Performance indicators for stability of slackline balancing

In this study we analyze slackline balancing, a task where the subject has to maintain balance on a narrow elastic ribbon that is mounted between two anchor points. We investigate a list of potential performance indicators for stability and their relationship to successful and unsuccessful slackline balancing. We captured the motions of 11 subjects of three different skill levels (beginners, sportive beginners and experts) and recorded a total of 205 standing and 180 walking motions on the slackline. We analyzed all trials fitting subject specific, dynamic rigid body models to the measured kinematic motions. The results show that experts are able to precisely control their angular momentum. They mainly balance in the transverse plane and reduce motion in the sagittal plane and around the vertical axis. We also found their walking style being adapted to these criteria. Further, experts are able to move their contact foot in the transverse plane while keeping their center of mass steady. They adjust their ankle and knee compliance in the stance leg. Sufficient center of mass acceleration control in the vertical direction was found to be a necessary skill to walk on a slackline. Additionally, we found consistent hand coordination patterns in all experts and already in sportive beginners.