long run pace not stable

Question

I'm preparing for a couple of 10km race taking place in 2 and 3 months. I've started 4 weeks ago a training program suggesting specific training based on heart rate, and each week I should do a long slow run.

The problem is that keeping a steady HR means that my pace go slowly down during the session. I run for more that 60 minutes, my heart rate should stay between 145 and 150, and in the first 20-30 min I can keep a peace lower than 6:30 min/km, later I should slow to 7:00 or even more to avoid my HR going over 155.

Is that normal? Am I training wrong or over-training? Am I simply not well trained yet and my body isn't used to run for more than 30 minutes or so?

Answer 1

Thats normal for me and I think in general its true. Heart rate is really a measure of effort though limited and dependent on many internal and external factors. Note that HR shouldnt necessarily be constant throughout and some coaches (Greg McMillian) say that its normal for it to increase over the total distance and for older runners like myself its even normal to be at a higher %maxHR. See Greg McMillian's book "YOU (Only Faster)", Chapter 13: The Endurance Zone, under "Nerdy Note: The Science of Endurance Zone Training" first paragraph.

For long runs at an easy effort I let myself run a little fast mainly just to establish a good easy effort. After 2-3 miles or about 20 minutes my heart rate stablizes and pace slows/stablizes but as the distance gets longer my pace will still drop slightly at the same HR & effort. Lately I have been trying to keep the same rhythm and pace near the end of a long easy run and even though it feels like Im pushing harder and harder to keep the same pace my heart rate only goes up slightly. Its kind of shocking because near the end I'll feel like Im running a faster run or a tempo pace.

If you look at race prediction times (which tend to follow the curve of world records scaled to your most recent race time) then you'll see there is a large change in pace for shorter distance than longer ones. I think effort over distance is kind of proportional to this curve, and can be used to understand how effort changes over distance in a single run.

Answer 2

Look into cardiac drift - what you're experiencing is a greater effort to maintain the same pace as you go on (so by keeping the same effort, the pace must drop). It's a perfectly normal phenomena, and affects everyone!

Answer 3

During a run there are great deal of influences on your heart rate:

• Wind: Wind can be a great factor in lowering/raising your HR. 2 days ago i did a run in 3 bft. Running with the wind/against the wind at a constant pace did change my heart rate by 5 to 10 bpms
• Fatigue: The further you run, the more fatigued the muscles get. This causes your running form to suffer (if you aren't trained well yet). this makes you run less efficient. Making you need more energy/effort to run the same distance at the same pace.
• Terrain: I don't need to explain to you that running uphill causes your heart rate to increase. Even a slight incline (barely noticable to the eye) can influence the effort needed/heart rate.
• Environment: The temperature & humidity can influence the heart rate. Although you shouldn't notice the difference after 30 min unless you're running in a very warm or cold region
• Dehyrdation: The losing of fluids causes the blood to thicken. Causes your heart to increase the effort to get the blood around and thus increasing the heart rate.
• HR-Monitor ineffectiveness: Some (older/cheaper) heart rate monitors can give faulty data over time. They tend to show inrelieable data.

There are some further factors (like illness,food intake before the run,...) but those are of course not an issue here.

From my personal experience I wouldn't stare blindly at your heart rate monitor. The most important thing is that you learn to listen to your body. And learn to run based on what your body is telling you.

Some great reads:

Factors affecting the heart rate

Factors affecting running heart rate

Factors that affect the heart rate during training