If I understand the issue, it sounds like your hamstrings. We tend to use quads more in cycling, which then creates an inbalance between the pair, (quads and hammies being opposing muscle groups) which is often my problem. I do hamstring curls at the gym once or twice a week to even this out. Try this: if you can do quad lifts with 40 pounds (or whatever works for you) but only manage 10 with hamstring curls, there is an inbalance!

Sheldon