The compressive properties of the heel pad during the heel strike when running (barefoot and shod, two subjects, 4.5 m s-1) were studied by 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 heel pad deforms to a maximal percentage deformation of 60.5 +/- 5.5%. In shod running the heel pad deforms only 35.5 +/- 2.5% and the nonlinear force-deformation relationship reflects an increasing stiffness when deformation rises. Although the amplitudes of the vertical ground reaction forces do not differ notably in both conditions, barefoot running implies a maximal deformation to the fatty heel tissue, reducing its functional role from shock reduction towards local protection of the heel bone. It is argued that embedding the foot in a well-fitting shoe increases the effective stiffness of the heel pad.