The fastest land animals are of intermediate size. Cheetah, antelope, greyhounds and racehorses have been measured running much faster than reported for elephants or elephant shrews. Can this be attributed to scaling of physical demands and explicit physiological constraints to supply? Here, we describe the scaling of mechanical work demand each stride, and the mechanical power demand each stance. Unlike muscle stress, strain and strain rate, these mechanical demands cannot be circumvented by changing the muscle gearing with minor adaptations in bone geometry or trivial adjustments to limb posture. Constraints to the capacity of muscle to supply work and power impose fundamental limitations to maximum speed. Given an upper limit to muscle work capacity each contraction, maximum speeds in big animals are constrained by the mechanical work demand each step. With an upper limit to instantaneous muscle power production, maximal speeds in small animals are limited by the high power demands during brief stance periods. The high maximum speed of the cheetah may therefore be attributed as much to its size as to its other anatomical and physiological adaptations.

跑得最快的陆生动物体型中等。据测量，猎豹、羚羊、灵缇犬和赛马的奔跑速度比报告的大象或象鼩快得多。这是否可以归因于身体需求的扩大和供应的明确生理限制？在这里，我们描述了每一步机械功需求的缩放，以及每个姿势的机械功率需求。与肌肉应力、应变和应变率不同，这些机械要求不能通过改变肌肉传动装置、骨骼几何形状的微小调整或对肢体姿势的微不足道的调整来规避。对肌肉提供工作和力量的能力的限制对最大速度施加了根本限制。给定每次收缩的肌肉工作能力上限，大型动物的最大速度受到每一步机械工作需求的限制。由于瞬时肌肉功率产生的上限，小动物的最大速度受到短暂站立期间的高功率需求的限制。因此，猎豹的最高速度可能与其体型以及其他解剖学和生理学适应性一样重要。