See: ‘Balancing on tightropes and slacklines’ Journal of the Royal Society, Interface (2012) 9, 2097–2108. A joint investigation by Dr. Paolo. Paoletti (now at Centre for Engineering Dynamics, School of Engineering, University of Liverpool), along with L. Mahadevan (Lola England de Valpine Professor of Applied Mathematics, Professor of Organismic and Evolutionary Biology, and Professor of Physics, at the Department of Organismic and Evolutionary Biology, and Wyss Institute for Bioinspired Engineering, Harvard University, Cambridge US.) [The professor was co-recipient of the 2007 Ig Nobel physics prize for studying how sheets become wrinkled.]
“Our analysis of the open and closed-loop dynamics shows the existence of an optimal rope sag where balancing requires minimal effort, consistent with qualitative observations and suggestive of strategies for optimizing balancing performance while standing and walking.”
Improbable Clarification : A tightrope is a slackline that’s too tight, and a slackline is a tightrope that’s gone loose.
Also see, related, but bigger : Lateral excitation of bridges by balancing pedestrians. Proc. R. Soc. A 8 April 2009 vol. 465 no. 2104 pp. 1055-1073