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Stein Mombaur 2019

Performance indicators for stability of slackline balancing

Tags: #slackline #dynamic_balance

Related: [[Learning to balance on a slackline - Mildren Et Al 2018]] [[Improved postural control after slackline training is accompanied by reduced H-reflexes - Keller Et. Al. 2011]] [[Balancing on Slacklines Modeling and Empirical Evaluation - Vallery Neumann 2013]] [[A Case Study on Balance Recovery in Slacklining - Huber Kleindl 2010]]

Key Definitions

  1. Virtual Pivot Point: Point where all external forces coincide. If this point exists and is above the COM it helps stabilize the system. The Virtual Pivot Point exists for slacklining but is below the center of mass (as long as the center of mass is above the anchor points)
  2. Effective Basin of Support: Area in which a slackliner can actively move their base of support

Key Takeaways

  1. Experiment details:
    1. 11 subjects split between beginners, sportive beginners, and experts
    2. Analyzed trials with subject specific dynamic rigid body models
    3. Subjects performed both single leg standing and walking tasks
    4. Used subject specific kinematic models
    5. Subjects were recorded with a Qualysis motion capture system
    6. Normalized kinetic energy by subject mass and angular momentum by subjects inertia
  2. Hypothesis is that the following factors are indicative of slackline stability performance:
    1. Angular momentum
    2. COM position, velocity, and acceleration
    3. Stance foot position, velocity and acceleration
    4. Projection of COM position in relation to foot position
    5. Kinetic energy
    6. Coordination of hand movement
  3. All slackliners attempt to minimize kinetic energy and overall movement
    1. Experts show lower kinetic energy while walking and start with lower kinetic energy
  4. Experts have lower COM and foot accelerations than beginners
    1. the reduced foot acceleration shows they can control the sideways contact force from the line, a slackline specific skill
    2. Control of vertical COM seems to be an important factor in maintaining balance on the line
    3. experts are able to control their vertical COM acceleration by being compliant in their knees
    4. Vertical COM movement is greater during walking, but it appears experts have adapted their walking style to slacklining.
  5. Expert slackliners are able to precisely control their angular momentum, and move their base of support while keeping their center of mass steady.
    1. Beginners show roughly 50% more angular movement than experts
    2. Experts angular momentum is confined to the frontal plane, while beginners show rotation in all three axes
  6. Experts are able to effectively move their base of support in order to maintain their COM over their BOS
    1. They call the area in which the subject can actively move their BOS to be the effective basin of support.
  7. Found consistent hand coordination patterns in experts and some sportive beginners
    1. Hand coordination shown in graphs of x vs y movements
    2. Correlated up and down movements may help minimize vertical COM acceleration
    3. Coordinated hand movements allow for generating angular momentum without creating COM acceleration
    4. The underlying physics of the hand movements is not well understood
  8. "Next steps" include adding force sensors to the line
  9. Discussion in appendix shows that markered motion capture performs significantly better than IMUs at recording slackline data

Limitations

  1. Used a particularly small slackline (3m on a slack rack)
    1. Line was too short to show characteristic shaking motion during learning
    2. Experts had to adapt back to shorter line
    3. "The validity of the results for bigger slackline setups is yet to be confirmed"
  2. Fewer expert sessions were recorded than beginner because expert trials lasted longer