When we talk about “how a surface feels” or “how safe it is,” we are really talking about impact mechanics. How energy is absorbed, transmitted, and returned when the body meets the surface. In practice, the sports surface industry relies on three main tiers of tests. Each provides useful information, but each answers a different question.
Direct impact and compaction tools such as the Clegg hammer
These are simple portable devices that drop a known mass and record peak deceleration to yield a single number.
What they do well: Track changes in the surface over time such as moisture, compaction, and maintenance effects. They guide grounds teams in day-to-day management.
What they do not do: Represent athletes. They do not replicate human load profiles or joint mechanics.
Think of them as “trend thermometers” for the surface. The Clegg test was first developed by Dr Baden Clegg at the University of Western Australia in the late 1960s and is now widely used in soils, pavements, and turf or grass contexts.
Uncontrolled fall safety tests such as Gmax and HIC
These answer the question: If a player or child falls and their head or body hits the surface, how severe is the impact?
Gmax is most common in the USA for sport. It measures peak deceleration and sets an upper compliance limit.
HIC or Head Injury Criterion integrates the acceleration and time curve to estimate head injury risk and define a critical fall height.
These are safety and compliance tools, not performance tools. Importantly, HIC is the only sports surface test with epidemiological data linking results to actual injury outcomes. This makes it uniquely valuable for welfare-driven standards especially in playgrounds and Rugby.
Player like impact tests such as the Artificial Athlete and Advanced Artificial Athlete
To approximate a runner’s footstrike we use tests that mimic the spring and mass behaviour of the lower limb.
The Artificial Athlete (AA) and its successor the Advanced Artificial Athlete (AAA) apply a spring and mass drop at a controlled energy.
They report shock absorption in percent, vertical deformation in millimetres, and sometimes energy restitution in percent (but this is not very well excepted due to inaccuracies and relevance).
This category sits closest to the real loads and contact durations experienced by athletes. It gives the most relevant mechanical proxy for on-foot feel and comfort.
Load rate dependence: why rankings can flip between tests
Sports surfaces are viscoelastic. Their behaviour depends on how fast they are loaded, how much force is applied, and the contact area.
This means a surface that feels soft under AAA may register as relatively hard under Gmax or HIC. Different test masses, energies, and pulse durations probe different aspects of surface response. This is not a contradiction but a reminder that method selection matters.
For that reason, a suite of tests is always recommended when designing systems to capture the multi-dimensional behaviour of the system.
Because of these viscoelastic properties, different players will also experience the same surface differently. Variations in body mass, running style, speed, and footwear change the way the surface is loaded, which is why athlete-centred testing is essential.
Note: Attempts to correlate the results of different devices are rarely helpful. Where correlations do exist they are usually only valid across a narrow operating range and not over the full spectrum of surface performance (in other words please don’t use Clegg or Gmax results to “predict CFH” its inaccurate and dangerous)
Practical takeaways for specifiers and operators
- Match the tool to the question.
- Expect method dependent rankings and classifications
- Use multiple tests, not a single number when validating systems in the lab or in the field
- MOST IMPORTANTLY: Close the loop with athletes.
Through my own research working directly with athletes, I have been able to capture insights into player–surface interaction that no mechanical device alone can reveal. These player-centred findings feed directly into surface design and deliver benefits that are completely missed if we rely only on simple impact testing.
Bottom line
All these tests are good and necessary. They are the foundation of compliance, maintenance, and surface monitoring but they should not be used in isolation. They are only the start of the question and not the answer. The only way to truly validate surface quality and safety and to deliver both what athletes want and what they need is through in-depth athlete-based testing
Note: In recent years, a wide range of new testing systems have been developed. These devices and services often use similar approaches and technologies to the established methods referenced in the article.
