Since I went back to the gym and was given a set of exercises to do including plank and other core exercises I have felt stronger while cycling, so it seems to make a difference.
I suffer from a back problem which I have been told by a surgeon that I will have to live with till it's too bad to suffer then surgery.
My problem was I couldn't do ab work because of my back and having poor abs/core was making my back pain and mobility worse.
I took it on myself to slowly build up my strength and it has worked wonders for my back but I was just going through the motions lately and starting to miss out on it again.
the ap I mentioned above, and probably the one Musa mentioned, has exercises I would never have considered before and I feel a huge benefit from it on and off the bike.