A lead body of an implantable medical electrical lead includes a first filler and a second filler, each extending within a proximal portion thereof. The first filler may have a stiffness that is less than that of the second filler, and may surround and isolate a plurality of conductors which are coupled to a connector terminal of the lead, which connector terminal extends proximally from a proximal end of the lead body. The second filler extends along a limited length, alongside the first filler, wherein the limited length extends distally from the connector terminal; the second filler may further extend into the connector terminal.