Get on your hands and knees on the floor. Tighten your stomach muscles by blowing all of your air out. Bring one leg up straight behind you and hold it in this position. Be careful not to rotate your pelvis. Start by holding this position for 5 seconds and as you gain strength, lengthen the time. Repeat this position with the opposite leg. As you further gain strength, raise the opposite arm in front of you and hold while raising your leg.