The effect of grass leaf height on the impact behaviour of natural turf sports field surfaces

The effect of three grass leaf height treatments (50 mm, 25 mm, < 1 mm) of two sports field rootzones (clay loam, sand) was assessed under controlled conditions using the 0.5 kg and 2.25 kg Clegg Impact Soil Testers (CIST) and the Dynamic Surface Tester (DST) device. Results were dependent upon the test device, impact energy, and drop number of the impact. The presence of grass was shown to be more important than specific grass heights in regulating impact behaviour, with no differences detected between 50 mm and 25 mm treatments. Peak deceleration was reduced (P < 0.05) by the presence of grass (50 mm and 25 mm treatments) for drop one, but not drop three of the 0.5 kg CIST missile, indicating grass leaves absorb some impact energy on lower energy single impacts but not when leaves are flattened under repeated loading. There was no difference in peak deceleration of the higher energy 2.25 kg CIST among leaf treatments for first drop, but was significantly lower (P < 0.05) for third drop on the < 1 mm treatment where the soil exhibited greater (P < 0.05) plastic displacement. Surface loading rate and energy absorption did not differ across treatments under athlete-specific impact stresses measured with the DST, suggesting grass leaves may not affect athlete impacts. Greater consideration is required for future impact testing to assess surfaces to specific impacts that occur in game situations through the use of appropriate test devices.