Effect of load and speed on the energetic cost of human walking.

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.

The energy cost of walking in children.

Size, morphology and motor skills change dramatically during growth and this probably has an effect on the cost of locomotion. In this study, the effects of age and speed on the energy expended while walking were determined during growth. The rate of oxygen consumption and carbon dioxide production were measured in 3- to 12-year-old children and in adults while standing and walking at different speeds from 0.5 m x s(-1) to near their maximum aerobic walking speed. Standing energy expenditure rate decreases with age from 3.42 +/- 0.48 W x kg(-1) (mean +/- SD, n = 6) in the 3- to 4-year-olds to 1.95 +/- 0.22 W x kg(-1) (n = 6) in young adults. At all ages the gross cost of transport has a minimum which decreases from 5.9 J x kg(-1) x m(-1) in 3- to 4-year-olds to 3.6 J x kg(-1) x m(-1) after 10 years of age. The speed at which this minimum occurs increases from 1.2 m x s(-1) to 1.5 m x s(-1) over the same age range. At low and intermediate walking speeds the net cost of transport is similar in children and adults (about 2 J x kg(-1) x m(-1)). In young children walking at their highest speeds the net cost of transport is 70% (3- to 4-year-olds) to 40% (5- to 6-year-olds) greater than in adults.

Speed, stride frequency and energy cost per stride: how do they change with body size and gait?

In this study we investigate how speed and stride frequency change with body size. We use this information to define 'equivalent speeds' for animals of different size and to explore the factors underlying the six-fold difference in mass-specific energy cost of locomotion between mouse- and horse-sized animals at these speeds. Speeds and stride frequencies within a trot and a gallop were measured on a treadmill in 16 species of wild and domestic quadrupeds, ranging in body size from 30 g mice to 200 kg horses. We found that the minimum, preferred and maximum sustained speeds within a trot and a gallop all change in the same rather dramatic manner with body size, differing by nine-fold between mice and horses (i.e. all three speeds scale with about the 0.2 power of body mass). Although the absolute speeds differ greatly, the maximum sustainable speed was about 2.6-fold greater than the minimum within a trot, and 2.1-fold greater within a gallop. The frequencies used to sustain the equivalent speeds (with the exception of the minimum trotting speed) scale with about the same factor, the -0.15 power of body mass. Combining this speed and frequency data with previously published data on the energetic cost of locomotion, we find that the mass-specific energetic cost of locomotion is almost directly proportional to the stride frequency used to sustain a constant speed at all the equivalent speeds within a trot and a gallop, except for the minimum trotting speed (where it changes by a factor of two over the size range of animals studied). Thus the energy cost per kilogram per stride at five of the six equivalent speeds is about the same for all animals, independent of body size, but increases with speed: 5.0 J kg-1 stride-1 at the preferred trotting speed; 5.3 J kg-1 stride-1 at the trot-gallop transition speed; 7.5 J kg-1 stride-1 at the preferred galloping speed; and 9.4 J kg-1 stride-1 at the maximum sustained galloping speed. The cost of locomotion is determined primarily by the cost of activating muscles and of generating a unit of force for a unit of time. Our data show that both these costs increase directly with the stride frequency used at equivalent speeds by different-sized animals. The increase in cost per stride with muscles (necessitating higher muscle forces for the same ground reaction force) as stride length increases both in the trot and in the gallop.

Energetic cost of carrying loads: have African women discovered an economic way?

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.

Mechanical work in terrestrial locomotion: two basic mechanisms for minimizing energy expenditure.

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.