Pneumatic artificial muscles have many advantages. They have high force mass ratio, high compliance and simple
structures. Especially, the flexibility contributes to compose novel mechanisms. The flexibility of pneumatic artificial
muscles releases us from the spatially strict design required for rigid mechanical elements: it could actualize compact
mechanisms made of a fewer parts. OurresearchgroupdevelopedathinMcKibbenactuator. ThethinMcKibbenactuators
is more flexible than conventional McKibben actuators. Authors succeeded in manufacturing novel soft mechanisms that
are made of only braided artificial muscles. We aim to establish a design method for these novel mechanisms. We already
have fabricated a cylindrical mechanism made of helical muscles. In this paper, we report a geometric model for the
cylindrical mechanism. We focus on only deformation of a side surface in the coordinate system of muscles to simulate
the movements of the cylindrical mechanism. The movement was verified by an experiment, and besides, it was simulated
successfully in the geometric model that was created. The deformation of height and radius directions changed according
to muscles alignment.