Running-related injuries among trail runners are very common and footwear selection may 7 modulate the injury risk. However, most previous studies were conducted in a laboratory 8 environment. The objective of this study was to examine the effects of two contrasting 9 footwear design, minimalist (MIN) and maximalist shoes (MAX), on the running 10 biomechanics of trail runners during running on a natural trail. Eighteen habitual rearfoot 11 strike trail runners completed level, uphill and downhill running at their preferred speeds in 12 both shod conditions. Peak tibial acceleration, strike index and footstrike pattern were 13 compared between the two footwear and slopes. Interactions of footwear and slope were not 14 detected for all the selected variables. There was no significant effect from footwear (F=1.23, 15 p=0.27) and slope (F=2.49, p=0.09) on peak tibial acceleration and there was no footwear 16 effect on strike index (F=3.82, p=0.056). A significant main effect of slope on strike index 17 (F=13.24, p<0.001) was found. Strike index during uphill running was significantly greater 18 (i.e., landing with a more anterior foot strike) when compared with level (p<0.001, Cohen's 19 d=1.72) or downhill running (p<0.001, Cohen's d=1.