Assessing the stability of footwear

Reporting on the results of an investigation into the practicalities of quantifying the stability of shoes.

Image © Syda Productions | Dreamstime

‘Stability’ has traditionally been viewed as a desirable quality in footwear, and is a factor that manufacturers often seek to enhance, as far as the style allows. However, certain styles of footwear are deliberately designed to create a sensation of instability for the wearer. Some of these products are marketed as promoting a more ‘natural’ way of walking, whilst some are based around a distinctive sole contour that forces the wearer to make a constant series of small involuntary movements in order to maintain an upright stance.

One rationale behind such designs is that shoes that make the wearer unstable encourage him or her to constantly expend more energy by making constant stabilising movements. The appeal being that such ‘micro-movements’ are made without any conscious effort, yet are beneficial, helping people to exercise without making a conscious decision to do so.

Clearly it is desirable to be able to quantify the level of (in)stability conferred by different footwear. This can help to validate marketing claims or assist manufacturers to develop products that more completely meet their customers’ expectations where stability, rather than instability, is the desirable characteristic. High-heeled women’s footwear is another obvious candidate for assessment, being one of the most inherently unstable styles of footwear. Therefore, testing to quantify the stability of new designs could be used to feed back into the design process to steer future improvements.

The Centre of Force trajectory

One of the ways in which we can quantify the level of stability is to use pressure-mapping software that displays a factor called the ‘Centre of Force’. This is an indication of the centre, or focal point, of all forces active on the in-shoe sensor we use. Its position in recordings taken of static subjects is indicated by a suitable icon, and by a series of hashed marks for recordings of moving subjects. The properties of these hashed marks describe the ‘Centre of Force trajectory’ which can be used to better understand the way that force is applied through the foot during walking. This is the distance and direction travelled by the Centre of Force for a moving foot.

By recording a pressure mapping ‘movie’ and analysing a composite image of the peak forces exerted during a pair of steps (left and right), this Centre of Force trajectory can be viewed.

A series of hashed lines indicate the speed and magnitude of these movements and allow for comparisons to be made between different test scenarios. Each hash mark represents a single frame of the recorded movie – the longer the hash mark, the greater the movement of the Centre of Force during that frame. Obviously, as each frame lasts for the same length of time, long hash marks indicate rapid movement and short hash marks indicate slower movement. The overall dimensions of the trace for the Centre of Force trajectory in a particular recording tell us about the overall velocity and magnitude of the movement of force on the sensor, and can be used to make judgements about the stability of the forces registered by the sensor.

For example, a person whose foot pronates (rolls inward) when walking exhibits a Centre of Force trajectory where the Centre of Force describes a curve bowing inwards toward the inside edge of the foot as it moves from the heel to the toe. Likewise, a person who supinates (rolls his or her foot outward) when walking would create a Centre of Force trajectory describing a curve bowing out towards the outside edge of the foot as the force moves from heel to toe.

A person’s natural balance, stability and ‘proprioception’ (defined as ‘the ability to sense stimuli arising within the body regarding position, motion, and equilibrium’) are all affected and controlled by a number of factors. They are governed by visual feedback, as well as by feedback from muscle responses – a person’s ‘proprioceptors’ and ‘sensorimotor’ systems (of or relating to motor activity caused by sensory stimuli).

SATRA’s research

Figure 1: The Centre of Force trajectory recorded when the subject stood as still as possible for the duration of the test. Even when stationary some movement of the force lines is evident, showing that the subject made some small involuntary movements associated with maintaining balance

A number of recordings were made to demonstrate the principles of how we can begin to quantify instability. During the first assessment (the results of which are shown in figure 1), our subject stood on a pair of calibrated in-shoe pressure sensors and stabilised himself before any recording was started. He then tried to remain as still as possible during the course of the test (20 seconds). The equipment was set to record at 100Hz to capture small, quick and intuitive movements. To create recordings, the wearer stood on top of a pair of pressure mapping sensors placed on top of 10mm thick strips of ethylene vinyl acetate (EVA) of 0.32 g/cm3 density. This simulated the sole of a sports shoe but, unlike most sports soles, it was flat to allow the wearer to remain as still as possible (it had no heel or curved profile that might have induced instability). The subject wore socks, but no shoes and a 20 second pressure-mapping movie was recorded at a sampling rate of 100Hz. This created a pressure distribution map with corresponding Centre of Force trajectory trace.

Figure 2: The Centre of Force trajectory for a person standing in the same position as in figure 1, but swaying slightly from side to side, trying to minimise front to back movements. Despite this level of conscious effort to regulate body movement, there are still clear front-to-back Centre of Force movements that have been logged

In a second test, the subject was asked to stand still with closed eyes – the only one requiring this. A third test saw the subject swaying slightly from side to side, trying to minimise front-to-back movement (figure 2), and in a fourth test the subject made exaggerated stabilising movements with his or her arms while standing still (figure 3). The black lines running approximately along the heel to toe axis on the images generated in these tests represent the movement of the Centres of Force. It is the varying length of these lines that can be measured and used to quantify instability.

Figure 3: The Centre of Force trajectory for a recording of a wearer making exaggerated stabilising movements with their arms whilst standing on the spot. Even though the feet were kept in the same place, the system has recorded the movement of the Centre of Force, reflecting the shifting of their weight and flexing movements made by the balancing muscles

To analyse the images generated by such procedures, the vertical distance between the big toe and the heel of the right foot is set as a calibration length equal to 100 (the units are irrelevant). It is then possible to take measurements of the movement of the Centre of Force and express them in terms of percentage of the foot length. This can be done both for the ‘y’ axis (for medial movements) and for the ‘x’ axis (for lateral movements), to quantify the degree of movement and instability exhibited by a person in a particular pair of shoes or boots.

It was seen that there is a definite increase in the magnitude of medial (front-to-back) movements of the Centre of Force when test subjects experience instability, even while standing still, as they subconsciously create micro-movements to maintain balance and regulate posture.

When standing still, closing the eyes had the effect of increasing medial movement of the Centre of Force by 190 per cent in the right foot and by 40 per cent in the left foot. This increased medial movement is an excellent indicator of instability. By blindfolding test subjects to deliberately exaggerate their instability in test footwear, any increased medial movement attributable to the footwear is exaggerated, making it easier to observe and quantify any instability promoted by the footwear.

Other indicators

SATRA has a range of other facilities that can be used to characterise factors of a subject’s gait, including stability.

High-heeled shoes about to be assessed for their stability

For example, the use of an energy-expenditure system to monitor the heart rate and activity level of a subject, which can be useful as another indicator of insecurity and instability while walking. As well as logging heart rate, the system also has a built-in accelerometer which logs movement of the chest, related to respiration, and number and frequency of footsteps, which is processed through software to provide a calculation of calorific energy expenditure. It is logical that a person walking in unstable or uncomfortable footwear would have to exert more energy than a person wearing comfortable, stable footwear. While the system we use is not sensitive enough to discriminate the small, intuitive movements made to maintain balance, it is possible to observe and record increases in a subject’s heart rate in addition to overall movement, brought about due to the increased effort of wearing unstable shoes.

As well as measuring a subject’s Centre of Force trajectory in a variety of fairly static tests, we can also record CofF from a subject negotiating an uneven or stepped surface while wearing test shoes. A comparison of how their Centre of Force trajectory differs from that recorded during a test walk across the same surface when wearing standard or control shoes will tell a skilled operator a lot about the footwear, and its effect on the gait of the wearer. Test exercises like these can be supplemented by the use of additional resources, such as our high-speed camera, or the heart rate and motion logger mentioned previously.

There are also other, less technical approaches to assessing the impact of a subject’s footwear on their stability. Wearers can be stood on an unstable platform such as a balance board used in a gym, and asked to try to achieve a balanced equilibrium as quickly as possible. The time taken for the subject to come to rest could be recorded and used to judge his degree of fine motor control, which would be affected by the factors such as the contours of the shoe soles and the size and style of any heel, as well as the overall fit of the shoe.

Where stability is a desirable quality in a sample of footwear, its performance can be validated by means of a protocol that incorporates some or all of the elements discussed here.

In conclusion, this kind of research and testing should be of interest both to manufacturers who wish their shoes to create a sensation of instability in the wearer and those who would like their footwear to be as stable as possible, subject to the limits imposed by the style (for example, high-heeled shoes).

How can we help?

Please email research@satra.com for further information on the assessment of the stability of footwear.

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