Shock attenuation characteristics of three different military boots during gait

Musculoskeletal injuries are related to the cushioning properties of boots in military populations. This study aimed to compare ground reaction force (GRF) and subjective perceived comfort from two different military boots supplied by the Brazilian Army with a commercial boot. Twenty army recruits volunteered for a GRF assessment during walking on a 10-m walkway and a perceived comfort test after 20 min walking on a treadmill. Both experiments were conducted with three different military boots: CC10 (styrene-butadiene rubber - SBR - midsole 30 mm thickness, 65 Shore A; 631.8 g weight; supplied by the Brazilian Army); CC13 (SBR midsole 20.6 mm thickness, 66 Shore A; 530.3 g weight; supplied by the Brazilian Army) and CAT (polyurethane - PU - midsole 31.7 mm thickness, 55 Shore A; 423 g weight; commercially available). GRF was analyzed in the time (principal component analysis - PCA) and frequency (Blackman-Tukey) domains. No difference was found for the first and second peak forces or loading rate; however, significant influence from the military boots' design on GRF was found by PCA and frequency analysis. Loading factor presented higher values at early stance with lower force for CC10 compared to CC13 at these epochs. CC13 also presented higher power spectral density compared to CC10 at higher frequency bands. However, CAT was significantly more comfortable than CC10. These results suggest that the thicker SBR midsole boot was more effective in reducing impact, while the lightest boot with softer midsole hardness made with PU was the most comfortable.