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Studies on Centre of Mass and Rocker soles.??

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Phil Rees, Aug 16, 2012.

  1. Phil Rees

    Phil Rees Active Member

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    Does anyone know of any studies that comment on the influence a rocker soled shoe may have on the spatial characteristics of the centre of mass of the body during walking.
    Any information would be very much appreciated.
    Thanks in anticipation
  2. efuller

    efuller MVP

    There is a lot written on postural sway which usually is measured with center of pressure excursion. Center of pressure is different from center of mass.

    In stance, with a rocker bottom shoe there is a smaller base of support and it is easier to "sway" to the point where the center of pressure of the foot moves distal to the rocker point and the person would tend to fall over, especially with peripheral neuropathy.

    Gait and rocker bottom shoes. Gait is dynamic and foot placement is key in maintaining balance. Gait is a controlled fall forward and you put your swing foot in a position to keep the fall under control. People can walk with peg legs, which is a pivot instead of a rocker. I would not expect much change in the location in center of mass with walking with rocker shoes. If there was a change, I would expect it to be very small.

  3. Phil Rees

    Phil Rees Active Member

    Thanks for replying Eric,
    I'm more interested in the influence a rocker may have on the sinusoidal pathway of the Centre of mass of the whole body. I'm particularly interested in the effect it may have on the magnitude of the vertical displacement.
  4. Some studies published on COM work and rocker shoes. I haven't read this fully for a while, but as I recall it talks about COM velocity too. http://www-personal.umich.edu/~artkuo/Papers/JEB06.pdf

    P.S. double integration of force plate data can be used to estimate COM displacement, so if you can find any force data you can always manipulate it yourself. Or, simply look at the GRF data and perform the double integration in your head to get a qualitative feel for the differences.- my maths brain isn't that good, I'm sure Daryl Phillips can do this though.

    I guess your f-scan data will give you an estimate of vertical displacement, Phil.
  5. efuller

    efuller MVP

    Doing some quick rocker drawings. Take a 10 inch sole with the rocker at 60% of foot length so that the rocker point is behind the metatarsal heads. If material is added to the midsole to make the rocker point 1 inch inferior to the tip of the shoe. From a quick drawing, without protracter, it looks to be about 20 degrees. For those with a calculator handy arcsine 1/4 = angle (remember radians vs degrees)

    A 20 degree angle would be pretty big angle for a rocker. Looking at the drawing the location of the metatarsal heads would drop less than a quarter of an inch with above dimensions. Whether or not the whole body drops that much will depend on many things: has the other foot contacted yet, is knee flexion altered and probably more. I don't know how much height excursion of center of mass there is in gait without a rocker. Phil, why are you interested in the excrusion?

  6. Phil Rees

    Phil Rees Active Member

    Simon .... thanks for the papers, at first glance they look to be what I'm looking for ... Cheers

    Eric, the geometry of our latest design of rocker sole is demonstrating consistant and significant ruductions in the vertical component of force when compared to other rocker designs. We know that the mass of the subject hasn't changed so we're focusing our attention on the acceleration of the mass . Acceleration, as you know has two components, distance and time. The time values we're recording are all very similar regarding time to 1st peak, 1st to 2nd peak 1st peak to midstance trough etc etc. but the magnitudes of the peaks we're seeing are approximatley 17% reduced at the rear foot peak and around 22% at the forefoot peak. So as you imagine the distance of the migration of the C of M is now of interest.
    Hope that all makes sense ...
  7. Admin2

    Admin2 Administrator Staff Member

  8. Phil Wells

    Phil Wells Active Member

    Hi Phil

    At the recent Delcam technology forum, Chrus Nester presented excerpts from work they have been doing on rocker location, angle and length and came up with some very interesting results.
    I am not sure if the work is ready to be published yet as it is part of a European funded footwear project but might be worth asking.
    This work is fairly close to be a prescriptive protocol for rocker where certain loading/un-loading patterns are required.


  9. Phil Rees

    Phil Rees Active Member

    Thanks Phil
    We were talking to Anita Williams about this a couple of weeks ago ... we plan on getting together pretty soon.
  10. Phil:

    I have been following this rather interesting discussion and I believe a few points need to be made or need to be reemphasized. As Eric noted, center of mass (CoM) is not the same as center of pressure (CoP). CoM is the point spatial location of where the mass of the body can be considered to be concentrated and CoP is the point location where the ground reaction forces acting on the plantar foot can be considered to be concentrated.

    During walking, the CoM of the body always progresses forward, does not ever move backwards, and tends to undergo fairly small accelerations and decelerations in all three body planes in order to keep the individual walking smoothly and efficiently. Rather, the CoP on the plantar foot will undergo rapid accelerations, may frequently move backwards or forwards rapidly, and will exhibit quite wide variance from step to step since the CoP represents the response of the individual's central nervous system that changes the magnitudes and temporal efferent activities to the muscles of locomotion in order to keep the CoM moving forward in a smooth and efficient manner.

    For these reasons, rocker soled shoes will likely have relative little effect on CoM position and motion but may have very profound effects on CoP position and motion. The following book by the late David Winter is short and concise, covers these topics very nicely and should be required reading for anyone undertaking a project such as the one you are involved in (Winter, David A.: A.B.C. (Anatomy, Biomechanics and Control) of Balance During Standing and Walking. Waterloo Biomechanics, Waterloo, Ontario, Canada, 1995).
  11. Phil Rees

    Phil Rees Active Member

    Kevin, I really appreciate you taking an interest in my question.

    The problem I'm trying to understand is, how can our rigid rocker soled shoe design have such a dramic effect on the vertical component of force?. The inshoe pressures, yes I can understand, and, have designed the internal plantar surface of the shoe and that of the last to do just that. But how is it effecting the force?.
    My thinking is this ... we all know that force is the product of Mass x Acceleration. The mass of the test subject has remained constant, so that leaves the acceleration of the mass. Acceleration has two components, the actual distance travelled by the mass and the time it took to get from A to B. From the test results we have, the timing of the events (time to 1st peak, time from 1st peak to 2nd peak, time from initial contact to toe off) are consitant between the two sets of data.
    So that leaves me with distance the mass is moving. I'm thinking that our rocker design is reducing the distance the mass of the body is migrating thereby reducing the vertical force. So I was wondering if looking at the vertical migration of the centre of mass would perhaps give me an answer.
    Any comments or thoughts would be appreciated
  12. Remember that flattening the COM vertical displacement may increase metabolic cost too.
  13. A further thought: if the vertical component is decreasing, perhaps the shear component is increasing proportionally- maybe not what you want in a diabetic population.

    How are you measuring vertical component and what are you comparing it to?
  14. Phil Rees

    Phil Rees Active Member

    Simon, that's what I need to avoid at all cost and why I need to understand what's happening with the force.
    We're using Tekscan to acquire our measurements and we're comparing different rocker geometries and sole profiles. We're also comparing our own design against other commercial prescription"rocker soled shoes

    Many thanks for the metabolic paper ... a very interesting study
  15. You need a "proper" force plate, Phil. That way you'll be able to see the vertical versus shear force components. Moreover, you need to couple that with 3D kinematic analysis to see what's happening with the joint moments in your shoes.
  16. Phil Rees

    Phil Rees Active Member

    Absolutley right Simon, we're planning that right now.
  17. http://www.jfootankleres.com/content/5/S1/O6
  18. Then please don't just have clinically normal people walking in a straight line over the force plate! Test them on the target audience, i.e. diabetic patients with neuropathic changes and have them perform activities of daily living such as sit to stand manoeuvres, step down, direction changes etc. Good luck with it.
  19. Phil:

    I assume that you are referring to the vertical component of the ground reaction force (GRF) vector when you ask "how can our rigid rocker soled shoe design have such a dram(at)ic effect on the vertical component of force? The vertical component of the GRF vector may be measured between the shoe sole and the ground by either a force plate (preferred method) or may also be calculated from a pressure mat.

    First of all, let's talk about what actually causes the characteristic double-humped curve during walking of the vertical component of GRF vs time (see illustration below). In walking, during the first part of stance phase, the center of mass (CoM) of the body is being accelerating upward as the heel loads the ground which causes the first hump of the double-humped curve. Soon after forefoot loading, before the middle of midstance, the CoM experiences an acceleration downward as the CoM reaches its peak height above the ground. This downward acceleration of the CoM causes the depression in the middle of the double-humped GRF curve until the GRF reaches about 85% body weight (while only one foot is on the ground. Then the second hump of the walking GRF curve is caused by the forceful pushing of the forefoot against the ground during late midstance and early propulsion. During the latter part of propulsion, the GRF rapidly decreases to zero after toe-off.

    Therefore, the GRF curve during walking will be affected not only by the upward and downward accelerations of the CoM relative to the ground but will also be affected by an increase in muscle forces which may more forcefully push the foot into the ground. With rocker soled shoes, if there is a decrease in gastrocnemius-soleus muscle activity during propulsion, or an inability of the forefoot to push as forcefully against the ground during late midstance and propulsion, this could cause a decrease in magnitude of or a shortening of the second hump of the GRF curve during walking.

    Hope this helps.
  20. Here is some of the information on CoP and CoM from a great article by the late David Winter that I mentioned in my posting from a few days ago. This material should help you better understand how CoP and CoM affect each other during both standing and walking.
  21. Phil Rees

    Phil Rees Active Member

    Kevin / Simon thank you both so much, you've both been a great help (as usual) .... much appreciated
  22. efuller

    efuller MVP

    Are you using the raw data from the Tekscan? A few years ago there was an algorithm that was used that "modified" the raw data. This may effect your total force measures when shifting from rocker to non rocker.

  23. Hi Eric, are you talking about the com'nalysis software that Daryl developed as an add on or something else?

    I had a chat with Daryl last October and he said that Tekscan were going to drop this software add in. Shame, I thought it would be good for calculating left versus right leg stiffness. I think there could be good mileage in research with this, but nobody seems to have picked it up and run with it. If only I could afford an F-scan...
  24. efuller

    efuller MVP

    It was something different. I believe they were having a problem with all of the sensors adding up to the "correct" total force. If I remember correctly they had the user plug in the body weight and the software "corrected" the raw data so that it equaled the "correct" total force. There might be some threads on the arena where this issue was discussed. Norm Murphey tried to explain it, and I don't ever recall being satisfied with the explanation.


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