Conserved mammalian muscle mechanics during eccentric contractions.

Skeletal muscle has a broad range of biomechanical functions, including power generation and energy absorption. These roles are underpinned by the force-velocity relationship, which comprises two distinct components: a concentric and an eccentric force-velocity relationship. The concentric component has been extensively studied across a wide range of muscles with different muscle properties. However, to date, little progress has been made in accurately characterising the eccentric force-velocity relationship in mammalian muscle with varying muscle properties. Consequently, mathematical models of this muscle behaviour are based on a poorly understood phenomenon. Here, we present a comprehensive assessment of the concentric force-velocity and eccentric force-velocity relationships of four mammalian muscles (soleus, extensor digitorum longus, diaphragm and digastric) with varying biomechanical functions, spanning three orders of magnitude in body mass (mouse, rat and rabbits). The force-velocity relationship was characterised using a hyperbolic-linear equation for the concentric component a hyperbolic equation for the eccentric component, at the same time as measuring the rate of force development in the two phases of force development in relation to eccentric lengthening velocity. We demonstrate that, despite differences in the curvature and plateau height of the eccentric force-velocity relationship, the rates of relative force development were consistent for the two phases of the force-time response during isovelocity lengthening ramps, in relation to lengthening velocity, in the four muscles studied. Our data support the hypothesis that this relationship depends on cross-bridge and titin activation. Hill-type musculoskeletal models of the eccentric force-velocity relationship for mammalian muscles should incorporate this biphasic force response. KEY POINTS: The capacity of skeletal muscle to generate mechanical work and absorb energy is underpinned by the force-velocity relationship. Despite identification of the lengthening (eccentric) force-velocity relationship over 80 years ago, no comprehensive study has been undertaken to characterise this relationship in skeletal muscle. We show that the biphasic force response seen during active muscle lengthening is conserved over three orders of magnitude of mammalian skeletal muscle mass. Using mice with a small deletion in titin, we show that part of this biphasic force profile in response to muscle lengthening is reliant on normal titin activation. The rate of force development during muscle stretch may be a more reliable way to describe the forces experienced during eccentric muscle contractions compared to the traditional hyperbolic curve fitting, and functions as a novel predictor of force-velocity characteristics that may be used to better inform hill-type musculoskeletal models and assess pathophysiological remodelling.

Differential expression of genes in the RhoA/ROCK pathway in the hippocampus and cortex following intermittent hypoxia and high-intensity interval training.

Structural neuroplasticity such as neurite extension and dendritic spine dynamics is enhanced by brain-derived neurotrophic factor (BDNF) and impaired by types of inhibitory molecules that induce growth cone collapse and actin depolymerization, for example, myelin-associated inhibitors, chondroitin sulfate proteoglycans, and negative guidance molecules. These inhibitory molecules can activate RhoA/rho-associated coiled-coil containing protein kinase (ROCK) signaling (known to restrict structural plasticity). Intermittent hypoxia (IH) and high-intensity interval training (HIIT) are known to upregulate BDNF that is associated with improvements in learning and memory and greater functional recovery following neural insults. We investigated whether the RhoA/ROCK signaling pathway is also modulated by IH and HIIT in the hippocampus, cortex, and lumbar spinal cord of male Wistar rats. The gene expression of 25 RhoA/ROCK signaling pathway components was determined following IH, HIIT, or IH combined with HIIT (30 min/day, 5 days/wk, 6 wk). IH included 10 3-min bouts that alternated between hypoxia (15% O2) and normoxia. HIIT included 10 3-min bouts alternating between treadmill speeds of 50 cm·s-1 and 15 cm·s-1. In the hippocampus, IH and HIIT significantly downregulated Acan and NgR2 mRNA that are involved in the inhibition of neuroplasticity. However, IH and IH + HIIT significantly upregulated Lingo-1 and NgR3 in the cortex. This is the first time IH and HIIT have been linked to the modulation of plasticity-inhibiting pathways. These results provide a fundamental step toward elucidating the interplay between the neurotrophic and inhibitory mechanisms involved in experience-driven neural plasticity that will aid in optimizing physiological interventions for the treatment of cognitive decline or neurorehabilitation.NEW & NOTEWORTHY Intermittent hypoxia (IH) and high-intensity interval training (HIIT) enhance neuroplasticity and upregulate neurotrophic factors in the central nervous system (CNS). We provide evidence that IH and IH + HIIT also have the capacity to regulate genes involved in the RhoA/ROCK signaling pathway that is known to restrict structural plasticity in the CNS. This provides a new mechanistic insight into how these interventions may enhance hippocampal-related plasticity and facilitate learning, memory, and neuroregeneration.

The contractile efficiency of the mantle muscle of European common cuttlefish (Sepia officinalis) during cyclical contractions.

Escape jet propulsion swimming in cuttlefish (Sepia officinalis) is powered by the circular muscles surrounding the mantle cavity. This mode of locomotion is energetically costly compared to undulatory swimmers. The energetic cost of swimming is determined by the mechanical power requirements and the efficiency with which chemical energy is transferred into useful mechanical work. One step in this energy transduction process is the transfer of energy from ATP hydrolysis into mechanical work by the muscles. Here, we determined the efficiency of this step, termed the contractile efficiency. Muscle preparations from the circular muscles of the mantle cavity were subjected to sinusoidal length changes at different cycle frequencies, and stimulated with a phase and duration that maximised initial net work. Changes in ATP, arginine phosphate and octopine content between control and exercised muscles were determined and used to calculate the energy released from ATP hydrolysis (Emet). The maximum contractile efficiency (the ratio of net work to Emet) was 0.37, occurring at the same cycle frequency at which mechanical power was maximal and that was used during jet propulsion swimming, suggesting that cuttlefish muscle is adapted to generate muscular power efficiently. The overall efficiency of cuttlefish jet propulsion swimming was estimated to be 0.17, which is broadly comparable to that measured during animal flight and human-powered pedalled locomotion, indicating the high energetic costs of jet propulsion swimming are not due to inefficient locomotion per se, instead, they result from the relatively high mechanical power requirements.

The importance of muscle activation on the interpretations of muscle mechanical performance.

The work loop technique was developed to assess muscle performance during cyclical length changes with phasic activation, simulating the in vivo conditions of many muscles, particularly during locomotion. To estimate muscle function in vivo, the standard approach involves subjecting a muscle to length trajectories and activation timings derived from in vivo measurements, whilst simultaneously measuring force. However, the stimulation paradigm typically used, supramaximal, "square-wave" stimulation, does not accurately reflect the graded intensity of activation observed in vivo. While the importance of the timing and duration of stimulation within the cycle on estimates of muscle performance has long been established, the importance of graded muscle activation has not been investigated. In this study we investigated how the activation pattern affects muscle performance by comparing square-wave, supramaximal activation with a graded in vivo activation pattern. First, we use in vivo electromyography derived activation patterns and fibre strains from the rabbit digastric muscle during mastication and replayed them in situ. Second, we used Hill-type musculoskeletal model derived activation patterns and fibre strains in a trotting mouse, replayed ex vivo in the soleus (SOL) and extensor digitorum longus (EDL) muscles. In the rabbit digastric muscle square-wave activation led to an eight-fold higher estimate of net power, compared to the in vivo graded activation pattern. Similarly, in the mouse SOL and EDL, supramaximal, square-wave activation resulted in significantly greater positive and negative muscle work. These findings highlight that realistic interpretations of in vivo muscle function rely upon more accurate representations of muscle activation intensity.

The functional role of the rabbit digastric muscle during mastication.

Muscle spindle abundance is highly variable in vertebrates, but the functional determinants of this variation are unclear. Recent work has shown that human leg muscles with the lowest abundance of muscle spindles primarily function to lengthen and absorb energy, while muscles with a greater spindle abundance perform active-stretch-shorten cycles with no net work, suggesting that muscle spindle abundance may be underpinned by muscle function. Compared with other mammalian muscles, the digastric muscle contains the lowest abundance of muscle spindles and, therefore, might be expected to generate substantial negative work. However, it is widely hypothesised that as a jaw-opener (anatomically) the digastric muscle would primarily function to depress the jaw, and consequently do positive work. Through a combination of X-ray reconstruction of moving morphology (XROMM), electromyography and fluoromicrometry, we characterised the 3D kinematics of the jaw and digastric muscle during feeding in rabbits. Subsequently, the work loop technique was used to simulate in vivo muscle behaviour in situ, enabling muscle force to be quantified in relation to muscle strain and hence determine the muscle's function during mastication. When functioning on either the working or balancing side, the digastric muscle generates a large amount of positive work during jaw opening, and a large amount of negative work during jaw closing, on average producing a relatively small amount of net negative work. Our data therefore further support the hypothesis that muscle spindle abundance is linked to muscle function; specifically, muscles that absorb a relatively large amount of negative work have a low spindle abundance.

Adaptations for extremely high muscular power output: why do muscles that operate at intermediate cycle frequencies generate the highest powers?

The pectoralis muscles of the blue-breasted quail Coturnix chinensis generate the highest power output over a contraction cycle measured to date, approximately 400 W kg- 1. The power generated during a cyclical contraction is the product of work and cycle frequency (or standard operating frequency), suggesting that high powers should be favoured by operating at high cycle frequencies. Yet the quail muscles operate at an intermediate cycle frequency (23 Hz), which is much lower than the highest frequency skeletal muscles are capable of operating (~ 200 Hz in vertebrates). To understand this apparent anomaly, in this paper I consider the adaptations that favour high mechanical power as well as the trade-offs that occur between force and muscle operating frequency that limit power. It will be shown that adaptations that favour rapid cyclical contractions compromise force generation; consequently, maximum power increases with cycle frequency to approximately 15-25 Hz, but decreases at higher cycle frequencies. At high cycle frequencies, muscle stress is reduced by a decrease in the crossbridge duty cycle and an increase in the proportion of the muscle occupied by non-contractile elements such as sarcoplasmic reticulum and mitochondria. Muscles adapted to generate high powers, such as the pectoralis muscle of blue-breasted quail, exhibit: (i) intermediate contraction kinetics; (ii) a high relative myofibrillar volume; and (iii) a high maximum shortening velocity and a relatively flat force-velocity relationship. They are also characterised by (iv) operating at an intermediate cycle frequency; (v) utilisation of asymmetrical length trajectories, with a high proportion of the cycle spent shortening; and, finally, (vi) relatively large muscles. In part, the high power output of the blue-breasted quail pectoralis muscle can be attributed to its body size and the intermediate wing beat frequency required to generate aerodynamic force to support body mass, but in addition specialisations in the contractile and morphological properties of the muscle favour the generation of high stress at high strain rates.

The hydrodynamics of jet propulsion swimming in hatchling and juvenile European common cuttlefish, Sepia officinalis.

Cuttlefish swim using jet propulsion, taking a small volume of fluid into the mantle cavity before it is expelled through the siphon to generate thrust. Jet propulsion swimming has been shown to be more metabolically expensive than undulatory swimming, which has been suggested to be due to the lower efficiency of jet propulsion. The whole-cycle propulsive efficiency of cephalopod molluscs ranges from 38 to 76%, indicating that in some instances jet propulsion can be relatively efficient. Here, we determined the hydrodynamics of hatchling and juvenile cuttlefish during jet propulsion swimming to understand the characteristics of their jets, and whether their whole-cycle propulsive efficiency changes during development. Cuttlefish were found to utilise two jet types: isolated jet vortices (termed jet mode I) and elongated jets (leading edge vortex ring followed by a trailing jet; termed jet mode II). The use of these jet modes differed between the age classes, with newly hatched animals nearly exclusively utilising mode I jets, while juveniles showed no strong preferences. Whole-cycle propulsive efficiency was found to be high, ranging from 72 to 80%, and did not differ between age classes. During development, Strouhal number decreased as Reynolds number increased, which is consistent with animals adjusting their jetting behaviour in order to maximise whole-cycle propulsive efficiency and locomotor performance. Although jet propulsion swimming can have a relatively high energetic cost, in cuttlefish and nautilus, both neutrally buoyant species, the whole-cycle propulsive efficiency is actually relatively high.

Heterogeneity in form and function of the rat extensor digitorum longus motor unit.

The motor unit comprises a variable number of muscle fibres that connect through myelinated nerve fibres to a motoneuron (MN), the central drivers of activity. At the simplest level of organisation there exist phenotypically distinct MNs that activate corresponding muscle fibre types, but within an individual motor pool there typically exists a mixed population of fast and slow firing MNs, innervating groups of Type II and Type I fibres, respectively. Characterising the heterogeneity across multiple levels of motor unit organisation is critical to understanding changes that occur in response to physiological and pathological perturbations. Through a comprehensive assessment of muscle histology and ex vivo function, mathematical modelling and neuronal tracing, we demonstrate regional heterogeneities at the level of the MN, muscle fibre type composition and oxygen delivery kinetics of the rat extensor digitorum longus (EDL) muscle. Specifically, the EDL contains two phenotypically distinct regions: a relatively oxidative medial and a more glycolytic lateral compartment. Smaller muscle fibres in the medial compartment, in combination with a greater local capillary density, preserve tissue O2 partial pressure (PO2 ) during modelled activity. Conversely, capillary supply to the lateral compartment is calculated to be insufficient to defend active muscle PO2 but is likely optimised to facilitate metabolite removal. Simulation of in vivo muscle length change and phasic activation suggest that both compartments are able to generate similar net power. However, retrograde tracing demonstrates (counter to previous observations) that a negative relationship between soma size and C-bouton density exists. Finally, we confirm a lack of specificity of SK3 expression to slow MNs. Together, these data provide a reference for heterogeneities across the rat EDL motor unit and re-emphasise the importance of sampling technique.

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.

Oxygen transport kinetics underpin rapid and robust diaphragm recovery following chronic spinal cord injury.

KEY POINTS: Spinal treatment can restore diaphragm function in all animals 1 month following C2 hemisection induced paralysis. Greater recovery occurs the longer after injury the treatment is applied. Through advanced assessment of muscle mechanics, innovative histology and oxygen tension modelling, we have comprehensively characterized in vivo diaphragm function and phenotype. Muscle work loops reveal a significant deficit in diaphragm functional properties following chronic injury and paralysis, which are normalized following restored muscle activity caused by plasticity-induced spinal reconnection. Injury causes global and local alterations in diaphragm muscle vascular supply, limiting oxygen diffusion and disturbing function. Restoration of muscle activity reverses these alterations, restoring oxygen supply to the tissue and enabling recovery of muscle functional properties. There remain metabolic deficits following restoration of diaphragm activity, probably explaining only partial functional recovery. We hypothesize that these deficits need to be resolved to restore complete respiratory motor function. ABSTRACT: Months after spinal cord injury (SCI), respiratory deficits remain the primary cause of morbidity and mortality for patients. It is possible to induce partial respiratory motor functional recovery in chronic SCI following 2 weeks of spinal neuroplasticity. However, the peripheral mechanisms underpinning this recovery are largely unknown, limiting development of new clinical treatments with potential for complete functional restoration. Utilizing a rat hemisection model, diaphragm function and paralysis was assessed and recovered at chronic time points following trauma through chondroitinase ABC induced neuroplasticity. We simulated the diaphragm's in vivo cyclical length change and activity patterns using the work loop technique at the same time as assessing global and local measures of the muscles histology to quantify changes in muscle phenotype, microvascular composition, and oxidative capacity following injury and recovery. These data were fed into a physiologically informed model of tissue oxygen transport. We demonstrate that hemidiaphragm paralysis causes muscle fibre hypertrophy, maintaining global oxygen supply, although it alters isolated muscle kinetics, limiting respiratory function. Treatment induced recovery of respiratory activity normalized these effects, increasing oxygen supply, restoring optimal diaphragm functional properties. However, metabolic demands of the diaphragm were significantly reduced following both injury and recovery, potentially limiting restoration of normal muscle performance. The mechanism of rapid respiratory muscle recovery following spinal trauma occurs through oxygen transport, metabolic demand and functional dynamics of striated muscle. Overall, these data support a systems-wide approach to the treatment of SCI, and identify new targets to mediate complete respiratory recovery.

Abnormal skeletal muscle blood flow, contractile mechanics and fibre morphology in a rat model of obese-HFpEF.

KEY POINTS: Heart failure is characterised by limb and respiratory muscle impairments that limit functional capacity and quality of life. However, compared with heart failure with reduced ejection fraction (HFrEF), skeletal muscle alterations induced by heart failure with preserved ejection fraction (HFpEF) remain poorly explored. Here we report that obese-HFpEF induces multiple skeletal muscle alterations in the rat hindlimb, including impaired muscle mechanics related to shortening velocity, fibre atrophy, capillary loss, and an impaired blood flow response to contractions that implies a perfusive oxygen delivery limitation. We also demonstrate that obese-HFpEF is characterised by diaphragmatic alterations similar to those caused by denervation - atrophy in Type IIb/IIx (fast/glycolytic) fibres and hypertrophy in Type I (slow/oxidative) fibres. These findings extend current knowledge in HFpEF skeletal muscle physiology, potentially underlying exercise intolerance, which may facilitate future therapeutic approaches. ABSTRACT: Peripheral skeletal muscle and vascular alterations induced by heart failure with preserved ejection fraction (HFpEF) remain poorly identified, with limited therapeutic targets. This study used a cardiometabolic obese-HFpEF rat model to comprehensively phenotype skeletal muscle mechanics, blood flow, microvasculature and fibre atrophy. Lean (n = 8) and obese-HFpEF (n = 8) ZSF1 rats were compared. Skeletal muscles (soleus and diaphragm) were assessed for in vitro contractility (isometric and isotonic properties) alongside indices of fibre-type cross-sectional area, myosin isoform, and capillarity, and estimated muscle PO2 . In situ extensor digitorum longus (EDL) contractility and femoral blood flow were assessed. HFpEF soleus demonstrated lower absolute maximal force by 22%, fibre atrophy by 24%, a fibre-type shift from I to IIa, and a 17% lower capillary-to-fibre ratio despite increased capillary density (all P < 0.05) with preserved muscle PO2 (P = 0.115) and isometric specific force (P > 0.05). Soleus isotonic properties (shortening velocity and power) were impaired by up to 17 and 22%, respectively (P < 0.05), while the magnitude of the exercise hyperaemia was attenuated by 73% (P = 0.012) in line with higher muscle fatigue by 26% (P = 0.079). Diaphragm alterations (P < 0.05) included Type IIx fibre atrophy despite Type I/IIa fibre hypertrophy, with increased indices of capillarity alongside preserved contractile properties during isometric, isotonic, and cyclical contractions. In conclusion, obese-HFpEF rats demonstrated blunted skeletal muscle blood flow during contractions in parallel to microvascular structural remodelling, fibre atrophy, and isotonic contractile dysfunction in the locomotor muscles. In contrast, diaphragm phenotype remained well preserved. This study identifies numerous muscle-specific impairments that could exacerbate exercise intolerance in obese-HFpEF.

Energy cost of ambulation in trans-tibial amputees using a dynamic-response foot with hydraulic versus rigid 'ankle': insights from body centre of mass dynamics.

BACKGROUND: Previous research has shown that use of a dynamic-response prosthetic foot (DRF) that incorporates a small passive hydraulic ankle device (hyA-F), provides certain biomechanical benefits over using a DRF that has no ankle mechanism (rigA-F). This study investigated whether use of a hyA-F in unilateral trans-tibial amputees (UTA) additionally provides metabolic energy expenditure savings and increases the symmetry in walking kinematics, compared to rigA-F. METHODS: Nine active UTA completed treadmill walking trials at zero gradient (at 0.8, 1.0, 1.2, 1.4, and 1.6 of customary walking speed) and for customary walking speed only, at two angles of decline (5° and 10°). The metabolic cost of locomotion was determined using respirometry. To gain insights into the source of any metabolic savings, 3D motion capture was used to determine segment kinematics, allowing body centre of mass dynamics (BCoM), differences in inter-limb symmetry and potential for energy recovery through pendulum-like motion to be quantified for each foot type. RESULTS: During both level and decline walking, use of a hyA-F compared to rigA-F significantly reduced the total mechanical work and increased the interchange between the mechanical energies of the BCoM (recovery index), leading to a significant reduction in the metabolic energy cost of locomotion, and hence an associated increase in locomotor efficiency (p < 0.001). It also increased inter-limb symmetry (medio-lateral and progression axes, particularly when walking on a 10° decline), highlighting the improvements in gait were related to a lessening of the kinematic compensations evident when using the rigA-F. CONCLUSIONS: Findings suggest that use of a DRF that incorporates a small passive hydraulic ankle device will deliver improvements in metabolic energy expenditure and kinematics and thus should provide clinically meaningful benefits to UTAs' everyday locomotion, particularly for those who are able to walk at a range of speeds and over different terrains.

Regional variation in the mechanical properties and fibre-type composition of the rat extensor digitorum longus muscle.

NEW FINDINGS: What is the central question of this study? Mammalian muscle is typically heterogeneous in fibre-type distribution, with distinct regional variation in composition. The effects this might have on mechanical performance are largely unknown. What is the main finding and its importance? Contractile properties vary regionally within a heterogeneous muscle. The mixed extensor digitorum longus muscle has phenotypically distinct compartments that differ in their isometric twitch kinetics, the optimal cycle frequency for maximal power generation and fatigue resistance. The mechanisms underpinning the decline in performance during fatigue differ between compartments. Regional variation in mechanical performance suggests that regions of the extensor digitorum longus muscle might be differentially recruited during locomotion, depending upon functional demand. Fibre-type composition is heterogeneous, and distribution varies spatially in many muscles, indicating that there might be regional variation in recruitment and mechanical output. The rat extensor digitorum longus muscle is composed of predominantly fast-twitch fibres and exhibits a gradient in phenotype, resulting in oxidative medial (areal composition 24.3% type I/IIa) and glycolytic lateral (92.4% type IIx/IIb) compartments. Here, we investigated the variation in mechanical performance between the medial and lateral compartments during isometric, isotonic and cyclical contractions. Isometric tetanic stress and force-velocity relationships were similar in both compartments, but isometric twitch kinetics were slower in the medial compared with the lateral compartment. The medial compartment also had a lower optimal cycle frequency for maximal net power generation (11 versus 15 Hz; P < 0.05) attributable to slower isometric kinetics, resulting in a lower level of activation and reduced net work generation at higher cycle frequencies, compared with the lateral compartment. The more oxidative, medial compartment had higher fatigue resistance, maintaining net power 26% longer than the lateral compartment. The predominant mechanisms underpinning the decrease in net power varied between the compartments, resulting from an increase in the work to extend the muscle and from a reduction in work during shortening in the medial and lateral compartments, respectively. Regional variation in mechanical performance and resistance to fatigue within a mixed muscle suggests that a differential recruitment pattern is likely during locomotion, with the medial compartment being used during slow-speed locomotion and the lateral compartment during burst activities.

Jet-paddling jellies: swimming performance in the Rhizostomeae jellyfish Catostylus mosaicus.

Jellyfish are a successful and diverse class of animals that swim via jet propulsion, with swimming performance and propulsive efficiency being related to the animal's feeding ecology and body morphology. The Rhizostomeae jellyfish lack tentacles but possess four oral lobes and eight trailing arms at the centre of their bell, giving them a body morphology quite unlike that of other free-swimming medusae. The implications of this body morphology on the mechanisms by which thrust is produced are unknown. Here, we determined the wake structure and propulsive efficiency in the blue-blubber jellyfish Catostylus mosaicus (order: Rhizostomeae). The animal is propelled during both bell contraction and bell relaxation by different thrust-generating mechanisms. During bell contraction, a jet of fluid is expelled from the subumbrellar cavity, which results from the interaction between the counter-rotating stopping (from the preceding contraction cycle) and starting vortices, creating a vortex superstructure and propulsion. This species is also able to utilise passive energy recapture, which increases the animal's swimming velocity towards the end of the bell expansion phase when the bell diameter is constant. The thrust produced during this phase is the result of the flexible bell margin manoeuvring the stopping vortex into the subumbrellar cavity during bell relaxation, enhancing its circulation, and creating a region of high pressure on the inner surface of the bell and, consequently, thrust. These mechanisms of thrust generation result in C. mosaicus having a relatively high propulsive efficiency compared with other swimmers, indicating that economical locomotion could be a contributing factor in the ecological success of these medusan swimmers.

Intra-specific variation in wing morphology and its impact on take-off performance in blue tits (Cyanistes caeruleus) during escape flights.

Diurnal and seasonal increases in body mass and seasonal reductions in wing area may compromise a bird's ability to escape, as less of the power available from the flight muscles can be used to accelerate and elevate the animal's centre of mass. Here, we investigated the effects of intra-specific variation in wing morphology on escape take-off performance in blue tits (Cyanistes caeruleus). Flights were recorded using synchronised high-speed video cameras and take-off performance was quantified as the sum of the rates of change of the kinetic and potential energies of the centre of mass. Individuals with a lower wing loading, WL (WL=body weight/wing area) had higher escape take-off performance, consistent with the increase in lift production expected from relatively larger wings. Unexpectedly, it was found that the total power available from the flight muscles (estimated using an aerodynamic analysis) was inversely related to WL. This could simply be because birds with a higher WL have relatively smaller flight muscles. Alternatively or additionally, variation in the aerodynamic load on the wing resulting from differences in wing morphology will affect the mechanical performance of the flight muscles via effects on the muscle's length trajectory. Consistent with this hypothesis is the observation that wing beat frequency and relative downstroke duration increase with decreasing WL; both are factors that are expected to increase muscle power output. Understanding how wing morphology influences take-off performance gives insight into the potential risks associated with feather loss and seasonal and diurnal fluctuations in body mass.

The elaborate plumage in peacocks is not such a drag.

One of the classic examples of an exaggerated sexually selected trait is the elaborate plumage that forms the train in male peafowl Pavo cristatus (peacock). Such ornaments are thought to reduce locomotor performance as a result of their weight and aerodynamic drag, but this cost is unknown. Here, the effect that the train has on take-off flight in peacocks was quantified as the sum of the rates of change of the potential and kinetic energies of the body (P(CoM)) in birds with trains and following the train's removal. There was no significant difference between P(CoM) in birds with and without a train. The train incurs drag during take-off; however, while this produces a twofold increase in parasite drag, parasite power only accounts for 0.1% of the total aerodynamic power. The train represented 6.9% of body weight and is expected to increase induced power. The absence of a detectable effect on take-off performance does not necessarily mean that there is no cost associated with possessing such ornate plumage; rather, it suggests that given the variation in take-off performance per se, the magnitude of any effect of the train has little meaningful functional relevance.

The energetic benefits of tendon springs in running: is the reduction of muscle work important?

The distal muscle-tendon units of cursorial species are commonly composed of short muscle fibres and long, compliant tendons. It is assumed that the ability of these tendons to store and return mechanical energy over the course of a stride, thus avoiding the cyclic absorption and regeneration of mechanical energy by active muscle, offers some metabolic energy savings during running. However, this assumption has not been tested directly. We used muscle ergometry and myothermic measurements to determine the cost of force production in muscles acting isometrically, as they could if mechanical energy was stored and returned by tendon, and undergoing active stretch-shorten cycles, as they would if mechanical energy was absorbed and regenerated by muscle. We found no detectable difference in the cost of force production in isometric cycles compared with stretch-shorten cycles. This result suggests that replacing muscle stretch-shorten work with tendon elastic energy storage and recovery does not reduce the cost of force production. This calls into question the assumption that reduction of muscle work drove the evolution of long distal tendons. We propose that the energetic benefits of tendons are derived primarily from their effect on muscle and limb architecture rather than their ability to reduce the cyclic work of muscle.

Flying fast improves aerodynamic economy of heavier birds.

A paradox of avian long-distance migrations is that birds must greatly increase their body mass prior to departure, yet this is presumed to substantially increase their energy cost of flight. However, here we show that when homing pigeons flying in a flock are loaded with ventrally located weight, both their heart rate and estimated energy expenditure rise by a remarkably small amount. The net effect is that costs per unit time increase only slightly and per unit mass they decrease. We suggest that this is because these homing flights are relatively fast, and consequently flight costs associated with increases in body parasite drag dominate over those of weight support, leading to an improvement in mass-specific flight economy. We propose that the relatively small absolute aerodynamic penalty for carrying enlarged fuel stores and flight muscles during fast flight has helped to select for the evolution of long-distance migration.

The impacts of muscle-specific force-velocity properties on predictions of mouse muscle function during locomotion.

Introduction: The accuracy of musculoskeletal models and simulations as methods for predicting muscle functional outputs is always improving. However, even the most complex models contain various assumptions and simplifications in how muscle force generation is simulated. One common example is the application of a generalised ("generic") force-velocity relationship, derived from a limited data set to each muscle within a model, uniformly across all muscles irrespective of whether those muscles have "fast" or "slow" contractile properties. Methods: Using a previously built and validated musculoskeletal model and simulation of trotting in the mouse hindlimb, this work examines the predicted functional impact of applying muscle-specific force-velocity properties to typically fast (extensor digitorum longus; EDL) and slow-contracting (soleus; SOL) muscles. Results: Using "real" data led to EDL producing more positive work and acting significantly more spring-like, and soleus producing more negative work and acting more brake-like in function compared to muscles modelled using "generic" force-velocity data. Extrapolating these force-velocity properties to other muscles considered "fast" or "slow" also substantially impacted their predicted function. Importantly, this also further impacted EDL and SOL function beyond that seen when changing only their properties alone, to a point where they show an improved match to ex vivo experimental data. Discussion: These data suggest that further improvements to how musculoskeletal models and simulations predict muscle function should include the use of different values defining their force-velocity relationship depending on their fibre-type composition.

Regional variation of the cortical and trabecular bone material properties in the rabbit skull.

The material properties of some bones are known to vary with anatomical location, orientation and position within the bone (e.g., cortical and trabecular bone). Details of the heterogeneity and anisotropy of bone is an important consideration for biomechanical studies that apply techniques such as finite element analysis, as the outcomes will be influenced by the choice of material properties used. Datasets detailing the regional variation of material properties in the bones of the skull are sparse, leaving many finite element analyses of skulls no choice but to employ homogeneous, isotropic material properties, often using data from a different species to the one under investigation. Due to the growing significance of investigating the cranial biomechanics of the rabbit in basic science and clinical research, this study used nanoindentation to measure the elastic modulus of cortical and trabecular bone throughout the skull. The elastic moduli of cortical bone measured in the mediolateral and ventrodorsal direction were found to decrease posteriorly through the skull, while it was evenly distributed when measured in the anteroposterior direction. Furthermore, statistical tests showed that the variation of elastic moduli between separate regions (anterior, middle and posterior) of the skull were significantly different in cortical bone, but was not in trabecular bone. Elastic moduli measured in different orthotropic planes were also significantly different, with the moduli measured in the mediolateral direction consistently lower than that measured in either the anteroposterior or ventrodorsal direction. These findings demonstrate the significance of regional and directional variation in cortical bone elastic modulus, and therefore material properties in finite element models of the skull, particularly those of the rabbit, should consider the heterogeneous and orthotropic properties of skull bone when possible.