Having to buy different running shoes for different conditions underfoot is a significant nuisance.
Today’s invention is a running shoe in the sole of which a bistable metal plate is embedded (blue in the diagram). A small change in pressure within the cavity in the sole causes the geometry to switch from a flat, road-going configuration (left) to a much more grippy trail-running shape, with prominent studs (right). The pressure change might be achieved by use of a small hand-operated bellows or even by winding a wedge in or out.
This approach has the added advantage that, on springing between different sole profiles, any dirt accumulated will tend to be spat off, improving grip generally.