Question on racing with a heart monitor - What is best strategy. Recently I ran a marathon with a heart rate marathon and was trying to maintain a constant heart rate by adjusting my pace - I found the first 10k to be much slower (~7:20 min/mile) than the 2nd and 3rd 10k (~7:00 min/mile) and the 4th 10k I had a hard time maintaining the heart rate and slowed down a bit (~7:25 min/mile). This is my 2nd marathon so pretty new at this - very happy to come down from 4:30 marathon last year to 3:10 this year.

Overall it was good enough to get me a qualifying time to go to Boston but now I want to get my best race in Boston. I'm fairly certain I can overcome the 4th 10k slowdown with a bit more pushing myself.

Question is - why is the 1st 10k slower than the 2nd and 3rd 10k for the same heart rate target - Does this indicate a problem with my warm-up before the race or should I be targetting a faster heart rate for the first 10k to even out the splits with my 2nd and 3rd 10k.

Any experience out there with this - appreciate your input?

I race with a heart rate monitor all the time. You've experienced exactly what you should expect for a constant heart rate. Heart rate drifts as your body heats up and you dehydrate, so your pace has to slow down to compensate. You're better off trying to hold mostly steady with a few beats drift for the first half, then slowly let it climb by 1-2 beats every couple of miles until around 22, and then kick it in with what you've got left. It takes many iterations to learn the best HR approach for a marathon. Most do not recommend using a monitor in a race, but I've found after running about 15 marathons with the monitor, I can use one to optimize my time. You'll have to be willing to experiment a bit. If you use anything near constant HR, you will always run positive splits. Here's what my splits with average HR per split were in my best marathon this year:

6:41/154, 6:53/163, 6:55/164, 6:52/165, 6:59/167,
6:54/167, 7:16/168, 7:03/168, 7:26/167, 7:08/167,
7:07/167, 7:19/168, 7:10/168, 7:10/170, 7:16/169,
7:17/171, 7:23/170, 7:25/173, 7:26/170, 7:30/173,
7:25/175, 7:30/176, 7:23/176, 7:29/177, 7:51/178,
8:06/180, 3:38(7:31/mile)/178

time 3:12:47, average HR overall: 170

In this race, my target was 3:15 and change, so once I saw I had it in the bank, I didn't bother pushing all out for the last couple of miles.

For further reference, my anaerobic threshold is 177 and my max heart rate is 210 (which is not really a relevant
quantity).

Thanks for the reply - your data does not show what I've been seeing which I find currious. I can see the drift from your time - appears you increased HR target and slowed down as you went. My trend was the reverse - without changing HR target (~160-162) - pace went up until ~18 miles - than I had a hard time maintaining the HR target - either too close to or above the threshold accidentally or just didn't push hard enough - not sure which - (knowing I was going to qualify for Boston - I had the mental excuse to slow down a bit). Here were my splits:

7:25/157, 7:20/162, 7:21/162, 7:17/162, 7:27/162, 7:11/161, 7:13/162,
7:10/160, 7:07/160, 6:55/161, 7:34/160, 7:04/160, 6:59/160, 6:50/161,
7:07/160, 7:07/160, 7:10/159, 7:20/158, 7:14/157, 7:21/157, 7:13/157,
7:33/155, 7:29/155, 7:50/154, 8:11/154, 7:20/160, 2:20/168 (sprinted in)

Anaerobic threshold - I think ~162 (never measured), MHR 190.

I've noticed the speeding up after 5-6 miles before in training runs - thought maybe it was just a part of the body warming up.

I'm currious what you do to warm-up before the race - maybe I'm spending the first few miles warming-up. The race above was pretty big so I didn't get to do a good warm-up - mostly trying to push my way closer to the starting line so I didn't get caught up with the 4 hr + marathoners.

By looking at your numbers, initially I would assume that your AT was around 165 or so, but on second thought, if it is about 162, it would explain why you gravitated towards a lower HR for the second half. One thing that is not obvious from the numbers is where the ups and downs were, which will obviously have a significant effect on the pace and HR.

I don't do anything to warm up before marathons and my numbers are quite strange, which may be an artifact of my training (all endurance training with no speed work or tempos). In particular, you'll notice that I'll be at a low HR for the first 5-8 miles in a marathon at a pretty good pace, at which point things tend to climb for about 5-10 beats and I stabilize. Hence I'll always get in some fast miles up front which for most people would lead to a crash later. Without the HR monitor, I'd never go nearly as fast at first and my overall time would be a good bit slower. Here's another race that I ran much more conservatively because the temps hit 84 and dewpoint was 63. In this one, I decided to kick it in at the end:

6:59/155, 6:56/164, 6:58/163, 7:12/164, 7:08/164,
7:14/165, 7:29/164, 7:32/164, 7:29/164, 7:07/163,
7:38/163, 7:31/162, 7:22/165, 7:29/166, 7:46/164,
7:39/166, 7:51/168, 7:48/169, 7:31/169, 7:46/171,
7:47/172, 8:06/169, 8:04/172, 7:48/175, 7:02/179,
7:06/185, 3:05/184(7:27/mi) (time - 3:17:35, HRavg=168)

No hills - Houston a very flat race - temps in the 50s whole race, well hydrated - even carried a few bottles to help keep hydrated and drank at every station - the after race weight was only a couple lbs less than the start.

It is interesting comparing the biometrics of an ultra-endurance runner as yourself vs the newby marathoner - me.

Observations -

Clearly the depth of your endurance gives you great splits at the beginning with comparibly little effort. Where I am working at 85% mhr to pull off below average splits for my race at the beginning - you are at 78% mhr running your fastest miles.

The mhr=210 would suggest by the simple 220-age you are 10 years old - I suspect you are older. Of course this formula is not real good - it suggests I'm 30 though I'm actually 39. I got a 190 mhr about 5 months ago when I was largely out of shape - I haven't been able to get it above 185 in recent months.

I wonder what your pace would be like if you stayed a few beats off your AT for the whole race. Guess you'd be flying way too fast at the beginning which I guess would slow you down at the end - but I wonder if it really would - could be you'd be way below 3 hours.

My AT is probably something I should go get tested to really optimize the performance. I'm wondering if I can sustain more than 162. I ran a 10 mile run this morning trying to maintain 7:00 pace to see what my HR did - see if it would slow down like in the race or speed up showing drift - it was completely constant without any clear direction up or down with all 1/2 mile splits averaging 163-167 after the first mile. True the last 6-7 miles of the marathon were slower heart rates but I knowing I had better than 3:10 pace going and only needing 3:15 to qualify for Boston - I'm more inclined to think I could have pushed to keep the heartrate/pace with little drift and pulled in another 5 minutes faster with proper motivation.

I really apprecate your responses.

It's nice that you have a flat race to form a baseline - that way you can really make out the trend. I'm 37, by the way (needed 3:15 for Boston as well). That's very interesting about your 10 miler, with even pace and even heart rate the whole time! It's amazing that you're seeing almost no drift at all. That's a great quality to have. Quite honestly, being your first (or near first) marathon, it's not clear you really need to get tested or make any major adjustments, unless you're a data freak like I am! 220-age is just a statistical average over a large sample, assuming a linear profile with age. Unfortunately, it's become accepted as a max heart rate predictor as well (which it doesn't do well). In reality, however, there's no much important about max heart rate, at least for endurance, as the heart beats that occur between the AT and the max really have little value, for training or racing.