Figure 1.

A sequence of simple biped models with different leg architectures. In each biped model, only one leg is shown; the second leg is identical to the one shown. All models have point-mass upper bodies and massless legs. (a) The leg is a telescoping actuator (muscle) with no spring (tendon) in series. (b) The leg has a knee joint, articulated by a muscle without a springy tendon in series. The two segments are of equal length. (c) The leg is a telescoping actuator (muscle) in series with a spring (tendon). (d) The leg has a knee joint, articulated by a muscle in series with a springy tendon. (e–h) Visual abbreviations for each of the four biped models are shown below the corresponding model. These visual abbreviations will be used in other figures to indicate what model those figures refer to.