Sports Testing 103: Vertical Deformation

This is from a series of publications on

The third property I want to introduce in this series is Vertical Deformation. This article outlines the methods and calculations used to evaluate the Vertical Deformation property of a sports surface. This property is considered important in the design and selection of  of basketball, volleyball, track and dance surfaces because it is thought to relate to foot stability. Soccer fields are often tested for vertical deformation but they use a different method and the results have a different meaning than the one covered in this paper. Vertical deformation is thought to be related to foot stability. It is also thought to be associated with elevated torque levels during pivoting on synthetic indoor sports surfaces.

While force reduction is an attempt to simulate impacts during the ‘passive’ phase of a landing, vertical deformation is designed to examine the deflection of the surface during the entire impact which includes both ‘passive’ and ‘active’ response periods. In case you’re interested the absolute maximum force normally occurs during the active phase of a landing.

There is great harmony in the standards with regard to this property. This means that conducting the test according to DIN 18032-2 (2001) will yield valid results for ASTM F2772, EN 14904, ANSI E1.26, and the FIBA™ (2014). These same results are used by the MFMA™ in their PUR™ certifications. The methods and equipment contained in these two standards have only been slightly modified from DIN 18032-2 (1991).

During this test a 20 kg mass is  dropped onto a spring from a height of 120 mm. This spring is much softer than the one used for force reduction (40 N/mm compared to 2,000 N/mm). The softer spring is thought to better represent the active portion of a landing. The spring rests on an impact foot. That impact foot has two horizontal projections that allow a displacement sensor (LVDT) to measure the deflections of the test foot during the impact. The photo below is from a research project involving deflection testing of an infilled synthetic turf system.

The signals from these two LVDT’s are averaged and they yield the deflection at the impact point. The force generated during the impact is also recorded. The maximum force generated during the first impact and the maximum deflection generated during that impact are used to compute vertical deformation using the following equation

This equation normalizes the deflections generated during the impact so that (VD) represents the vertical deformation that would occur at 1500 N or 337 lbs. Force Reduction and Vertical Deformation are generally related such that when Force Reduction levels increase the Vertical Deformation level tends to increase.

The most serious limitations to this method, and frankly to the force reduction test, are the relatively light mass and low energy input into the surface. This standard only involves a mass of 20 kg, and a drop height of 120 mm (4.75″). It is certainly possible for a floor system to bottom out or reach a point where it takes very large forces to create even small deflections under real loads. These low mass/energy impacts may not predict actual biomechanical response, but they provide a repeatable, portable, non-destructive basis for comparison and thus I believe they have value in differentiating sports surfaces.

A follow up article exploring the requirements for Vertical Deformation by various standards and governing bodies is being developed.Check back for that information if you’re interested in learning more.

If you are looking for more detailed information regarding this property please visit ASET Services’ library. Contact ASET Services if you want to know how your new, or old, sports surface is performing ( or [email protected]).

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