Here’s What You Need To Know About Shoe Fit
Shoe fit is your first and most important blister prevention consideration. Too small and the extra pressure causes shear to become excessive with less cycles, resulting in blisters sooner.
Too big and your foot floats around. The larger bony excursions results in abrasions, deroofed blisters and other problems.
Either way, the magnitude of shear is higher than it needs to be. All this can be avoided (or at least minimised) with a shoe that fits perfectly. Optimal shoe fit for blister prevention revolves around length, width and adjustability.
The 'rule of thumb' is to have the width of your thumb between the end of the longest toe (usually the 2nd toe - but not always) and the end of the shoe upper. Measure this while you're standing (because your foot will elongate a bit) and make sure your heel is right at the back of the shoe.
Width is an important consideration too. What you don't want is the side of your foot overhanging (bulging over) the side of your shoe. By the same token, you don't want too much width. If you tie your laces and the facings (the material the laces go through) are touching one another, you've got nowhere to go if your shoe stretches. It's going to get too loose. Again, check this while you're standing.
It's not just about the length and width. You could have a perfectly sized shoe but if your laces are too tight or too loose, you've wasted your time. You might as well not have bothered getting the perfect sized shoes!
The whole reason we have laces is to optimise shoe fit at all times. Only when the foot fits snuggly in the shoe does the shoe provide the support it's able to and help prevent injury, like blisters. If your shoes are too loose:
first make sure you've used the last pair of eyelets - the holes your laces go through
next try the lace-lock lacing technique (video below) - it's a winner
if that doesn't work, check Ian's Shoelace Site for a lacing technique that suits you
At the other extreme, to avoid your shoes getting too tight, be prepared to adjust your laces as you go. Foot volume will increase with temperature and longer duration activity, even in the leanest and fittest athletes.
The Importance of Shoe Fit for Blister Prevention
Perfectly fitting shoes won't always stop foot blisters. Richie (2010) cites three studies that have not confirmed optimal shoe fit as an effective blister prevention strategy and suggests factors other than shoe fit must be implicated.
However, when shoe fit is as good as it can be, bony excursions are minimised and peak pressures are reduced. These two things go towards minimising blister-causing shear. Neglecting good shoe fit makes it harder for other blister prevention interventions to be effective. My advice is to get professional help from a specialist footwear retailer to get the right shoe for the job and for your foot. And use your laces to maintain optimal fit.