ENERGETICS OF BIPEDAL RUNNING II - LIMB DESIGN AND RUNNING MECHANICS

Compared with quadrupeds, bipedal runners of the same weight have long er legs, take longer steps and can presumably use slower, more economi cal muscle fibers. One might predict that bipedal running is less expe nsive, but it is not. We hypothesized that bipeds recruit a larger vol ume of muscle to support their weight, eliminating the potential econo my of longer legs and slower steps. To test our hypothesis, we calcula ted the relative volume of muscle needed to support body weight over a stride in small dogs (Canis familiaris) and wild turkeys (Meleagris g allopavo) of the same weight. First, we confirmed that turkeys and dog s use approximately the same amount of energy to run at the same speed , and found that turkeys take 1.8-fold longer steps. Higher muscle for ces and/or longer muscle fibers would require a greater volume of acti ve muscle, since muscle volume is proportional to the product of force and fascicle length. We measured both mean fascicle Length and mean m echanical advantage for limb extensor muscles. Turkeys generated appro ximately the same total muscle force to support their weight during ru nning and used muscle fascicles that are on average 2.1 times as long as in dogs, thus requiring a 2.5-fold greater active muscle volume, Th e greater volume appears to offset the economy of slower rates of forc e generation, supporting our hypothesis and providing a simple explana tion for why it costs the same to run on two and four legs.