Head Injury Criterion (HIC) is the most comprehensive single metric for evaluating traumatic brain injury risk from falls onto athletic surfaces. It appears alongside Gmax in field certification requirements and independent test reports, yet it is less frequently understood by field owners, specifiers, and procurement teams.
HIC is not a simpler version of g-max, nor is it interchangeable with it. The two metrics measure different aspects of the same impact event, and a field can produce an acceptable g-max while still generating a concerning HIC value. Understanding the difference between the two, and why both matter, is essential for anyone responsible for specifying, procuring, or managing a synthetic turf field.
What HIC Measures
HIC measures the probability of traumatic brain injury resulting from a fall onto a surface. It is calculated from the full deceleration curve recorded during a standardized impact test, not just the peak value. Specifically, HIC integrates the area under the deceleration-time curve over the interval that produces the highest calculated value, weighted by the duration of that interval.
In practical terms, HIC accounts for both the magnitude of the impact force and the duration over which that force is applied. A short, sharp impact and a longer, lower-magnitude impact can produce the same g-max but very different HIC values, because HIC captures the time dimension of the impact event that g-max does not.
How HIC Differs From Gmax
g-max and HIC are both derived from the same drop test and the same deceleration curve. The difference is in what each metric extracts from that curve:
g-max— Peak Value Only
g-max takes the single highest point on the deceleration curve. It tells you how hard the surface hit back at its worst moment — but nothing about how long that moment lasted.
HIC – Curve Integration Over Time
HIC calculates the area under the deceleration curve over the interval that produces the maximum result. It tells you how much total energy was transmitted to the head over the duration of the impact, a more complete picture of injury risk than peak force alone.
A surface with a Gmax of 160g and a very brief peak produces a lower HIC than a surface with the same g-max but a broader, flatter deceleration curve. Both pass the g-max threshold, but they represent meaningfully different levels of head injury risk.
How HIC Is Tested
HIC is measured using the same standardized drop test protocol as g-max, ASTM F355 and EN 1177. The same missile drop produces both metrics simultaneously. Accelerometers record the full deceleration profile, from which both the peak value (Gmax) and the integrated curve value (HIC) are calculated.
As with g-max, HIC testing must be performed by ISO 17025-accredited independent laboratories to be considered independently verified. Act Global publishes HIC results from Firefly Sports Testing, Labosport, and Sports Labs exclusively.
HIC Thresholds by Standard
ASTM F1936 – General Athletic Surfaces
Maximum HIC: 1,000
This is the baseline threshold for athletic surfaces in the United States. A surface producing HIC above 1,000 is considered unsafe for athletic use under ASTM standards.
FIFA Quality Program
Maximum HIC: 1,000 for FIFA Quality
Maximum HIC: 1,000 for FIFA Quality Pro
FIFA applies the same HIC threshold across both certification levels, consistent with ASTM F1936.
Critical Fall Height
Critical fall height is a related metric derived from HIC testing. It defines the maximum height from which a person can fall onto a surface without exceeding HIC 1,000. It is particularly relevant for multi-sport fields and recreational surfaces used by children, where fall heights from play equipment or aerial athletic movements are a design consideration.
Why HIC Matters Beyond g-max
In the context of growing attention to head injury risk in sport, particularly concussion research across football, soccer, and rugby, HIC provides a more nuanced and defensible safety metric than g-max alone.
A procurement specification that references only Gmax thresholds is incomplete. A field can pass g-max at 180g while producing a HIC value that approaches or exceeds 1,000 depending on the shape of its deceleration curve. Specifiers, architects, and field owners who reference both metrics in their procurement documents are applying a more rigorous and defensible standard of care.
HIC and Lifecycle Management
Like g-max, HIC is not static. The same mechanisms that drive g-max upward over time, infill compaction, fiber flattening, base settlement, affect the shape of the deceleration curve and therefore HIC values. Annual independent testing that captures both g-max and HIC provides a more complete picture of surface safety than g-max alone.
Act Global Perspective
Act Global includes HIC as a standard output in all independent laboratory testing, not as an optional metric. Every published test report from Firefly Sports Testing, Labosport, and Sports Labs includes both Gmax and HIC values, because both are necessary to characterize surface safety completely.
Act Global systems are engineered to produce HIC values well within ASTM F1936 and FIFA Quality thresholds throughout the field’s service life, not just at installation. System-level specification of fiber density, infill type and depth, shock pad, and base design are optimized together to manage both peak force and impact duration.
Field owners and specifiers who request Act Global test data will receive complete deceleration curve data including both Gmax and HIC values from ISO 17025-accredited independent laboratories. This data is available through Act Global’s Test Report Center.
Frequently Asked Questions
What is a safe HIC value for a synthetic turf field?
ASTM F1936 and FIFA Quality standards both set the maximum acceptable HIC at 1,000. Fields producing HIC values below 700 are generally considered well within the safe range. Values between 700–1,000 are technically compliant but warrant monitoring, particularly as the field ages and infill compacts. Values above 1,000 indicate an unsafe surface under current standards.
Can a field pass g-max but fail HIC?
Yes. This is one of the most important practical implications of understanding the difference between the two metrics. A surface with a g-max of 175g, within FIFA’s 60–180g range, can still produce a HIC above 1,000 if the deceleration curve is broad enough. This is why both metrics should be specified and tested independently, not treated as interchangeable.
Is HIC tested separately from g-max?
No, both metrics are derived from the same drop test. The same missile impact produces the full deceleration curve from which both g-max and HIC are calculated. Requesting both metrics adds no additional testing cost or time, it simply requires that the laboratory report both values from the same test data.
How does infill type affect HIC?
Infill type and depth significantly affect the shape of the deceleration curve and therefore HIC values. Denser, less compressible infill materials tend to produce sharper, higher-peak curves, higher g-max and potentially higher HIC. More compressible infill materials produce broader, lower-peak curves, lower g-max and lower HIC. The interaction between infill, fiber, and shock pad determines the final curve shape, which is why system-level specification and testing is essential.
Should HIC be included in synthetic turf procurement specifications?
Yes. Any procurement specification for a synthetic turf athletic surface that references only g-maxis incomplete. Including HIC thresholds, at a minimum, ASTM F1936’s limit of 1,000, provides a more rigorous and defensible safety standard. Specifiers, architects, and facility managers who include both metrics in their procurement documents are applying current best practice in athletic surface safety specification.
Related Resources
What Makes Synthetic Turf Safe? A Science-Based Definition
Gmax Explained — Surface Hardness and Impact Attenuation
Rotational Resistance & Traction — Lower-Extremity Injury Risk (coming soon)
Act Global Test Report Center
The content in this article reflects Act Global’s interpretation of publicly available independent test data, ASTM standards, FIFA Quality Program documentation, and peer-reviewed research on athletic surface safety. It is provided for educational purposes only and does not constitute medical, legal, or engineering advice. HIC thresholds cited reflect published standards as of the date of this article, refer to the relevant governing body for current certification requirements. Refer to original sources and accredited testing laboratories for complete methodology and findings.
