The present invention relates to mechanical transmission system (100, MTS) having a proximal (20) and distal end (40), for a longitudinal steerable tube (500), which MTS (100) comprises a set of longitudinal members, LM, (110) arranged in a longitudinal direction around a fictive tube (180) maintained at an essentially constant circumferential and radial position with respect to the fictive tube (180) and being slidable relative to the fictive tube (180), and has a transmission bendable proximal part (134, TBPP), transmission bendable distal part (130, TBDP), and transmission shaft region, TSR (132) between the TBDP (130) and TBPP (134) wherein movements of the TBPP (134) are transmitted to the TBDP (130) along the TSR (132) by the LMs (110), wherein the MTS (100) comprises a sub-region that is a transmission amplifier region, TAR, (135) in which the fictive tube (180) contains at least one plane-section (182) larger than that (184) of the TBDP (130).