The double contact phase in walking children.

During walking, when both feet are on the ground (the double contact phase), the legs push against each other, and both positive and negative work are done simultaneously. The work done by one leg on the other (W(int,dc)) is not counted in the classic measurements of the positive muscular work done during walking. Using force platforms, we studied the effect of speed and age (size) on W(int,dc). In adults and in 3-12-year-old children, W(int,dc) (J kg(-1) m(-1)) as a function of speed shows an inverted U-shaped curve, attaining a maximum value that is independent of size but that occurs at higher speeds in larger subjects. Normalising the speed with the Froude number shows that W(int,dc) is maximal at about 0.3 in both children and adults. Differences due to size disappear for the most part when normalised with the Froude number, indicating that these speed-dependent changes are primarily a result of body size changes. At its maximum, W(int,dc) represents more than 40% of W(ext) (the positive work done to move the centre of mass of the body relative to the surroundings) in both children and adults.