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Fig. 3. Step angle as a function of speed for `walking' ducks [A, determined as
having energy recovery (ER) 0.2] or `running' ducks (B,
ER<0.2), and walking and running humans [C, walking in red
(N=11 subjects), running in blue (N=5 subjects)], with
running at below maximum possible walking speed with grey fills). Ducks are
coloured according to breed, with symbols denoting subject, as for
Fig. 1. Underlying black and
grey lines indicate the compass gait boundary and step frequencies described
for Fig. 2. The vertical grey
broken line denotes  =0.7
(Fr=0.5), usually identified as the walk–run transition speed
for humans. Vertical broken black lines in C mark the range in maximum speeds
predicted if humans walked both with passive swing limbs and while constrained
by compass gait mechanics.
 nat,human indicates the
frequency consistent with a passive, pendular swing leg action for humans.
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), the low-speed
constraint is lost (red area); without any passive potential-kinetic energy
interchange (ER=0) higher step angles and speeds are possible (blue
line) until 