An apparatus for guiding a flexible instrument includes a first plurality of linkages forming a first side of a channel of a support assembly and a second plurality of linkages forming a second side opposite the first side. In an elongated configuration, each linkage of the first plurality of linkages is interlocked between two adjacent linkages of the second plurality of linkages. Each linkage of the first plurality of linkages is hingedly coupled to an adjacent linkage of the first plurality by a bridging element that maintains a spacing between each linkage of the first plurality of linkages and each respective adjacent linkage. The support assembly transitions from the elongated configuration to a separated configuration as the support assembly is advanced along a longitudinal axis defined by the channel. In the separated configuration, each linkage of the first plurality of linkages is unlocked from between the two adjacent linkages.