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Two factors that may account for the compliant legs and large stance angles used by running animals. (a,b) A model that includes mechanical work of the legs and hysteresis losses from bouncing ‘viscera’ (a), suggests that compliant legs are favourable for economy. (b) For infinite viscera stiffness (black line), equivalent to a point mass model, mechanical cost of transport (MCoT) increases with stance half-angle (Φ), and impulsive running with an infinite kleg (Φ = 0) is energetically optimal. If the viscera dissipate energy (green lines), however, compliant legs become favourable for economy. (c,d) Compliant legs also provide robust stability in uneven terrain. Normalized maximum drop (NMD) estimates the maximum drop relative to leg length (ΔHmax/Lleg) before the leg misses stance entirely ((i) in c,d). For a fixed running speed, swing period and mean leg retraction velocity (, shown as dimensionless ) compliant legs ((ii) in c,d) have higher NMD than stiff legs ((iii) in c,d). Grey box in (b,d) indicates approximate Φ range used by animals (Farley et al. 1993).