsive horizontal ground reaction forces, with no braking forces, and (ii) they produced purely positive work during stance, with no decrease in the mechanical energy of the body during the step. The braking and propulsive forces ordinarily developed during steady-speed running are important for balance because they align the ground reaction force vector with the center of mass. Increases in acceleration in turkeys correlated with decreases in the angle of limb protraction at toe-down and increases in the angle of limb retraction at toe-off. These kinematic changes allow turkeys to maintain the alignment of the center of mass and ground reaction force vector during accelerations when large propulsive forces result in a forward-directed ground reaction force. During the highest accelerations, turkeys produced exclusively positive mechanical power. The measured power output during acceleration divided by the total hindlimb muscle mass yielded estimates of peak instantaneous power output in excess of 400 W kg-1 hindlimb muscle mass. This value exceeds estimates of peak instantaneous power output of turkey muscle fibers. The mean power developed during the entire stance phase increased from approximately zero during steady-speed runs to more than 150 W kg-1muscle during the highest accelerations. The high power outputs observed during accelerations suggest that elastic energy storage and recovery may redistribute muscle power during acceleration. Elastic mechanisms may expand the functional range of muscle contractile elements in running animals by allowing muscles to vary their mechanical function from force-producing struts during steady-speed running to power-producing motors during acceleration.

A motor and a brake: two leg extensor muscles acting at the same joint manage energy differently in a running insect

The individual muscles of a multiple muscle group at a given joint are often assumed to function synergistically to share the load during locomotion. We examined two leg extensors of a running cockroach to test the hypothesis that leg muscles within an anatomical muscle group necessarily manage (i.e. produce, store, transmit or absorb) energy similarly during running. Using electromyographic and video motion-analysis techniques, we determined that muscles 177c and 179 are both active during the first half of the stance period during muscle shortening. Using the in vivo strain and stimulation patterns determined during running, we measured muscle power output. Although both muscles were stimulated during the first half of shortening, muscle 177c generated mechanical energy (28 W kg–1) like a motor, while muscle 179 absorbed energy (–19 W kg–1) like a brake. Both muscles exhibited nearly identical intrinsic characteristics including similar twitch kinetics and force–velocity relationships. Differences in the extrinsic factors of activation and relative shortening velocity caused the muscles to operate very differently during running. Presumed redundancy in a multiple muscle group may, therefore, represent diversity in muscle function. Discovering how muscles manage energy during behavior requires the measurement of a large number of dynamically interacting variables.

The mechanical power output of the pectoralis muscle of blue-breasted quail (Coturnix chinensis): thein vivolength cycle and its implications for muscle performance

Sonomicrometry and electromyographic (EMG) recordings were made for the pectoralis muscle of blue-breasted quail (Coturnix chinensis) during take-off and horizontal flight. In both modes of flight, the pectoralis strain trajectory was asymmetrical, with 70 % of the total cycle time spent shortening. EMG activity was found to start just before mid-upstroke and continued into the downstroke. The wingbeat frequency was 23 Hz, and the total strain was 23 % of the mean resting length.

Bundles of fibres were dissected from the pectoralis and subjected in vitro to the in vivo length and activity patterns, whilst measuring force. The net power output was only 80 W kg–1 because of a large artefact in the force record during lengthening. For more realistic estimates of the pectoralis power output, we ignored the power absorbed by the muscle bundles during lengthening. The net power output during shortening averaged over the entire cycle was approximately 350 W kg–1, and in several preparations over 400 W kg–1. Sawtooth cycles were also examined for comparison with the simulation cycles, which were identical in all respects apart from the velocity profile. The power output during these cycles was found to be 14 % lower than during the in vivo strain trajectory. This difference was due to a higher velocity of stretch, which resulted in greater activation and higher power output throughout the later part of shortening, and the increase in shortening velocity towards the end of shortening, which facilitated deactivation.

The muscle was found to operate at a mean length shorter than the plateau of the length/force relationship, which resulted in the isometric stress measured at the mean resting length being lower than is typically reported for striated muscle.

The effects of speed on thein vivoactivity and length of a limb muscle during the locomotion of the iguanian lizardDipsosaurus dorsalis

The caudofemoralis muscle is the largest muscle that inserts onto the hindlimb of most ectothermic tetrapods, and previous studies hypothesize that it causes several movements that characterize the locomotion of vertebrates with a sprawling limb posture. Predicting caudofemoralis function is complicated because the muscle spans multiple joints with movements that vary with speed. Furthermore, depending on when any muscle is active relative to its change in length, its function can change from actively generating mechanical work to absorbing externally applied forces. We used synchronized electromyography, sonomicrometry and three-dimensional kinematics to determine in vivo caudofemoralis function in the desert iguana Dipsosaurus dorsalis for a wide range of speeds of locomotion from a walk to nearly maximal sprinting (50–350 cm s–1). Strain of the caudofemoralis increased with increasing tail elevation and long-axis rotation and protraction of the femur. However, knee extension only increased caudofemoralis strain when the femur was protracted. The maximum and minimum length of the caudofemoralis muscle and its average shortening velocity increased from the slowest speed up to the walk–run transition, but changed little with further increases in speed. The times of muscle shortening and lengthening were often not equal at higher locomotor speeds. Some (20–25 ms) activity occurred during lengthening of the caudofemoralis muscle before footfall. However, most caudofemoralis activity was consistent with performing positive mechanical work to flex the knee shortly after foot contact and to retract and rotate the femur throughout the propulsive phase.

Pectoralis muscle performance during ascending and slow level flight in mallards (Anas platyrhynchos)

In vivo measurements of pectoralis muscle length change and force production were obtained using sonomicrometry and delto-pectoral bone strain recordings during ascending and slow level flight in mallards (Anas platyrhynchos). These measurements provide a description of the force/length properties of the pectoralis under dynamic conditions during two discrete flight behaviors and allow an examination of the effects of differences in body size and morphology on pectoralis performance by comparing the results with those of a recent similar study of slow level flight in pigeons (Columbia livia). In the present study, the mallard pectoralis showed a distinct pattern of active lengthening during the upstroke. This probably enhances the rate of force generation and the magnitude of the force generated and, thus, the amount of work and power produced during the downstroke. The power output of the pectoralis averaged 17.0 W kg(−)(1)body mass (131 W kg(−)(1)muscle mass) during slow level flight (3 m s(−)(1)) and 23.3 W kg(−)(1)body mass (174 W kg(−)(1)muscle mass) during ascending flight. This increase in power was achieved principally via an increase in muscle strain (29 % versus 36 %), rather than an increase in peak force (107 N versus 113 N) or cycle frequency (8.4 Hz versus 8.9 Hz). Body-mass-specific power output of mallards during slow level flight (17.0 W kg(−)(1)), measured in terms of pectoralis mechanical power, was similar to that measured recently in pigeons (16.1 W kg(−)(1)). Mallards compensate for their greater body mass and proportionately smaller wing area and pectoralis muscle volume by operating with a high myofibrillar stress to elevate mechanical power output.

The influence of temperature on power production during swimming. II. Mechanics of red muscle fibres in vivo

We found previously that scup (Stenotomus chrysops) reduce neither their stimulation duration nor their tail-beat frequency to compensate for the slow relaxation rates of their muscles at low swimming temperatures. To assess the impact of this ‘lack of compensation’ on power generation during swimming, we drove red muscle bundles under their in vivo conditions and measured the resulting power output. Although these in vivo conditions were near the optimal conditions for much of the muscle at 20 degrees C, they were far from optimal at 10 degrees C. Accordingly, in vivo power output was extremely low at 10 degrees C. Although at 30 cm s(−)(1), muscles from all regions of the fish generated positive work, at 40 and 50 cm s(−)(1), only the POST region (70 % total length) generated positive work, and that level was low. This led to a Q(10) of 4–14 in the POST region (depending on swimming speed), and extremely high or indeterminate Q(10) values (if power at 10 degrees C is zero or negative, Q(10) is indeterminate) for the other regions while swimming at 40 or 50 cm s(−)(1). To assess whether errors in measurement of the in vivo conditions could cause artificially reduced power measurements at 10 degrees C, we drove muscle bundles through a series of conditions in which the stimulation duration was shortened and other parameters were made closer to optimal. This sensitivity analysis revealed that the low power output could not be explained by realistic levels of systematic or random error. By integrating the muscle power output over the fish's mass and comparing it with power requirements for swimming, we conclude that, although the fish could swim at 30 cm s(−)(1) with the red muscle alone, it is very unlikely that it could do so at 40 and 50 cm s(−)(1), thus raising the question of how the fish powers swimming at these speeds. By integrating in vivo pink muscle power output along the length of the fish, we obtained the surprising finding that, at 50 cm s(−)(1), the pink muscle (despite having one-third the mass) contributes six times more power to swimming than does the red muscle. Thus, in scup, pink muscle is crucial for powering swimming at low temperatures. This overall analysis shows that Q(10) values determined in experiments on isolated tissue under arbitrarily selected conditions can be very different from Q(10) values in vivo, and therefore that predicting whole-animal performance from these isolated tissue experiments may lead to qualitatively incorrect conclusions. To make a meaningful assessment of the effects of temperature on muscle and locomotory performance, muscle performance must be studied under the conditions at which the muscle operates in vivo.

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.

Contractile properties of muscles used in sound production and locomotion in two species of gray tree frog

The sound-producing muscles of frogs and toads are interesting because they have been selected to produce high-power outputs at high frequencies. The two North American species of gray tree frog, Hyla chrysoscelis and Hyla versicolor, are a diploid-tetraploid species pair. They are morphologically identical, but differ in the structure of their advertisement calls. H. chrysoscelis produces very loud pulsed calls by contracting its calling muscles at approximately 40 Hz at 20 degrees C, whereas, H. versicolor operates the homologous muscles at approximately 20 Hz at this temperature. This study examined the matching of the intrinsic contractile properties of the calling muscles to their frequency of use. I measured the isotonic and isometric contractile properties of two calling muscles, the laryngeal dilator, which presumably has a role in modulating call structure, and the external oblique, which is one of the muscles that provides the mechanical power for calling. I also examined the properties of the sartorius as a representative locomotor muscle. The calling muscles differ greatly in twitch kinetics between the two species. The calling muscles of H. chrysoscelis reach peak tension in a twitch after approximately 15 ms, compared with 25 ms for the same muscles in H. versicolor. The muscles also differ significantly in isotonic properties in the direction predicted from their calling frequencies. However, the maximum shortening velocities of the calling muscles of H. versicolor are only slightly lower than those of the comparable muscles of H. chrysoscelis. The calling muscles have similar maximum shortening velocities to the sartorius, but have much flatter force-velocity curves, which may be an adaptation to their role in cyclical power output. I conclude that twitch properties have been modified more by selection than have intrinsic shortening velocities. This difference corresponds to the differing roles of shortening velocity and twitch kinetics in determining power output at differing frequencies.

Some advances in integrative muscle physiology

Integrative muscle physiology has evolved from black box correlations to an understanding of how muscular systems are designed at the molecular level. This paper traces some of the obstacles facing integrative muscle physiology and some of the intellectual and technological breakthroughs which led to the field's development. The ability to determine (1) which fiber types are active, (2) over what sarcomere lengths and velocities they shorten during locomotion and (3) their respective force-velocity relationships, enabled us to show that many muscular systems are designed so that muscles operate at optimal myofilament overlap and at optimal V/Vmax (where maximum power is generated). The ability to impose the in vivo length change and stimulation pattern on isolated muscle has further showed that fish muscle has a relatively slow relaxation rate, and thus rather than generating maximum power during swimming, the muscle appears designed to generate power efficiently. By contrast, during the single shot jump, frog muscle remains maximally activated during shortening and generates maximum power. Recently biophysical techniques have shown that relaxation rate can be altered during evolution by changing (1) Ca2+ transient duration; (2) Ca(2+)-troponin kinetics, and (3) crossbridge kinetics. New technologies will soon enable us to better appreciate how different animal designs evolved.

A predicted in vivo muscle force – velocity trajectory

The in vivo stress–strain and stress – strain rate relationships of the adductor muscle in a swimming scallop (Placopecten magellanicus) were predicted on the basis of detailed measured swimming movements and a recently developed dynamic model that integrates all important mechanical elements in the process. The in vivo behaviour of the muscle was found to be quite different than the in vitro properties measured on isolated muscle fibres, which suggests that in general the latter might not be directly used to predict the in vivo mechanical events.

Muscular force in running turkeys: the economy of minimizing work

During running, muscles and tendons must absorb and release mechanical work to maintain the cyclic movements of the body and limbs, while also providing enough force to support the weight of the body. Direct measurements of force and fiber length in the lateral gastrocnemius muscle of running turkeys revealed that the stretch and recoil of tendon and muscle springs supply mechanical work while active muscle fibers produce high forces. During level running, the active muscle shortens little and performs little work but provides the force necessary to support body weight economically. Running economy is improved by muscles that act as active struts rather than working machines.

Jet-propelled swimming in scallops: swimming mechanics and ontogenic scaling

Scallop locomotion was investigated on the basis of an analysis of fluid forces acting on the body and the balance of the forces during swimming. A hydrodynamic model for unsteady jet propulsion was developed in which propulsion performance is characterized by three nondimensional parameters: the storage/discharge volume ratio, reduced clapping frequency, and reduced discharge frequency. Pulsed jet propulsion is designed to achieve high thrust, although not necessarily with low hydrodynamic propulsive efficiency, as was previously widely considered. Swimming in scallops is realized by orientating the body at a certain angle of attack and maintaining a minimum swimming speed to prevent sinking. The working frequency of the locomotor system is determined and adjusted by the swimming strategy (angle of attack, swimming speed, and trajectory angle). For Placopecten magellanicus, the optimum angle of attack is about 6° – 12°, at which swimming requires the lowest energy input (lowest frequency) and hydrodynamic behaviour is ideal (without severe separation and stall). To maintain level swimming, P. magellanicus, during almost all their life, must swim at 5 – 7 body lengths per second if postured at a 6° – 12° angle of attack. The estimated Froude efficiency decreases during growth from about 0.5 to 0.3 for level swimming and from about 0.4 to 0.2 for climbing at an angle of 25°. It is suggested that the heavy body and inferior hydrodynamic characteristics (low aspect ratio and imperfect planform shape) have prevented scallops from becoming good swimmers. These problems are enhanced as the animals grow.

Built for jumping: the design of the frog muscular system

Frogs must generate a high level of mechanical power when they jump. The muscular system of frogs that jump is presumably designed to deliver these high powers. The length changes and activation pattern that muscles undergo during jumping were measured, and isolated muscle bundles were driven through this in vivo pattern. During jumping, muscles generated maximum power. Specifically, the muscle fibers (i) operated at optimal sarcomere lengths, (ii) operated at optimal shortening velocities, and (iii) were maximally activated during power generation. Thus, many different parameters must have evolved in concert to produce a system capable of this explosive jumping movement.

How fish power swimming

It is thought that fish generate the power needed for steady swimming with their anterior musculature, whereas the posterior musculature only transmits forces to the tail and does negative work. Isolated red muscle bundles driven through the length changes and stimulation pattern that muscles normally undergo during steady swimming showed the opposite pattern. Most of the power for swimming came from muscle in the posterior region of the fish, and relatively little came from the anterior musculature. In addition, the contractile properties of the muscle along the length of the fish are significantly adapted to enhance power generation.