Straight leg lifting, this method is mainly to exercise the muscles around the knee joint. Keep lying on your back, support your body with your elbows and keep your body stable. Then bend your left foot, straighten your right foot and hook your toes up. Then slowly lift your feet up so that your heels are about 15 cm above the plane. Hold this position for a few seconds, and then slowly put it down. Repeat this action several times with your right foot, and then change your left foot. If you want to increase the difficulty, you can hang a few bags of salt on your ankle.