Heart Rate Variability

[UPDATE 12/30/2015] While going over the additional data, I realized that I had a typo in Excel formula while computing RMSSD (Root Mean Square of Successive Differences). As a result, instead of average of all RR intervals, I was using average of only the first and last RR intervals. Oops. So I updated the RMSSD numbers and my concluding comment.


One of the features of Polar V800 is RR Recording. It measures heart rate variability (HRV). Heart rate variability is the differences in consecutive heart rates. For example, if heart beats at t=0, 1, 1.9, and 2.85 second, the heart rate would be 60, 66.7, and 63.2 beats per minute, and the HRV would be 6.7 and -3.5 bpm.

It’s also often expressed as RR-intervals, which is short for R-wave-to-R-wave interval. R-wave is the wave created by heart beat where the peak is found. RR-interval is also known as Inter-beat Interval (IBI). In the above example, RR-intervals (or IBIs) would be -0.1 and 0.05 second. (For more information on R-wave, visit here.)

It is nothing but a calculated value from straight off heart beat measurement. So what’s the deal? Why would anyone bother measuring their HRV?

It turns out that the HRV has been used in psychology experiments to represent how relaxed the subjects are, and how ready they are to deal with external stresses. The greater the HRV is (meaning the greater the heart rate varies from second to second), the more relaxed the subject is. On the other hand, the smaller the HRV is (i.e. the heart rate is more or less constant), the more stressed she is.

If anyone cares to recall their human anatomy class, HRV measures whether sympathetic or parasympathetic nervous system is kicking in. Sympathetic nervous system has to do with fight-or-flight response. It increases heart rate, and dilates pupil among other stress-related physical responses. Parasympathetic nervous system has to do with slowing down physical responses, such as decreased heart rate, stimulating digestive system, etc.

Because the HRV provides a simple measurement of how relaxed and ready we are, some athletes are using it to see whether they are over training. Thinking is that if the HRV shows little variation, we are under stress, and may not be the right time to start the training.

When I found out about HRV, it seemed counter intuitive. I would have thought that if I’m relaxed, I would see little variation on my heart rate.

Another thing that seemed odd to me was that I couldn’t find a scale (even relative scale) of what typical HRV range would be.

So I decided to experiment.

Minutes before my 15-mile run yesterday, I took my HRV measurement to establish my baseline. Not knowing what to expect, I wanted to get a good sample size, and I figured 5+ minutes would have enough sample points (about 300+ heart beats). Also to minimize the effect of other stresses, I took the measurements while lying down.

Here’s what my RR intervals looked like before my run.


And here’s what they looked like an hour and 45 minutes after my 15-mile run.



As predicted by literature, my heart rate varied noticeably more before my long run, compared to the one after my long run.

To quantify the variability, I ran the Root Mean Square of Successive Differences (RMSSD) of the 337 heart beats. (When I was rested, I had about 337 heart beats in 5 minutes.) BTW, if you want to find out about how to compute RMSSD, you can reference this article that I found.

And to measure how fast I was recovering, I took two additional 5-minute measurements, 23 hours and 45 hours after the run, and computed the RMSSD.

  • RMSSD 00:25 before 15-mile run: 25.551 msec
  • RMSSD 01:45 after 15-mile run   :    4.505 msec
  • RMSSD 23:00 after 15-mile run   : 14.396 msec
  • RMSSD 45:00 after 15-mile run   : 42.032 msec

It looks like my HRV drops to about 18% after my long run. Surprisingly I seem to get full recovery of HRV (plus some more!) within 45 hours of the long run!

That’s way faster than what my V800 estimates as my long run recovery time.

Oh well, even if my heart may be willing, my sore feet say I still needed a rest today. 🙂


15-Mile Run

After my 20-mile run last week, I thought of making a couple of changes to how I do my long run.

  1. Take an energy gel every hour
  2. Start slow and aim to do negative split

To see how they would affect my practice runs, I thought I would start building my mileages up again. So I did 15-mile run today while paying attention to the two points.

Did the two lessons affect my overall performance?

Well, it did affect how I felt after the run, but performance wise it was just about the same.

I ran 15.18 miles in 2:09:39, averaging 8:32 minute/mile. Those numbers are not much different from what I saw in late November when I did 16.14 miles in 2:18:18, averaging 8:34 minute/mile. (I did not take any energy gel during that run.)

But one difference was how I felt. I felt good after the run, and thought that I had enough reserve to go for an extra couple of miles. I don’t remember feeling that way after my 16-miler in November.

I am suspecting that what I need to work on is improving my stride length, and keeping it nice and long during the run.

Oh, and about the negative split. I realized that my training route ends with climb. It was not obvious to me because when I start my run, it didn’t look like a downhill. But when coming back, I could definitely the slowdown due to the incline. Plus I had to wait for the signal to change at the end.

I made stops to wait for the signal to change. I think I lost about 30 seconds in the beginning and about 45 seconds in the end.

At any rate, here’s how my run turned out to be by each mile. If I could have shaved off 45 seconds of signal wait time, I did about even split.




It was 37 degree in the morning. That was a good enough excuse for me to skip my long run today. So I decided to do a fartlek instead.

Fartlek literally means ‘speed play’ in Swedish. It’s about running fast and hard in one minute, and slowing down to catch a breath in another. There are lots of variations of fartlek that people have come up with.

There are two flavors of fartlek that I picked up over the years. One is what’s known as 10-20-30. I stumbled across it on the web while searching for running videos. I like it for its simplicity.

The idea is to run at maximum pace for 10 seconds, followed by medium pace for 20 seconds, then slow pace for 30 seconds. (Hence the name 10-20-30.) Repeat the routine 5 times, and they make one set. Take a couple of minutes of break, and go for another set. I usually do 4 sets, and I found it enough to tire me out. (You can check out the video that I saw here.)

Another variation is 1-minute interval training. I usually do this on a treadmill because the speed does not change as much as 10-20-30. The main idea is to do high pace running (for me about 10-11 miles/hr pace) for one minute, followed by 1 minute jogging or walking (between 4-6 miles/hr pace works for me). I repeat the routine about 12-14 times with warm up and cool down, and that seems to do it for me.

Today, for some reason I didn’t want to do either. Maybe it’s because of the cold weather, or maybe because I broke my routine by taking a few photos before my run for the first time.

Instead I did a totally free-style fartlek. Here’s what I ended up doing.


I ran about 23 minutes ranging from 3.5 to 14.7 miles/hr. I was a bit surprised that I was able to push myself to 11-12 miles/hr. I don’t think I saw those numbers earlier. It must be my 1-minute interval training and long runs working.

Overall stride length (the second row) and cadence both (the third row) moved in lock steps with the speed (the first row). Not surprising from my earlier analysis that speed comes from increase in cadence and stride length.

However, after looking at the chart a bit carefully, there was a few interesting patterns to note.

Let me highlight what I saw.


The first four squares on stride length graph show that I gained my speed from increases in stride length. But then later in the run, when I pushed myself, I gained my speed from increases in cadence.

This seems to suggest that my legs are not opening as wide when I get tired, and tend to rely on higher cadence to run faster.

What does this mean?

I think I need to work on bringing my knees up higher consistently. It seems strengthening the hip flexor muscles would help maintain the high knee raises.


20-Mile Run

4:50am. Alarm goes off. I reach for my iPhone, and turn the alarm off.

4:52am. Pick up the phone again. Open to see the weather report. It has not started raining. 60% chance of raining starting 8am. Gotta head out now to get back home before rain starts. What if it rains while I’m out on my run? Just do it.

5:05am. Drink a cup of water and fill up the water bottles.

5:10am. Take out three Gu gels from the cupboard. Open and eat one Gu gel. Very thick. A full cup of water to wash it down. I make a mental note to conserve water while running. I need at least two gulps to get the sugary taste out.

5:16am. Start the run. Very mild weather. Mile 1 at 8:01 min/mile pace. A good start.

5:45am. Get to Iron Horse Trail. No light. Crossing the Amador Valley Blvd, there are very dim lights coming out from houses near by. Very strange feeling. Imagine how ultra-marathon runners might feel. Running solo on a dark trail, not seeing the end, no running companions. Just listening to my footsteps, feeling the wet ground under my feet, staying on the trail. Cannot see where my feet are landing. Can only make out the gray silhouette of the trail. This might as well be a dream.

6:14am. Mile 7 complete. Average pace at 8:20 min/mile. Reach in for the second Gu. It takes 4 bites to chew and swallow the Gu, and another 3 gulps of water to settle them in my stomach. Eating a Gu takes 4+ minutes.

6:20am. Rain rolls in. Starts out a little bit by bit. But within a few minutes it develops into a decent rain. Wonder if I can complete 20 miles in this weather. Consider turning back. 18-mile run is not a bad long run. But there is only a little more than a mile left. Get it done!

6:25am. Rain becomes quite heavy. It is now showering on everything. Difficult to keep the eyes open because of raindrops. Long-sleeve shirt starts to stick to my arms and chest. I’m getting soaked. I wonder if I made a right call to continue pressing on.

6:30am. There! Half way mark. As soon as I get to 10 mile mark, I turn around. Rain tapers off a bit. I get the renewed energy. It must be the gel from 7-mile mark that I had. It’s working.

6:42am. At a major intersection. Iron Horse Trail meets Bollinger Canyon Rd. I see cross traffic doing 40 miles per hour in the rain. I stop to press the button to get a walking light. I stop moving my legs. A Subaru is slowing down as it pulls up near me. In a pouring rain, a guy standing outside with water belt tied across his waist waiting for the light to change. I would think the same thing. What a nut job. As soon as the light turns to walking sign, I run. Yes, I came out to run in this rain.

7:04am. Mile 14. Time for my second Gu. Saving the best for last, it’s berry flavor. I take out the gel from my water belt pouch. Hmm… Something’s wrong. I cannot open it. My hands are numb, and slippery from the rain. Plus I just cannot get any gripping power on my fingers. I concentrate to tear the Gu open, but it keeps on slipping. All my glucose must be going down to my legs. I need this Gu! After third or fourth try, I rip out the tab. Even then, it’s a struggle to squeeze the Gu out. (Mental note: Try chewable next time.) I taste the berry a bit, then it quickly turns into sugary aftertaste, same as the caramel and vanilla bean Gu. I wash it down with water.

7:22am. Mile 16. My right foot feels funny. I feel my toes twitching. Shoot. I have a cramp. Still have 4 miles to go. I cannot stop. But each time I land on my right foot, my toes are spreading out. Salt. Is that it? Low sodium? Don’t think that is it. Remember reading that there are enough sodium in energy gels for long runs. What could it be? Have to slow down the pace. There goes my average pace. Running turns to walking. For a moment, think about running it out. But then remember the age old advice: Listen to your body. Don’t get injured.

7:40am. Down to home stretch. It’s never easy to finish the last couple of miles. I am landing on my mid-foot to avoid cramping. Legs are heavy. No regards to running form. Just thinking about treading the rest of miles without getting cramped right foot. My watch reads 9:45 mins/mile. How did I do 8 mins/mile earlier? My legs feel as though they are tied to a tire.

8:15am. At last. I see my home. No more rain. I stop my watch. Total running time: 2:59:25. What a run. It’s time to recover.


Mile splits from 20-mile run. It fell apart at Mile 17 when I started to get a cramped right foot. Need to work on avoiding cramping. But mid-run snacks worked out great.

Two Ways To Run Faster

When thinking about running speed, it becomes obvious that there are two components.

One is how fast the legs are switching, that is, stride rate (also known as cadence). The other is how long each step travels, that is, stride length.

Everything else that a runner does, such as posture, upper body movement, breathing, etc. are all meant to increase either stride rate or stride length, thereby increasing the speed.

Now then the how-to-run-faster question becomes a multiple-choice problem. To increase the speed, a runner can either

  1. Increase the stride rate,
  2. Increase the stride length, or
  3. Increase both

From my reading of discussion threads and blog articles, there are many debates among runners regarding optimal stride rate. There are several articles that seem to suggest that optimal stride rate is 90 strides per minute (180 steps per min). Does that mean if I were to increase my speed, I have to increase my stride length?

Let’s figure it out by analyzing my own interval training data, where I vary my running speed.

How am I varying my speed? Is it by controlling stride rate, or stride length, or both?

Thanks to my stride sensor, I have the following data from my interval training the other day.


Red line is showing the stride rate (cadence) on the left Y axis, and purple line is showing stride length on the right Y axis. What’s interesting is that as I increased my running speed, I increased both stride rate and stride length. (This was done on a treadmill while controlling the speed. I remember the initial speed was at 6 miles/hr, and I jumped to 10 miles/hr. If you do the math, you’ll notice that my stride sensor was reporting my speed a bit lower. I’ll talk more about that later.)

When I increased the speed from 10 to 10.5 miles/hr toward the end (two wider peaks), I was increasing stride rate instead of stride length on the top end of the speed.

On the lower end of speed, when I slowed down to 4 miles/hr walking pace around the last two peaks, I was modulating my stride rate while keeping my stride length constant. This meant that my jogging (about 6 miles/hr) stride length is just about the same as my walking (about 4 miles/hr) stride length. What accounted for slower speed was my stride rate, which dropped from mid-80’s to low-60’s.

This is easier to see when stride rate is plotted against the speed.


In the above graph, the X axis is showing the stride rate (cadence), the Y axis is showing the speed, and the color of each circle is showing the stride length, where the darker the color, the longer the stride length is.

Most of my interval running had the stride rate between 85 and 95. And the speed within this stride rate zone was primarily controlled by stride length.

This shows I’m using my stride rate to vary my speed at the lower end, and primarily using the longer strides to pick up speed at the higher end. But in the very top end, I tend to rely on stride rate to increase the speed even further.

If I were to plot out these three phases, they would look like the following.


Yellow is the walking to jogging where I’m varying stride rate with minimum stride length. Orange is the jogging to running where both stride length and stride rate are increasing. Red is the final phase where stride length is staying constant yet stride rate is increasing by a bit.

It is interesting that increasing stride rate is where I can gain the last bit of speed. Reading about how a professional runner used higher stride rate in the home stretch to win a race, I somehow feel validated.


18-Mile Run

Yesterday was my 18 mile run.

It started out great. I have been watching how to improve my stride length by folding my legs faster and bringing my feet closer to my butt, and it must have helped in the beginning. I was doing about 8:15 minute a mile, and felt great.

Oh, I should also mention that I learned a lesson from my 17 mile run, and ate a banana before heading out. It must have counted for something.

Going up to San Ramon was a breeze. I was doing about 8:25 minute a mile all the way up to 9 mile mark. In fact when I reached my half way mark, I felt like I could go for another mile to make it 20 mile run. At least that’s what I felt like at that time.

It was nice going until about Mile 13.
After that, my legs turned into bricks.

Coming back was a different story. Starting 13 miles, I could feel that my legs were getting wobbly. Breathing and upper body were okay, but my legs were getting heavier and not moving. I thought I was switching my legs at the same pace as the first half, but on my watch it was showing 9+ minute per mile pace. Even when the road was clear, each crosswalk gave me an excuse to stop and wait for the light to turn green. When it turned green, I jogged. In my mind, I was already visualizing how good it would feel to lie down on the couch and kick up my feet.

By the time I got to the final 3 miles, I barely jogged. Mostly it was walking, and waiting for the light to change.


Comparing it with the earlier week, it was not much different. I remembered how difficult it was at the end of my 17 mile run. Yesterday’s run was no better.


I now think that I must have depleted glycogen. For my 18 mile run, I was out on the road for 2 hours 49 minutes. I should have gotten some snack, either energy gel or sports drinks. All I had was three 10 oz bottles, and a banana that I had before the run. Looking back, the banana that I ate before the run must have kicked in during my run because at the mid mark I felt like I could go for 20 mile run.

So I ordered some energy gel for the first time. I read online that you should eat a gel every 1 hr or about every 6 miles or so. Boy, no wonder I felt out of energy. I was running close to 3 hours without any snack.

Let’s see what kind of difference energy gel will make on my next long run.


Stride Sensor

After I tried Garmin Forerunner 225, I realized how much training data I will be missing if I did not measure treadmill running. Although I ended up not keeping Forerunner 225, I remember how it tracked cadence as well as estimated distance without GPS tracking. I say estimated because it often got the distance off, but considering that it was measuring my steps while strapped on to my swinging right arm, it was a neat feature to have.

Anyway, a few times that I tried Forerunner 225 on my treadmill running I knew I had to measure my treadmill running better.

So when I got Polar V800, I was disappointed to find that the watch did not track running cadence. Although there was a mention of cadence tracking feature coming out soon (Polar V800 has built-in accelerometer), as of now V800 does not track cadence yet. And because it doesn’t track cadence, it does not estimate the distance. Ugh.

Considering it tracks steps as part of daily activity tracker features, I know the hardware is capable. I guess the V800 product managers thought that they needed to make the watch more appealing to casual users. But that meant V800 in treadmill running mode only captured heart rate with chest-strapped heart rate monitor. What a waste of hardware.

That’s missing the entire side of equation. Without distance, I wouldn’t be able to track the weekly mileage, and see how many miles I can cover on my weekend long run. Given that I cover 3 to 4 miles per each workout, and I do about 3 workouts a week, that’s about 10 miles that is missing from my weekly mileage. Cadence would be useful to measure also to see whether I’m moving my feet fast enough.

Once I realized that I was missing lots of data, I started to look into stride sensor. Stride sensor is a stand-alone accelerometer that clips on to your shoe lace, and measures the number of steps and step length. Based on the two measurements, a running watch calculates the speed and distance covered.

Thankfully Polar had its own stride sensor that is compatible with V800: Polar Stride Sensor. It supported auto-calibration with V800, which meant that I could just pair Polar Stride Sensor with V800, and V800 would automatically adjust the multiplication factor based on the GPS distance that was measured by the watch.


So I started to monitor Polar Stride Sensor price. Normally it was sold for around $55. For some reason it is more expensive on Amazon at around $67 as of now, but when I saw the price dropping below $50 on Amazon, I pulled the trigger.

Now I do all my in-door treadmill runs with my stride sensor. It pairs well with V800, and tracks my speed, distance, and cadence.


Oh No! My Marathon Partner Got Injured

I just got a message from my marathon running partner. He just got a confirmation that he has meniscus tear on his knee, and won’t be running the Napa Valley Marathon with me.

This reminded me of the three pieces of advice that I got from a running YouTube video.

There are three ways to get better at running

  1. Run more
  2. Run faster
  3. Don’t get injured while doing it

(Here is the full video if you are interested in watching: https://www.youtube.com/watch?v=1wdqKh6pvos)

I am now starting to read what to do to avoid getting injury. It seems like the street wisdom says

Don’t ramp up the weekly mileage too fast

Vary the training style

Rotate running shoes

I should pay more attention to these as I increase my long-run mileage.