Based on the biological structure of the Elephant Trumpet class, a soft body actuator that can satisfy the above requirements is designed by combining the extension PAM and contraction PAM. This will allow the lumbar spine assisted rehabilitation exoskeleton robot to provide support function to reduce the compression force of the lumbar spine joints and to meet the required output force requirement. The actuator is a variable stiffness pneumatic artificial muscle (VSPAM) with both extension and contraction functions. A new mathematical model of the VSPAM’s output force is developed using the energy conservation principle after the kinematic analysis of the device is completed. A strong correlation is found in quasi-static studies examining the relationship between the output force experimental data of VSPAM and the new mathematical model. Stiffness experiment results confirm the VSPAM’s ability to have variable stiffness at a range of particular lengths, showing that the stiffness of the VSPAM can be changed without reference to a particular length. Ultimately, an exoskeleton robot prototype for lumbar spine assisted rehabilitation is built for lumbar rehabilitation training. The experimental results show that the VSPAM has several advantages over traditional soft actuators, such as the capacity to change the stiffness at particular lengths and the ability to contract and extend relative to the VSPAM’s initial length to generate contraction and extension forces.