THE MECHANICAL CHARACTERISTICS OF THE HUMAN HEEL PAD DURING FOOT STRIKE IN RUNNING - AN IN-VIVO CINERADIOGRAPHIC STUDY

The compressive properties of the heel pad during the heel strike when running (barefoot and shed, two subjects, 4.5 m s(-1)) were studied b y means of a high-speed two-dimensional cineradiographic registration (150 frames s(-1)) of an actual running step. Vertical ground reaction forces were measured with a force platform. In barefoot running the h eel pad deforms to a maximal percentage deformation of 60.5 +/- 5.5%. In shed running the heel pad deforms only 35.5 +/- 2.5% and the nonlin ear force-deformation relationship reflects an increasing stiffness wh en deformation rises. Although the amplitudes of the vertical ground r eaction forces do not differ notably in both conditions, barefoot runn ing implies a maximal deformation to the fatty heel tissue, reducing i ts functional role from shock reduction towards local protection of th e heel bone. It is argued that embedding the foot in a well-fitting sh oe increases the effective stiffness of the heel pad.