Soft wearable actuators can help connect machines and humans, providing a personalized, ergonomic, and cooperative physical interface between people and their world. Until now, the torque of these interfaces has been limited, restricting their ability to assist the completely paralyzed. This article presents a method for realizing a soft structure that stably and comfortably applies a knee extension torque to the body that is sufficient for sit-to-stand (STS). The structure, the pleated pneumatic interference actuator (PPIA), is based on pleated inflatables; is lightweight, collapsible, and clothing integratable; and generates torque from buckling of a constrained fabric-reinforced rubber tube. Multiple PPIAs were integrated into a soft orthosis, the soft lift assister for the knee (SLAK). The SLAK was inflated to a pressure of 320 kPa, and it produced a maximum 324 Nm torque at a flexion angle of 82°. This exceeds the peak 180 Nm torque required for STS and torques required for other everyday tasks. The SLAK met the torque requirement for STS, which is more than 93% of the STS motion when worn by a test leg. Worn by a human, it shows potential for complete support, which is more than 100% of the motion. The PPIA's theoretical model overestimated torque at low to moderate flexion angles and underestimated PPIA torque at high flexion angles. Further development of the PPIA will focus on testing the SLAK with human subjects; increasing the PPIA's speed and flexibility; reducing the PPIA's bulk; and improving the PPIA's model accuracy.