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Biomedical subjects

R M Alexander

Publications and source records attributed to R M Alexander.

At least 19 recordsLinked to original sources
PubMed1995

Leg design and jumping technique for humans, other vertebrates and insects.

Humans, bushbabies, frogs, locusts, fleas and other animals jump by rapidly extending a pair of legs. Mathematical models are used to investigate the effect muscle properties, leg design and jumping technique have on jump height. Jump height increases with increased isometric force exerted by leg muscles, their maximum shortening speeds and their series compliances. When ground forces are small multiples of body mass (as for humans), countermovement and catapult jumps are about equally high, and both are much better than squat jumps. Vertebrates have not evolved catapult mechanisms and use countermovement jumps instead. When ground forces are large multiples of body mass, catapult jumps (as used by locusts and fleas) are much higher than the other styles of jump could be. Increasing leg mass reduces jump height, but the proximal-to-distal distribution of leg mass has only a minor effect. Longer legs make higher jumps possible and additional leg segments, such as the elongated tarsi of bushbabies and frogs, increase jump height even if overall leg length remains unchanged. The effects of muscle moment arms that change as the leg extends, and of legs designed to work over different ranges of joint angle, are investigated.

Animals↗
PubMed1992

A model of bipedal locomotion on compliant legs.

Simple mathematical models capable of walking or running are used to compare the merits of bipedal gaits. Stride length, duty factor (the fraction of the stride, for which the foot is on the ground) and the pattern of force on the ground are varied, and the optimum gait is deemed to be the one that minimizes the positive work that the muscles must perform, per unit distance travelled. Even the simplest model, whose legs have neither mass nor elastic compliance, predicts the changes of duty factor and force pattern that people make as they increase their speed of walking. It predicts a sudden change to running at a critical speed, but this is much faster than the speed at which people make the change. When elastic compliance is incorporated in the model, unnaturally fast walking becomes uncompetitive. However, a slow run with very brief foot contact becomes the optimum gait at low speeds, at which people would walk, unless severe energy dissipation occurs in the compliance. A model whose legs have mass as well as elastic compliance predicts well the relationship between speed and stride length in human walking.

Adult↗
PubMed1991

Optimization of gut structure and diet for higher vertebrate herbivores.

A generalized herbivore gut is modelled as (i) a well-stirred anterior chamber in which microbial fermentation occurs; (ii) a tubular reactor in which digestion but no fermentation occurs; and (iii) a posterior fermentation chamber. The rate at which the herbivore gains metabolizable energy is calculated for diets that can be eaten at different rates and contain different energy densities of easily digested cell contents, and of cell wall materials that can be fermented but not digested. The optimum gut structure for each diet is determined. Chewing probably speeds digestion and fermentation but reduces eating time. Optimal chewing times are determined for particular diets and guts. Herbivores often have a choice between poorer food that can be eaten fast and richer food that can only be eaten more slowly. Energy costs may be incurred in travelling between patches of the richer food. Optimal diet choices are predicted for herbivores with particular gut structures.

Animals↗
PubMed1991

Optimum timing of muscle activation for simple models of throwing.

In diverse throwing activities, muscles contract in sequence, starting with those furthest from the hand. This paper uses simple mathematical models, each with just two muscles, to investigate the consequences of this sequential contraction. One model was suggested by shot putting, another by underarm throwing and the third by overarm throwing, but all are much simpler than real human movements. In each case there is an optimum delay between activation of the more proximal muscle and of the more distal one, that maximizes the speed at which the missile leaves the hand. If the delay is shorter than optimal, the throw is completed sooner and less time is available for contraction of the proximal muscle: it may shorten faster, exerting less torque, or through less than its full range of movement, and so do less work. If it is longer than optimal, less time is available for contraction of the distal muscle, which therefore does less work. The optimal delay is in some cases longer than would maximize total work because the delay influences the proportion of the work that appears as kinetic energy of the missile.

Arm↗
PubMed1991

Energy-saving mechanisms in walking and running.

Energy can be saved in terrestrial locomotion in many different ways. The maximum shortening speeds (Vmax) of the muscles can be adjusted to their optimum values for the tasks required of them. The moments exerted by the muscles at different joints can be adjusted to keep the ground force in line with the leg so that muscles do not work against each other. The joints of the legs can be kept as straight as possible, minimizing muscle forces and work requirements. Walking gaits should be selected at low Froude numbers (a dimensionless speed parameter) and running gaits at high Froude numbers. Tendon and other springs can be used to store elastic strain energy and to return it by elastic recoil. This paper aims to show how these energy-saving mechanisms work and to what extent mammals exploit them. Arguments based on our rather limited knowledge of the relationship between the mechanical performance of muscle and its metabolic energy consumption are used throughout. They suggest that muscles that are optimally adapted for their tasks in running should do positive work with constant efficiency.

Animals↗
PubMed1991

Elastic mechanisms in primate locomotion.

Tendons that stretch elastically and recoil, as the forces on them rise and fall, can save energy in running by enabling the animal to make do with shorter or slower muscle fascicles, that can generate force more economically. Non-human primates have rather long fascicles and thick tendons in their distal leg muscles and so seem poorly adapted to save energy in this way. Additional savings are made possible by the elastic compliance of ligaments in the foot. Though tendon and ligament compliance tend to save energy, the compliance of branches tends to increase the energy cost of arboreal locomotion.

Animals↗
PubMed1991

Ratios of cross-sectional areas of muscles and their tendons in a healthy human forearm.

The muscles and tendons in the forearm and hand of a young man, amputated after an accident, have been weighed and measured. The physiological cross-sectional areas of those muscles that had long tendons were 35 +/- 9 (mean and standard deviation) times the cross-sectional areas of the tendons. The mean is very close to the optimum calculated from the theory of Ker, Alexander & Bennett (1988). It implies that the tendons experience stresses of about 11 MPa and strains of about 1.3%, when the muscles exert their maximum isometric forces. Very much larger forces would be needed to break the tendons.

Adult↗
PubMed1990

Optimum take-off techniques for high and long jumps.

High jumpers run at moderate speeds and set down the foot, from which they take off, well in front of the body. Long jumpers run up much faster and place the foot less far forward, with the leg at a steeper angle. A simple model, which takes into account the mechanical properties of muscle, predicts optimum take-off techniques that agree well with those used by athletes. These predictions are remarkably insensitive to the numerical values assigned to the physiological parameters.

Biomechanical Phenomena↗
PubMed1990

The influence of temperature on muscle velocity and sustained performance in swimming carp.

The aim of this study was to evaluate how fish locomote at different muscle temperatures. Sarcomere length excursion and muscle shortening velocity, V, were determined from high-speed motion pictures of carp, Cyprinus carpio (11-14 cm), swimming steadily at various sustained speeds at 10, 15 and 20 degrees C. In the middle and posterior regions of the carp, sarcomeres of the lateral red muscle underwent cyclical excursions of 0.31 microns, centered around the resting length of 2.06 microns (i.e. from 1.91 to 2.22 microns). The amplitudes of the sarcomere length excursions were essentially independent of swimming speed and temperature. As tail-beat frequency increased linearly with swimming speed regardless of temperature, the sarcomeres underwent the same length changes in a shorter time. Thus, V increased in a linear and temperature-independent manner with swimming speed. Neither temperature nor swimming speed had an influence on tail-beat amplitude or tail height. Our findings indicate that muscle fibres are used only over a narrow, temperature-independent range of V/Vmax (0.17-0.36) where power and efficiency are maximal. Carp start to recruit their white muscles at swimming speeds where the red muscle V/Vmax becomes too high (and thus power output declines). When the V/Vmax of the active muscle falls too low during steady swimming, carp switch to 'burst-and-coast' swimming, apparently to keep V/Vmax high. Because Vmax (maximum velocity of shortening) of carp red muscle has a Q10 of 1.63, the transition speeds between swimming styles are lower at lower temperatures. Thus, carp recruit their white anaerobic muscle at a lower swimming speed at lower temperatures (verified by electromyography), resulting in a lower maximum sustainable swimming speed. The present findings also indicate that, to generate the same total force and power to swim at a given speed, carp at 10 degrees C must recruit about 50% greater fibre cross-sectional area than they do at 20 degrees C.

Animals↗
PubMed1989

Foot strike and the properties of the human heel pad.

Many force-plate records of human locomotion show an impulse (the foot strike) shortly after ground contact. The authors' hypothesis is that this results from the rapid deceleration of a mass (the 'effective foot') under forces which compress the heel pad. The quantitative implications are investigated through an illustrative calculation. The observations used are (a) the peak force reached in foot strike (b) the vertical velocity of the foot immediately before ground contact and (c) the properties of the heel pad in compression. Data for (a) and (b) are available in the literature; measurements for (c) are presented here. The deductions are: (a) the time taken to reach peak force is about 5.4 ms, which agrees with published measurements; (b) the mass of the effective foot is about 3.6 kg. The effective foot thus includes a substantial portion of the leg: this seems reasonable. The models used for the calculations clarify the relationship between the foot strike and the shock wave, which it generates.

Biomechanical Phenomena↗
PubMed1988

Why animals have different muscle fibre types.

Animals have different muscle fibre types: slow fibres with a low maximum velocity of shortening (Vmax) and fast fibres with a high Vmax. An advantage conferred by the use of different fibre types during locomotion has been proposed solely on the basis of their in vitro properties. Isolated muscle experiments show that force generation, mechanical power production and efficiency are all functions of V/Vmax, where V is the velocity of muscle shortening. But it is not known whether animals actually use the different fibres at shortening velocities that are optimal for mechanical power production and efficiency. Here we compare the V of muscle fibres during locomotion with their Vmax. This comparison shows that during slow locomotion, the slow fibres shorten at a velocity that gives peak mechanical power and efficiency and the fast fibres shorten at their optimal velocity when powering maximal movements. Our results also show that maximal movements are impossible without fast fibres because the slow ones cannot shorten rapidly enough.

Animals↗