Should I Be Aiming To Build Up The Callouses On My Feet?
Here's what the research shows about time spent in your shoes, training and blister incidence:
Van Tiggelen et al (2009) found previous hiking or military experience offered some protection to blister formation
Brennan et al (2012) determined that troops who did not “break in” their boots were more likely to suffer with blisters
Paterson et al (1994) found that military cadets who wore their boots less than 20 hours per week in the 2 weeks leading up to training were more likely to get foot blisters
In testing double sock systems against standard military issue socks, Thompson et al (1993) found reduced blister incidence was most noticeable early on in recruit training, when “recruits are adapting to the rigors of physical training and acquiring their military skills.”
Gardner and Hill (2002) found hikers who had not preconditioned their footwear were more likely to get blisters (32% versus 25%)
Your Skin Adapts To Blister-Causing Forces
Skin adapts to the forces placed upon it. It has been found (MacKenzie, 1974; Jagoda et al, 1981; Sanders et al, 1995; Knapik et al, 1996) that when the skin is subjected to repeated low level frictional forces, the following changes occur:
Epidermal cell turnover is faster
Cells are more resistant to frictional forces
And the epidermis becomes thicker
What's more, these adaptive changes in the epidermis take place sooner than you might think. MacKenzie (1974) examined changes to the skin of mouse ears that were rubbed every day 10 times with a moderate force (so as not to cause skin damage). Skin characteristics were observed at day 1, 7, 14, 28 and 35. What MacKenzie found was there were more cells in the epidermis and they were larger and more resistant to mechanical damage, compared to ears that received no rubbing. The interesting thing to note was, the changes at 7 days were identical to those at 14, 28 and 35 days. And this was with just 10 rubs of moderate force per day!
The First Few Days
Reported in the graph below are the number of blisters that developed per day during a 21 day, 580km road march in Korea (Choi et al, 2013).
Of the 142 college student participants, 135 suffered with blisters - that's a massive 95.1%. Ninety percent of blisters occurred in the first five days, with day two having the highest incidence. After that, there was a significant tailing off. While there were less than ideal aspects to this event (shoes were provided to all participants and for some reason, cotton socks were worn in spite of an average humidity of 93%), an adaptive response of the skin to the frictional demands of the activity has to be at play here. Similar results were found by Reynolds et al (1999) who studied injuries in a group of 218 male light infantry soldiers on a 5-day 161km cross country march. Forty three of 45 blisters occurred on day two of the march.
Practical Advice For Adapting Your Skin To Blister-Causing Forces
Spending time in your shoes for as little as a week will kick-start a natural protective adaptation of the skin to blister-causing forces. You can extend this to socks, insoles, running distance and whatever activity you're involved in. Make changes small and slow - to benefit from the gradually increasing skin resilience to blisters.
Depending on your individual blister threshold, this protective mechanism may not provide full blister protection. The research results quoted above show a somewhat reduced blister incidence, not a zero blister incidence. Look again at the results of Gardner and Hill (2002) - even after "breaking in" footwear, there was still a 25% blister incidence (compared to 32% without breaking them in).
But the adaption that comes with "breaking in" shoes and with other aspects of training is a step in the right direction and should not be neglected!
You could say these adaptive changes "toughen" the skin. But "skin toughening" is a phrase more commonly used to describe the effect of preparations like Compound Benzoin Tincture, alcohol, salt water, black tea and others (Read, 1990; Vonhof, 2012). An exact blister prevention "skin toughening" mechanism of action is not clear (in fact it hasn't even attracted any expert commentary, that I can find). And there's no blister or skin friction research that has tested these preparations either (Knapik et al, 1995). One thing I can tell you is these preparations are typically ‘astringent’ agents and have a "skin drying" effect. This means lower friction levels. And we know that means less blisters. Read this article for more on skin drying strategies.
Take Home Message On Skin Adaption
New shoes, activities, distances and terrains exert new shear loads to the skin. Getting your skin "used to" these new forces by making gradual and incremental changes can help protect your skin and make it more able to resist blister formation ... up to a point!