I have been riding since 1971, touring and racing. This year something has crept up on me that I never experienced before. If I sprint for a stop sign or do hard pulls at the front or jam a short hill my legs feel like they are about to explode. Not sure if it is lactic acid build up or something else.
It is to the point that I am forced to rest after a sprint or hill, and my pulls are very short indeed.

Physical Age 55. Is this what is suppose to happen? I stretch every day and before/after each ride. What is going on? Ideas?

Something else.

The lactic acid build-up theory was discredited earlier in the century.

https://www.nytimes.com/2008/02/12/he...musc.html?_r=0

https://www.livestrong.com/article/51...ter-a-workout/

My guess, based only on my own personal experience and the very limited information you've given, is that you need to do some training, and keep at it.

Hills and sprints take work. When I just ride along, I quickly lose my sprinting and climbing abilities. Within a month, calls to the engine room are answered with giggles and burning. I have to keep up with it.

Three-quarters of my riding is commuting. While I prefer my long loops to work because they let me stretch out and crank, I have to mix it up. If I stick only to the long loop, I can rack up impressive miles. (As I did in June, my third highest mileage month.) But I suffer on the weekends when rolling hills come into play on club rides. And forget sprinting.

I have to use my hilly route, which is a series of short, steep, punchy hills chained together. I have to take my short, stop-and-go route to race the cars for my intervals.

In spring, after commuting on my short stop-and-go route all winter, I can probably out-sprint Mark Cavendish. This time of year, not so much.

In late August when training for a very hilly ride, I use my hilly route twice a day for a few weeks, and do hill training again on the weekend. Then on that very hilly ride, I can climb like a goat. I lose that pretty quickly again afterwards when I go back to miles-based riding.

So it's the age old story. If you want to get better at climbing hills, climb more hills. Same for sprints. And same for distance. If you want to be good at all three, you have to practice at all three and keep up with it.

Thanks for the links. Good reading and informative. Back in the day as a racer we followed the plan laid out in a book that made the argument for lactic build up in muscles. Used HR monitors to peak training, not over train, etc. I guess the book was written on an incorrect assumption and premise.

I think I will need you to come along side once and a while to light a fire to get me moving! I have been riding with intensity and purpose lately and feel my legs coming back. Can't go like stink, but I can go!

Thanks, Coach.

12 years or so ago I experienced what I thought was too much lactic acid, with incredible burning sensations and pains in the legs muscles--it turned out my thyroid decided to go South!

Anyway a blood analysis will reveal if any problems!

Good luck!