Physiology. Walking made simple.
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Biomechanical Phenomena↗
Biomedical subjects
R McNeill Alexander
Publications and source records attributed to R McNeill Alexander.
Models and the scaling of energy costs for locomotion.
To achieve the required generality, models designed to predict scaling relationships for diverse groups of animals generally need to be simple. An argument based on considerations of dynamic similarity predicts correctly that the mechanical cost of transport for running [power/(body mass x speed)] will be independent of body mass; but measurements of oxygen consumption for running birds and mammals show that the metabolic cost of transport is proportional to (body mass)-0.32. Thus the leg muscles seem to work more efficiently in larger animals. A model that treats birds as fixed wing aircraft predicts that the mechanical power required for flight at the maximum range speed will be proportional to (body mass)1.02, but the metabolic power is found to be proportional to (body mass)0.83; again, larger animals seem to have more efficient muscles. A model that treats hovering hummingbirds and insects as helicopters predicts mechanical power to be approximately proportional to body mass, but measurements of oxygen consumption once again show efficiency increasing with body mass. A model of swimming fish as rigid submarines predicts power to be proportional to (body mass)0.5 x (speed)2.5 or to (body mass)0.6 x (speed)2.8, depending on whether flow in the boundary layer is laminar or turbulent. Unfortunately, this prediction cannot easily be compared with available compilations of metabolic data. The finding that efficiency seems to increase with body mass, at least in running and flight, is discussed in relation to the metabolic energy costs of muscular work and force.
Animals↗
Hitching a lift hydrodynamically--in swimming, flying and cycling.
Swimming animals set the water around them moving, and flying animals generate air movements. Other animals traveling with them can save energy by exploiting these movements of the fluid medium; similarly, a cyclist can save energy by riding close behind another. A new study of dolphin mothers and calves exemplifies the advantages of moving in concert.
Animals↗
Evolution. Enhanced: A rodent as big as a buffalo.
The largest living rodent is the South American capybara, a creature the size of a sheep that unlike smaller rodents stands on relatively straight legs. However, as Alexander explains in his Perspective, a new fossil find in Venezuela (Sánchez-Villagra et al.) reveals that the capybara would be dwarfed by Phoberomys, a giant rodent the size of a buffalo that lived during the Miocene Epoch.
Animals↗
Biomechanics: frogs in and out of phase.
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Animals↗
Tendon elasticity and muscle function.
Vertebrate animals exploit the elastic properties of their tendons in several different ways. Firstly, metabolic energy can be saved in locomotion if tendons stretch and then recoil, storing and returning elastic strain energy, as the animal loses and regains kinetic energy. Leg tendons save energy in this way when birds and mammals run, and an aponeurosis in the back is also important in galloping mammals. Tendons may have similar energy-saving roles in other modes of locomotion, for example in cetacean swimming. Secondly, tendons can recoil elastically much faster than muscles can shorten, enabling animals to jump further than they otherwise could. Thirdly, tendon elasticity affects the control of muscles, enhancing force control at the expense of position control.
Animals↗
Dimensions and moment arms of the hind- and forelimb muscles of common chimpanzees (Pan troglodytes).
This paper supplies quantitative data on the hind- and forelimb musculature of common chimpanzees (Pan troglodytes) and calculates maximum joint moments of force as a contribution to a better understanding of the differences between chimpanzee and human locomotion. We dissected three chimpanzees, and recorded muscle mass, fascicle length, and physiological cross-sectional area (PCSA). We also obtained flexion/extension moment arms of the major muscles about the limb joints. We find that in the hindlimb, chimpanzees possess longer fascicles in most muscles but smaller PCSAs than are predicted for humans of equal body mass, suggesting that the adaptive emphasis in chimpanzees is on joint mobility at the expense of tension production. In common chimpanzee bipedalism, both hips and knees are significantly more flexed than in humans, necessitating muscles capable of exerting larger moments at the joints for the same ground force. However, we find that when subject to the same stresses, chimpanzee hindlimb muscles provide far smaller moments at the joints than humans, particularly the quadriceps and plantar flexors. In contrast, all forelimb muscle masses, fascicle lengths, and PCSAs are smaller in humans than in chimpanzees, reflecting the use of the forelimbs in chimpanzee, but not human, locomotion. When subject to the same stresses, chimpanzee forelimb muscles provide larger moments at the joints than humans, presumably because of the demands on the forelimbs during locomotion. These differences in muscle architecture and function help to explain why chimpanzees are restricted in their ability to walk, and particularly to run bipedally.
Animals↗
Energetics and optimization of human walking and running: the 2000 Raymond Pearl memorial lecture.
Humans seem to adjust their walking and running gaits to minimise the metabolic energy cost of locomotion. The walking speed that we tend to prefer is the one that minimises energy cost per unit distance, though faster speeds might seem preferable when time is valuable. At speeds up to 2 m/s, walking requires less energy than running, and we walk. At higher speeds, running is more economical, and we run. At each speed we use the stride length that minimises energy costs. A computer model that predicts metabolic rates for all conceivable gaits of a simple biped helps to understand these and other features of human gait. The energy cost of walking is increased on uphill slopes and also on soft ground. Consequently, zigzag paths should be preferred to straight ones, up hills of more than a critical gradient. Also, it may be more economical to divert a path around a hill than to travel along a straight line. Simple theories of optimum diversions are presented, both for hilly ground and for ground interrupted by marshy patches, on which costs of walking are increased. Energy costs are also increased by heavy loads, though it seems possible in some circumstances to carry moderate loads without measurable extra cost.
Biomechanical Phenomena↗