Using the STM 528 Pedatron to simulate running

Revealing how SATRA has developed additional valuable uses for its Pedatron test machine.

by David Smith

SATRA originally developed the STM 528 Pedatron test machine to provide an alternative to user wear trials over an extended period of time for soling wear assessment. However, with the benefit of SATRA’s ongoing research programme, as well as extensive Pedatron testing experience and developments in the machine, it is now also used to assess more features of footwear in addition to soling wear.

In order to achieve a realistic wear assessment, Pedatron has been developed to closely replicate the action of walking, with parameters determined from biomechanical studies of walking gaits. The introduction of improved artificial foot forms (figure 1) gives a closer correlation with underfoot pressures measured from actual walking gaits. In addition, these foot forms reproduce wear characteristics on internal footwear components typical of those found in products due to user wear.

The SATRA Pedatron test is a whole shoe test. It reproduces the flexing, shear loads, compression shock and turning motions which occur in footwear use. Under the influence of these loading conditions, the effect of the shoe construction on the soling wear can be demonstrated. The repeatable nature of a Pedatron test allows for the assessment of how alternative constructions – for example, in the upper or the method of assembly – influence soling wear.

Visitors to SATRA seeing the Pedatron in action often ask what else it can do – whether it can be made to run, walk uphill, or recreate the gait of different subjects, such as women, children, or walkers of heavier stature.

While the current machine cannot be programmed with different gaits, it can be adjusted to reproduce the forces associated with a variety of gaits, and set up to allow a variety of novel test scenarios, including running.

In order to establish what changes could be made to the Pedatron to recreate the effects of running, it was necessary to consider the effects on lower leg biomechanics of walking or running at speed. This raised a number of questions. Obviously, the rate of footstrike is quicker, but are the underfoot forces correspondingly higher? What happens when we run? What is it about running that more quickly abrades or damages footwear?

It is not necessarily the speed involved, although obviously a greater number of steps in the same time period will accelerate any wear. It is more likely to be the increased forces and pressures underfoot, caused by landing heavily and pushing off with correspondingly greater force, as well as more pronounced flexing of the footwear.

SATRA research

SATRA has conducted research into running gait and measured the forces created underfoot by runners. In order to determine the forces that would be desirable to reproduce with the Pedatron, gait tests were carried out – both shod in running footwear and barefoot – in order to measure underfoot forces in both conditions, using subjects in a range of bodyweights running across a tri-axial force plate.

Measurements were taken from the whole foot, heel and forepart, which were then related to subject bodyweight. Forces were calculated and expressed in terms of a factor (multiplier) of the subject’s bodyweight. Data from all subjects was then compared to force data previously obtained from the Pedatron’s default gait, to establish what differences exist, and identify how the Pedatron might be altered to create a different force pattern.

The most noticeable difference between human and machine force profiles is the timing. The gait cycle (heel strike to toe-off) of the Pedatron is five times the length of a running human gait cycle. As yet, it has not been considered practical to increase the speed of the Pedatron, so we continue to keep a constant, controllable and repeatable pace, and focus instead on accurately reproducing the forces applied during running. In particular, forces under the forepart of the foot increase dramatically during a running gait.

Adjusting the Pedatron’s gait

Recently, SATRA researchers carried out some experimental work to adjust the action of one of our Pedatrons in order to create a more toe-dominant gait pattern. This was in response to an enquiry from a customer, who wanted SATRA to simulate the increased wear on footwear likely to be encountered by regular cross-country runners, particularly when running up or down hills.

Three approaches were considered as means of increasing the underfoot forces in the gait of the Pedatron: i) adding more weight to the leg shaft, ii) adjusting the pressure of the air supply to individual pneumatic cylinders within the machine’s mechanism, or iii) fitting angled adaptors to the prosthetic foot being used.

Previous experiments in increasing the Pedatron’s force by physically adding weight to the leg shaft of the machine showed that this approach increases forces underfoot at all parts of the gait cycle – heel, mid-stance and toe-off. Increasing the forces applied by the pneumatic cylinders in the system (see figure 2) by increasing the pressure of the air supplied to them allows us to fine-tune forces at different stages of the gait cycle.

SATRA now routinely uses realistic prosthetic feet on the Pedatron, which provide lifelike wear and abrasion on any footwear tested. Being prosthetics, and designed for use by amputees with very differing medical needs, a range of accessories are available to customise their mounting and positioning. Using angled adaptors for the prosthetics, the angle of the foot relative to the leg shaft can be altered so that the foot meets the surface at a steeper pitch, either heel-down or toe-down.

Using a combination of an angled adaptor, an increase in the air pressure in one pneumatic cylinder and the addition of an angled adaptor to the Pedatron’s prosthetic foot causes it to tilt forward relative to the floor. Increasing the pressure of the compressed air supply to the cylinders driving the foot has the effect of changing the angle at which the sole first meets the floor. This firstly changes the angle at heel strike, lowering the relative height from the toe to the floor, giving greater emphasis on the toe or forepart area of the foot. Such adjustment also increases the horizontal angle between the sole and the floor surface at toe-off as well as the time during which the foot is in contact with the floor.

The machine was also adjusted so that the height from shoe to floor was set and measured from the toe rather than from the heel as normal. In addition, the internal angle to which the footwear is flexed can be altered by the use of angled adaptors which increases the flexing angle. This makes the test more aggressive by increasing the strains on the upper.

Through this series of adjustments, the vertical component of the underfoot forces under the forepart of the prosthetic foot on the Pedatron was significantly increased. The toe force was successfully increased in this way to almost 1,400N.

So, while the current version of the Pedatron cannot yet recreate the speed of a running subject, we can alter the contact and flexing angle of the foot and increase the forces exerted by the Pedatron so that they are representative of those exerted by a runner. When the forces from both runners and Pedatron were analysed, with reference to the multiplying factor of bodyweight, it was seen that our adjusted toe-dominant Pedatron could be said to mimic the underfoot forces of a runner weighing around 70kg. As the deadweight attached to the ‘leg’ shaft of the Pedatron is 75kg, this force profile is quite acceptable.

Publishing Data

This article was originally published on page 42 of the May 2016 issue of SATRA Bulletin.

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