Excuse me if this is discussed often. I searched several times with different combinations of the most obvious search terms, and didn't find anything.

So, my question is about dead legs. Yesterday while riding my speed felt pretty normal. I have a hill that I have to climb out of my neighborhood, and was getting 15-16 mph up the hill. That's about average for me. On the flat, or slight incline, after the hill I was averaging about 18-20, also average. I got about 20 commuting miles in yesterday. I did not ride my rollers yesterday, so my workload was less than usual, almost a rest day. Today I put in some time on the rollers, and was struggling to muster any effort to push it above 18mph. I usually spin about 23mph (ride fixed, no option to change gearing so this is around 100-110rpm) for an hour and a half. When I left the neighborhood, I was just so listless and slow. About 12 on the hill and 16 on the flats. I didn't feel specifically tired, just felt like I had no energy. I can't blame it on change in diet or anything. This happens to me occasionally, and I want to know why. I also want to know if there is something I can do to avoid it, or work through it, or even plan for it on a day when I can afford to not ride. Thanks.

Edit: I had the wind at my back today, and yesterday it was calm. I should have been faster today.

Either you were bonked, or you are overreaching and need a rest, or you're fighting a cold/flu.

Hmm. I did feel slightly off this morning. Not like I was sick, but like I might be getting something. That may be it. I'm on my way back up and about. I'll report back.

You’re not always going to have great training days. Life is filled with distractions, and you’re going to get stressed out. Combine that with a bad night’s sleep or a lack of food, overtraining and you’re looking at a lot of things that can potentially go wrong. The bottom line here is that you’re not going to have great days all the time. Everybody has less than stellar days, in fact if you train very hard, your probably going to have more bad than good especially mentally.

Yeah I totally get that. I'm just wondering what the actual cause is, physically, and what, if anything, I can do to avoid or lessen it.

Anyway, when I went out today (about another 20 commuting miles) I started out just where I left off yesterday. To try and expound on the feeling just a little more: It was like my legs would push weakly from the thighs, but then my calves were just jelly. It didn't feel like I could get the power to the pedal. It was just getting lost somewhere, like I had a backed up pipe or something, if that makes any sense. But once I hit the hill I came out of it and felt great. I was back to normal speed and effort, and everything seemed to be firing on all cylinders. Weird. Well, time to jump on the rollers.

You're overreaching a bit. When you go hard and get your HR up, that happens because your glands release hormones that make that all happen on your command. (sheesh, what crappy construction, oh well) But you get that, right? So your glands get a little tired. That can happen from a series of high HR rides, or it can happen on a long, strenuous ride, or it can happen just from continuous training stress. But you're just overreaching a little, which means you aren't dead yet.

So when you reached that hill, you got some mental burst, got the glandular release to which so many of us are addicted, and away you went. However, that's a bad sign, that you had to dig for it. Pretty soon, when you dig for it, it won't be there and then you'll have some serious resting to do. I've seen this scenario a many times in myself, where I'll be just beat during a ride, but something will come up and I'll attack and my HR will come up and away I'll go.

As other commenters have suggested, the scenario I outlined may not be true in your case. It could easily be something else. But watch out for it. The way most of us avoid it is to have 1 week in 4 that's an easy week, or for the ancient among us, even 1 week in 3. If you're tired, that good feeling won't come back with just one day off.

To combat this, because I'm terrible about going hard too much and too often, I got a recording HRM years ago, download to my computer, and keep track of my time in zones every week.