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Swing-leg retraction: a simple control model for stable running
1 Artificial Intelligence Laboratory, Cambridge, MA 02139, USA
2 Harvard/MIT Division of Health Sciences and Technology, Cambridge, MA 02139,
USA
3 ParaCare Laboratory, Balgrist Hospital, University Zurich, CH-8008 Zurich,
Switzerland
4 Department of Physical Medicine and Rehabilitation, Harvard Medical School,
Spaulding Rehabilitation Hospital, Boston, MA 02114, USA
* Author for correspondence (e-mail: a_seyfarth{at}yahoo.com)
Accepted 22 February 2003
In running, the spring-like axial behavior of stance limbs is a well-known and remarkably general feature. Here we consider how the rotational behavior of limbs affects running stability. It is commonly observed that running animals retract their limbs just prior to ground contact, moving each foot rearward towards the ground. In this study, we employ a conservative spring-mass model to test the effects of swing-leg retraction on running stability. A feed-forward control scheme is applied where the swing-leg is retracted at constant angular velocity throughout the second half of the swing phase. The control scheme allows the spring-mass system to automatically adapt the angle of attack in response to disturbances in forward speed and stance-limb stiffness. Using a return map to investigate system stability, we propose an optimal swing-leg retraction model for the stabilization of flight phase apex height. The results of this study indicate that swing-leg retraction significantly improves the stability of spring-mass running, suggesting that swing-phase limb dynamics may play an important role in the stabilization of running animals.
Key words: biomechanics, legged locomotion, return map, spring-mass model, swing phase
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