Fig. 1.

Mechanical work to redirect the center of mass during walking. The body center of mass (COM) moves atop pendulum-like legs, so that motion during single support requires relatively little work. The COM moves in an arc, and its velocity () must be redirected from a downward-and-forward direction to an upward-and-forward direction when transitioning from one stance leg to the next. Each leg produces a ground reaction force (largely directed along the leg) to effect this transition, and consequently performs work on the COM. The addition of an extra load (M_load) increases the total mass (M_total) that must be redirected, with a proportional increase in work. That work is hypothesized to require additional metabolic energy expenditure. M, body mass.