like some have mentioned solid core is necessary for a cyclist. after spending a winter working on my core i was able to ride longer and harder, with less pain and/or discomfort in the neck, shoulder, and back. i noticed the difference for sure on the bike and off the bike. my routine is simple consisting of exercises done at home.

push ups, sit ups, dips, plank, side plank, pedal plank, bird dog, sicossor kicks, ect....

typically i do this on a non riding day or after a very easy effort. later.