The mechanical work done by children during walking at different speeds
The mechanical work done by children during walking at different speeds
The motion analysis system: infrared emitters are delimiting the body segments.
Their movements are recorded by four infrared cameras.The total mechanical work performed during walking is generally evaluated as the sum of the external work (Wext, i.e. the work required to move the center of mass of the body relative to the surroundings) and the internal work (Wint,k, i.e. the work required to move the limbs relative to the center of mass of the body) (Willems et al. , 1995).
In this classical method, the work done by one leg against the other (Wint,dc) is not counted. Indeed, 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 is done simultaneously.
Using force platforms, we studied the effect of speed and age (size) on Wint,dc. In adults and in 3 to 12 year-old children, Wint,dc as a function of speed shows an inverted U shape curve, attaining a maximum value which is independent of size but which occurs at higher speeds in larger subjects. Normalising the speed with the Froude number shows that Wint,dc (J kg-1 m-1) 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, Wint,dc represents more than 40% of Wext (the positive work done to move the centre of mass of the body relative to the surroundings) in both children and adults.
The relative importance of Wint,dc in the total work performed during walking has also been evaluated in children and in adults (see picture). We measured simultaneously Wext, Wint,k and Wint,dc in children and in adults walking at different speeds, and calculate the total mechanical work and the efficiency of positive work production. A paper will be published soon.
More information: Bénédicte SchepensMain publications of the lab on that topic :
SCHEPENS B., BASTIEN G., HEGLUND N.C. & WILLEMS P.A. (2004) Mechanical work and muscular efficiency in walking children. J. Exp. Biol. 207: 587-596BASTIEN G., HEGLUND N.C. & SCHEPENS B. (2003) The double contact phase in walking children J. exp. Biol., 206: 2967-2978
WILLEMS P.A., CAVAGNA , G.A. & HEGLUND N.C. (1995) External, internal and total work in human locomotion. J. Exp. Biol., 198: 379-393
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