The portable powered ankle–foot orthosis (PPAFO) is capable of providing bidirectional-assistive torque at the ankle joint [1]. It was designed for untethered operation away from the confines of a lab or treadmill by use of a portable pneumatic power supply. To create the ankle torque, the original design (Fig. 1) utilized an off-the-shelf rotatory pneumatic actuator (PRN30D-90-45, Parker Hannifin, Cleveland, OH) capable of generating up to 12 N·m torque output at an operating pressure of 120 psig (0.83 MPa). This value is 10–20% of the peak torque generated by healthy abled-bodied individuals (Fig. 2). Increased torque may help to improve gait function.

Since this off-the-shelf rotatory actuator was not designed for this application, it had a large lateral profile. The bulkiness of the actuator made it difficult to be concealed underneath clothing and increased the risk of interfering with the surrounding...

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