Thursday, 7 November 2013

Bad News and Blisters

Well, not such disastrous news really. Just that I went to the physio a couple of weeks ago and have been banned from running until my calves and hamstrings are in a bit better shape. Hopefully I can start again in a week or two, but probably have to work back up slowly. I'm not too bothered, it's the right time of year to sort out this kind of thing. I've also been to see the knee guy, scan fixed for Friday though I expect it will be a similar story, general wear and tear, not much to be done, figure out some sort of coping strategy. If I can get going again by the New Year I'll be happy enough. Plenty of decorating and gardening to do in the mean time, it's an ill wind and all that.


But I need to keep my hand in on the blog or I'll forget how to string a sentence together, so I got to thinking about a conversation I had with a runner back in the summer. He'd had to pull out of a race he'd trained long and hard for because of blisters. I've seen it happen to others over the years, and I've also seen runners put up with a lot of pain and submit to monumental tape jobs just to get them to the finish. I don't get it. I don't mean that I don't get blisters (although I don't), I just don't see why people put up with the whole blister thing to the extent that they do. I've also noticed that in quite a few events I've entered recently, one of the kit requirements is to have a basic first aid kit with "blister plasters" as an essential part of it. Again, I don't get it. Once you put a piece of Compeed ("other brands are available") on top of an already formed blister, the only way it's going to come off is attached to most of the skin it's covering, and that's not pleasant. Been there, got the teeshirt (or more accurately the several days of discomfort while the skin grows back again). Once you're treating blisters, the battle is already lost, your feet are going to hurt for a while. Why not stop them forming in the first place?

I can't guarantee that if you listen to me you won't get blisters. Everyone is different so the precise solution that works for you will be individual. But if blisters are a problem for you, you ought at least to be able to work out why the problem arises and have some logical strategy for preventing it.

What causes blisters - the engineering

This first bit may be hard going.  You can skip it and go directly to "What causes blisters - summary" if you like, but I always like to know where someone's theories have come from, so here are mine. Now as I've said many times on these pages, I have no medical knowledge whatsoever, but then I believe that blisters are not a medical problem  -  it's just down to engineering (but then what isn't?). I'm just going on what I've observed, and applying a few simple rules of analysis:-

1. A blister forms when skin is rubbed. Actually, I suspect that what does the damage is when the layer of skin being rubbed moves over the tissues immediately underneath it, but we can leave that medical nicety to those who understand these things, for our purposes it doesn't matter; all we need to know is that blisters come from rubbing.

2. Rubbing comes when the material next to your foot (normally your sock, unless you're one of these strange runners who don't wear any) moves along the surface of your skin. You can also get rubbing when individual toes move against each other, but let's park that for simplicity and come back to it later. For now we'll concentrate on the sock to foot movement.

3. Whether, or how quickly, rubbing from this movement produces a blister depends on three factors:-
  (i) How hard is the rubbing -a gentle caress that you don't notice or a discomfort that is apparent from the first step?
 (ii) How big is the movement - does your sock slide relative to your foot by one millimeter or ten?
(iii) How long does it go on for  -  fifteen minutes or fifteen hours?

Let's look at these in reverse order (mainly because it's easier that way!)

(iii) How long the rubbing goes on for is not really within your control. OK, you could run a little faster and complete your race in eleven hours in stead of ten, but we're talking ultras here and their nature is to go on a bit; in these terms a 10% reduction in time is unliklely to make a difference.  The real point is that a foot/sock/shoe combination that causes a blister to appear in a 50 mile race might have been completely trouble free over 10k. Or to put it the other way, until you start doing longer distances, you don't know whether you have a problem or not. The key learning here is to get out and do the distance, or somewhere near to it, before you get to an event that means something to you. You may be able to step up from a marathon to a 50 miler quite successfully without doing a distance in between; on the other hand you may not, and you won't know whether you're crossing your "blister threshhold" until you try. So this is the first key point  - understand whether you have a problem or not.

(ii) How big the rubbing movement is will ultimately depend on the freedom that your foot has to move within your shoe. There are a lot of complex force transfers going on inside your shoe when you run - just think about two or three of the simpler ones:-

- To move forward you must transfer a muscular force to the ground beneath you. This is transferred partly through a reactive force on the back upright piece of your shoe (which will then compress slightly), and partly through sole/sock/shoe friction inside the shoe, then ultimately through friction between the underside of your shoe and the ground. All this forces your foot backwards inside the shoe. As soon as you have pushed off from the ground, all this force is removed and your foot will "relax" slightly forward inside your shoe. Throw in some uphills and down hills and this potential for your foot to move forwards/backwards inside your shoe increases dramatically.

-As you lift your foot off the ground on each step you're hoping that your shoe will come up with it, so there must be some force holding it on. But the shoe also has weight (gravity)  holding it down, so there is potential for your shoe to lag behind your foot as you lift it up, then for your foot to sink back into your shoe as you return it to the ground.

- On anything other than a perfectly level surface, the ground will impose some (however small) sideways force on your shoe as it lands and pushes off. You try to keep your legs upright as much as possible so the shoe effectively "rolls" around your foot as the forces are balanced.

All these forces (and many others) want to make your shoe to move relative to your foot. How far it moves depends on how much you let it  -  that is, how tight your shoes are. Park this thought for now.

(i) To understand what affects how hard the rubbing force is at any point on your foot, you need to go back to a bit of mechanics you probably learnt at school.  The rubbing force is friction, and there is a simple relationship which says that the magnitude of a frictional force between two surfaces is equal to the normal force between the surfaces (the force holding them together) and the coefficient of friction between the two materials (F = mu x R, remember that one?), Rather than worrying about the equation, imagine (or carry out, if you've a mind to) a little experiment. Put a finger inside a sock and rub the back of your hand with it. The harder you press, the more you feel the rubbing; this is because you are increasing the normal force (R). Now wet the sock and repeat the experiment. You feel the rubbing more because you have increased the coefficient of friction (mu); you could decrease the coefficient of friction again by say smearing a bit of vaseline on your hand; simple. Now a final experiment. Rub with one finger inside the sock as hard as you can, and then compare the effect with rubbing with four fingers inside the sock as hard as you can. It hurts more with the single finger because you have concentrated the same force into a smaller area, and this is one of the most important things to remember when thinking about blisters. They are nearly always caused by the unintentional creation of a point load, in conjunction with a point at which your sock can move around on your foot.

Probably time for a G&T while we pause to summarise where we've got to here.

What causes blisters  - summary

- Blisters are caused by your sock rubbing against the skin of your foot
- The likelihood and/or speed at which a blister forms is governed by
     - how much the sock can move relative to the skin (in the direction along the surface)
     - the force holding the sock against the skin
     - the coefficient of friction (the "stickiness") between sock and skin
The longer the rubbing goes on, the more likely a blister will form, but in a race of a definite length we can't do much about that.

Blister Prevention

OK, so now we have some idea of why we get blisters, how do we stop it happening. Let's look at the causes I summarised above in turn:

(a) Cutting down the movement between skin and sock.

When you run, your shoe stops momentarily when it hits the ground, but your foot wants to carry on moving. Any spare space you have inside your shoe will let your foot move inside it, and what your sock then does (at this point in the discussion) is anyone's guess. The more your foot moves, the more the potential for rubbing, and so for blisters. The only real corrective action for this  is a no-brainer, but I still wonder about it when I see some discussions on forums and facebook groups. You have to have shoes that fit. This is simply the most important step by a long way in blister prevention. Now this doesn't just mean shoes that are nominally  the right size; you need shoes that match your foot shape. So forget about picking a shoe model because someone else has recommended them. It might be perfect for them, but unless your feet are the same shape as theirs there is no reason whatever that it will be equally perfect for you.

In my book, a shoe should allow you about 10mm in front of your toes, and a couple of millimeters above them in the toebox (both of these to allow for expansion during longer races and for an inevitable bit of movement when going down steep hills), but the lacing should allow you to make them a completely snug fit everywhere else. No lifting of the heel inside the shoe as you step off, no sideways movement when you stand on a lateral slope, toes not touching the front of the shoe when going down a steep slope. If you can't get this with the shoe you're trying, move on to another shoe until you can. Salomon, Hoka, Innovate, Brooks, etc, will all be perfect for some people and useless for others. Models vary even within the brands; Hoka Stinsons are near perfect for me but Hoka Mafates are a waste of time. Forget about how brilliant the technical claims are, if they don't fit your foot then all you're buying is pain. You may even have to mess about with different insoles and tongues, but I can't emphasise too much how important shoe fit is.

But even with a perfect fit, there will always be some movement, what is the next stage of prevention?

(b) Cutting down the force holding the sock against the skin

This force is there because it's necessary to transmit muscular effort to movement over the ground, so you can't take it away. But remember the little experiment we considered earlier - one finger in the sock hurt more than four. This means that you have to spread this force as evenly as you can over the whole sock area, and avoid "pressure points" which carry more than their fair share of the load. So how do we do this? Yes, I hope you're there already, you have to have shoes that fit. It really is a double whammy.

Now sometimes you can't get this absolutely  perfect. I have quite protruding heel bones (they wear holes in the back of my shoes before anything else goes) so there is an inevitable pressure point here for which the subsequent prevention methods are needed. And again, however much you even out the pressure force on your socks, it is still there. So although we might have minimised them we still have some movement and some force. So where do we go from here?

(c) Cutting down the coefficient of friction

Socks rub. Wet socks rub worse. Think what is happening here. The foot moves inside the shoe, but what happens to the sock? Either the sock grips the inside of the shoe and moves against the foot, or the sock grips the foot and moves inside the shoe (or maybe a bit of both!) We want the sock to grip the foot and move inside the shoe (no rubbing if this happens), but in general the outside of your foot is smoother than the inside of your shoe so the sock is likely to stick to the shoe.

One tactic I've used with success on occasions is to wear two pairs of socks, a fairly thin tight-fitting pair next to the foot and a thicker pair on top. The movement is then taken between the two pairs as they run fairy easily against each other. This got me round the 200 mile Tor des Geants with no blisters at all, when I saw many runners in a lot of pain and even having to retire in the same event from blisters. However this method has a real downside, in that in the very wet conditions underfoot typical in the UK the two pairs of wet socks just stick to each other and you lose all the benefit of having two pairs.

So, at the end of the day we are left with trying to minimise the coefficient of friction between sock and foot, because this is the only area that we have some chance of really controlling across a range of conditions. A number of tactics are available:

  i) Taping. This can be successful, but we need to remember what we are trying to do. We are not simply "covering up" skin, we are trying to reduce friction. So if you put a smallish piece of tape over a potential "hot spot" and the tape has a rough outer surface (like a fabric plaster, zinc oxide tape, or Kinesio style tape), then you will stop the sock rubbing against that bit of skin, but you won't stop the sock dragging the tape and the skin underneath it backwards and forwards. That's how you get a blister under the tape you've applied. If you use tape, my view is that you need to tape a substantial area to anchor the skin to prevent it moving, and preferably go for a shiny top surface to reduce friction  -  that's why a lot of the US runners favour duck tape ("gaffer tape") in preference to medical products. I used to use tape but I rarely do these days, I think other methods work better for me. Tape systems can also be less effective when your feet get very wet.

 ii) Lubrication. Again, remember what we're doing here  -  reducing the friction between foot and sock. I used vaseline for years with some success but then went on to Sudocrem, a product designed for preventing nappy rash (and therefore designed to work when wet!). It's very white and a bit messy, but a good coating on your feet before you start a race should see you through 24 hours or so, no matter how wet you get. These sort of products are also one way to deal with blisters you may get between your toes, the result of skin on skin rubbing (see, I hadn't forgotten about that).  Again, I believe the best starting point is to get shoes that fit properly, to minimise this movement in the first place, but if this type of blister is a real problem for you then you may find Injinji type socks (with individual toe pockets) a good plan - the principles are still the same, they prevent rubbing by holding each toe in a pocket that moves with it.

iii) Magic Socks. There have been a number of socks claiming to prevent blisters on the market in recent years, but one that has really cracked it for me is Drymax. The design principles are really sound, there is a Teflon type material woven into the sock to reduce friction (between foot and sock and between sock and shoe), and the material also draws water away from the foot (they insist it's capillary rather than wicking but the effect is the same), so preserves the effect of the Teflon even in wet conditions. They do require a bit of an act of faith because they say that if you use lubrication (vaseline etc) you compromise their effectiveness, but after biting the bullet on this a year or so ago I haven't used anything else in race since then. No tape, no lubrication, just well fitting shoes and good socks!

So there you have it, if you've made it this far, for I've realised this has turned into quite a lengthy piece.  I'm sure all of us will still from time to time have the odd blister, because we didn't do something quite right, or a different set of conditions caught us out. But they are certainly not an inevitable consequence of running long distances; with a bit of understanding and planning, you can always look after your feet.


Dave T said...

Wow! Will there be a test on this?

Rob said...

Blisters - one general point that I would assume is commonplace: that if you do a lot of distance running, the skin on your feet will get toughened up and you'll gradually get less and less blisters. Anyone disagree with that?
These days, I find the only blisters I get are under my toenails. Presumably these occur when the shoe forces the nail to move relative to the flesh beneath, so the nail becomes detached?
Mind you, I'm "one of these strange runners who don't wear any" (socks), so perhaps my experience is untypical. But if I had to adapt to such newfangled devices, I'll bear in mind your comment about Magic Socks!

Anonymous said...

Hello, thanks for the great and informative article. What is your opinion / experience about the Injinji Toe Socks ? Greetings Hanz.

Anonymous said...

Hi Andy,

My evolution to blister free running went along your lines - start with the wrong shoe brand (Aisics for me) and taping the blister area. Then moving onto the right shoe brand (NB) but the wrong socks coupled with taping and finally to the right socks: Sole twin skin for most stuff and drymax for long boggy ultras.
They are a bit bulky.

Rebecca Rushton said...

This is a great piece Andy. Your engineering background is actually really helpful in understanding the physics of blisters and you have explained many concepts better than most - really well done!
Hanz, the toesocks are popular with ultrarunners and often do enough for interdigital blisters. They're a good place to start.

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