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 skinsock interface and the sockshoe 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 andor 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. Microtears occur within this layer and when sufficient.