Perspectives in Physiology

Abstract

he obvious way to make a humanlike robot walk is to provide it with motors to drive every joint, and a computer to control them. The computer tells every joint what its angle should be, at every stage Enhanced online at www.sciencemag.org/cgi/ content/full/308/5718/58 Walking Made Simple of the stride. Many successful robots have been made in this way. The best ones imitate a human walk quite well, but require complex, fast, precise control mechanisms, and use far more energy than a walking human would. In contrast, passive-dynamic walking robots are simple mechanical devices composed of rigid parts connected by joints that are able to walk in a stable fashion down a slope even though they have no motors or controllers. In a recent Science paper (1), Collins et al. describe their design of several new robots inspired by passive-dynamic walkers. These new robots have much simpler control systems than those of powered robots, but walk at least as well as they do, and at lower energy cost. To understand and appreciate these new robots, we need to know something about human walking and its energy cost. The cost of transport for human (or animal) locomotion is defined as (energy cost)/ (body weight × distance traveled). It may seem perverse to use weight rather than mass in this formula, but it makes the cost of transport dimensionless. The energy cost may be defined as the food energy consumed (giving the metabolic cost of transport), or as the mechanical work performed (the mechanical cost of transport). Measurements of oxygen consumption show that for humans walking at the most economical speed (about 1.3 m/s), the metabolic cost of transport is about 0.2. The corresponding mechanical cost of transport is about 0.05 (our muscles work with efficiencies of around 0.25) (2). We have to do work, when we walk, to overcome friction in our joints (3) and to counter air resistance (4), but the work needed for these purposes is far too small to explain the observed costs of transport. In principle, no other work is needed to travel