g-max Testing - An Overview
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For a comprehensive overview, read on...
When a player falls, the impact is absorbed either by the
surface or the player's body. The "harder" the surface, the greater the
amount of the
impact absorbed by the player's body; and, not surprisingly,
greater the likelihood the fall will
result in injury. This is
especially true with respect to concussions - traumatic
injuries to the brain - which can occur when the player's head hits the
playing surface. Concussions are an insidious form of injury. The
effects of individual concussions are most often fairly mild. In the
case of multiple concussions, however, the effects can be cumulative
and the consequences can be long lasting - even permanent.
Additionally, if a player has already experienced one concussion,
they are more likely to experience another; and subsequent concussions
carry a much higher risk of serious injury - up
to, and including, death. If for no other reason than to lessen
players' exposure to this type of injury, it is important to monitor impact-related characteristics of sports surfaces.
Impact testing (commonly referred to as g-max testing)
measures the shock-attenuation performance of sports surfaces -
including synthetic (artificial) turf and natural turf athletic fields. Test results (g-max values) are a ratio: the ratio of the maximum acceleration
(deceleration) experienced during an impact, to the normal rate of
to gravity. The higher the g-max value, the poorer the shock-attenuation performance of the surface. Measuring impact-attenuation is a fundamental tool of
athletic field safety
testing. It is also useful in assessing the playability of
Testing g-max values involves measuring the shock-attenuation performance of a
playing surface in situ,
and comparing the results against a standard. The most commonly used
standard is the one established by ASTM International (ASTM).
surfaces, ASTM requires that the reported g-max value for every
test point be less than 200g's (as measured in
accordance with ASTM Standard F355-A and ASTM Specification F1936). If the requirement
met, the field is
considered unsafe and remediation is needed.
While the maximum threshold value of 200g's is the de facto
standard for evaluating the safety of an artificial field,
architects set different standards for the fields they design. These
alternate standards typically involve a range of acceptable g-max values. The range sets an
upper limit that addresses safety, and a lower limit
that focuses on
playability; the operative assumption being that fields that are too
"hard" are dangerous, while fields that are too "soft" contribute to
excessive fatigue and poor player performance. The range of acceptable
values will vary in accordance with the materials and methods used to
build a field, and also as a matter of the architect's personal
often should you test?
It is important to note that g-max measurements are not static. On
synthetic turf surfaces, g-max values will generally increase as the
field ages. The amount of the increase, and the rate at which it
occurs, are tied to construction methods, materials used,
environmental conditions, and the frequency and types of maintenance.
On a natural turf field
these same factors are at work, along with the moisture content of the
soil, the type of turfgrass, and the density of coverage. This suggests
that regular g-max testing is important - but how often is often enough?
Everyone agrees that new synthetic fields should be tested before they
are accepted from the builder. Beyond that, opinions differ: some
advocate testing at least twice a year; some suggest an annual
test; others say that testing can be less frequent. Among
researchers and other industry experts, there is a general
consensus that an annual test is most desireable:
- Annual testing demonstrates a serious and proactive
- Fields rarely become unsafe overnight, so testing once a
year should provide adequate warning of emerging safety issues
- Significant changes in g-max readings can be an early sign
problems are developing within the turf system - an annual test can
alert you to these problems before they become critical
- Annual testing generates a meaningful historical record -
a record that can be important if warranty or liability issues arise
- Testing once a year isn't a "budget buster"
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If you make the commitment to have your synthetic field(s) tested, you
should also commit to having the tests done properly:
- You should insist on using the services of
independent test provider. While many manufacturers will offer to do
the testing, there is an inherent conflict of
interest if they do it themselves.
- Your test provider should be knowledgeable of, and in
compliance with, applicable ASTM standards. Your provider should be
to show you copies of current standards and discuss them with you.
- Equipment used to test the field should
to ASTM Standard F355-Procedure A (F355-A) which requires
a cylindrical missile with a circular, flat, metal impacting
surface. The weight of the missile must be 20lbs (+/- 0.11 lb).
- The accelerometer used in the test equipment should
have a current calibration certificate, and the calibration should
be NIST traceable.
- Procedures used to test an artificial field,
plus the number and location of individual
conform to ASTM Specification F1936.
- Do not accept values determined with a "Clegg" tester -
on a natural turf field tested in accordance with ASTM Standard F1702.
Both a Clegg tester and a "F355" tester can be used to calculate g-max
the numbers are not
comparable. (Be wary of anyone who tells you they are!)
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But what does this mean
- As the owner
of a field, you have an obligation to provide safe facilities. Where
standards exist, you should insure that your facilities are in
compliance and that you and your organization aren't exposed to
unnecessary liability if an injury occurs.
- As an athletic
director or coach, you have a duty of care to the
athletes under your supervision. Your duty of care
encompasses the obligation to make sure the facilities you use are
inherently safe and properly maintained.
- As a concerned
parent or player, you owe it to your child
and/or yourself to know that a field has been built in accordance with
established standards. You also want to know that it is
regularly tested, by an
independent test provider, to insure continuing compliance with those
- As an architect
you should insure that g-max standards for safety and playability are
clearly defined in the specifications you write. You should require
that all tests of synthetic surfaces be conducted by an independent test provider, in accordance with ASTM standards F355-A
and F1936. And, you should
specify that fields be evaluated prior to acceptance and at least annually throughout the warranty period. Click here to view a sample specification in pdf format. (You will need Adobe Reader to view this file.)
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