Heart rate spike during intervals & fatigue

Hey all, I’ve been using the 32 week resistance training + sweat spot off season plan. Just started sweet spot part 2 and had something strange happen to me during the “Sweet Spot Burts 3 x 15 minutes” workout this past Tuesday. During the second 15 minute interval, my heart rate went from 150 bpm and shot up to 200 bpm. Also, I felt I couldn’t produce the power anymore and started feeling a bit light headed. I slowed down to recover and my heart rate went down to 160 bpm, which is still quite high for me. I started the 3rd interval, but my heart rate shot up again and I decided to cut the ride short and ride home easy.

Here’s a screenshot of my ride analysis (heart rate spike is around minute 46):

I found this ride surprising as I don’t think this has ever happened to me before (high heart rate & not being able to complete the intervals). Usually, I find the efforts challenging, but doable. In the days after, I felt very tired. I decided to skip the Wed. and Thurs. rides as I was feeling pretty wiped out. On the positive side, I was able to ride again on Saturday and Sunday and felt good.

After doing some research, I’m thinking I just bonked. Does that sound right? I’ve been riding for years and think I’ve bonked before, but nothing this severe.

My typical fueling routine for a workday ride is…

  1. head out without eating
  2. maybe eat a gu on the ride

Sounds like based on these fascat articles on pre-ride nutrition and during-ride nutrition I should be eating more.

By heading out without eating when is your last meal? Are you at least eating 3 hours before you start a workout like this? 3 x 15 minute sweet spot burst workout is a huge workout and you should be properly fueled before a ride like this.

I haven’t seen anything quite this extreme. I’m glad you did not continue the workout. Sometimes heart rate monitors can give off false readings but sounds like this was legit as you were not only seeing the higher number, but you felt it and also were losing power. And it also repeated the same spike as soon as you started the third effort. The fact you were feeling light headed, this is unnatural for you and you felt weak is good you stopped.

There could be many reasons for a heart rate spike or increase, but with how dramatic it was my guess would be full on bonk. Just nothing left. I know medication can play a factor so not sure if you take anything, but usually that would be higher from the start and seems first interval was very much in control. Also heat can play a factor and that is maybe not as high from the start but builds but not so sharply.

Did you ride after this at all? Hopefully heart rate was back to normal on the following days. I’d be careful before pushing on to making sure this does not happen again. Check your resting heart to make sure that is regular.

Hey, @Jake. Thanks for the reply. I was doing this workout around 6AM. Last meal was dinner the night before. I don’t take any medication. Temperature wise, it was a cool morning around 45 degrees F.

My next ride after this was Saturday…4 days later. Heart rate seemed totally back to normal (highest HR was 169 bpm on a long climb). In terms of resting heart rate, it looks pretty consistent to me - around 44 bpm except it was 50 bpm on the day of this ride I asked about and 48 bpm the day after.

Do you think it would be a good idea to reach out to my MD to ask about it?

It wouldn’t hurt to check in as we are not doctors. Sounds like it might have just been a bonk since you returned to normal the next day.

I would just find someway to eat more before you ride, even if that early. Again sweet spot burst 3 x15 minute is a big workout and need to fuel for that. Find something that works for you even though its that early. Banana with some nut butter, pre made rice, oatmeal, yogurt / granola, or even just a Clif Bar. But try to take it some calories. Could even sneak in some while you ride.