Mechanical work and muscular efficiency in walking children.

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 external work, W ext (i.e. the work required to move the centre of mass of the body relative to the surroundings), and the internal work, W int (i.e. the work required to move the limbs relative to the centre of mass of the body, W int,k, and the work done by one leg against the other during the double contact period, W int,dc). Above 0.5 m s(-1), both W ext) and W int,k, normalised to body mass and per unit distance (J kg(-1) m(-1)), are greater in children than in adults; these differences are greater the higher the speed and the younger the subject. Both in children and in adults, the normalised W int,dc shows an inverted U-shape curve as a function of speed, attaining a maximum value independent of age but occurring at higher speeds in older subjects. A higher metabolic energy input (J kg(-1) m(-1)) is also observed in children, although in children younger than 6 years of age, the normalised mechanical work increases relatively less than the normalised energy cost of locomotion. This suggests that young children have a lower efficiency of positive muscular work production than adults during walking. Differences in normalised mechanical work, energy cost and efficiency between children and adults disappear after the age of 10.