chins are a great mass builder for the back.
go to the chinning bar, grasp it with an thumbs over-hand grip, hang from the bar, and cross your legs this will protect your lower back and stabalize your spine.
From this position pull your body up with the power of those lats, bring yourself up so your chest, which is puffed out when you are approaching the bar, nearly touches the bar, keep your elbows as far back as you can.squeeze the lats at this point, then slowly return to where ypu strated. dont just let your self fall down, controll yourself.
peace