6. External, Internal and Total Mechanical Work Done During Locomotion

This chapter explains why and how the interaction between moving body and ground in legged terrestrial locomotion necessarily requires mechanical work to be done by muscles: external work to sustain the motion of the center of mass of the body relative to the surrounding and internal work to sustain the motion of the limbs relative to the center of mass. The procedure used to measure external work from the force exerted on the ground, measured by means of a force-platform, is described in detail with the errors it involves. The two basic mechanisms adopted in nature to minimize the metabolic energy expenditure due to external work performance, the pendular mechanism of walking and the bouncing mechanism of running, hopping and trotting, are introduced here and will be fully treated in Chaps. 7 and 8 respectively. The mechanical constraint causing execution of internal work in legged terrestrial locomotion is explained. The division of the total work in external plus internal is shown to be consistent with a physical principle (the Koenig’s theorem). The possibilities of error made in measuring internal work by a cinematographic procedure and total work as the sum of external plus internal work are discussed.