Opposite arm & leg raise - upper and lower back strength exercise
This exercise can be done over a stability ball for additional challenge or over a chair as shown here. Let your arms hang down so that your fingertips rest on the floor. Make sure to keep your head in neutral alignment, neither looking up nor allowing it to hang down. Your back should be perfectly flat and level. Maintain this alignment while you lift your opposite arm and leg. Your goal is to be able to lift the arm and leg to a position level with (but not above) your back. Initially you may not be able to attain this straight alignment, but with time and practice you should be able to. Make sure to maintain your back and neck in their neutral alignment as you lift the arm and leg; avoid the temptation to look up.