Straight Leg Raises
If your knee’s not at its best, start with an easy strengthening exercise for your quadriceps, the muscles in the front of the thigh. This move puts little to no strain on the knee: Lie on your back on the floor or another flat surface. Bend one knee and place your foot flat on the floor. Keeping the other leg straight, raise it to the height of the opposite knee. Repeat 10-15 times for three sets.