During walking, the centre of mass of the body moves like that of a 'square wheel': with each step cycle, some of its kinetic energy, E(k), is converted into gravitational potential energy, E(p), and then back into kinetic energy. To move the centre of mass, the locomotory muscles must supply only the power required to overcome the losses occurring during this energy transduction. African women carry loads of up to 20% of their body weight on the head without increasing their energy expenditure. This occurs as a result of an unexplained, more effective energy transduction between E(k) and E(p) than that of Europeans. In this study we measured the value of the E(k) to E(p) transduction at each instant in time during the step in African women and European subjects during level walking at 3.5-5.5 km h(-1), both unloaded and carrying loads spanning 20-30% of their body weight. A simulation of the changes in E(k) and E(p) during the step by sinusoidal curves was used for comparison. It was found that loading improves the transduction of E(p) to E(k) during the descent of the centre of mass. The improvement is not significant in European subjects, whereas it is highly significant in African women.