Stable running with segmented legs

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 spring-mass model to 1.5 m s(-1) for almost straight joint configurations, which is below the preferred transition speed from human walking to running (approximate to 2 m s(-1)). At moderate speeds the tolerated range of landing angle is largely increased (17 degrees at 5 m s(-1)) compared with the linear leg spring model (2 degrees). 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.