An implantable medical lead has a header assembly having an inner header channel and a radial aperture. An inner lead assembly includes a helical active fixation element connected to an inner conductor. The inner lead assembly is coaxially housed within the header channel. A post is inserted into the radial aperture after insertion of the inner lead assembly into the channel. The post projects radially inwardly into the header channel to become interposed between adjacent turns of the helical fixation element. At this position the post is able to transform a rotation of the helical fixation element to a longitudinal movement of the fixation element.