You've set the bike up on the turbo, started Rouvy and gotten ready to go. A nice warm-up, a few efforts to prime the legs and you know what you've got to do today. Five x 3-mins at VO2max. You know you can do it, it's 'just' three minutes.
The warm-up concludes, and it's time to roll it hard. You crush the pedals, your heart rate goes up, your breathing intensifies, time slows down, 30-seconds, 60-seconds, and before you know it you're finishing the effort. Bang on at the correct power. Interval 2 starts and the recovery period felt all too short, but you crack on. This one is biting more, and now at the conclusion of this interval you're 10 W down on where the plan said you should be. No worries you'll rescue this in the next interval. Damn… this one is worse it's 15 W down. And, the pain is intensifying even more.
You keep telling yourself that you'll solve this in the next interval. You don't. You finish the session, with each interval effort being less than it should be. You're drenched in sweat, you're exhausted and the numbers stare back at you and tell you that you're shit. The power meter is a double-edged sword. Marvellous. You're a failure. Or, are you?
We've all been there. An absolutely pants session, where you feel like you've failed. Depending on the intervals you're doing, there's multiple ways to read this. For most intervals, yes, you may have a specific minimum power level you want to hit and which has been prescribed to you. But your body doesn't work like that. If you're attempting, say, a VO2max interval and you end up 5 or 10 W down, it simply isn't a fail. You'll still have achieved a heavy workload, and you'll still be knackered! Sure, it may have been better if you'd have hit the exact (or better still 10 W more!) number you were meant to hit, but it's impossible to be 100% for every session (and if you are, this likely means you're undertraining – as you've obviously got too much time to recover). Some days you're off.
Why you might be off
There are multiple reasons why you may not be firing on all cylinders. The most likely reason is accumulated fatigue from prior sessions. This is normal, to be expected, and something you want. The idea that you should be fresh for every session isn't warranted and has no basis in physiology.
The next most likely reason is inadequate nutrition – this could be related to prior days with insufficient carbohydrates and protein, or it could be short-term such as you've failed to eat for 5 hours before training, and your carb stores are depleted. For most sessions, you want to be fuelled. That doesn't mean you need a plate of pasta and 16 gels before starting (although that may be fun!) it may just mean you need some toast and jam an hour or so beforehand. Really, it depends on the session intensity and duration.
Perhaps you slept badly or had an argument with your partner or at work – for many people these will result in depressed power. There's not a lot you can do to resolve things for this and some riders will just need to accept a marginally lower power, others may be better off changing the session to an easy spin (looking at myself for these issues, not that I've ever argued with my partner. Hahaha!).
Also, you'll want to check your training environment – as I write this it's autumn and freezing (for me), but conversely, it may be that it's baking hot and the heat stress will result in your power being lower (if you're not well adapted to it). If you're indoors, make sure there's a fan blowing and there's some air flow (of course if you're actually doing heat training then you'll need to ignore this and suck it up).
Maybe you're basing your power numbers on outdated tests, you know, from the middle of the summer when you were flying and now your FTP is down 20 W and you're lying to yourself about where you are, so you're suffering like a dog, missing the target and beating yourself with the proverbial shitty stick. Get some testing done if it's been a long time or your fitness has changed (check my calculator here).
Lastly, and not to understate things, biological variation. Normal day to day variation is in the region of +/-2 to 3%, so it's quite possible that you're just a little off with no real explanation. If all is ok in the next few days just don't worry.
285 W or 200 W? The line that matters
Of course, the above ideas presume that you're a few watts off. If your target was 300 W and you averaged 285 W for them then the above ideas are valid. On the other hand, if all you did was scrape together 200 W then something else is possibly up, or most likely you haven't achieved the training stimulus you wanted. That happens as well.
So, if you're at 285 W or whatever, crack on if you can with the session. It'll likely be fine, but if you're at 200 W (when you're meant to be at 300 W) then yes, you'll want to either stop the session or change it dramatically. It's possible you could be becoming ill (or are ill), or you're nowhere near it mentally today (I find this happens to me – you know, when I don't argue with the missus and there's about as much chance as hitting the numbers as winning next year's Tour de France. Shame really, because it'd be pretty cool if a 58 year old codger could do that. Be a great headline: balding, old, masters fatty kicks Tadej's arse. Only got to add about 140 W onto my FTP, could be doable by July).
If anything such as dizziness, significant or unusual pain is occurring then these would be reasons to definitely stop immediately.
Like most people, I've had good sessions, average sessions, and sessions so bad I've wondered what's the point? But as a coach I've seen a ton of people have the same reactions. One or two bad sessions is totally fine and absolutely nothing to worry about. Continued bad sessions is something to worry about.
So before you beat yourself up about the next one, check the numbers are right first. Run your last test through my calculator here — https://www.cyclecoach.com/calculator — so that you know whether you're actually 15 W down, or just chasing a target from last summer that doesn't fit you anymore.