SATRA STM 507 Dynamic footwear stiffness tester
In bio-mechanical trials organised by SATRA, running in a stiff shoe was found to involve more muscle activity in the lower leg and 5 per cent greater energy usage than in a flexible shoe.
Longitudinal flexibility, therefore, is a desirable property for sports shoes and especially running shoes. Flexibility is also desirable in children's footwear, where undue strain should not be placed upon the still-growing and developing foot.
In operation, the footwear is flexed about a defined line drawn across the forepart, the bending moment required to flex the footwear and the maximum angle achieved are recorded and used to calculate the SATRA stiffness index.
Torsional stiffness is a measure of the resistance to twisting of footwear about its longitudinal axis. The footwear is clamped in the toe area, forces are applied through the heel clamp such that the heel and backpart is twisted first inwards and then outwards up to a maximum angle. The torque required to achieve the twist is used to calculate the torsional stiffness values.