Fig. 2. Modified Hildebrand diagram showing distribution of the 295 gait cycles for which diagonality (D) and duty factor (S_f) were calculated (i.e. in which deviations from symmetry were 10% or less for both forelimbs and hindlimbs). Gait cycles plotted within the blue triangles lack a whole-body aerial phase. In the upper blue triangle (diagonality >50%), animals are using diagonal-sequence ambles. In the lower blue triangle (diagonality <50%), animals are using lateral-sequence ambles. Rectangular bands indicate running trots (in red) (diagonality equal to or near 50%) and paces (in yellow) (diagonality equal to or near 100% or 0%). The diagonal black line represents the equation diagonality=100-forelimb Sf [forelimb duty factor is the relevant duty factor in this case, based on the mathematical model used (Cartmill et al., 2002)]. Those gait cycles plotting directly on that line have maximal duration of bipedal support periods. Primate ambling strides plot above but close to the line, thus eliminating the whole-body aerial phase while preserving near-maximal periods of bipedal support. Note that some of the data represent running trots (mostly for Callithrix jacchus).