Abstract: A linearly infinitely variable mechanism comprises first and second portions which are relatively axially positionable to establish the effective length of the mechanism. A rotary element is axially captured on the second portion. The operative connection between the first portion and the second portion is via a threaded connection of the first portion with the rotary element. The axial capture of the rotary element comprises a twist lock which, when locked, forcefully abuts the ends of the rotary element to preclude its rotation and prevent a change in the effective length of the mechanism provided that the first and second portions are constrained against relative rotation. When unlocked, the rotary element can rotate within its axial capture so that axial forces applied to the mechanism are effective to change its length.