| Spring-like leg behavior is found in both humans and animals when
running. In a spring-mass model, running proves to be self-stable in
terms of external perturbations or variations in leg properties (for
example, landing angle). However, biological limbs are not made of
springs, rather, they consist of segments where spring-like behavior
can be localized at the joint level. Here, we use a two-segment leg
model to investigate the effects of leg compliance originating from the
joint level on running stability. Owing to leg geometry a non-linear relationship
between leg force and leg compression is found. In contrast
to the linear leg spring, the segmented leg is capable of reducing the
minimum speed for self-stable running from 3.5 m s-1 in the springmass
model to 1.5 m s-1 for almost straight joint configurations,
which is below the preferred transition speed from human walking to
running (≈2 m s-1). At moderate speeds the tolerated range of landing
angle is largely increased (17° at 5 m s-1) compared with the
linear leg spring model (2°). However, for fast running an increase in
joint stiffness is required to compensate for the mechanical disadvantage
of larger leg compression. This could be achieved through the
use of non-linear springs to enhance joint stiffness in fast running. |
