The energetic cost of generating muscular force was studied by measuring the energetic cost of carrying loads in rats, dogs, humans, and horses for loads ranging between 7 and 27% of body mass.

Oxygen consumption increased in direct proportion to mass supported by the muscles, i.e.

VOO2,L/VOO2/mL/m = 1.01 ± S.D. ± 0.017, where VOO2,L is the oxygen consumption of the animal running with a load, VOO2 is the oxygen consumption at the same speed without a load, mL, is the mass of the animal plus the load, and m is the mass of the animal.

Stride frequency, average number of feet on the ground over an integral number of strides, the time of contact of each foot relative to the other feet, and the average vertical acceleration during the contact phase were not measurably changed by the loads used in our experiments. From these observations we conclude that the average accelerations of the centre of mass of the animal are not changed by carrying the loads, and that muscular force developed by the animal increases in direct proportion to the load.

It follows that the rate of energy utilization by muscles of an animal as it runs along the ground at any particular speed is nearly directly proportional to the force exerted by its muscles.

The energetic cost of generating force over an interval of time (∫ F dt) increases markedly with running speed.

An important consequence of the direct proportionality between increased oxygen consumption and mass of the load is that small animals expend much more energy to generate a given force at a given speed than large animals.

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