Actuators for physical human-robot interaction, such as series elastic actuators, have been investigated for use in assistive systems. The authors have explored a pneumatic actuator working as a passive spring with controllable stiffness and equilibrium point. This actuator has the potential to allow assistive devices to balance the competing goals of maximizing user assistance while minimizing the energy consumption of the devices. This paper introduces a novel switching control framework for assistive devices that use the pneumatic actuator to achieve these goals. A simulation study is conducted to confirm the performance of the proposed framework. The results suggest the proposed framework can find the required assistance without extraneous air consumption, which reduces the user's joint torque by 13% compared with the torque in the no-assist case. Further, the assistive device can reduce the user's joint torque by half at the expense of some air consumption. From these observations, we conclude that the proposed framework could be utilized to balance the amount of assistance and energy consumption according to the user's situation.