I started doing old fashioned sit ups, to strengthen the hip flexors. No idea if that's working but I can do almost 50 sit ups at a rapid pace versus 20 before

Seriously though I think that working the lats, the pull up muscles in the arm (biceps? but not really, if your hand is holding a bar like the drops of a bike), lower back, etc help with out of saddle sprinting efforts.

And another observation. Usually when I lift I immediately feel some benefit, even just the pumped muscles right after the workout. I feel energetic, strong, etc. I usually see some reasonable strength gain at first (plateaus to known levels). This year is the first year where I've lifted and such and my muscles feel downright flabby, even after a couple weeks of work. It's really annoying. Same weights, curling (bar pointing forward, like the way the drops are) 22-23 lbs, but without the snap I used to have. Is this called "getting old"?