Dead Intervals
 

Tonight my workout was supposed to be 3-4 15 minute intervals in zone 4 hr.....I did these two weeks ago and nailed them perfectly, but tonight I bombed.......I couldn't hold zone 4 and barely held zone 3 on the first interval.....On the second interval I pushed hard but never got into zone 4 and subsequently aborted at about 12 min.....I didn't attempt another and just did a high cadence spin in zone 1 for the duration of the workout...

What causes this?? ... why was I unable to get my hr into zone 4 and maintain when it was no problem previously?

A myriad of reasons.
How is your nutrition?
How is your hydration?
Are you resting?
Are you getting enough sleep?
Are you under any stress?
Was it windy?
Do you just need a day, or even a week off?

+1 You prob need rest, a day off, easy day, etc.

It depends on what happened between last week and this week. It could be illness, overtraining, new training, injury, you ate something different...

I'd take it easy for a day or two. Then get back to training normally. Don't try to make up for this workout or anything, just take note that it was a bad day.

at least you did the smart thing and aborted.

tomorrow, skip whatever workout you had planned and take a nap instead.

OP - I had a day like that on Tuesday. I was suppose to do 3x15 @ threshold wattage intervals. I couldnt get my hr even close to LT. I lost 5 lbs last week and I didnt get much sleep Sunday and Monday. So I knew why I was less than good. I rode the rest of that night at Tempo, since thats all the higher my HR was going anyway. I ended up hitting new PRs for 20 and 30 min power anyway. Wednesday I rode pretty easy for an hour. I was good to go for last nights hammer fest. HR was shooting back up where it was supposed to. I ended up doing close to an hour averaging 3 beats below my LT. It was a hard frigging ride. I almost made the whole way without getting dropped. But I got farther last night than I had since before my bad crash in June 2006 when I was making the selections and duking it out for the final sprints (the reason i crashed in 2006). Today I hurt like a mofo.

Take a day off. Take 2. Get some sleep. Eat a meal. Have a beer. Its worked for me. It'll work for you too.

did your position change? cadence different? cooler temperatures? any of these might also effect your hr vs fatigue.

how was your perceived exertion? HR is fickle and subject to lots and lots of "noise". As others have stated it could be cumulative fatigue from prior days/weeks training, hydration, weather, nutrition, amount of sleep or rest or time spent on your feet non-bicycling that contributed.

I've had workouts like you're describing when training with HR as primary metric, and found it frustrating. Also had workouts when using HR where my heart rate was soaring straight through target zones effortlessly for no real reason I could identify. That's a reason that I took the HRM off last year and just trained with sensations, speed, critical distance, and this year am using power + those other metrics, but find myself still primarily relying on sensations. Don't read too much into it, it's one day. Take a few zone 2-ish days, hydrate, focus on sleep/rest, eat well, and you'll be flying again in no time.

Also, if you're really overloading yourself, you'll have days where you just cant hit your target HR or power #s, usually happens to me at end of a 3 week build, then you know it's time to chill for a few days.

+1