Back-Stretchable McKibben Muscles: Expanding the Range of Antagonistic Muscle Driven Joints

McKibben artificial muscles are used in many musculoskeletal robots because they have characteristics similar to those of natural skeletal muscles in terms of the contraction ratio and force generated. However, they differ from natural skeletal muscles in that they cannot perform passive elongation movements (referred to as “back-stretching” in this study), in which the muscle is stretched by an external force from its natural length while deactivated. Therefore, when using McKibben artificial muscles for antagonist muscle drives, it is necessary to use artificial muscles that are longer than the linear distance between the origin and insertion point of the muscle (causing them to sag while deactivated). However, when acting as an agonist muscle, such an artificial muscle does not fully utilize its contractile capacity and reduces the range of motion of the joint. This is different from natural skeletal muscles, which are placed along the bone without sagging under the skin. In this study, we proposed a back-stretchable McKibben muscle that contracts like a conventional McKibben artificial muscle when air pressure is applied and back-stretches like a natural skeletal muscle with no applied pressure. In addition, we developed a design method based on a mechanical model. In comparative experiments on a robotic arm, we achieved a 1.22-fold increase in the antagonist muscle driving range compared with conventional McKibben artificial muscles without causing the muscles to sag significantly.