Equipment used for comfort testing – part 1

Exploring the test methods and equipment available to assess footwear comfort.

by Peter Allen

Footwear comfort is a widely promoted attribute in marketing footwear and can be applicable across a wide range of footwear categories. Comfort is a key influence in achieving customer satisfaction, and can be an important factor in brand loyalty. The elimination of all points of discomfort would be a success in its own right. However, the promotion of specific benefits – for example, 'staying cool', 'reduced weight' or 'improved shock absorbency' gives an opportunity to market a positive attribute.

Fitting trials and user trials have been (and remain) valuable means of assessing comfort. However, physical tests and measurements can yield objective scientific data allowing improvements to be identified and targeted for improving foot comfort.

This article, the first of two, looks at how a number of SATRA test machines and devices can be used in the process of assessment of footwear characteristics, materials and completed products, and so contribute to an improvement in foot comfort. In this article, the focus is placed on two aspects of footwear comfort – ‘fit’ and ‘thermo-physiological’ aspects of footwear comfort.

The contribution of footwear fit to foot comfort

The number one comfort factor is fit. Fit is primarily about determining the dimensional properties of the footwear that will best accommodate the shape and size of the foot. Extensive foot surveys, in which thousands of feet are measured, allow the size, shape and size distribution for a population’s feet to be determined. Some of the typical parameters yielded by a foot size survey include the foot length, joint and instep girth, heel and joint width, big and little toe positions and depths. Modern foot scanning technology (currently being used in SATRA’s global foot survey) allows much more dimensional information to be captured. With access to reliable data on the distribution of foot sizes, a skilled lastmaker can develop lasts appropriate to the size and style of the required shoe for the target market. To aid the process of ensuring lasts are correctly shaped and dimensioned, SATRA manufactures a digital last assessment jig – SATRA STD 223. This can be used in conjunction with a number of SATRA last assessment test methods, which cover men’s, women’s and children’s footwear, to carry out a dimensional assessment of a last. In use, the last is aligned within the jig. This allows accurate, repeatable dimensions to be obtained from the last, using digital readouts incorporated into the jig.

An appropriately shaped and dimensioned last makes a good starting point to obtain footwear which gives a comfortable fit relative to the category of footwear being produced. There are, however, a number of factors associated with the materials used and the footwear manufacturing processes which can influence the final dimensions of the product. As an aid to assessing finished footwear, SATRA produces internal shoe size gauge (SATRA STD 225) and associated variants. Another valuable measuring instrument is the STD 206 quarter and back height gauge. Incorrect footwear length and incorrect quarter and back part heights can be a direct cause of discomfort.

There are a number of other considerations regarding fit and its influence on foot comfort. SATRA offers a number of services relating to fit – for example, a fit assessment, last assessment and comfort index.

Thermo-physiological effects on foot comfort.

Fundamental to human life is the ability of the body to regulate its temperature. The body is very effective in being able to determine a change of heat loss or gain, which we often interpret as being able to detect temperature. For instance, if subjects are placed in an environment of 40ºC and 10 per cent relative humidity (rh), they will feel reasonably comfortable. By contrast, if they are then placed in an environment of 40ºC and 90 per cent rh, they will quickly report feeling very hot and uncomfortable. This is because at the lower humidity, the body is able to easily regulate its heat through the evaporation of sweat. In the high humidity condition, however, subjects will sense that they are not regulating their body temperature effectively, as sweat is not so readily evaporated due to the high humidity. This will cause the body to increase the rate of sweat production. In a similar manner, a fan blowing air over a person’s face is interpreted by the subject as lowering the air temperature whereas, in fact, the air temperature is the same. The subject is interpreting the higher rate of heat loss caused by the moving air as a lower air temperature.

So, how does this relate to footwear comfort? Subjects will feel thermally comfortable if their bodies can readily regulate heat gain or loss. Discomfort is felt when it becomes harder to regulate body temperature. This discomfort applies to both excessive heat gain or heat loss. Even at rest, the body generates heat, which needs to be lost to the environment – including from the hands and feet. As the external temperature rises or the user’s activity levels increase, more heat loss is required.

The two main mechanisms of body temperature control are blood flow and perspiration (sweat) rate. Increased blood flow to the skin surface facilitates heat loss, and the process of sweat evaporation causes a cooling of the skin surface, as energy is required to evaporate liquid sweat to vapour. Most types of footwear enclose the foot to a greater or lesser extent. In so doing, the footwear can inhibit these body temperature regulation systems. Firstly, the footwear construction materials will provide a degree of insulation to the foot. This can be an advantage in cold conditions, but also presents a barrier to heat loss when the body is seeking to lose heat. Secondly, the footwear construction can inhibit the process of sweat evaporation and the associated cooling this would deliver. As a foot gets warmer, it also swells in size. This has the effect of making footwear more tightly fitting, which can lead to discomfort for the wearer as increased pressure is applied to the foot. An inability to lose heat can lead to increased sweat levels. If the sweat is not managed by the footwear, the high humidity and retained sweat can lead to secondary discomfort effects. Such effects include softening of the skin, which can make the skin susceptible to soreness and abrasion, along with fungal growth and odour. The tightening due to foot swelling can be compensated in the design of a shoe by minimising fitting pressure and accepting a more generous fit standard.

SATRA produces the SATRA STM 175 Permeability and Absorption Test Machine and the STM 567 Endofoot Test Machine, both of which are very effective in making assessments of the moisture management capabilities of materials and whole footwear.

SATRA test method TM47:2002 (water vapour permeability and absorption), conducted using the STM 175 test machine, is SATRA’s preferred method for the assessment of the moisture management capabilities of footwear materials, with respect to absorbed moisture and transpired moisture. This test allows the contribution of the complete assembly of upper, lining and hose to be made. It also is used to assess the amount of moisture absorbed (retained) by each component, as well as the water vapour transpired (escaping to the environment). The test is a simulation of wear conditions, with the interior of the materials exposed to both a humidity and temperature representative of actual in-shoe conditions.

Having selected the materials of construction, it is also important to check the performance of the completed footwear with respect to moisture management. Poor details of design and manufacture can undermine the performance of the completed footwear. SATRA test method TM376:2009 – ‘Advanced moisture management test’ conducted using the SATRA STM 567 Endofoot Test Machine permits the performance of the whole footwear to be assessed for its ability to manage internally-generated moisture.

The STM 567 Endofoot Test Machine comprises a heated footform which is held at body temperature onto which a skin simulant, standard hose and the footwear to be tested are placed. The footwear is fastened as it would be in wear. The test is conducted in a conditioned environment, and a controlled rate of airflow is passed over the footwear. A measured amount of water is introduced into the footform via small-bore pipes at a controlled rate to simulate sweat. The test machine allows the rate of ‘sweat’ to be varied. All the components (hose, footwear and any removable insock) are weighed at both the start and at the end of the test.

As the amount of water introduced during the test is measured, it is possible to identify where the introduced water has gone – both the water absorbed into each component and that which has transpired to the external environment. From these results, it is possible to determine an overall assessment of moisture management for the footwear – both short-term, when water is initially absorbed, and long-time, when the ability of footwear to lose water vapour via transpiration or ventilation will be important to maintain foot comfort. Maintaining a dry hose (sock) is a key means to deliver footwear comfort.

Another important footwear comfort test which can be conducted using the STM 567 Endofoot Test Machine is a determination of the thermal insulation properties of whole footwear. The test method is SATRA TM436:2010 – ‘Determination of whole shoe thermal insulation value and cold rating’. When conducting this test, the conditioned footwear is installed onto the heated footform after fitting the skin simulant and hose. The test is run without the introduction of any moisture, but with the controlled airflow operational. After a period of stabilisation, the electrical energy required to maintain the foot temperature is recorded. From this energy value (in watts), the temperature gradient from the footform to the external test environment and the surface area of the foot form, a thermal resistance called the ‘R’ factor is calculated for the footwear.

Having obtained this R value, a simple calculation is used to determine the external temperature and associated user activity level at which the footwear will be comfortable to wear. The activity level gives the amount of heat which is required to be lost while the temperature gradient needed to lose this heat allows the determination of the outside air temperature at which the footwear will be comfortable. For footwear used at low external air temperatures, matching the thermal insulation to the activity level is important.

Too much insulation at a high activity level will lead to a build-up of sweat which, when a period of inactivity follows, can result in excess loss of heat and associated discomfort. The STM 567 Endofoot Test Machine delivers a level of accuracy which allows the effect of different hose, or alternative lining materials to be assessed. The test can be used to determine the suitability of the level of insulation for hot as well as cold external temperatures.

These two market-leading machines (STM 175 and STM 567) provide a valuable means to assess and develop the performance of footwear materials and whole footwear, to provide thermal comfort to the user. Other SATRA tests and their associated test equipment are available to assist in the assessment of materials – such as for wicking performance.

Click here for comprehensive information on SATRA's range of test equipment.

Publishing Data

This article was originally published on page 36 of the December 2014 issue of SATRA Bulletin.

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