midsole section 14 are provided with a mating corrugated surfaces comprised of a plurality of interfitting ribs the use of such a mating surface assists to keep alignment of the above described elements of the footbed 13 and the midsole section 14 where mating corrugated surfaces are used a constant depth of the channels 50 can be obtained by providing a plurality of channel structures 58 in the dips between ribs of the corrugated surface of the upper surface of the midsole section 14 in more advanced embodiments valve means may be provided to control the flow of air into and or out of the cavities or through the orifices 27 28 or along the channels 26 the valve means could be one way mechanisms or could rely on compressive closure of the openings due to weight transfer during the walking cycle one example of a sole unit having a valve arrangement is shown in fig4 to 6 the heel area of the sole unit incorporates an air inlet member 40 which is located generally in the midsole section 14 and which extends laterally in this embodiment the inlet member 40 is generally tubular and is open to the atmosphere at.