Originally Posted by
brawlo
It could just be your muscles getting used to things and working hard. Sprinting on the bike would typically involve muscles working in a different plane than you would hit them in a gym workout. That means that even though they may be worked, the change of action is enough to make them hurt and get DOMS. Just like DOMS though, the pain should dissipate once you're used to it. If it doesn't, then there could be bigger things at play that you should consider getting looked at
This would be my guess, too.
Chainsnapper, standing starts and rolling accelerations involve a lot of pulling of the arms which pull across the back and neck area.
Maybe some supplemental deadlifts (or similar pulling exercises) will tone up that area and "overbuild" them and have them able to handle the load.
You never know, maybe your legs are stronger than your arms/neck and the latter is limiting what you are capable of.