Clegg hammer measures and human external landing forces : is there a relationship?

Ground hardness is deemed an important consideration for player safety for sports played on natural turf surfaces. Currently, a ground hardness measure is being determined using a Clegg hammer, with the suitability for play dependent on an acceptable reading. This study aimed to examine whether a relationship between Clegg hammer readings and ground reaction forces (GRF's) generated by a human during a drop landing exist. Fifteen male community level Australian football players were recruited for the study. Participants performed a single leg drop landing on the right leg from a 45cm box onto the force plate to record GRF's. Ten trials were conducted for three conditions: no shock pad, thin shock pad (15mm) and thick shock pad (50mm) under a synthetic turf sample. Four consecutive Clegg hammer readings were recorded following each set of ten trials. Variables of interest were maximum vertical GRF (Max vGRF), maximum rate of loading (Max RoL) and Clegg hammer (CH) readings. Pearson's Correlation Coefficient was conducted to examine the relationship between variables and conditions. Slight to fair relationships were found between the Max vGRF and any of the four CH drops (0.181 ≤r≥ 0.189; p ≤ 0.01). This finding was similar to the relationship with Max RoL (0.209 ≤r≥ 0.217; p ≤ 0.01). When analysed for the specific shock pad condition, the relationships remained poor (r <0.1; p ≥ 0.29), with the exception of the Max RoL and the CH readings on the thick shock pad (0.1 ≤r≥ 0.2; p ≥ 0.03). The results of this study show that the ground reaction forces experienced by a human on different levels of surface hardness are significantly different to the forces on impact of the Clegg hammer. Consequently, the Clegg hammer may not be the most appropriate device for relating surface hardness to player safety, thus it is possible that the Clegg hammer alone is insufficient in globally determining ground safety.