Question for the 41 gurus. Today I was doing 2x20 threshold intervals. The 2nd felt much harder than the first. The 2nd interval also had a much lower heart rate than the first, but the heart rate for both intervals was always in Zone 4 (my LTHT is 172 - first interval was mid-upper Zone 4, while second interval was low Zone 4), yet both intervals ended up with the same avg speed and distance covered. I don't understand how that can be.

Here's the ride
https://app.strava.com/rides/10682708

Laps 2 and 4 are where the intervals are. I felt pretty good during the first interval and pushed hard. After recovering for 20 minutes, I slowly ramped up the pace again to just under Zone 4 in a little under 10 minutes, then started the 20 minute threshold interval. I felt like I wasn't going quite as fast but honestly wasn't all that focused on speed. I was just trying to go as hard as I could for the duration of the interval.

But when I look at the avg speed and the mileage covered for both intervals, they're essentially the same. So my questions are, why did the 2nd interval seem much harder than the first, why did I have a lower heart rate even though it felt harder, and yet somehow had the same avg speed/distance covered? I'm not sure how this could be.

Occam's razor: the simplest answer is usually the right one.

For me, sometimes my Garmin readout simply goes out of sync.

I don't have a Strava membership, so all I saw was the profile and time, no HR or speed data.

My experience is that at then end of an interval workout, I'll sometimes see my HR not get up as high as it's supposed to regardless of how hard I push. According to what I've read, this is due to accumulated fatigue. Since I usually do these after 8-10 hours at work, it's not surprising.

Other possibilities have to do with eating and drinking. You were out there for probably two hours and change. If you hadn't eaten anything before and/or during your ride, you might've just been running out of fuel. That would account for the tiredness; I don't know whether that would depress your HR. If you weren't hydrating enough, that could do it too. Everyone has different needs for hydration, but I'd go through almost three bottles on a ride like that.

Another thing that might have influenced your situation is how long you took to get up to the designated HR for the second interval. The program I follow involves getting your HR to the target range in 45-60 seconds. You took ten minutes, which implies that you were riding close to the target range for some time before you started the interval, so your body got more than the 20 minutes planned...you don't have an internal switch that goes on and off at the designated HR.

If you used a higher cadence and lower gearing for the first interval than the second, that would have influenced the way you felt as well.

I'm not a guru; these are just SWAGs I'm throwing out there.

How can that be? Heart Rate and perceived effort both fluctuate with a bunch of factors other than power output, and are comparitively poor measures to train by.

This is why people buy power meters.

If a heart rate monitor is all you got, its better than nothing, just don't get to obsessed with the numbers, and realize they are going to fluctuate.

HR is not very accurate for repeated intervals for the reasons you have noticed. Do some google searches for training with power vs heart rate and you will understand why.

Check your cadence.

A harder geared, slower cadence can produce the same speed as an easier geared, higher cadence but at a lower HR.

That said, there are a million other variables that could cause this result.

Thanks for all the info, everyone. I see what you mean that there are a lot of factors that could change the expected result. A power meter would be nice and I'm saving up for one eventually, but for right now I'll have to rely on perceived effort and heart rate. I understand heart rate is nearly useless for short intervals, but I thought long intervals like 20 minutes should be fine. I uploaded the same ride to Garmin Connect, so maybe there might be more info there. https://connect.garmin.com/activity/188434218

Just checked for the two interval laps (laps 2 and 4), and avg cadence was 95 on first interval and 94 on the 2nd. Since the avg speed and distance were also comparable, it seems that I was using essentially the same gears. Just that the 2nd interval felt harder than the first, and also had a lower heart rate.

I see. I have seen this happen to me as well on other occasions. Fatigue could be part of it, but this workout was one of my first attempts at trying to reduce that. Last hard ride was 3 days before (Saturday), and the day before I did a 2 hr recovery ride where I didn't let my heart rate out of Zone 1 at all. Still working on getting enough sleep but at least the 1st interval felt really good. Not so much on the 2nd. Maybe fatigue was still a factor.

An hour or so before the workout I had a large bowl of oatmeal. Between the two intervals, I had 1 cliff bar. I weigh 145lbs. During the workout, I went through 1 1/2 24-oz bottles of water. Maybe I needed to drink more, but did take a pee break during the workout so more water might have made me need to stop again. Don't know if needing to pee means I was drinking enough or not but there it is.

I've been trying to make adjustments in my workouts to get more improvements and this was one of them. Before, I would do the same thing of taking under a minute to get to my target range and felt that the effort taxed me too much, leaving my muscles burning too much during the interval and forcing me to back off a little bit while they recovered. So I thought that I would try ramping up the pace over a longer period of time and settling in for the interval. Even though it felt easier getting up to the target range, maybe the impact is that even though my muscles aren't burning during the interval, I've used too much energy getting there and now feel like there's not enough gas in the tank...

A lot of what you guys are saying in the replies makes sense. Maybe it's a combination of all of these factors that together made it feel the way it did... One day, I'll have a power meter and hopefully all of this wishy-washy-ness will disappear.

You're overthinking this. Using HR worked fine for your intervals and it looks like you had a decent workout. It's normal for the 2nd interval to feel harder than the first and the 3rd even harder!

If you had a powermeter you'd probably find your power on the 2nd interval was a little lower than the first. The speed could change or not depending on wind, traffic and your riding position.

Yeah, you're probably right. Makes sense that later intervals feel harder than the first. HR is still within reason of the target so I'm probably overthinking it.

Everyone posted a lot of good info on things that could affect results while training with heart rate, so now it's time for me to do a lot of reading while I save my pennies

Thanks all