The work done during each step to lift and to reaccelerate (in the forward direction) and center of mass has been measured during locomotion in bipeds (rhea and turkey), quadrupeds (dogs, stump-tailed macaques, and ram), and hoppers (kangaroo and springhare). Walking, in all animals (as in man), involves an alternate transfer between gravitational-potential energy and kinetic energy within each stride (as takes place in a pendulum). This transfer is greatest at intermediate walking speeds and can account for up to 70% of the total energy changes taking place within a stride, leaving only 30% to be supplied by muscles. No kinetic-gravitational energy transfer takes place during running, hopping, and trotting, but energy is conserved by another mechanism: an elastic "bounce" of the body. Galloping animals utilize a combination of these two energy-conserving mechanisms. During running, trotting, hopping, and galloping, 1) the power per unit weight required to maintain the forward speed of the center of mass is almost the same in all the species studied; 2) the power per unit weight required to lift the center of mass is almost independent of speed; and 3) the sum of these two powers is almost a linear function of speed.

The stride frequency at which animals of different size change from one gait to another (walk, trot, gallop) changes in a regular manner with body mass. The speed at the transition from trot to gallop can be used as an equivalent speed for comparing animals of different size. This transition point occurs at lower speeds and higher stride frequencies in smaller animals. Plotting stride frequency at the trot-gallop transition point as a function of body mass in logarithmic coordinates yields a straight line.

1. During each step of running, trotting or hopping part of the gravitational and kinetic energy of the body is absorbed and successively restored by the muscles as in an elastic rebound. In this study we analysed the vertical motion of the centre of gravity of the body during this rebound and defined the relationship between the apparent natural frequency of the bouncing system and the step frequency at the different speeds. 2. The step period and the vertical oscillation of the centre of gravity during the step were divided into two parts: a part taking place when the vertical force exerted on the ground is greater than body weight (lower part of the oscillation) and a part taking place when this force is smaller than body weight (upper part of the oscillation). This analysis was made on running humans and birds; trotting dogs, monkeys and rams; and hopping kangaroos and springhares. 3. During trotting and low-speed running the rebound is symmetric, i.e. the duration and the amplitude of the lower part of the vertical oscillation of the centre of gravity are about equal to those of the upper part. In this case, the step frequency equals the frequency of the bouncing system. 4. At high speeds of running and in hopping the rebound is asymmetric, i.e. the duration and the amplitude of the upper part of the oscillation are greater than those of the lower part, and the step frequency is lower than the frequency of the system. 5. The asymmetry is due to a relative increase in the vertical push. At a given speed, the asymmetric bounce requires a greater power to maintain the motion of the centre of gravity of the body, Wext, than the symmetric bounce. A reduction of the push would decrease Wext but the resulting greater step frequency would increase the power required to accelerate the limbs relative to the centre of gravity, Wint. It is concluded that the asymmetric rebound is adopted in order to minimize the total power, Wext + Wint.

It is well established that the energy cost per unit distance traveled is minimal at an intermediate walking speed in humans, defining an energetically optimal walking speed. However, little is known about the optimal walking speed while carrying a load. In this work, we studied the effect of speed and load on the energy expenditure of walking. The O(2) consumption and CO(2) production were measured in ten subjects while standing or walking at different speeds from 0.5 to 1.7 m s(-1) with loads from 0 to 75% of their body mass (M(b)). The loads were carried in typical trekker's backpacks with hip support. Our results show that the mass-specific gross metabolic power increases curvilinearly with speed and is directly proportional to the load at any speed. For all loading conditions, the gross metabolic energy cost (J kg(-1) m(-1)) presents a U-shaped curve with a minimum at around 1.3 m s(-1). At that optimal speed, a load up to 1/4 M(b) seems appropriate for long-distance walks. In addition, the optimal speed for net cost minimization is around 1.06 m s(-1) and is independent of load.

When travelling in East Africa one is often surprised at the prodigious loads carried by the women of the area. It is not uncommon to see women of the Luo tribe carrying loads equivalent to 70% of their body mass balanced on the top of their heads (Fig. 1). Women of the Kikuyu tribe carry equally large loads supported by a strap across their foreheads; this frequently results in a permanently grooved skull. Recent experiments on running horses, humans, dogs and rats showed that the energy expended in carrying a load increased in direct proportion to the weight of the load for each animal at each speed, that is, carrying a load equal to 20% of body weight increased the rate of energy consumption by 20% (ref. 1). The purpose of the present study was to determine whether these African women use specialized mechanisms for carrying very large loads cheaply. We found that both the Luo and Kikuyu women could carry loads of up to 20% of their body weight without increasing their rate of energy consumption. For heavier loads there was a proportional increase in energy consumption, that is, a 30% load increased energy consumption by 10%, a 40% load by 20% and so on. We suggest that some element of training and/or anatomical change since childhood may allow these women to carry heavy loads economically.

Human feet and toes provide a mechanism for changing the gear ratio of the ankle extensor muscles during a running step. A variable gear ratio could enhance muscle performance during constant-speed running by applying a more effective prestretch during landing, while maintaining the muscles near the high-efficiency or high-power portion of the force-velocity curve during takeoff. Furthermore, during acceleration, variable gearing may allow muscle contractile properties to remain optimized despite rapid changes in running speed. Forceplate and kinematic analyses of running steps show low gear ratios at touchdown that increase throughout the contact phase.

This review addresses a simple question: How do muscles use the energy they consume during terrestrial locomotion? Using a comparative approach, it was found that the mass-specific rate of metabolic energy consumption changes by more than ten-fold with body size, while the mass-specific rate at which the muscles performed mechanical work did not change at all. It was also found that the rate of metabolic energy consumption increased linearly with speed, while the rate at which muscles performed mechanical work increased curvilinearly with speed (oc V1.53). We conclude from these observations that the rate at which animals consume metabolic energy during terrestrial locomotion is not determined by the rate at which their muscles perform mechanical work. Instead, the metabolic cost of generating muscular force over time (integral of F dt) appears to determine the metabolic cost of terrestrial locomotion. The cost of generating force increases with increasing speed and decreases with increasing body size in exactly the same manner as cost of locomotion. It is suggested that the metabolic cost of generating muscular force may be determined by the intrinsic velocity of shortening (i.e proportional to rates at which the cross-bridges between actin and myosin cycle) of the muscle motor units that are active during locomotion. Faster motor units are used both as animals increase speed and in equivalent muscles of smaller animals moving at the same speed. This suggestion is testable and future studies should determine whether or not it explains the higher costs of generating muscular force with increasing speed and decreasing body size.

Children consume more energy per unit body mass to walk at a given speed than do adults (DeJaeger et al., 2001). The difference in the net mass-specific metabolic energy cost per unit distance (i.e. the cost of transport, the energy required to operate the locomotory machinery) between adults and children is greater the higher the speed and the younger the subject. For example, at a speed of 1·m·s -1 , a 3-4-year-old has a net oxygen consumption 33% greater than adults. This difference disappears by the age of 11-12·years.In order to take into account the difference in size between children and adults, the speed of progression can be normalised using the dimensionless Froude number, V -f 2 /(gl), where V -f is mean walking speed, g is acceleration of gravity and l is leg length (Alexander, 1989). In this case, the difference in the cost of transport between children and adults for the most part disappears. This indicates that, after the age of 3-4·years, the difference in the cost of transport may be explained mostly on the basis of body size (DeJaeger et al., 2001).As previously observed in running (Schepens et al., 2001), body size can also affect the positive muscle-tendon work (W tot) performed during walking. Wtot naturally falls into two categories: the external work (Wext), which is the work necessary to sustain the displacement of the centre of mass of the body (COM) relative to the surroundings, and the internal work (W int), which is the work that does not directly lead to a displacement of the COM. Only some of Wint can be measured: (1) the internal work done to accelerate the body segments relative to the COM (Wint,k) and (2) the internal work done during the double contact phase of walking by the back leg, which generates energy that will be absorbed by the front leg (Wint,dc). On the contrary, the internal mechanical work done for stretching the series elastic components of the muscles during isometric contractions, to overcome antagonistic cocontractions, to overcome viscosity and friction cannot be directly measured (although this unmeasured internal work will affect the efficiency of positive work production; Willems et al., 1995).Walking is characterised by a pendulum-like exchange between the kinetic and potential energy of the COM. In children, the 'optimal speed' at which these pendulum-like transfers are maximal increases progressively with age from 0.8·m·s -1 in 2-year-olds up to 1.4·m·s -1 in 12-year-olds and adults (Cavagna et al., 1983). At all ages, the optimal speed is close to the speed at which the mass-specific work to move the COM a given distance, Wext, is at a minimum. Above the optimal speed, the energy transfers decrease. This decrease is greater the younger the subject. The decreased transfers result in a greater power required to move the COM: at 1.25·m·s -1 , The effect of age and body size on the total mechanical work done during walking is studied in children of 3-12·years of age and in adults. The total mechanical work per stride (W tot ) is measured as the sum of the ex...