What is slip resistance?

Describing the important principles behind footwear slip.

by Tom Bayes

The onset of a slip event is brought about by a loss of ‘traction’ (also described as ‘grip’) and what happens after this is important. A lack of traction is clearly an important factor. Certainly, when aware that the surface they are walking on is slippery, wearers will modify their gait accordingly and take great care with each step. They will be more positive than usual in placing their feet, even testing the traction before taking their next step.

Excessive traction can also be an issue and can result in a tripping event if the footwear grips very strongly on the surface. Most wearers of occupational footwear will want to walk with a confident gait, as they normally have a set time in which to perform their task – except for when they are in visually recognised hazard areas. Therefore, when walking on a ramp that may ultimately lead to a slip, wearers are likely to modifying their gait accordingly, even if in a test environment in which the subject is attached to a safety line to prevent any mishaps or actual injury.

In general, slips and trips suddenly materialise. They are unexpected and accidental events for which a person is unprepared and these often result in a fall and some related injury. These events can lead to very serious consequences. They can leave the wearer incapacitated and unable to move – for example, after breaking a hip – perhaps even unable to call for help. This can be especially serious in cold environments.

Some definitions

Let us consider some definitions on which the experts at SATRA have agreed. ‘Traction’ is a measure of static friction, and there are many traction tests. These involve planting the footwear purposely and stably on a known surface (knowing it is slippery) and exerting a horizontal force without moving.

As mentioned, the footwear’s loss of friction with the surface is quite often sudden and unexpected. Even if stationary on a ramp, assuming that the normal force vector will be vertical (of the body mass), there will be a horizontal force of some kind generated. The event can be very sudden, lifting the feet from the ground.

Of course, relying on traction while, for example, walking up a ramp will put the onus on the forepart of the tread pattern. The design of tread patterns is important. Many patterns are designed to assist forward locomotory forces, with backward-facing saw tooth designs being very good at achieving this and commonly seen in the athleisure market. However, when the wearer is walking (or running) down a ramp, some designs of tread will clearly fall short in slip resistance performance.

Slip resistance

‘Slip resistance’ is not traction – it is not a static event and so is very different. It is a dynamic measure of the friction (involving movement) that occurs once a slip event has taken place and is in progress. The concept of slip resistance is actually quite simple, yet is often misinterpreted and misunderstood.

The slip resistance of a sole unit is its ability to attenuate and halt a slip event once it has occurred. Traction is how well the sole grabs the surface, slip resistance is all about what happens following the failure of traction and the event is occurring. The concept is that a slip progression will be determined by the dynamic ‘coefficient of friction’ (CoF).

This is entirely different to the static friction measure (the traction). The measurement is made by forcing a slip event to happen and then measuring the CoF during that slip, dynamically, over a pre-determined time while the slip event is in progress. SATRA has spent many years determining this value, as well as a higher limit guideline to prevent sole units becoming tripping hazards (such as catching a high-traction sole on any surface which is not slippery).

SATRA TM144

The SATRA TM144:2021 – ‘Friction (slip resistance) of footwear and floorings’ test method is regularly reviewed in light of new innovation and new materials to ensure it is fit for purpose. As a result, it has been adopted in many international standards. Any surface can be used – including carpet, laminate flooring, different versions of ice, oil, powder contaminates and even a mud simulant.

The ice test is worth a further mention. Many polymers are susceptible to the cold, as they get harder with a drop in temperature. Hardness of the polymers is a very important factor in slip resistance. These materials are purposely cooled in a shallow water bath to just above freezing prior to testing to ensure a hardness in context with a cold environment.

It is important to reiterate that SATRA TM144 tests the ability of the sole and tread to attenuate a slip event and control it to reduce the prospect of injury. Rather than being a ‘traction’ test, it determines what happens when traction is lost and the subsequent effects of this situation.

Gait

Gait is heavily involved in slip resistance. Traction is one side of the discussion – for example, trying to move a heavy load such as a delivery box or mobile rack which requires horizontal forces to be manually applied involving certain and confident contact with the walking surface. However, the majority of occupations simply require walking from one location to another. Unless the person is infirm, this is done quite casually and confidently.

It is true that we visually recognise walking hazards, spills, ice and areas that may require some extra care, and we subsequently alter our gait accordingly. Nevertheless, the vast majority of accidents occur when we are not even aware of the danger.

Most occupations need confident mobility, but a confident gait when approaching a slip or trip hazard can be disastrous. This is exactly why the SATRA TM144 test method and the STM 603 slip resistance tester are designed the way they are – to recreate unknown and unforeseeable events and measure the behaviour of the footwear sole and tread pattern to attenuate the incident so that no injury results.

In contrast to SATRA TM144, ramp-based tests have their place, but these are traction-based. While such tests are valuable, the results they produce will not correlate with a dynamic and forced slip event, which would be in a different mode, with the injuries that occur mostly as the result of not being able to control the subsequent slip event.

Publishing Data

This article was originally published on page 10 of the November 2025 issue of SATRA Bulletin.

Other articles from this issue »

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