Always consult a therapist first, to determine if this low back exercise is good for you.
Starting position: Seated on two chairs
Procedure: Place your leg stretched on the other chair. Push the back of your knee downwards, focus on contraction of the thigh muscles. Hold this muscle contraction for 5 seconds.
Sets: 3x
Repetitions: 10-15x
Rest between sets: 10-20 seconds