The existence of an organized directional structure to the connective and elastic tissue fibers of skin was demonstrated by examining cross sections of skin along and across the Langer's lines. The converse is true with cuts and stitches across the Langer's lines. Tension along these lines varies with sex and with obesity and, of course, with the location on the body. Description of the mechanical behavior of the skin depends on a little engineering background. First, we must define some terms.
Stress is the force per unit area and has been traditionally expressed in pounds per square inch.
Strain is the distance a material is stretched divided by the original length of the material. Strain is expressed as a unitless quantity or when multiplied by 100 is denoted as per cent elongation of the test specimen. If a material is monitored when stretched, the stress developed can be plotted against the strain and this becomes the standard engineering stress-strain plot. Figure 2 shows an initial shallow increase in the stress of the skin as a force is applied with an increasing stiffness and stress development as further force is applied. The shallowness of the initial stress-strain curve indicates the reason that at small displacements or stretching the flexibility of the skin is quite large.