54 Journal of Applied Biomechanics, 2011, 27, 54-63 © 2011 Human Kinetics, Inc. Victoria H. Stiles (Corresponding Author) and Sharon J. Dixon are with Sport and Health Sciences, College of Life and Envi- ronmental Sciences, University of Exeter, Exeter, U.K. Igor N. Guisasola and Iain T. James are with the School of Applied Sciences, Cranfeld University, Cranfeld, U.K. Biomechanical Response to Changes in Natural Turf During Running and Turning Victoria H. Stiles, Igor N. Guisasola, Iain T. James, and Sharon J. Dixon Integrated biomechanical and engineering assessments were used to determine how humans responded to variations in turf during running and turning. Ground reaction force (AMTI, 960 Hz) and kinematic data (Vicon Peak Motus, 120 Hz) were collected from eight participants during running (3.83 m/s) and turning (10 trials per condition) on three natural turf surfaces in the laboratory. Surface hardness (Clegg hammer) and shear strength (cruciform shear vane) were measured before and after participant testing. Peak loading rate during running was signifcantly higher (p < .05) on the least hard surface (sandy; 101.48 BW/s ± 23.3) compared with clay (84.67 BW/s ± 22.9). There were no signifcant differences in running kinematics. Compared with the “medium” condition, ffth MTP impact velocities during turning were signifcantly (RM-ANOVA, p < .05) lower on clay (resultant: 2.30 m/s [± 0.68] compared with 2.64 m/s [± 0.70]), which was signifcantly (p < .05) harder “after” and had the greatest shear strength both “before” and “after” participant testing. This unique fnding suggests that further study of foot impact velocities are important to increase understanding of overuse injury mechanisms. Keywords: ground reaction force, kinematics, sports surfaces Despite the growth of artifcial surfaces, traditional sports such as football, rugby, cricket, hockey, tennis and lacrosse are still frequently played on natural turf surfaces at a variety of sporting levels. However, a natural turf surface does not withstand the rigors of frequent multi- sport use, is highly infuenced by changes in the weather and requires a large area of ground to rotate pitch usage. Therefore, there is a need to continue to develop natural turf surfaces to a) protect green spaces and playing felds in the built environment and b) preserve the fundamental playing characteristics for sports that would otherwise change if they became accustomed to play on artifcial turf surfaces. Several studies have illustrated the danger of engi- neering artifcial sports surfaces on the basis of surface performance and durability, without considering human interaction (Torg et al., 1974; Andreasson & Olofsson, 1983; McCarthy, 1989). Engineering natural turf surfaces for more intensifed use and use within enclosed stadium environments has already resulted in signifcant changes in mechanical properties. Mechanical properties of the impact interface have been found to infuence player injury risk, for example, a greater incidence of overuse injuries has been found while running in harder shoes and on harder surfaces (Andreasson & Olofsson, 1983). A harder surface can lead to damage of the cartilage (Orchard, 2001) whereas a too compliant surface can lead to early leg-muscle fatigue (Millet et al., 2006). There is some research evidence that increased ground reaction forces (levels of impact and rates of loading) and altered joint movement patterns (kinematics) yielded when performing on harder surfaces can cause overuse injury (James et al., 1978; Nigg et al., 2003). Peak rate of loading in particular has been shown to increase with increases in shoe or surface hardness (Clarke et al., 1983a, 1983b; Hennig et al., 1996; Stiles et al., 2007). Kinematic adjustments in the form of increased initial knee fexion, reduced heel impact velocity, reduced initial foot sole angle relative to the horizontal and variations in joint angular velocities have been reported in response to running on surfaces of increased hardness (Bobbert et al., 1992; De Wit & De Clercq, 1997; De Wit et al., 2000; Dixon et al., 1998; Dixon et al., 2000). Increased understanding of player-shoe-surface interaction in relation to impact attenuation and lower limb movement is vital to inform and reveal biomechani- cal mechanisms of overuse injury (Torg et al., 1974; Nigg & Segesser, 1988). Some analysis of natural turf proper- ties has been achieved in the feld, for example the assess- ment of traction performance during cutting maneuvers (Coyles et al., 1998) and plantar pressures underfoot during sports specifc movements (Eils et al., 2004, Ford et al., 2006). Overcoming the challenges of incorporat- ing natural soil media in the biomechanics laboratory to enable more sophisticated laboratory-based equipment