Nate's right on about the "hard" aspect,...I didn't address that part, just the recruitment part. Thanks Nate, good point,.... even with good training of your obliques, if your diet isn't good and your bodyfat isn't low, you'll just look soft.
Helene, I don't think I understand what you're talking about re: doing obliques on a back
extension machine?
Ultimately, if you train them like the rest of your body,...consider what
training style you're applying to what muscles.
By that I mean, if you're lifting to build, and you lift hard and heavy,...then your muscles reflect that (according to your genetics that is).
So if you do one set of 50 reps of side bends with a 5lb DB.... your "development" won't be the same as doing 6-8 reps to muscle failure per set, with very heavy DB's for 4 sets.
((Of course, diet, cardio, bodyfat, etc., being equal across the board))
I know trainers who do not subscribe to this philosophy (re: how doing heavy weighted side bends affect oblique development), so please understand this is just based on my experience, what I've seen and learned......and I understand that everyone develops and responds differently. So the only way to "know" for sure is through trial and error.
I prefer to focus on "cross over" movements, not lateral bends (for development).
Lateral bends are, however, a healthy and natural movement.