A variably flexible insertion device includes a hollow body having a proximal end with an entrance for receiving an instrument and a distal end with a tip for protrusion of the instrument. A device transitions the hollow body between a relatively flexible condition and a relatively stiff condition. Tendons within the hollow body maintain the hollow body in the relatively flexible and relatively stiff conditions. A method for variably flexing an insertion device for receiving an instrument, includes providing a hollow body having inner and outer sleeves defining a space therebetween in which tendons are disposed. Suction is applied to create a vacuum in the space for frictionally locking the tendons in place between the sleeves in a relatively stiff condition of the hollow body. The vacuum is relieved to release the tendons in a relatively flexible condition of the hollow body.