A medical instrument system comprises an instrument manipulator configured to control a position of a medical instrument with respect to a base. The instrument manipulator comprises an instrument carriage comprising a medical instrument connector configured to engage the medical instrument. The instrument carriage is configured to translate along a linear axis to advance or retract the medical instrument with respect to the base. The medical instrument system also comprises an insertion stage slideably engaged with the instrument carriage along the linear axis. The insertion stage has a drive assembly comprising a drive belt and a drive motor configured to drive the drive belt. The base is fixedly coupled to the drive belt. The medical instrument system also comprises a connecting element having a distal end fixedly coupled to the drive belt and a proximal end fixed to the instrument carriage.