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Niiler 2016

Measures to Determine Dynamic Balance

Tags: #balance #biomechanics #control #dynamic_balance

Related: [[The Condition for Dynamic Stability - Hof Et Al 2005]] [[Extrapolated COM - Hof 2007]] [[N-Step Capturability - Koolen Et Al. 2012]]

Key Definitions

Balance Triad: The primary influencers of the balance system: the vestibular system, vision system, and somatosensory system.

Vestibular System: Uses fluid levels in the inner ear as a type of gyroscope, giving feedback on the head's orientation.

Vision System: Uses the eyes' view of the world to relate the body's internal sense of position to the outside world.

Somatosensory System: Uses the body's sensation of ground reaction forces and body positioning as a reference for posture orientation.

Postural Control: The ability of a subject to maintain a stable posture while balancing. Two subjects may be able to stand on one leg without falling for the same amount of time, but the one who barely moves while balancing has better postural control than the one who movies erratically while balancing.

Margin of Stability: The shortest perpendicular distance between the extrapolated (velocity adjusted) COM and the boundary of the base of support.

Interfoot Line: Line drawn between the center of each foot during gait. Balance may be measured by the distance of the projected COM from this interfoot line.

Autocorrelation: Can be used to measure how similar each cycle of the gait is.

Lyapunov Analysis of Stability: Takes a signal such as mediolateral COM movement and time delays it, looking for divergence. If a signal has a positive mean divergence, that means the movement is increasingly different and therefore unstable.

Key Takeaways

  1. Static balance is well understood from a physics perspective, but does not easily extend to dynamic balance. Numerous methods have been devised for assessing dynamic balance, including both quantitative and qualitative methods.
    1. The quantitative methods, which we are interested in, include Lyapunov exponents, autocorrelation, margin of support, and deviation of the COM from the interfoot line.
    2. The quantitative methods either calculate the bodies COM, or surmise dynamic balance through gait analysis.
    3. They require expensive and dedicated hardware as well as staff capable of analyzing and interpreting the results, but there is potential for new technology like the Kinect (or FreeMoCap!) to revolutionize balance assessment and training
  2. Balance is typically not hit or miss, rather it is a continuum, therefore a quantifiable metric is necessary.
    1. Dynamic balance could be defined as not falling, as maintaining a line of motion, as maintaining posture, etc.
    2. It can be quantified by using force plates, accelerometers, motion capture
      1. There is also some indication that smartphone accelerometers can be used for research
  3. Balance takeaways:
    1. Large variations in COM position indicate both more sway in the posture, and larger corrective forces
      1. This may also be an indication of weaker muscles, as weak muscles take longer to correct an action than stronger ones
    2. A study with accelerometers placed on subjects torso found increased variability of measured acceleration distinguished the fallers from the non-fallers
      1. Perhaps torso acceleration is worth investigating as a quantifiable metric of balance?
    3. A half-width walking study of non-fallers found higher mediolateral COM velocity across the board compared to full-width walking. This indicates it could be a more fine grained approach for assessing balance deficits.
      1. This means COM velocity could be a good quantifiable metric for determining dynamic balance.
      2. Difficult but measurable tasks in general can be good for assessing balance deficits (slacklining as a no-width walking study.)
    4. A dual task assessment using concurrent balance and cognitive tasks is also more sensitive to balance impairments than a single task approach.
    5. When we balance on a slackline, is our BOS the whole foot, the part of the foot touching the slackline, or somewhere in between?
    6. Balance is often studied mediolaterally, but many falls occur either forwards or backwards (tripping or slipping). Studying balance 3 dimensionally is then key.
  4. Quantitative methods allow the analysis of data as a means of assessing dynamic balance, not necessarily dependent on protocols and tools. They tend to follow a more physics based approach.
    1. Margin of Stability accounts for movement in balance by seeing whether the velocity-adjusted projected COM lies within the base of support. This is a very popular method, especially with motion capture labs, although validation has been somewhat mixed. The mixed validation results may in part be explained by balance impaired individuals guarding their balance, by putting more effort into remaining within the bounds of stability
    2. Interfoot Line is a line drawn between the center of each foot during gait. Balance may be measured by the distance of the projected COM from this interfoot line. One major benefit of this system is that it uses the body's own reference system, rather than the labs.
    3. Autocorrelation can measure how similar each cycle of the gait is. The thinking is that a stable gait is an indicator of stability while walking, and that fluctuations in the gait are indicators of instability.
    4. Lyapunov Analysis of Stability takes a signal such as mediolateral COM movement and time delays it, looking for divergence. If a signal has a positive mean divergence, that means the movement is increasingly different and therefore unstable. For example, if after a perturbation while walking a subject continues to diverge from its prior gate, it was not able to compensate for the perturbation and find stability. But if it does not continue to diverge, it was able to deal with the perturbation and get back to stability. This has the limitations of needing a large number of gait cycles to calculate, and may over-punish a single, corrected for misstep.
  5. Different measures of dynamic stability have been tested by putting balance impaired populations against healthy controls, testing to identify correlations with fall history, or by testing one method against another.