A reversible, heat-set covered fiber is described, the covered fiber comprising: A. A core comprising an elastic fiber comprising a substantially crosslinked, temperature-stable, olefin polymer, and B. A cover comprising an inelastic fiber. The fiber is heat-set by a method comprising: (a) Stretching the covered fiber by applying a stretching force to the covered fiber; (b) Heating the stretched covered fiber of (a) to a temperature in excess of the crystalline melting point of the olefin polymer for a period of time sufficient to at least partially melt the olefin polymer; (c) Cooling the stretched and heated covered fiber of (b) to a temperature below the crystalline melting point of the olefin polymer for a period of time sufficient to solidify the polymer; and (d) Removing the stretching force from the covered fiber.