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.
http://connect.garmin.com/activity/188434218
Originally Posted by
topflightpro
Check your cadence.
A harder geared, slower cadence can produce the same speed as an easier geared, higher cadence but at a lower HR.
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.
Originally Posted by
revchuck
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.
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.
Originally Posted by
revchuck
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.
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.
Originally Posted by
revchuck
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.
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.