The invention provides an extendible member which is configurable between a coiled form and an extended form. The extendible member comprises: a primary member comprising a sheet of material resiliently biased in a slit tube form, wherein the slit tube can be opened out at the slit to assume an open form in which it has a flattened cross section; at least one resilient secondary member having first and second connections to the primary member at respective different circumferential positions on the primary member, wherein in the extended form, the primary member is in its slit tube form and the resiliency of the secondary member causes at least part of the secondary member to displace towards the slit in the primary member to provide torsional and axial stiffness to the primary member, and wherein in the coiled form, the primary member is in its open form and the secondary member conforms to the flattened cross section of the primary member so that primary and secondary member can be co-coiled. Corresponding methods are also provided.