These are strengthening exercises for the quadriceps muscle -
Sit on a firm surface with the back well supported at a slight angle.
Keep one leg straight out on the surface, and bend the other to 90 degrees.
Then lift the heel of the bent knee off the bed a few inches and hold in that position for 10 seconds.
Lower the leg to midway, and again ensure that the heel is a few inches off the surface. Hold for 10 seconds.
Lower the leg nearly all the way, so it is nearly straight but still with the heel not touching the bottom, and hold it there for 10 seconds.
Repeat this several times a day.