The mechanisms for minimizing energy expenditure in human locomotion

In walking and in running the progression of the body involves at each step changes in kinetic energy, Ek, due to acceleration and deceleration, and changes of potential energy, Ep, due to vertical displacement. The energy costs of walking and running are minimized by two different mechanisms. In walking an alternate exchange of Ek and Ep takes place at each step, so that the muscles have only to restore the small part of the energy that is not recovered. The most economical speed of walking is that at which this recovery is maximal. In addition, at each speed there is an optimal frequency at which the total (external plus internal) mechanical power, and hence the metabolic cost, is minimal. In running the changes of Ek and Ep are almost completely in phase, implying a greater energy dissipation than in walking: part of this energy is stored by stretching the elastic elements of the previously contracted muscles and recovered during the following cycle, increasing the overall efficiency of the progression. The energy cost of walking with a load increases proportionally with the load. However, walking at low speed with a load not exceeding 5-10 per cent of the body weight is not more expensive than unloaded walking. Moreover, it has been observed that African women walking at their optimal speed can carry on their heads loads of up to 20 per cent of their body weight without any extra cost. A possible explanation of this finding could be that a different distribution of the body mass, with a higher position of the centre of gravity of the body, further increases the recovery of energy at each step.