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

No - we;re looking back in 'admiration' at the muscle development. Backs do tend to get neglected when thinking 'body image' - but perhaps not when thinking 'fitness for sport'... there's a lesson in their somewhere. Keep your back in mind, guys, when you're thinking of hitting the gym.