There is provided a sole unit having a midsole section and an outsole section. The midsole incorporates a number of cavities which open at their top ends and which are wider at their bottom ends. The outsole section incorporates a number of diaphragm elements which close off the bottoms of the cavities. In some arrangements there is a footbed which has in its underside a number of recesses disposed above each of the cavities, the recesses being linked by channels and incorporating orifices leading to the inside of an article of footwear.