Influence of footwear and soil density on loading within the shoe and soil surface during running.

Using pressure transducers in soil surfaces prepared in controlled conditions, the vertical stress was recorded at 100 mm, 200 mm, and 350 mm depth within two soil surfaces of 1460 kg/m3 and 1590 kg/m3 for five subjects of 747–843 N body weight running at 4 m/s (5 per cent). Simultaneous in-shoe pressure data were collected to investigate the influence of soil density on loading experienced by the player and to provide information on the load applied to the surface. For each soil density, the subjects wore three different footwear types: soccer boots with traditional studs, boots with moulded studs, and boots designed for synthetic turf. For the mean of all subjects, there was no significant difference in the maximum vertical soil stress or loading rate between surfaces at any depth but within each surface there was a significant reduction of 32 kPa between –100 mm and the other depths. The peak loading rate was two orders of magnitude greater at –100 mm than at –200 mm or –350 mm. The variation in maximum vertical stress at –100 mm for different subjects was significant (p<0.001) and increased with increasing subject weight (R2 = 0.87); at –200 mm and –350 mm there was no significant subject or density effect; a similar pattern was observed with the peak loading rate, with a linear relationship between the loading rate and the subject weight. In-shoe pressure data revealed no significant differences in the peak force or loading rate between surfaces, but a significantly lower heel pressure for the soft (1460 kg/m3) surface compared with the hard (1590 kg/m3) surface (p<0.05). Wearing of different footwear had no influence on the peak force or pressure but revealed a lower rate of loading of force for the moulded boot than for the studded boot when performing on the hard surface. There was a low and non-significant relationship between the peak input force and the peak force experienced within the surface (R2 = 0.01; p > 0.05), however, peak resultant pressure data were used successfully to model the vertical stress distribution during running using a linear elastic model of soil behaviour. This novel approach to understanding the behaviour of the soil surface and the player has revealed a complex relationship between the input load and the load experienced by the surface. Future models will seek to understand this relationship further