What is the best exercise for my back is a question I am frequently asked. You will not be surprised that my answer starts with “that depends”. It depends on why you asking, what your goal is, what you expect as an outcome, your training and injury history, and also what you consider to be your back.
If you are struggling with an achy and stiff back, as opposed to very specific pain issues, then try these exercises out and see how you get on. They focus on the main movements of the spine; flexing forwards, backwards and sideways, and rotation, as well as some bracing, which we use throughout our daily lives without realising it.
Flex 
and Extend 
External Rotation 
Internal Rotation 
‘Superman’, Opposite Arm/Leg Balance
Flex and extend the spine
On all fours, with hands under shoulders, and knees under hips, lift your head, stick your bum in the air, and squeeze your shoulder blades together. As you continue to extend through your back you will feel gentle tension all the way along your spine. As you change direction and begin to arch your back focus o lifting from the middle of your back and tuck your pelvis underneath you, so you get more tension in your upper hip and stomach. move back to a neutral position, and slow wiggle your hips from side to side, aiming to draw your hip towards the side of your ribs.
Rotation
Same starting position on all fours, with hands under shoulders, and knees under hips. Place one hand behind your neck then, keeping your eye on your elbow, squeeze the shoulder blades and swing your elbow up before dropping back down and drawing your elbow towards your opposite knee.
Superman balance
Named for very obvious reasons, but you do not need a cape, or to wear your underpants on the outside. Again beginning on all fours, with hands under shoulders, and knees under hips, press your hands, knees and forefeet into the ground. As you do so you will feel the stomach starting to brace. Push down hard so you feel solid, then drive one heel straight back behind you, feeling tension in your bum, before reaching the opposite arm as far out in front of you as you can.
My YouTube version is here from 2:41 to 6:07
