Transformation of dynamical fluctuation into coherent energy

Studies of noise-induced motions are showing that coherent energy can be extracted from some kinds of noise in a periodic ratchet. Recently, energetics of Langevin dynamics is formulated by Sekimoto [J.Phys.Soc.Jpn, 66 1234 (1997)], which can be applied to ratchet systems described by Fokker-Planck equation. In this paper, we derive an energetics of ratchet systems that can be applied to dynamical-noise-induced motion in a static potential. Analytical efficiency of the energy transformation is derived for the dynamical noise in an overdumping limit of the system. Comparison between analytical and numerical studies is performed for chaotic noise.Comment: 3 pages, 2 figures; submitted to Phys. Rev. Let

Exact solution of a linear molecular motor model driven by two-step fluctuations and subject to protein friction

We investigate by analytical means the stochastic equations of motion of a linear molecular motor model based on the concept of protein friction. Solving the coupled Langevin equations originally proposed by Mogilner et al. (A. Mogilner et al., Phys. Lett. {\bf 237}, 297 (1998)), and averaging over both the two-step internal conformational fluctuations and the thermal noise, we present explicit, analytical expressions for the average motion and the velocity-force relationship. Our results allow for a direct interpretation of details of this motor model which are not readily accessible from numerical solutions. In particular, we find that the model is able to predict physiologically reasonable values for the load-free motor velocity and the motor mobility.Comment: 12 pages revtex, 6 eps-figure

Energetics of Forced Thermal Ratchet

Molecular motors are known to have the high efficiency of energy transformation in the presence of thermal fluctuation. Motivated by the surprising fact, recent studies of thermal ratchet models are showing how and when work should be extracted from non-equilibrium fluctuations. One of the important finding was brought by Magnasco where he studied the temperature dependence on the fluctuation-induced current in a ratchet (multistable) system and showed that the current can generically be maximized in a finite temperature. The interesting finding has been interpreted that thermal fluctuation is not harmful for the fluctuation-induced work and even facilitates its efficiency. We show, however, this interpretation turns out to be incorrect as soon as we go into the realm of the energetics [Sekimoto,J.Phys.Soc.Jpn.66,1234-1237(1997)]: the efficiency of energy transformation is not maximized at finite temperature, even in the same system that Magnasco considered. The maximum efficiency is realized in the absence of thermal fluctuation. The result presents an open problem whether thermal fluctuation could facilitate the efficiency of energetic transformation from force-fluctuation into work.Comment: 3pages, 4sets of figure

Gait disturbances in football, rugby players and skiers following anterior cruciate ligament reconstruction

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Biomechanics of predator–prey arms race in lion, zebra, cheetah and impala

The fastest and most manoeuvrable terrestrial animals are found in savannah habitats, where predators chase and capture running prey. Hunt outcome and success rate are critical to survival, so both predator and prey should evolve to be faster and/or more manoeuvrable. Here we compare locomotor characteristics in two pursuit predator–prey pairs, lion–zebra and cheetah–impala, in their natural savannah habitat in Botswana. We show that although cheetahs and impalas were universally more athletic than lions and zebras in terms of speed, acceleration and turning, within each predator–prey pair, the predators had 20% higher muscle fibre power than prey, 37% greater acceleration and 72% greater deceleration capacity than their prey. We simulated hunt dynamics with these data and showed that hunts at lower speeds enable prey to use their maximum manoeuvring capacity and favour prey survival, and that the predator needs to be more athletic than its prey to sustain a viable success rate

Remarkable muscles, remarkable locomotion in desert-dwelling wildebeest

Large mammals that live in arid and/or desert environments can cope with seasonal and local variations in rainfall, food and climate1 by moving long distances, often without reliable water or food en route. The capacity of an animal for this long-distance travel is substantially dependent on the rate of energy utilization and thus heat production during locomotion—the cost of transport2,3,4. The terrestrial cost of transport is much higher than for flying (7.5 times) and swimming (20 times)4. Terrestrial migrants are usually large1,2,3 with anatomical specializations for economical locomotion5,6,7,8,9, because the cost of transport reduces with increasing size and limb length5,6,7. Here we used GPS-tracking collars10 with movement and environmental sensors to show that blue wildebeest (Connochaetes taurinus, 220 kg) that live in a hot arid environment in Northern Botswana walked up to 80 km over five days without drinking. They predominantly travelled during the day and locomotion appeared to be unaffected by temperature and humidity, although some behavioural thermoregulation was apparent. We measured power and efficiency of work production (mechanical work and heat production) during cyclic contractions of intact muscle biopsies from the forelimb flexor carpi ulnaris of wildebeest and domestic cows (Bos taurus, 760 kg), a comparable but relatively sedentary ruminant. The energetic costs of isometric contraction (activation and force generation) in wildebeest and cows were similar to published values for smaller mammals. Wildebeest muscle was substantially more efficient (62.6%) than the same muscle from much larger cows (41.8%) and comparable measurements that were obtained from smaller mammals (mouse (34%)11 and rabbit (27%)). We used the direct energetic measurements on intact muscle fibres to model the contribution of high working efficiency of wildebeest muscle to minimizing thermoregulatory challenges during their long migrations under hot arid conditions

Effect of phosphate and temperature on force exerted by white muscle fibres from dogfish.

Effects of Pi (inorganic phosphate) are relevant to the in vivo function of muscle because Pi is one of the products of ATP hydrolysis by actomyosin and by the sarcoplasmic reticulum Ca pump. We have measured the Pi sensitivity of force produced by permeabilized muscle fibres from dogfish (Scyliorhinus canicula) and rabbit. The activation conditions for dogfish fibres were crucial: fibres activated from the relaxed state at 5, 12, and 20°C were sensitive to Pi, whereas fibres activated from rigor at 12°C were insensitive to Pi in the range 5-25 mmol l. Rabbit fibres activated from rigor were sensitive to Pi. Pi sensitivity of force produced by dogfish fibres activated from the relaxed state was greater below normal body temperature (12°C for dogfish) in agreement with what is known for other species. The force-temperature relationship for dogfish fibres (intact and permeabilized fibres activated from relaxed) showed that at 12°C, normal body temperature, the force was near to its maximum value

Inferring cost of transport from whole-body kinematics in three sympatric turtle species with different locomotor habits

Chelonians are mechanically unusual vertebrates as an exoskeleton limits their body wall mobility. They generallymove slowly on land and have aquatic or semi-aquatic lifestyles. Somewhat surprisingly, the limitedexperimental work that has been done suggests that their energetic cost of transport (CoT) are relatively low.This study examines the mechanical evidence for CoT in three turtle species that have differing degrees ofterrestrial activity. Our results show that Apolone travels faster than the other two species, and that Chelydra hashigher levels of yaw. All the species show poor mean levels of energy recovery, and, whilst there is considerablevariation, never show the high levels of energy recovery seen in cursorial quadrupeds. The mean mechanical CoTis 2 to 4 times higher than is generally seen in terrestrial animals. We therefore find no mechanical support for alow CoT in these species. This study illustrates the need for research on a wider range of chelonians to discoverwhether there are indeed general trends in mechanical and metabolic energy costs

Mechanisms of attenuation of pulmonary V'O_{2} slow component in humans after prolonged endurance training

In this study we have examined the effect of prolonged endurance training program on the pulmonary oxygen uptake (V'O2 ) kinetics during heavy-intensity cycling-exercise and its impact on maximal cycling and running performance. Twelve healthy, physically active men (mean\ub1SD: age 22.33\ub11.44 years, V'O2peak 3198\ub1458 mL \ub7 min-1 ) performed an endurance training composed mainly of moderate-intensity cycling, lasting 20 weeks. Training resulted in a decrease (by 3c5%, P = 0.027) in V'O2 during prior low-intensity exercise (20 W) and in shortening of \u3c4 p of the V'O2 on-kinetics (30.1\ub15.9 s vs. 25.4\ub11.5 s, P = 0.007) during subsequent heavy-intensity cycling. This was accompanied by a decrease of the slow component of V'O2 on-kinetics by 49% (P = 0.001) and a decrease in the end-exercise V'O2 by 3c5% (P = 0.005). An increase (P = 0.02) in the vascular endothelial growth factor receptor 2 mRNA level and a tendency (P = 0.06) to higher capillary-to-fiber ratio in the vastus lateralis muscle were found after training (n = 11). No significant effect of training on the V'O2peak was found (P = 0.12). However, the power output reached at the lactate threshold increased by 19% (P = 0.01). The power output obtained at the V'O2peak increased by 14% (P = 0.003) and the time of 1,500-m performance decreased by 5% (P = 0.001). Computer modeling of the skeletal muscle bioenergetic system suggests that the training-induced decrease in the slow component of V'O2 on-kinetics found in the present study is mainly caused by two factors: an intensification of the each-step activation (ESA) of oxidative phosphorylation (OXPHOS) complexes after training and decrease in the "additional" ATP usage rising gradually during heavy-intensity exercise