Iain T. James

Natural turf surfaces: the case for continued research

It is well documented that health and social benefits can be attained through participation in sport and exercise. Participation, particularly in sports, benefits from appropriate surface provisions that are safe, affordable and high quality preferably across the recreational to elite continuum. Investment, construction and research into artificial sports surfaces have increased to meet this provision. However, not all sports (e.g. golf, rugby and cricket) are suited to training and match play on artificial turf without compromising some playing characteristics of the games. Therefore, full sport surface provision cannot be met without the use of natural turf surfaces, which also have an important role as green spaces in the built environment. Furthermore, a significant number of people participate in outdoor sport on natural turf pitches, although this is a declining trend as the number of synthetic turf surfaces increases. Despite natural turf being a common playing surface for popular sports such as soccer,rugby and cricket, few biomechanical studies have been performed using natural turf conditions. It is proposed that if natural turf surfaces are to help meet the provision of sports surfaces, advancement in the construction and sustainability of natural turf surface design is required. The design of a natural turf surface should also be informed by knowledge of surface related overuse injury risk factors.This article reviews biomechanical, engineering, soil mechanics, turfgrass science, sports medicine and injury related literature with a view to proposing a multidisciplinary approach to engineering a more sustainable natural turf sport surface. The present article concludes that an integrated approach incorporating an engineering and biomechanical analysis of the effects of variations in natural turf media on human movement and the effects of variations in human movement on natural turf is primarily required to address the longer-term development of sustainable natural turf playing surfaces. It also recommends that the use of ‘natural turf’ as a catch-all categorization in injury studies masks the spatial and temporal variation within and among such surfaces, which could be important.

The effect of maintenance on the performance of sand-filled synthetic turf surfaces

The effect of infill quantity and contamination on the performance of second generation sand-filled synthetic turf sports surfaces was investigated in a laboratory study. Three 1 m2 test surfaces were constructed by placing synthetic turf over a stone–tar-macadam–rubber shockpad sub-base. Ball rebound, ball roll, surface rebound hardness and rotational resistance of a dimpled rubber sole were measured for a range of infill quantities (0–35 kg/m2) and infill contamination concentrations (0, 10 and 20%). Increasing infill quantity increased hardness, reduced ball rebound and reduced rotational resistance linearly (p < 0.01). Ball deceleration increased up to 10 kg/m2 after which there was no further significant increase in the range tested. An optimum infill quantity of 25–30 kg/m2, based on performance characteristics and the length of fibre above the infill, was identified for the synthetic turf surface tested. Increasing contamination also increased ball deceleration and reduced infiltration rate and kept surfaces wetter for longer during drying (p < 0.001), resulting in conditions suitable for moss and algae formation. Maintenance, including regular brushing and monitoring of infill quantity, is required to ensure even distribution of the correct quantity of infill and the minimization of infill contamination in all infilled synthetic turf surfaces.

An Initial Investigation of Human-Natural Turf Interaction in the Laboratory

It is essential to provide high quality, safe and affordable sports surfaces in order to attain the health and social benefits from sports participation. Investment, construction and research into artificial sports surfaces have increased to meet this provision (Kolitzus, 1984; Nigg & Yeadon, 1987). Full provision cannot be met without natural turf surfaces, which also have an important role as greenspaces in the built environment. For improved access to sports facilities, there needs to be a significant improvement in the durability of natural turf surfaces and thus greater understanding of the human-natural sports surface interaction. Research into human interaction with natural surfaces is complicated by integrating natural soil media and sustaining turf growth in the laboratory environment. This study describes and provides data on methodology incorporating the biomechanical assessment of natural turf in the laboratory. Practicalities of using natural turf in the laboratory were overcome by using 10 portable plastic trays (0.57 m×0.38 m×0.08 m), turfed with ryegrass in a sand rootzone. Trays were positioned lengthways in the laboratory on non-slip matting (6 mm thick) to form a continuous runway and cover the force plate (AMTI, 960Hz). Ground reaction force (GRF) data were collected from two subjects wearing football boots (artificial turf/hard pitch design) for running, turning, and acceleration from rest. Mean GRF values compared well with the range of magnitudes presented in the literature for similar movements (Stucke, Baudzus & Baumann, 1984; Munro, Miller and Fuglevand, 1987; Miller, 1990) demonstrating Baumann, 1984; Munro, Miller and Fuglevand, 1987; Miller, 1990) demonstrating that the incorporation of natural turf in the laboratory environment has been achieved successfully. Compared to running (subject 1, −0.41±0.06 BW; subject 2, −0.34-±0.04 BW), peak horizontal force increased for turning (subject 1, −0.50±0.06 BW; subject 2, −0.90±0.01 BW) and accelerating from rest (subject 1, −0.52±0.05 BW; subject 2, −0.44±0.09 BW), reflecting greater braking and propulsive requirements for the respective movements for both subjects. Peak vertical impact forces were 1.89 BW (±0.24) and 2.01 BW (±0.26) for subjects 1 and 2 respectively during running and 1.40 BW (±0.02) and 2.57 BW (±0.37) respectively during turning. To improve human-natural turf interaction, future studies will assess multiple subjects, movements, footwear and a range of natural turf condition using the methodology developed here.

Using the GoingStick to assess pitch quality

Mechanical behaviour of natural turf sports pitches is commonly assessed using the Clegg Impact Soil Tester and the studded disc apparatus under benchmark frameworks. Using the studded disc is time consuming and laborious, which restricts the frequency at which data on surfaces can be collected. To address this, the GoingStick® was evaluated for use as a surface assessment tool. The device was originally developed for testing horseracing tracks, and quantifies both the penetration resistance and shear resistance of the turf surface. Data were collected on three sports pitches (rugby union and football) of varying sporting level and soil texture over two seasons of sport. A laboratory experiment was also conducted assessing data from the GoingStick and the Clegg Impact Soil Tester for four soil treatments. The first season data highlighted that the maximum measurable value was too low on the device, owing to sports pitches being harder than race tracks. This issue was also found for the harder soil treatments in the laboratory study. The development of a new sports pitch calibration resolved this issue for the second season, where the entire range of resistance was successfully measured. Linear relationships were evident between penetration resistance measured with the GoingStick and impact hardness measured with the third drop of the 2.25 kg Clegg Impact Soil Tester (r2 = 0.75), and between shear resistance measured with the GoingStick and peak torque resistance measured by the studded disc (r2 = 0.88). The results of the study indicate the potential for the GoingStick to efficiently quantify the mechanical behaviour of natural turf pitches. Further work should aim to determine benchmark ranges for the measured parameters and incorporate the device within decision-support frameworks for surface management.

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.

Development of a simplified dynamic testing device for turfed sports surfaces

The response of natural turf surfaces to loading changes with the force and loading rate applied. Quantification of surface behaviour to athlete loading is complicated by the lack of devices that replicate forces, stresses and loading rates of athletes that can be specifically used on natural turf. To address this issue, a vertical dynamic impact testing device, the DST, was developed. The DST consists of a compressed air-driven ram that vertically impacts a studded test foot on to the surface using data from biomechanical studies. The vertical dynamic stress of athlete foot strike during running is replicated, using peak force and mean boot contact area data. The ram pressure is adjustable to allow variation of the stress applied upon impact, potentially replicating a range of athlete–surface interactions. Initial laboratory testing indicated that the device was sensitive to changes in soil condition due to variations in impact data. Total penetration time and distance, and surface energy absorption were all significantly greater in prepared ‘soft’ soil treatments (p < 0.05). The loading rate in the first 50 ms after impact was significantly greater in the ‘hardest’ soil treatment (p < 0.05). Future research work will determine in situ behaviour of actual playing surfaces, compare device loading rates to those of athletes, and assess surfaces to a range of stresses.

An Investigation into the Link Between Soil Physical Conditions and the Playing Quality of Winter Sports Pitch Rootzones

Playing quality standard development failed to demonstrate how set targets could be achieved, resulting in Groundsmen being unable to manage pitches to optimize playing quality. This research linked easy-to-measure pitch parameters to the outcome of tests for player-surface interaction quality, to enable this to be monitored in real-time and ensure appropriate pitch management options are selected. 25 pitches were tested three times over an 18-month period. Tests for surface traction and hardness were conducted, along with a range of soil and grass factors; multiple linear regression was used to generate prediction models. R2 values varied with soil type and weather conditions, although increased sand content generally reduced the reliability of the prediction equations. It was concluded that top-dressing may have skewed the data; suggesting more sand-based pitches than actually existed, or that sand-dominated rootzones varied little in playing quality. The production of significant regression equations has demonstrated which easily-influenced pitch factors can be manipulated to alter player-surface interaction quality and ultimately, lower the risk of injury.

Kinematic Response to Variations in Natural Turf During Running (P96)

Important health and social benefits can be gained from participation in sports and exercise. Appropriate surface provision that aids sports participation, cannot be met by artificial surfaces alone — it requires natural turf surfaces to be utilised. Considerable improvement in the durability of natural turf surfaces and thus, a greater understanding of the human-natural sports surface interaction is required. Ground reaction force data have been used to help quantify how human participants respond to changes in natural turf properties during running and turning. A kinematic analysis would further this understanding. This EPSRC/UK funded study analyses kinematic response to variations in natural turf during running. Three different rootzone conditions (clay, sandy and rootzone) were constructed in portable plastic trays (0.60 m × 0.40 m × 0.08 m) and turfed with ryegrass. Trays were positioned in the laboratory on non-slip matting (6 mm thick) to form a continuous runway. Three-dimensional kinematic data (Vicon Peak, automatic, opto-electronic system 120 Hz) were collected for nine subjects wearing football boots (studded natural turf design) during running (3.83m.s−1). Group mean data for initial and peak ankle and knee angles and peak joint angular velocities were statistically compared using an analysis of variance with repeated measures (ANOVA R.M, p<0.05). Mechanical measures of surface hardness (Clegg Hammer) and shear were taken before and after subject testing and assessed using a paired t-test (p<0.05). Moisture content was also assessed. Kinematic data were found to be representative of typical running values presented in the literature. While mechanical measures revealed that natural turf conditions were not identical, changes in surface did not yield any significant kinematic differences. The consistent production of ankle and knee joint kinematics with changes in mechanical surface properties could suggest that humans prefer to maintain similar geometries when running on a variety of natural turf surfaces. Alternatively, the mechanical properties of the natural turf conditions may not have been sufficiently different to elicit changes in human response during running.

The Measurement of Applied Pressure at Depth with Two Natural Soil Surfaces at Different Densities

To advance the engineering of natural turf sports surfaces it is necessary to characterize the stress states and paths of the loads applied by athletes during activity. Such loads are transitory and dynamic. In order to characterize the pressure distribution in a natural soil sports(?) surface a novel experiment was conducted in the 20 m long, 1.8 m wide, 1.0 m deep soil dynamics laboratory at Cranfield University. Two soil surfaces of 1460 kg m−3 and 1590 kg m−3 were constructed from a sandy loam soil (66% sand, 17% silt and 17% clay). Hardness (0.5 kg Clegg impact hammer) was 125 and 235 g, and maximum penetration resistance 1200 and 1800 kPa, respectively. Seven subjects (57–85 kg body mass) were asked to run at a constant speed of 4 m s−1 (±5%) over each surface, three times, in three different types of footwear used in soccer. Loading and unloading of the soil surface was measured using a ceramic membrane pressure transducer of 19 mm diameter, aligned to the vertical and buried at 100, 200 and 350 mm below the surface. Pressure data were recorded at 5 kHz and processed to determine peak pressure and loading and unloading behaviour of the soil surface. ANOVA determined maximum pressure for the two surfaces was significantly lower at 350 mm (7–15 kPa) and 200 mm (2–3 kPa) than at 100 mm (52–61 kPa) depth (p=0.05) but that there was no significant difference between the two surfaces at any particular depth. Maximum pressure at 100 mm depth was linearly correlated with subject weight (for Subjects 3–7). Loading and unloading behaviour of the soil showed a pattern of bimodality, caused by heel strike and push-off, similar to biomechanics running experiments conducted with force plates. These results suggest that in soil surfaces, pressure distribution at and below 200 mm is independent of surface density or subject, but that mechanical properties such as density and stiffness must be considered in the top 100 mm of a surface. This research also demonstrates the applicability of in-surface pressure transducers in integrated soil mechanics and biomechanics testing.

The Mechanical Behaviour of Cricket Soils During Preparation by Rolling

The nature of the ball — surface interaction in cricket has been identified as critical to the quality and safety of the sport. The requirement for even ball bounce and good pace from a clay loam soil cricket pitch has been successfully characterized and has been observed to be related to soil properties such as dry bulk density, moisture content and organic carbon content. To achieve the required mechanical properties, practitioners manage the compaction of a cricket pitch through the use of smooth steel-wheeled rollers. The relationship between moisture content and the compaction and shear strength was determined for a typical clay loam soil and was found to be significant. The effect of subsequent passes of 4.75 and 5.71 kN on soil dry bulk density was also determined in the soil dynamics laboratory. Maximum dry bulk density was achieved after 20 and 10 passes of each roller, respectively. The roller did not have a significant effect on dry bulk density below 50 mm in the profile.