This is an effective exercise to strengthen the leg muscles on the front (anterior) of the leg.
Sit toward the front of your chair with knees bent and feet flat, holding on to the sides for balance.
Keeping your left foot planted and upper body still, straighten your right knee until your leg is parallel with the floor. Hold for 2 counts, making sure the front of your thigh is engaged, and then lower it back to the floor. Do 5 reps on the right, then repeat on the left.