Effect of midsole geometry on foot-strike pattern and running kinematics

Abstract

A minimalist shoe could be described as a shoe with minimal midsole thickness at the heel (heel height) and a minimal positive difference between heel and metatarsus heights (drop). This study aimed to analyse the acute effect of heel height, drop and combinations of the two on the foot-strike pattern and running kinematics. Twelve healthy rearfoot males ran for 1 min at 3.9 and 4.7 m.s−1 on a treadmill in 16 different shoe conditions across which heel height and drop of the midsole varied independently. The foot-strike pattern was determined from high-speed video (240 Hz) by measuring the angle between the bottom of the foot and the horizontal at ground contact. Running kinematics were measured by an optical system (1000 Hz) placed on each side of the treadmill by which step frequency, duty factor, and leg and vertical stiffness were measured. Results showed that the lower the heel height and/or drop, the flatter the foot at ground contact, which characterised a foot-strike pattern tending toward a midfoot-strike pattern. Running kinematics was directly affected by drop but not by the heel height: drop was positively correlated to contact time and duty factor and negatively to flight time and leg stiffness. The foot-strike pattern alteration induced by a low drop and/or heel height was correlated to changes in running kinematics toward a running pattern previously associated with low foot–ground impact, and in turn to a ‘safer’ running pattern regarding stress fractures.