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How does a stone get into your shoe when walking or running?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by wdd, Aug 21, 2012.

  1. wdd

    wdd Well-Known Member

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    I went for a walk yesterday along an old, dusty, dry, unmade/unsurfaced but flat road.

    I was wearing a non-descript pair of trainers.

    I walked about four miles and had to stop on three occasions to remove stones from my shoe. On each occasion it was from the same shoe.

    While dancing about on one foot trying to remove the trainer and stone and then replace the trainer without putting my shoeless foot on the ground (ensuring that another stone didn't stick to my foot) I wondered about the biomechanics related to getting a stone in a shoe during walking or running?

    Initially I tried to imagine a scenario where the stone was thrown up by a shoe and ended up in that shoe. I gave up that idea as it seemed to require a miracle?

    Then I focused on the idea of one shoe throwing the stone in the air and the other catching it. That seemed more reasonable but seemed to generate more questions than answers. So I thought I would ask the experts to clarify things for me.

    my questions are:

    What are the necessary conditions, during walking and running, for getting a stone into your shoe?

    Why one shoe more than the other?

  2. David Smith

    David Smith Well-Known Member

    I like this question, very twee but just the sort of thing we should be thinking about especially in terms of diabetes and injury risk. :dizzy:

    So, assuming there is not a miraculous manifestation of stones in our shoes, just in order to teach us that lesson of - removing it, leaving it at the roadside and carry on our walk un-bothered and unhindered and not letting a small thing cause a unnecessarily large and debilitating injury, then what are the conditions necessary to allow the stone into the shoe?

    1st you need to be actually wearing a shoe!

    2nd you need stones, if your walking thru Asda / Walmart then you probably will never get a stone in your shoe

    On the other hand if your walking along a country lane wearing your 14 hole DM's laced up to the knee the you will not get a stone in your shoe.

    If your wearing your sandals on a pebble beach then you will get many stones in your shoe almost immediately. Unless the pebbles are larger than say a pigeons egg then you would not.

    The small stone in the normal shoe or trainer while walking along a stony country lane is in fact just the right combination of conditions required:

    Just enough room to let a stone in but no way out again.

    Another condition required is probability: The more stones of the right size under and around the feet while walking the greater the probability that one will bounce into the open gap of the shoe vamp.

    The way you walk would be another condition, so if you walk slowly and in an apropulsive manner it is unlikely that you will kick up any stones and so the probability that one will find its way into the gap is low.

    On the other had if you skip along like a school child then many stone would be kicked up and the probability is increased that one could find its way into the shoe.

    I don't think the mechanics of kicking up stones needs to be discussed much since you only need to kick your foot into a gravel drive and watch the stones fly just apply Newtons laws here) The back foot kicks stones into the front foot or the front foot flicks stones back into the rear foot or stones fly thru the air and the foot moves to just the right position to allow the two trajectories to have a common meeting point.

    Why on more than the other, well how many people have you tested? this is a low power bias error. E.G. Maybe one of your feet pronates more than the other and opens a larger gap in the medial shoe vamp and so the probability of a stone entering is increased.

    Aahh this what Pod Arena should be all about!!;)

    Regards Dave Smith
  3. wdd

    wdd Well-Known Member

    Hi Dave,

    Thanks for taking the question seriously. It is, possibly an odd question but hopefully it will interest one or two 'like-minded' people.

  4. davidh

    davidh Podiatry Arena Veteran

    The sequence of events is possibly:
    1. Stone thrown up by stance-phase foot as it lifts into the air after toe-off.
    2. Stone is also moving forward in the same direction as body progression.
    3. Body is moving forward at a similar speed to the stone.
    4. Contra-lateral foot is now in stance-phase.
    5. Stone miraculously lands in gap between trainer and foot.

    You could narrow the odds a little by hopping for four miles. If you don't pick up a stone then I contend that my hypothesis is probable, rather than possible. Don't forget to hop along the same old, dusty, dry, unmade/unsurfaced but flat road.

    Good question.
  5. Bill and Dave:

    I think you need to get a slow motion video of people waking and running on trails to see how much debris is flying about with each foot step. I think you would then be surprised at how few stones make it into our shoes after walking and/or running on such terrain.
  6. Boots n all

    Boots n all Well-Known Member

    Did any of you think to ask if there was a hole in the sole of the shoe:confused:

    No need to have airborn stones and it would explain why it only happened to the one shoe every time.

    To the mathematicians here, l wonder what the odds are of an airborn stone being flicked up at the right time, the gap(?) to open at the right time, for the stone to hit at the right time and angle to be correct for it to go in ??????

    If there is a hole in the sole at the met heads/flex point of the sole, the hole in the sole at the flex point opens wider at the propulsive stage, picking up a stone off the road, as it closes, it pushes the stone further into the sole cavity and pushes it out through the soft fabric insole of the trainer.

    Your over thinking the "hole" thing possibly?

    As with any diagnosis its about asking the right questions to start with
  7. David Smith

    David Smith Well-Known Member


    Ah yes good thought, outside of the shoe box as it were:D


    where are those video's, I'm onto You Tube first thing:rolleyes:

  8. RobinP

    RobinP Well-Known Member

    I don't know why this tickled me but the comparison to a pigeon egg is hilarious.

    I can imagine a stone small enough to get inside a shoe but I must confess to having no idea whatsoever about the size of a pigeon's egg
  9. David Smith

    David Smith Well-Known Member

    Well its bigger than a cuckoo's egg but smaller than hens egg egg at about 41mmx30mm


    informative stuff eh!

  10. wdd

    wdd Well-Known Member

    Pressed the post button way too soon.
  11. blinda

    blinda MVP

    D'you know, I'm not in the least surprised you knew that, Dave. I can see you as a pigeon fancier.
  12. wdd

    wdd Well-Known Member

    Thanks for all of your replies.

    Each day since my first post I have taken the same walk in the same trainers with markedly different results. The difference in the number of stones per day combined with your suggestions has helped me to formulate a hypothesis on one possible method by which a stone can get into your shoe.

    As kevin says I need to take a slow motion video. To test my current hypothesis the video would need to be set up at below ankle height so that the sole of the airborne shoe could be seen along with the upper rim of the upper of the grounded shoe.

    Davidh's possible sequence made me ask myself some 'useful' questions.
    Thanks DavidS for the idea of a hole in the sole of the shoe. No the shoes were almost brand new but they do have an interesting ridge pattern on the sole which could be described as a partial hole and might fulfill some of the grab and suck characteristcs of a hole.

    After reading the comment about pigeon eggs I measured some of the stones underfoot. They are predominantly irregularly shaped, about 5mm long and wide and a maximum of 2mm thick.

    Back to the first paragraph of this post. I have done the same walk each day since my first post on this subject. This morning is the first morning that I managed to get another stone in my shoe. It hasn't rained since my first post so I think that the ground is just as dry and therefore the stones are just as likely to get thrown up.

    What has changed is that the temperature has dropped markedly. On day one the temperature was about 37 degrees C and it dropped dramatically the next day by about 10 degrees C.

    My hypothesis goes something like this.

    Stones 'stick' to the soles of the stance phase shoe. The number of stones sticking is determined by the type and size and shpe of the stones underfoot and the temperature of the stones and the sole of the shoe and the position of the stones relative to the sole ridge pattern.

    As the stance phase foot is lifted stones start to fall off. The likelyhood of a stone falling of will be determined by its size and its level of adhesion with the sole of the shoe.

    During the swing phase stones continues to fall off the sole of the shoe. As stones fall of their direction of travel is determined by the direction of travel the area of the sole to which they are adhered, the compressional characteristics of the underlying sole and the nature of the adhesion and gravity.

    For a stone to have the possibility of getting into the shoe it must be released from the sole of the swing phase shoe when the sole of that shoe is higher than the Upper Rim of the Upper of the Shoe (URUS) of the stance phase shoe. It must also have a direction and sufficient velocity of travel to allow it to collide with or just above the URUS without rebounding. The risk of rebound will affected by hosiery.

    The potential gap between URUS and the skin/sock, opens at some point during the stance phase and voila the stone has made its way into the shoe.

    The length of the URUS which can potentially receive a stone is remarkably long and while it obviously includes all of the medial URUS of the stance phase shoe it is envisageable that stone could also enter around the lateral aspect of the heel posteriorly and anteriorly to the medial/lateral division of the URUS at least.

    Just noticed the sun's come out. Time for me to take a little walk.

    All the best

  13. Bill:

    I think your analysis is a good one. My bet is that the stones that are making it into our shoes during walking and running are nearly always coming from the contralateral foot/shoe as it accelerates into swing phase.
  14. Dr. Steven King

    Dr. Steven King Well-Known Member


    I agree with Kevin that a majority of stones will come from the contralateral foot during swing phase.

    While field testing Kingetics new army boots on our Maui trails i often get stones "flicked" into my shoes. Often the stones are sharp jagged lava stones about the size of a BB ". The chances of getting "rock foot" increases on loose sand and gravel trails where the swing foot may carry the rock foward during swing phase. The chances also increase on downslopes where the rock does not need to be lifted as high.

    Most rocks enter on the medial side inferior to the medial maleolus when the foot is creating a small gap along the upper. I also often notice on long trial runs smaller debree and dirt covers the backs of my lower calfs, likely by the same mechanism.

    As an interesting side to this i have often had to make slits in my prototype boots for fitting purposes. A vertical slit in the posterior heel and longitudinal slit to open up the toe box. I rarely get stones enter these areas again most stones enter from above. Now when the stone gets in it is fun to track where that stone migrates to during gait, walking is differrent that running with running allowing the stone to migrate more. Often the stones will go the the toe box and self Kick-out there or at the posterior heel opening. Believe it or not one solution for your stone in shoe problem may be to make some holes in your shoes...

    Another solution would be to wear higher socks that you can fold over the top of your shoe, this is easier with high tops and army boots. This will cover the opening/gap that developes on the shoes as the heel starts to lift off. Abet you may get a few stones in your sock but if the sock is snug enough it will not migrate down to the footbed. When we have deep powder snow we often use gators to keep snow from getting into our ski boots, you could try some of those as well.

    A Hui Hou,

    Kingetics- Lasting You Stronger...TM

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