Many studies have quantified orbital dynamic stability (measured using Floquet Multipliers) in walking. The question is should running also be orbitally stable?
Hi Sina,
If you assume walking (or running, for that matter) is a periodic process, it would seem that it is orbitally stable (i.e. under normal circumstances, we do not fall). However, (1) walking (or running) may not be strictly periodic (in which case Floquet multipliers cannot be used), and/or (2) Floquet multipliers have thusfar shown little correlation to "actual" stability, i.e. the probability of falling, not even in simple actual cyclic models of gait (see also our recent review Bruijn SM, Meijer OG, Beek PJ, van Dieën JH (2013) Assessing the stability of human locomotion: a review of current measures. J R Soc Interface. 10(83):20120999). Thus, I would say; yes, of course you can calculate them, but what your results mean is very unclear at the moment.
Best,
Sjoerd
Hi Dr. Bruijn,
Thank you for your comments. As you mentioned, Floquet multipliers might have little correlation to "actual" stability (e.g. probability of falling). One potential solution might be to calculate Floquet multipliers for locomotion patterns that are less stable in physical sense, i.e. backward walking or running. If the pattern show less orbital dynamic stability than forward locomotion measured by FMs, then it might be indicative that FMs can have correlation to physical sense of falling.
Best,
Sina
Hi Sina,
I think your reasoning is not quite correct here; you must assume that backward walking is less stable in order for your reasoning to hold. What would you do if you find FM to be lower during backward walking? Say they don't work, or that backward walking is actually more stable? Since we don't know for real about backward walking, I'd go for something much easier, that is, a model. Even in such a model, FM do not appear to work. Also, in other experimental manipulations that were aimed at reducing stability, FM show little, or even opposite effects to expected. Indeed, there is one study that shows that fallers have higher FM, but this study has only 4 subjects in each group.... This, in my opinion, makes the use of FM rather doubtful....
Best,
Sjoerd
My assumption that backward walking is less stable was partly based on a valuable paper you and your colleagues published within which it was shown that backward walking is less stable than forward walking in terms of LyE (I don't remember the exact citation, and I think it's the only paper on local dynamic stability in backward walking).
However, your concern that what if FM has lower value in backward walking and how it can be interpreted is exactly feasible.
Another problem with this assumption might be that can we say if a moving system (e.g. backward walking) is less locally (LyE) stable is also less orbitaly (FM) stable?
Best,
Sina,
Hi Sina,
Good point, forgot about that one :). Indeed, given that paper, I think it would be safe to say backward walking is less stable (LyE have gone trough much more testing, and come out better). And yes, you're right, less locally stable doesnt necesary mean less orbitally stable. Actually, the other results I mentioned make me wonder wether orbital stability matters at all...
Best,
Sjoerd
- Badges
- Science topic