Equipment used for comfort testing – part 2

SATRA machines and related tests used to assess the essential characteristic of comfort in footwear.

This second of two articles (read part 1 here) continues our consideration of test equipment which can be used in assessing some footwear characteristics associated with foot comfort. The topics covered in this article are i) shock absorption and energy return, ii) shoe flexibility, iii) cushioning and iv) ground insulation. This is not an exhaustive list of comfort related assessments, although those described here are important characteristics which should be considered in relation to comfort.

Shock absorption and energy return

High impact forces are produced at heel strike in walking and running. Depending on the speed involved, these forces may be up to three times the person’s body weight. The impacts send shockwaves up through the foot, leg and body, which can jar bones and joints. A number of health problems have been linked to shock loading (a sudden and drastic increase of load similar to a ‘hammering’ effect) – both one-off and repeated. These problems include back pain, joint disorders and heel bone fractures. Clinical trials have shown that the use of shock absorbing underfoot footwear materials can help to alleviate symptoms, and can reduce pain and discomfort from injuries. SATRA TM142:1992 – ‘Falling mass shock absorption test’ provides a means of assessing the amount of shock absorption capability of footwear and footwear materials.

Whole sole and heel constructions can be assessed, as can individual components – for example, insocks. When conducting a test to SATRA TM142 on the SATRA STM 479 Dynamic Shock Absorption Test Machine, a sample is held in position on the machine base and a known mass is allowed to fall onto the footwear from a defined height. The test method specifies the dimensions of the impactor. By the use of an accelerometer and displacement transducer incorporated into the test machine, a value for the peak deceleration for the falling mass is determined. A low peak deceleration indicates good shock absorption characteristics.

Output from research conducted with this test method and test machine has demonstrated that providing sufficient thickness of shock absorbing materials has a significant effect on improving footwear’s ability to accommodate impact loading. Insocks can significantly improve shock absorption capacity for hard-soled shoes, but may offer only limited extra benefit when used in footwear with good inherent shock absorption capability.

Figure 1: The SATRA STM 479 test machine

Another characteristic which can be assessed from the SATRA TM142 test is the energy return or rebound provided by the heel construction, footwear material or insock. There is no direct relationship between shock absorption and energy return, and materials can be engineered to give different combinations of energy return and shock absorption. Energy return is a separate comfort factor to shock absorption, and concerns what happens after the heel or material has been compressed by a heel impact. Having a means to provide a scientific assessment of this characteristic allows manufacturers to develop products which incorporate a definable amount of energy return. The SATRA STM 479 test machine (figure 1) is supplied with software that allows both graphical and numerical outputs of test results for both shock absorption and energy return to be reported.

Figure 2: SATRA’s STM 512 Repeated Compression Test Machine

If both new footwear and footwear conditioned by a simulated period of use are assessed for shock absorption and energy return, an assessment can be made of the footwear’s capability to maintain its performance during use with respect to these characteristics. A simulated period of use can be achieved using SATRA’s STM 512 Repeated Compression Test Machine (figure 2) to conduct the SATRA TM156:2002 – ‘Resistance of heel assemblies to repeated compression’ test (figure 3). This machine provides a repetitive impact load to the sample representing repeated heel strikes.

Figure 3: Conducting the SATRA TM156 test

Shock absorption and energy return tests (SATRA TM142) conducted before and after a period of simulated heel impacts (SATRA TM156) will indicate any fall off in footwear performance for these two characteristics over a period of simulated use. A further advantage of using the STM 512 test machine to simulate a period of use is that it can be applied equally to all types of cellular sole units, heel assemblies or sheet material, as well as completed footwear.

Shoe flexibility

Figure 4: The SATRA STM 507 Dynamic Footwear Stiffness Test Machine

Another characteristic which has a bearing on foot comfort is the flexibility of the footwear – both longitudinal flexing in the joint region and torsional flexibility (twisting) in the waist area. Greater flexibility facilitates natural foot function, whereas higher longitudinal shoe stiffness inhibits foot mobility and movement. The resistance from stiffer footwear can be more tiring, as muscles have to work harder in flexing the footwear. There are situations when shoes need to provide additional foot support – for instance, in certain types of outdoor or sports activities or in the case of users with certain medical conditions. In these cases, higher stiffness may be required. Whether the footwear needs to provide more or less longitudinal or torsional stiffness, it is important to be able to assess these footwear characteristics. The SATRA STM 507 Dynamic Footwear Stiffness Test Machine (figure 4) allows both the longitudinal flexing stiffness and the torsional stiffness to be determined for completed footwear. Two SATRA test methods can be conducted using the STM 507 test machine: SATRA TM194:2004 – ‘Longitudinal stiffness of footwear’, and SATRA TM256:2002 – ‘Torsional stiffness of footwear’.

Cushioning

Underfoot cushioning is an important comfort factor for footwear, as it minimises the risk of underfoot soreness from a long day walking on hard surfaces. In addition, even when first put on, footwear with good underfoot cushioning engenders a sense of comfort to the wearer. One method which can be used to give a quantifiable value to underfoot cushioning is SATRA TM183:2001 – ‘Whole shoe cushioning assessment’. This assessment involves a full body-weight force being applied to the shoe bottom. The load being applied from a tensile tester acts in compression with the load applied through a profiled presser footform (SATRA STD 501 Cushioning Assessment Test Device). The STD 501 test foot can be used in conjunction with SATRA’s STM 566 Tensile Test Machine.

The compression energy absorbed is measured in joules to give an indication of the cushioning performance. The benefit of fitting an insock or footbed can be assessed by comparing the results of the test with and without the additional component fitted on top of the shoe bottom. It is worth noting that a percentage of the population will report very soft underfoot cushioning as being uncomfortable. For this reason, SATRA recommends an upper limit to underfoot cushioning values.

Ground insulation

Ground insulation is the amount of insulation provided by the footwear against ‘feel through’ from stones or rough or uneven ground underfoot. Too much feel through can lead to discomfort for the wearer, whereas too much ground insulation could isolate the user from picking up valuable characteristics of the underfoot conditions. The amount of ground insulation is influenced by the thickness, flexibility and softness of the sole.

There are two main mechanisms that contribute to ground insulation. ‘Bridging’ is when a very stiff or rigid shoe bottom bridges across bumps and underfoot irregularities, distributing the applied load across a large area of the foot. ‘Absorption’ is a mechanism in which a thick compressible shoe bottom moulds around a small object, absorbing much of its substance within the material. As the object is pressed into the bottom of the sole, the forces are dissipated, thus reducing the peak pressure transmitted through the shoe bottom. SATRA TM190:2002 – ‘Ground insulation index’ allows a way to assess both the bridging and absorption properties of footwear. The results of this assessment need to be considered in the context of the footwear application – such as everyday footwear, safety boots, sports shoes or for particular categories of users. The test is conducted using a tensile tester operating in compression, along with a specified indentor, support table, support ring, pressure mapping sensor and other ancillaries defined in the test method.

While the perception of comfort is influenced by the subjectivity of the footwear user, these two articles have overviewed a number of physical tests which allow brands, footwear manufacturers or component suppliers to obtain quantifiable data which can be used to assist in the development of products delivering comfort to their customers.

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How can we help?

For more information on the SATRA test equipment mentioned in this article, please contact test.equipment@satra.com or email footwear@satra.co.uk for more information on using SATRA test methods.

Publishing Data

This article was originally published on page 40 of the February 2015 issue of SATRA Bulletin.