It's those lats again (and all the other stuff!) A bit of training: For the lower back:  Rowing machine is always good:      

Lat-erally - in other words,  latissimus dorsi   - the muscle which give you the 'wings' - assisted in strength terms by deltoids in the shoulders, rhomboids and those little muscles and tendons which contribute to shoulder mobility, collectively known as rotator cuff... ...all of which add up to some pretty impressive results when properly included in training. As shown here. Examples of training:    Check out these 5 different holds for the cable pulldown to focus on specific areas of the bacl: The young 'uns know what they need to do: Ninja training is perfect!!