The article explores the peculiar allometry of maximum running speed in animals, which increases up to a critical body mass and then decreases, with the fastest animals being of intermediate size. The authors introduce the physiological similarity index (Γ), a dimensionless number that captures the competition between kinetic energy capacity and work capacity in musculoskeletal systems. Γ transitions from a dominance of muscle forces in small animals to a dominance of inertial forces in large animals. The scaling of Γ defines conditions of "dynamic similarity" that enable comparisons and estimates of locomotor performance across different species. The study challenges the traditional use of the Froude number as the primary dynamic similarity index and suggests that the differential growth of muscle and weight forces is less important for most terrestrial animals. The authors also discuss the implications of these findings for comparative analyses of locomotor systems and the evolution of musculoskeletal adaptations.The article explores the peculiar allometry of maximum running speed in animals, which increases up to a critical body mass and then decreases, with the fastest animals being of intermediate size. The authors introduce the physiological similarity index (Γ), a dimensionless number that captures the competition between kinetic energy capacity and work capacity in musculoskeletal systems. Γ transitions from a dominance of muscle forces in small animals to a dominance of inertial forces in large animals. The scaling of Γ defines conditions of "dynamic similarity" that enable comparisons and estimates of locomotor performance across different species. The study challenges the traditional use of the Froude number as the primary dynamic similarity index and suggests that the differential growth of muscle and weight forces is less important for most terrestrial animals. The authors also discuss the implications of these findings for comparative analyses of locomotor systems and the evolution of musculoskeletal adaptations.