The mechanics of walking in children.

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Abstract
The work done at each step, during level walking at a constant average speed, to lift the center of mass of the body, to accelerate it forward, and to increase the sum of both gravitational potential and kinetic energies, was measured at various speeds in children 2-12 yr of age, with the same technique used previously for adults (Cavagna, 1975; Cavagna, Thys and Zamboni, 1976). The pendulum-like transfer between potential and kinetic energies (Cavagna et al. 1976) reaches a maximum at the speed at which the weight-specific work to move the center of mass a given distance is at a minimum (optimal speed). This speed is about 2.8 km/h at 2 yr of age and increases progressively with age up to 5 km/h at 12 yr of age and in adults. The speed freely chosen during steady walking at the different ages is similar to this optimal speed. At the optimal speed, the time of single contact (time of swing) is in good agreement with that predicted, for the same stature, by a ballistic walking model assuming a minimum of muscular work (Mochon and McMahon, 1980). Above the optimal speed, the recovery of mechanical energy through the potential-kinetic energy transfer decreases. This decrease is greater, the younger the subject. A reduction of this recovery implies a greater amount of work to be supplied by muscles: at 4.5 km/h the weight-specific muscular power necessary to move the center of mass is 2.3 times greater in a 2-yr-old child than in an adult.

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