The present invention provides a helmet, comprising first and second components having a sliding interface between them, wherein the sliding interface is provided between respective sliding surfaces of the first and second components, and the first component comprises (a) a mixture of (i) an olefin polymer, (ii) a lubricant, and optionally one or more further agents; or (b) an ultra high molecular weight (UHMW) polymer having a density of ≤960 kg/m3, which UHMW polymer is preferably an olefin polymer.