Abstract: An extensible boom assembly which has a tubular outer section, a tubular intermediate section and a tubular inner section supported in a telescoping relationship, and containing a flexible power conducting line which extends through the assembly from adjacent the forward end thereof to adjacent the rearward end thereof. An articulated, hollow link track member which is supported within the boom assembly and which has one end portion doubled-back upon itself, receives and protects the power conducting line from damage during the relative movement of the several boom sections.