In this video Alex is demonstrating Seated Knee Flexion with the Vive PROFlex Strap. This is 2-in-1 leg lifter and exercise strap is helpful in getting your leg on/off the bed, as well as assisting with exercises after surgery. This exercise is assisted with your arms in order to help gain knee bending motion after surgery. (Disclosure -- Vive PROFlex Strap was designed by Cindy, PT with Adaptive Equipment Corner for her knee and hip clients:).
Vive PROFlex Strap (2-in-1 leg lifter/exercise strap) https://amzn.to/2zMcSFQ
This exercise can be performed with a sheet or long belt as well.