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As you know, blisters are caused by shear ... the stretching and distortion of soft
tissues. Blisters can occur in many places on the foot.
Not only the weight bearing plantar surface but also the top of the toes, the back of
the heel and potentially anywhere. But blisters in any area of the foot have 2 things in common
... there is high friction between the skin, sock and internal shoe surface to keep them
stuck together ... and there is movement of the adjacent bone relative to the stationary
skin. These two factors cause shear. Let's have a look at shear at the metatarsal
heads, a common location for blisters. Shear occurs in the presence of 2 factors:
1) Firstly, high friction exists at the two interfaces: the skin-sock interface and the
sock-shoe interface causing them to stick together.
2) Secondly, there is a movement force. The movement force comes from the metatarsal heads.
Imagine as the forefoot contacts the ground with each step. The skin becomes stationary
while the metatarsal heads skid forward relative to the skin. And the opposite occurs at propulsion.
These two conditions of high friction and moving bones exist as part of normal foot
function. Shear is normal and our tissues are quite able to deal with it ... to a point.
I'm sure you can imagine that as our activity increases in intensity and-or duration, shear
may reach a point where it becomes excessive ... more than the skin can handle. At this
point, damage occurs. This damage occurs within the stratum spinosum -- also known as the
prickle layer of the epidermis. Micro-tears occur within this layer and when sufficient
tears have occurred, a dedicated cavity exists and fluid fills the cavity. This is a blister.
Now you know how blisters are REALLY caused .. and I've got a feeling you're already thinking
about blister prevention in a different way ... and that's a good thing.