Chapter 2: The Injury Mechanism
In this chapter:
- What causes bisters – the blister injury
- Influencing factors
- Good friction / bad friction
WHAT CAUSES BLISTERS – THE BLISTER INJURY
Step 1 Place the tip of your right index finger on the back of your left hand.
Step 2 Wobble it back and forth but keep it stuck to the same bit of skin. Notice how your skin stretches? This is shear and this is what causes blisters. Keep wobbling as you read:
Shear might look like rubbing but it’s not. Notice how your finger tip has not moved relative to the skin of the back of your hand? But the skin on the back of your hand has moved relative to the underlying bone. Shear is the sliding of tissue layers over one another and it happens internally, below the skin’s surface (whereas rubbing happens on the outer skin surface). It’s that last little bit of shear that is damaging, when there is maximum skin stretch. When shear is excessive and repetitive, blisters form.
Here’s the significance of friction – friction is what keeps the tip of your finger stuck to the back of your hand! Shear needs high friction to be able to approach blister-causing levels. The video below shows you what shear looks like at the back of the heel, a common site for blisters. The foot remains stationary in the shoe as the heel bone moves up and down. This causes the soft tissues (skin, fatty tissue, fascia, ligaments etc) between the skin surface and bone to shear (stretch).
Contrary to popular belief, you don’t need rubbing to cause blisters.¹² ⁴⁸ ⁷³ This is one of the main misconceptions of blister formation.
Rubbing underneath the skin causes blisters, not on top of it! As the bone moves one way, the force of friction keeps the surface of the skin stationary. And the skin in between is made to stretch (shear). When shear is excessive and repetitive, it causes a tear just under the skin surface – the initial blister injury. Then it fills with fluid to become a blister. Rubbing underneath the skin (between skin layers) causes blisters. Rubbing on top of the skin causes the blister to deroof.
The “rub” doesn’t cause the blister. Shear is the blister injury. Rubbing (in the presence of high friction) causes further abrasive skin damage. “The injurious effects of friction on the skin and the underlying tissues can be divided into two classes, those without slip [no rubbing] and those with slip [rubbing]. The former may rupture the epidermis and occlude blood and interstitial fluid-flows by stretching or compressing the skin [blisters]. The latter adds an abrasion to this damage [deroofed blisters].” ⁴⁸
To put it simply, if you’re trying to prevent blisters, start thinking less about what’s rubbing on the skin surface; and more about how you’re going to stop the rubbing underneath the skin!
The four requirements for blister-causing shear are:
- A certain type of skin
- High friction and pressure (the coefficient of friction)
- Moving bone
Friction is the force that resists movement of one object over another. Skin friction is different to other types of friction because skin is a living and compressible tissue and so skin friction has two components: surface adhesion and tissue deformation. The degree to which one predominates over the other depends on the characteristics of the two surfaces and is the subject of debate amongst tribologists. ¹⁰ ¹⁷ ¹⁸ ²² ⁴⁸ ⁶² ⁷⁹ ⁸² ⁸⁴ ⁸⁶ ⁸⁹ ⁹⁵ ¹¹⁴ [This gets a bit technical – if you would like more information, Hendricks and Derler⁸⁹ is a recommended read].
The common expression for frictional behaviour is the coefficient of friction (COF).¹¹⁴ It is a dimensionless number (a ratio) that represents the ‘slipperiness’ or ‘stickiness’ between two surfaces and is defined by the equation: Coefficient of friction = Friction force / Force of contact. The table below gives you an idea of the coefficient of friction of several pairs of materials. There are two different COF values because the friction coefficient will be different when the surfaces are in stationary contact (static) and when they are moving relative to each other (dynamic – think rubbing). Apart from Teflon on Teflon, see how friction is higher when there is no movement!
Figure 6: Coefficient of friction data. The static COF is always higher, except for Teflon on Teflon – adapted from Townsend (Ref 59)
GOOD FRICTION BAD FRICTION
Without friction, your foot would slide around too much in your shoe causing injuries to your feet (eg: black toenails, toenails falling off) and making your muscles work harder for balance, propulsion and overall functional efficiency. Your foot requires traction within the shoe and it gets it from friction. This concept of ‘not all friction is bad’ is a very important one.⁵⁶ ⁷³ ⁸⁸ By design, socks, insoles and shoe linings provide high friction – this is good. Friction only becomes bad when it’s high enough to cause skin injury (and the threshold for injury will be different for each person). And it usually exists in small localized areas, not the whole foot.
“A certain amount of frictional force is necessary on the plantar surface of the foot (sole) in order to develop traction and stability for propulsion.”⁸⁸
Friction is a force parallel to the skin surface. It works in the opposite direction to the movement force from the bone. While the bone acts to pull the skin and soft tissue one way, the force of friction resists it, kind of pulling them the other way. Then there are compressive forces that are perpendicular to the skin surface – one from above (body weight) and one from the ground below – that cause pressure. These forces combine to define the shear load borne by the skin and soft tissue (shown below).
Figure 7: The horizontal forces determining soft tissue shear. The perpendicular compressive forces are not labelled but are inferred.
“Shear forces are applied to the human foot during walking and running because of the mechanics of foot alignment during contact and propulsion. The foot approaches the ground at a tangential angle (not a purely vertical angle) and then pushes off in a similar tangential direction. The foot [bones] must skid to a stop and then push into the ground to propel forward. The skidding will occur in both an anterior-posterior and medial-lateral direction, depending on the activity and demands of the sport.”⁸⁸