When playground operators and facility managers talk about playground safety, the conversation quickly turns to technical terms: impact attenuation, HIC scores, and Gmax. These terms appear in Australian Standards, inspection reports, and surfacing specifications — but they’re rarely explained clearly.
This guide demystifies impact attenuation and the measurements used to quantify it, so you can make informed decisions about your playground surfacing and confidently interpret compliance documentation.
Impact attenuation is the ability of a surface to absorb and dissipate energy from an impact — specifically, the energy generated when a child falls from playground equipment and strikes the ground.
Think of it this way: when a child falls, they accelerate under gravity until they hit the ground. At that moment, the kinetic energy built up during the fall must go somewhere. If the ground is hard (like concrete or asphalt), almost all of that energy transfers into the child’s body in a very short time — resulting in a large impact force that can cause serious injury, particularly head injury.
An impact-attenuating surface — bark chips, rubber mulch, wet pour rubber — compresses under impact. This compression extends the time over which the energy is transferred, spreading the force over a longer period and reducing its peak. The result is a much lower peak deceleration force on the child’s head and body.
Australian Standard AS 4422 sets out the requirements for impact-attenuating playground surfacing, including the test methods and performance thresholds that surfacing must meet to be considered compliant.
HIC stands for Head Injury Criterion. It is a mathematical measure of the likelihood of head injury resulting from an impact, derived from the deceleration profile measured during an impact test.
During impact attenuation testing, a headform (a hemispherical mass that represents a child’s head) is dropped from a specified height onto the test surface. A triaxial accelerometer inside the headform records the deceleration over time during the impact event.
The HIC value is then calculated from this acceleration-time record using the following formula:
HIC = max { (t₂ – t₁) × [(1/(t₂-t₁)) ∫ a(t)dt]^2.5 }
In plain English: the HIC is calculated by finding the time interval during the impact that maximises the product of the average deceleration and the duration of that interval. It accounts for both the magnitude and duration of the deceleration — because both factors contribute to the risk of head injury.
Under Australian Standard AS 4422, playground surfacing must achieve a HIC value of 1,000 or less when tested at the critical fall height of the equipment it is protecting.
A HIC of 1,000 corresponds approximately to a 5% probability of severe head injury in adults. For children, whose heads are proportionally heavier and whose skulls are less mature, the threshold provides a meaningful safety margin.
To put this in context:
Concrete, for example, typically produces HIC values in the tens of thousands at playground fall heights — which is why it is entirely unsuitable as playground surfacing.
Gmax is the peak deceleration recorded during an impact test, expressed as a multiple of gravitational acceleration (g). It represents the maximum instantaneous force experienced during the impact.
Under Australian Standard AS 4422, the Gmax of playground surfacing must not exceed 200g when tested at the critical fall height.
Both Gmax and HIC measure aspects of impact severity, but they capture different things:
A surface could theoretically have a low Gmax (no single extreme spike in deceleration) but a high HIC (sustained elevated deceleration over a longer period) — or vice versa. This is why both measurements are required under AS 4422. Compliant surfacing must meet both the HIC ≤ 1,000 and Gmax ≤ 200g thresholds simultaneously.
Force attenuation is a related but slightly broader concept. While impact attenuation specifically refers to the ability of a surface to attenuate the impact forces during a fall, force attenuation can refer more generally to the reduction of forces through a system — including through the body itself, through equipment components, or through surfacing.
In the context of Australian playground standards and surfacing, impact attenuation and force attenuation are often used interchangeably, with impact attenuation being the more technically precise term.
Impact attenuation testing under AS 4422 follows a specific procedure. There are two contexts in which testing is conducted:
Type testing is conducted in a laboratory setting to characterise the performance of a surfacing material. A headform of specified mass is dropped onto a prepared sample of the surfacing material from a range of heights. The resulting HIC and Gmax values are recorded, building a performance profile across different drop heights and surface depths.
Type testing is used by surfacing manufacturers to establish the performance claims for their products and to determine the minimum depths required to achieve compliance at various critical fall heights.
Site testing is conducted on installed surfacing to verify that it performs as required in its actual installed condition. This is particularly important for loose-fill materials that may compact over time, as well as for any surfacing that has experienced significant use or weathering.
Under Australian practice, site testing using portable field impact test equipment is conducted by qualified playground inspectors as part of comprehensive playground inspections. The results confirm (or otherwise) that the installed surfacing continues to meet the HIC and Gmax requirements at the critical fall height of the equipment it protects.
The impact attenuation performance of playground surfacing is not static — it can change over time and under different conditions. Key factors include:
For loose-fill materials, impact attenuation performance is directly related to depth. As the material compacts and is displaced by use, the effective depth decreases — and with it, the impact attenuation performance. Regular depth checks and top-ups are essential for loose-fill materials.
Loose-fill materials can compact due to use, rainfall, and the weight of the material itself. Compaction reduces the ability of the material to deform under impact, increasing HIC and Gmax values. High-use areas directly under equipment (particularly under swings and at the base of slides) tend to compact fastest.
Water content significantly affects the impact attenuation performance of some materials, particularly sand. Wet sand is significantly harder (higher Gmax and HIC) than dry sand, and dry sand that has been disturbed can be very loose and uneven. The moisture content at the time of testing should be noted and considered when interpreting results.
For rubber-based surfacing (wet pour, rubber tiles, rubber mulch), temperature affects the material’s stiffness and therefore its impact attenuation performance. In Australian conditions, where summer temperatures can be extreme, rubber surfacing can become significantly stiffer in hot conditions, potentially affecting its HIC and Gmax performance. This is why Australian-specific testing in relevant temperature conditions is important.
Over time, surfacing materials can degrade. UV exposure, temperature cycling, and biological activity (particularly for organic materials like bark) all affect material properties. Regular inspection and periodic impact testing are important to ensure ongoing compliance.
Understanding impact attenuation has direct practical implications for how you manage your playground:
When designing or refurbishing a playground, ensure that the specified surfacing has been type-tested and shown to comply with HIC ≤ 1,000 and Gmax ≤ 200g at the critical fall height of your equipment. Ask your supplier for test data — not just a claim of compliance.
For loose-fill materials, establish a regular depth-checking regime. Many operators check surfacing depth as part of their monthly routine inspection. Have a maintenance plan that includes topping up materials when depth falls below minimum requirements.
Annual comprehensive inspections, conducted by a qualified playground inspector with portable impact test equipment, will verify that your surfacing continues to perform as required. Don’t wait for a visible problem to commission testing.
High-use areas — directly under swings, at slide exits, and at the base of popular climbing structures — are where surfacing compacts fastest. These areas may need more frequent top-up between annual inspections.
At Kidzspace, with over 10 years in the industry and more than 1,000 playground projects across Australia, impact attenuation is central to every surfacing recommendation we make. Our approach includes:
The Kidzspace team can help you specify the right impact-attenuating surfacing for your playground — one that complies with AS 4422, suits Australian conditions, and fits your budget.
Copyright © Kidzspace. All Rights Reserved.
Copyright © Kidzspace. All Rights Reserved.