In Medieval Europe, soldiers wore steel plate armour for protection during warfare. Armour design reflected a trade-off between protection and mobility it offered the wearer. By the fifteenth century, a typical suit of field armour weighed between 30 and 50 kg and was distributed over the entire body. How much wearing armour affected Medieval soldiers' locomotor energetics and biomechanics is unknown. We investigated the mechanics and the energetic cost of locomotion in armour, and determined the effects on physical performance. We found that the net cost of locomotion (C(met)) during armoured walking and running is much more energetically expensive than unloaded locomotion. C(met) for locomotion in armour was 2.1-2.3 times higher for walking, and 1.9 times higher for running when compared with C(met) for unloaded locomotion at the same speed. An important component of the increased energy use results from the extra force that must be generated to support the additional mass. However, the energetic cost of locomotion in armour was also much higher than equivalent trunk loading. This additional cost is mostly explained by the increased energy required to swing the limbs and impaired breathing. Our findings can predict age-associated decline in Medieval soldiers' physical performance, and have potential implications in understanding the outcomes of past European military battles.
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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 (E(i)). 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. E(i) was very similar for Saw50% and Saw75% cycles at all frequencies (approximately 0.27 at 5 Hz). Saw25% cycles had E(i) 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.
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The metabolic cost of the negotiation of obstacles, and the influence that this has on route selection, are important determinants of an animal's locomotor behaviour. We determined the gross metabolic cost of locomotion on slopes of different gradients, ranging from -90 to +90 deg, in leaf-cutter ants (Acromyrmex octospinosus) in a closed-circuit respirometry system. Ants were able to select their preferred speed for each gradient. The grossmetabolic energy expenditure per unit distance travelled on the slope (C(path)) was calculated from the rate of CO(2) production and the speed of locomotion. These data were used to predict the optimal slopes for minimising the vertical cost of locomotion and vertical journey time. The gross rate of CO(2) production was approximately constant (1.7 ml g(-1) h(-1)) and was not significantly affected by slope. Ants moderated their speed with slope (P<0.05), travelling the fastest during level locomotion (2.0±0.1 cm s(-1), N=20) and increasingly slowly with increased gradient (both on an incline and a decline). C(path) varied significantly with slope, being lowest during level locomotion (646.0±51.2 J kg(-1) m(-1)) and increasing with increasing gradient. These results suggest that ants adapt their locomotor behaviour to keep metabolic rate constant despite changing mechanical demands. It is predicted that when undertaking a journey involving vertical displacement that ants will select routes with a gradient of between 51 and 57 deg during ascent and with a gradient of between -45 and -51 deg during descent, in order to minimise both vertical journey time and vertical cost of locomotion.
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Take-off in birds at high speeds and steep angles of elevation requires a high burst power output. The mean power output of the pectoralis muscle of blue-breasted quail (Coturnix chinensis) during take-off is approximately 400 W kg(-1) muscle, as determined using two independent methods. This burst power output is much higher than has been measured in any other cyclically contracting muscle. The power output of muscle is determined by the interactions between the physiological properties of the muscle, the stimulation regime imposed by the central nervous system and the details of the strain cycle, which are determined by the reciprocal interaction between the muscle properties and the environmental load. The physiological adaptations that enable a high power output to be achieved are those that allow the muscle to develop high stresses whilst shortening rapidly. These characteristics include a high myofibrillar density, rapid twitch contraction kinetics and a high maximum intrinsic velocity of shortening. In addition, several features of the strain cycle increase the power output of the quail pectoralis muscle. First, the muscle operates at a mean length shorter than the plateau of the length/force relationship. Second, the muscle length trajectory is asymmetrical, with 70 % of the cycle spent shortening. The asymmetrical cycle is expected to increase the power output substantially. Third, subtle deviations in the velocity profile improve power output compared with a simple asymmetrical cycle with constant lengthening and shortening rates. The high burst power outputs found in the flight muscles of quail and similar birds are limited to very brief efforts before fatigue occurs. This strong but short flight performance is well-suited to the rapid-response anti-predation strategy of these birds that involves a short flight coupled with a subsequent sustained escape by running. These considerations serve as a reminder that the maximum power-producing capacities of muscles need to be considered in the context of the in vivo situation within which the muscles operate.
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Blue-breasted quail (Coturnix chinensis) were filmed during take-off flights. By tracking the position of the centre of mass of the bird in three dimensions, we were able to calculate the power required to increase the potential and kinetic energy. In addition, high-speed video recordings of the position of the wings over the course of the wing stroke, and morphological measurements, allowed us to calculate the aerodynamic and inertial power requirements. The total power output required from the pectoralis muscle was, on average, 390 W kg(-1), which was similar to the highest measurements made on bundles of muscle fibres in vitro (433 W kg(-1)), although for one individual a power output of 530 W kg(-1) was calculated. The majority of the power was required to increase the potential energy of the body. The power output of these muscles is the highest yet found for any muscle in repetitive contractions. We also calculated the power requirements during take-off flights in four other species in the family Phasianidae. Power output was found to be independent of body mass in this family. However, the precise scaling of burst power output within this group must await a better assessment of whether similar levels of performance were measured across the group. We extended our analysis to one species of hawk, several species of hummingbird and two species of bee. Remarkably, we concluded that, over a broad range of body size (0.0002-5 kg) and contractile frequency (5-186 Hz), the myofibrillar power output of flight muscles during short maximal bursts is very high (360-460 W kg(-1)) and shows very little scaling with body mass. The approximate constancy of power output means that the work output varies inversely with wingbeat frequency and reaches values of approximately 30-60 J kg(-1) in the largest species.
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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.
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http://www.sebiology.org/meetings/Abstract_Archive/abstracts/main-8435.pdf
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