The dynamic loading response of surfaces encountered in beach running.

The purpose of this study was to measure the response to dynamic loading of sand surfaces typically encountered in beach running. An instrumented drop test rig was constructed and used to guide a drop mass through impact with two surfaces (i) dry, uncompacted sand; and (ii) wet, compacted sand. Four drop masses (3.86, 7.24, 10.62 and 14.0 kg) were chosen and dropped from four different drop heights (100, 200, 300 and 400 mm) to represent the kinetic energies typically experienced during heelstrike in running. Accelerations were measured using a piezoelectric accelerometer and the trajectory of the drop head was measured using a displacement transducer. The following response variable were calculated for each trial: (i) peak impact force, (ii) mean impact force, (iii) impulse, (iv) total impact time, (v) rise time, (vi) fall time, (vii) maximum penetration, (viii) energy absorbed by the surface, and (ix) surface stiffness. Mean and peak impact forces were approximately 4 times greater for the wet surface while penetration, impact time and rise time were approximately 3-4 times greater for the uncompacted surface condition. The wet surface was also found to be 6 times stiffer than the uncompacted surface indicating the presence of water substantially altered surface compliance. Results are discussed in terms of their implications for performance and the potential for injury to athletes who run on these surfaces.