Terrestrial Locomotion

Chapter 8 Terrestrial Locomotion CHAPTER 83TERRESTRIAL LOCOMOTION 8.12Introduction In Chapters 5–7, the structure and properties of each of the major tissues of the musculo-skeletal system were examined separately but, in living systems, the skeleton, muscles, tendons and other connective tissues are always elaborately and intimately associated; coordinated, energetically efficient movements cannot be attributed to any single tissue, but to the integration of properties of all the components of the musculo-skeletal system and its control by the nervous system. The matching of the complementary properties of the different tissues so that they work together efficiently is most clearly demonstrated in natural activities such as locomotion.The additional energy for locomotion is normally the single largest increase of metabolic rate above BMR (Section 3.3). Nearly all animals (other than parasites) have to move to obtain food, to escape from predators and often to migrate as well. Therefore, any change in the energy cost of travel makes a large difference to an animal’s total energy budget. Walking and running seem such natural, straightforward activities but the mechanics are in fact very complicated and have only recently been studied in detail. The physical concepts are not easy to grasp and the theory is complicated and very mathematical: this chapter concentrates on the principles involved, referring wherever possible to common experiences or observations that exemplify the phenomena to be explained. We also return to the theme mentioned earlier: the diversity of structure and function within basic uniformity. If the mechanical principles of locomotion are the same, why do different kinds of animals move in different ways and achieve such different feats? Why do some hop while others trot or gallop, or can only walk, and why can some species climb, or jump, or migrate long distances but very few do all these activities? Answering these questions requires reference to some of the concepts developed in Chapter 3 as well as to Chapters 5–7. This chapter is specifically about how vertebrates with legs travel on land, but many of the same principles also apply to flying and swimming. 8.22Locomotion on legs Useful information about the magnitudes and directions of the forces generated during walking can be obtained from the analysis of photographs and records of forces exerted on the ground measured with a force platform. Figure 8.1 (overleaf) summarizes calculations based upon such observations. One use of mechanical energy is obviously for swinging the legs back and forth, but the weight of the body is also raised at each step, as shown in Figure 8.1a, requiring work to be done against gravity, and then allowed to fall back again. − On land, body weight in newtons (N) is body mass (kg) × gravity (9.81m s 2), although of course, in water all organisms weigh much less and many fish are neutrally buoyant (i.e. weightless in water). The vertical movements are much more pronounced at faster gaits, as anyone who has sat on a trotting or galloping horse knows well. 227 S324 Book 3 Size and Action 123 123 Figure 8.13(a) Diagram of the path of the centre of mass (also called the centre of gravity) of a walking man. (b) and (c) Leg movements and forces in human walking (b) and running (c), based upon films and records from a force platform that measured vertical forces. The arrows show the direction of forces in newtons (N) exerted on the ground by the man’s (b) 850N 600N(c) 1080N 1 210N right leg. The forward velocity of the body is also not constant: at phase 1 of the stride shown in Figure 8.1b, the force exerted between the leg and the ground is directed backwards against the direction of travel. Far from pushing the body forward, this leg is acting as a brake. So, to maintain the average forward velocity, the body has to be accelerated again at each stride, as is happening in phase 3 of the stride shown in Figure 8.1b. As well as moving in the vertical plane (Figure 8.1a), the centre of mass also tilts from side to side, as each foot takes its turn to support the body’s weight. A major advance in our understanding of terrestrial locomotion using legs was the recognition (in the 1970s) that the energy involved in all these different movements alternated between kinetic energy (i.e. motion) and gravitational potential energy, as it does in a pendulum. A swinging pendulum is going fastest at the bottom of its arc of swing, i.e. its kinetic energy is greatest when its potential energy is lowest. At the two extremes of its arc, a swinging pendulum moves more and more slowly, i.e. it loses kinetic energy, but it gains potential energy because its centre of mass is higher, i.e. further away from the Earth. The path of the hip in walking (Figure 8.1a) is like an upside down pendulum. This exchange provides about 60–70% of the energy changes that raise and re- accelerate the centre of mass of the body, so active contraction of the muscles need only contribute the remaining 30–40%. Quadrupedal walking also involves discontinuous forward movement and vertical and lateral forces although, in ungulates and other large mammals, the rise and fall of the centre of mass and swaying from side to side are small 228 Chapter 8 Terrestrial Locomotion Figure 8.23The power used by a man while walking, running and cycling on a smooth, level treadmill in still air, based upon measurements of oxygen consumption. compared to those of humans. Children usually find they need to hang on less tightly when sitting on the back of a walking pony (or cow or elephant) than when on the shoulders of a walking parent. The comparison in Figure 8.2 of the mechanics of walking (or running, or riding a horse) with riding a bicycle helps to illustrate the importance of these forces in terrestrial locomotion that depends upon legs. When riding a bicycle along a smooth, level road, there are no vertical movements of the body trunk (i.e. one’s hip is at a constant height above the Earth’s surface), and, if one is pedalling steadily, the velocity of forward movement is nearly constant, i.e. there are no alternating periods of acceleration and braking, and, at least for experienced cyclists, very little wobbling from side to side. You can appreciate the difference by watching the heads of cyclists and runners as they pass a horizontal surface such as window-ledge: the runners’ heads bounce up and down but those of the cyclists glide past like ghosts. The elimination of these sources of energy expenditure is the main reason why, as illustrated by the data in Figure 8.2, it is possible to convey one’s own mass plus that of the bicycle faster and for less energy by cycling than by walking or running. I How do the energetics of cycling change (a) on a bumpy road; (b) when going up hill; (c) in a strong headwind? (a)3Vertical movements of the body (and bike) are re-introduced, because when the tyre hits a bump, part of the forward kinetic energy is converted into upwards movement, or deflects the vehicle sideways (or both). On a very bumpy road, such energy losses may be so high that cyclists find it is easier to dismount and push than to try to ride. (b)3Going up hill means doing work against gravity to lift both one’s own mass and that of the bicycle. Such work is always directly proportional to the mass that is raised (Section 3.3.3). (c)3Energy losses from moving into a wind are proportional to the frontal area exposed to the wind, and to the relative velocity of oneself and the wind. The area of exposure is slightly greater for cyclists than for walkers, and cycling is faster, so the combined velocity of the headwind and cyclist is greater, but the main reason why a headwind is a greater impediment to cycling than to walking is that when cycling in still air, one travels much faster for less effort so the 229 S324 Book 3 Size and Action reduction in forward velocity (or the increased effort required to maintain normal velocity) is a greater proportion of the total energy expended. Only a small fraction of the energy used for running is work against the environment (i.e. wind, friction on the ground). Most of it is used to lift the body and accelerate the limbs. Thus, compared to riding bicycles, locomotion using legs seems to use energy extravagantly. The wheel is among the few basic engineering principles of which there is no equivalent among multicellular animals, possibly because of the problems associated with growth and physiological maintenance of structures that rotate on small axles. However, as just mentioned, wheels are efficient only when operated on smooth ground; on rough ground, wheeled vehicles lose energy by doing work against gravity as legged animals do, and they are useless for jumping and climbing. Legs evolved for walking on uneven terrain and can be instantly redeployed for jumping, climbing and many other uses. Detailed studies of the magnitude and time-course of the forces exerted on the ground during walking and running, combined with measurements of the mechanical and physiological properties of the musculo-skeletal system have revealed several energy-saving mechanisms that make locomotion using legs more efficient. One such mechanism is the principle of the pendulum; another is elastic energy storage (Section 7.2.2), which can also be thought of as an exchange between kinetic and potential energy, but in this case the potential energy is strain energy stored in a stretched tendon, as in a catapult or an elastic band.

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