SATRA's dynamic footwear stiffness test machine

When assessing product comfort, footwear stiffness is an important consideration.

by Peter Allen

Stiff shoes are tiring to wear and impose a high level of pressure on the foot while forcing the shoe to flex. A degree of stiffness may be beneficial in certain types of footwear – for example, in some types of walking boots, where support and protection are required.

Both dynamic longitudinal and torsional stiffness of footwear can be quantified by the use of SATRA’s STM 507 dynamic footwear stiffness test machine (shown in the photograph above). The SATRA TM194:2004 – ‘Longitudinal stiffness of footwear’ test, conducted using SATRA’s STM 507 test machine, measures the force required to flex a shoe and the angle to which that shoe can be flexed comfortably. The result of the test is a ‘stiffness index’ which can be related to wear performance and wearer expectations. A low stiffness index indicates a high level of footwear flexibility.

In biomechanical 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 a flexible shoe. Longitudinal flexibility is, therefore, a desirable property in sports shoes – especially running shoes. However, varying sports and activities may require different levels of performance, as stiffness also contributes to foot support and stability. Flexibility is also desirable in children’s footwear where undue strain should not be imposed on the still growing and developing foot.

When conducting the SATRA TM194 test to determine the longitudinal stiffness of footwear, the test footwear is flexed about a defined line drawn across the forepart joint region. The footwear is flexed 25 times, and the mean of the peak value from the last five flex cycles, together with the maximum flexing angle (up to a limit angle of 50 degrees), are used to calculate the stiffness index.

Footwear torsional stiffness

Another footwear characteristic which can be determined using the STM 507 test machine (with the addition of a torsion attachment), is footwear torsional stiffness. Torsional stiffness is a measure of the resistance to twisting of footwear about its longitudinal (heel to toe) axis. The SATRA TM256:2002 – ‘Torsional stiffness of footwear’ test method can be used to determine this characteristic. The footwear is clamped in the toe region forward of the joint flex line, and a horseshoe-shaped clamp is secured around the outside of the heel in line with, and parallel to, the insole. Forces are applied through the heel clamp, so that the heel and backpart is twisted (first inwards and then outwards) about the heel-toe axis, up to a maximum angle of 10 degrees. The torque required to achieve the twist is used to calculate torsional stiffness values. Torsional stiffness contributes to foot support and stability – low stiffness facilitates normal foot function and mobility, while high stiffness tends to inhibit foot function but provides greater support and stability.

Determination of the longitudinal and torsional stiffness of footwear are valuable characteristics which can be used to assess footwear as part of footwear product development. As indicated in this article, there are cases where control of one or both of these footwear stiffness characteristics can make a significant contribution to the footwear’s overall performance and user comfort.

Publishing Data

This article was originally published on page 12 of the January 2019 issue of SATRA Bulletin.

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