It can be good or bad on your back.

For me, i found my back likes the lower down "racing" position. The upright powition puts a good amount of strain on it, so I do need a springer seat for upright riding.

Pretty much, there is a component to make any riding style comfortable.

As for developing yout abs...which are part of your core....go do some specific abdominal excercises...cycling is not really a very good ab workout, unless you are doing trick riding, which is NOT reccomended for those with bad backs.

Cycling is primarily a cardiovascular and lower body workout.