To make the seated knee flexion exercise more challenging…
An exercise band can be securely attached in front of you, for example to the leg of a table or sturdy chair, and around the ankle of the leg you are training (first image).
Bend your knee against the resistance by pulling the foot backwards on the floor as shown in the second image. In a controlled manner, without letting it spring back, return to the starting position.
Repeat 10 times.