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Fig. 6. Stability plot of a spring–mass simulation dependent on angle of
attack (
TD) and spring stiffness
(
). Stable running requires a proper
adjustment of angle of attack to spring stiffness
(Seyfarth et al., 2002). The
black J-shaped area guarantees at least 30 following contacts (end of
simulation) and is referred to as the self-stable area. The circles (first
contact) and squares (second contact) represent the data from the track types
i=1–3 of a typical subject running at 4.8±0.16 m
s–1. Two distinct regions of stiffness and angle of attack
combinations were found. From the first to the second contact both stiffness
and angle of attack decrease in accordance with the results of the simulation.
However, in most cases the experimental results do not fit into the area of
self-stability but, rather, into an area that guarantees at least five
subsequent contacts. Initial parameter of simulation: horizontal component of
the initial velocity 
x,0=4.8 m
s-1, initial apex height y0=1.0 m, body mass
m=80 kg, initial leg length l0=1 m. The grey
scale on the right of the graph indicates how many subsequent steps of stable
running can be made with the chosen combination of angle of attack and spring
stiffness.
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