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Heart Rate Variability - Part 2 (How to use it)

In Part 1, the bare essentials of the science of Heart Rate Variability (HRV) was presented. In Part 2, I wanted to share my personal experience with HRV as an athlete and also my perspective as a coach working with athletes that measure HRV.

To recap, we can use HRV to help us understand how much stress we are under from training and life. Stress comes in many forms, not all of it bad, but does add up into the equation of how much training load we can handle. We can use HRV info to help us understand how stressful a flight can be or a tough day at work. We might find that running around getting ready the day before a race adds significant stress to our bodies that has a negative impact on performance. We might find that dinner and laughs with good friends helps with recovery. We can also use it to understand how we are recovering from our recent training or how hard certain workouts are for us. Trying to measure and quantify all forms of stress on the body is very helpful to use when trying to be our best as athletes and in daily life. A little stress makes us stronger. A lot of stress leads to break down.

There are a few options for measuring HRV that I am aware of at this time. There are wearables such as wrist straps (WHOOP) and rings (Oura Ring) that are meant to be worn 24/7. There are also apps that use an approved heart rate strap, finger sensor, or iPhone camera to measure HRV and resting heart rate (RHR) once a day, first thing in the morning (EliteHRV, HRV4Training, Ithlete). There are more options and I listed the ones I have looked into or have experience with. Once HRV, RHR and potentially other readings are available, the apps use algorithms and your data history/feedback to give recovery scores and suggestions about training or resting on the given day. So even though HRV may seem complicated, it is quite easy to use for training without needing to know the science. The apps do it for you and tell you green light - go, yellow light - proceed with caution and red light - stop. All of the apps have excellent, simple to understand resources to help you understand how to use your chosen HRV tool.

I have noted some pros and cons with wearables and the once a day sensors for your consideration. Most HRV apps sync automagically with Training Peaks (TP) so all of your data can be in one place. I think both are valid to use, but am partial to WHOOP at this time because the recovery scores are more in line with subjective feedback from athletes compared to the other apps I have direct experience with. I note the once a day morning reading recovery scores may be higher than WHOOP with similar HRV numbers for the same athlete. So one app says you are good to go and the other says take a rest day. That is a big difference to me. When I look closer, I almost always find WHOOP makes sense if I look at recent training, athlete feedback and trends. Much of WHOOP’s technology is proprietary so I am not sure of why the differences, but can say I have been very impressed over the last seven months of use and am not affiliated with WHOOP. If I had to rank HRV apps that I have worked with, it would be as follows: 1st - WHOOP, 2nd - Elite HRV, 3rd - ithlete, 4th - HRV4Training.

Wearable HRV Device Pros:

  • Sleep data

  • Measure strain for the entire day using HR and HRV data

  • Calorie estimation

  • RHR and HRV measurements are taken during deep sleep cycles and not affected by conscious thoughts or distractions

  • Prompts for user input for sleep, workout perceived effort, workout performance

  • Auto-detects sleep, naps, elevated heart rate consistent with workouts

  • Sleep and recovery recommendations and tips given by the app

Wearable HRV Device Cons:

  • Have to wear these things all of the time or at least while sleeping

  • Measurements are taken when a gadget thinks you are in deep sleep - might not be accurate?

  • The sensor could move and have poor contact with your skin overnight resulting in bad data

  • Battery dies overnight - no sleep/HRV data

  • Strap placement and contact with skin is critical for accurate data

  • Over-estimates calories expended (not by much if the strap has good contact, but can be quite off if the strap falls down too close to the wrist resulting in inaccurately high HR readings.

  • Riding a bike on rough roads may yield bad data for that time period

  • Can only view data on the phone - no display on the wrist strap

  • Cost?

Once a Day HRV Sensor Pros:

  • No need to wear anything 24/7

  • You can take readings the same time each day

  • You can take multiple readings to compare accuracy

  • Lower cost?

Once a Day HRV Sensor Cons:

  • No sleep data

  • No calorie estimation

  • No daily strain/stress scores based on 24/7 readings

  • Taking a reading first thing in the morning may be inferior to during a deep sleep cycle?

Okay, so now you have chosen a sensor to use. Now it is time to get to collecting data just like you probably did when you first got a power meter. I bet many of you remember looking at the power number bouncing all over the place and trying to make heads and tails of this new fangled thingy that everyone said you had to have to be a cyclist. It can be overwhelming at first, but the key is to start collecting data and see what the app says and how it relates to how you feel and your training. Again, it is the trends, so if your HRV drops one day, then you can reflect back on the previous day(s) and see if anything jumps out that may be linked to the lower score - fight with a friend, stress at work, in-laws in town for the month, alcohol, checking social media in bed, etc. Same with higher scores - better hydration, nutrition, foam rolling, meditating, more time in bed, improved fitness from better aerobic development to be able to absorb higher training loads, and so on. You still need to be a detective and it can be a fun game to see if you can get higher HRV scores by better taking care of the recovery details.

When I first started with a WHOOP strap in my preparation phase for the 2019 season, I was getting yellow recovery scores each day. A little tired but okay to proceed with training with caution and keep things in check. That felt right as I was loading a little fatigue on day after day with lower intensity work and did not feel snappy, but also did not feel run down. What jumped out for me was that my recovery scores plummeted the day following strength work in the gym. The first time, I just noted it and then I noted the same trend the day after strength training. I think an important point is that I felt like the app said - tired and it was not a one-off, but a consistent finding over time. I am not yet ready to trust a computer over what my body and brain feel like, but I am willing to let HRV weigh in to make an informed decision. So now I knew the toll that strength training had on my body and could adjust my training accordingly to make sure I was recovering properly in between workouts. It helped me plan which day to do what better and guides my weekly training. If I have hard intervals planned and HRV is lower than normal AND I feel like the app says it thinks I do, then I pass on the hard intervals. I still need to train when HRV is lower and need to make some decisions on my own, but HRV gets more and more credibility from me for my own training as it has helped me over time.

As a coach, it is hard to know how hard an effort was for an athlete and how they are recovering from day to day. Only the athlete truly knows how hard they tried in a workout and how they feel. HRV gives me some insight when coupled with power files and subjective feedback from the athlete. Again, I am looking for trends and not one-offs. A common occurrence with endurance training is that we take 1-2 days of recovery after some hard training. The first day back to training can feel awful with heavy dead legs with aches and pains. This can be because we are ‘too’ rested and need to ‘flush out the carbon’ and get the systems going again, but can also mean that we are still not recovered and a few more days of rest are needed. Enter HRV to help decide if pressing on is the ticket or keep on recovering with easy rides until HRV increases again indicating your body is truly rested. I can also learn how much back to back training, long rides or intense rides take out of an athlete. Athletes that recover very well from day to day might be good at stage racing and can handle more training load. Athletes might be able to absorb an intense workout, but a long ride is very hard for them and vice versa. HRV helps identify that with trends. If you note your HRV drops significantly the day after a certain type of workout consistently, then you have a trend and can better plan when to do this type of ride and how long it will likely take you to recover from it. Or decide it may be too much cost for the potential rewards.

Another interesting thing I have noted is muscle soreness and excellent recovery scores. So the athlete is feeling quite sore and not recovered from a recent hard session and the HRV app is saying green-light-go. I suspect that the athlete’s cardiovascular system is recovered but not the musculoskeletal system? Or the HRV data could be inaccurate. I am not sure of the cause, but think resting is best in this scenario and I think that is pretty straight forward with a lot of soreness.

Another interesting trend I have noted is when an athlete tapers for the big event. It is not uncommon for HRV to start to drop as an athlete becomes more fresh - maybe too fresh. I am still learning about this aspect of HRV and looking for trends with different athletes to see what is normal for them. I have noted that an ‘opener’ type workout that has a little intensity, but not a lot, helps maintain higher HRV scores as compared to just riding easy into the big event. ideally, you can do a practice run with a practice taper and event well before the big show to better know how you respond to a taper and using HRV can be valuable here to identify what works and maybe what does not work for you.

To be honest, I question the accuracy of a lot of data I see from power meters, not to mention HRV. With the WHOOP app, I often wonder if I am truly in deep sleep when the readings are taken. My recent experience with an injury may help shape your opinions. It has mine. The gist is I rode pretty hard when WHOOP said to take it easy and ended up with a minor knee injury. Likely a combination of things led to the injury, but maybe HRV saw it coming and maybe if I had rested instead of riding as the app suggested, I would have avoided an injury. Hard to say, but HRV has dropped when I have gotten sick, after travel and when I have more life stress than normal and can be just the nudge the hard working endurance athlete needs to convince us to take it easy when our brains want to go but our bodies need a day or two.

As I mentioned in Part 1, I initially started with HRV as I was concerned about atrial fibrillation for myself and other masters athletes I work with. I noted my heart was beating hard in my chest after some training sessions and races last season and I did not recall that happening in the past - maybe once or twice a year when lying in bed after a hard evening crit like nationals, which I could explain with all of the adrenaline of the day. My heart rate and rhythm was normal, but the beating felt intense inside my chest if that makes sense. This is all anecdotal, but I can say that those sensations are gone since using HRV and the WHOOP app to help me better understand how much stress my body is carrying at any given time. My training density has decreased - so volume is similar, but intensity is lower, often only completing one interval session per week +/- racing. Many weeks have no intervals if racing on the weekends. I have been an athlete most of my life and found less is more for me compared to others when it comes to training stress and find it interesting that HRV data supports this.

Lastly, we can use HRV to better use the PMC (Performance Management Chart) within Training Peaks, created by Dr. Andy Coggan. There are plenty of articles about the PMC. Check out the new 3rd edition of Training and Racing with a Power Meter where many of these concepts are discussed in detail. I liked the updates. I am surprised at how many serious athletes use the PMC but are not aware of the nuances and how to individualize it. The gist is that the PMC is trying to quantify the stress from training and also how long it takes to recover based on human performance modeling. An accurate FTP is critical for utilizing the PMC and I see most athletes have their FTP over-estimated by 20-40 watts. Do a 40 min TT or Zwift race and take your average power if you want to estimate FTP without a lab. OK, I know I get ranting about FTP, back to the PMC. TSB or Training Stress Balance is trying to quantify fatigue. We can use TSB along with HRV to individualize the PMC better. We can change CTL and ATL time constants within the chart to impact the PMC. CTL is set at 42 days and I would keep it there. ATL is set at 7 days and may be more accurate for an individual if set to 4 days or event 10+ days. See below.

Here are some images to show what I look at:

In the above TP app image - yellow line is TSB (fatigue), green line is HRV, red dots are Training Stress Score (TSS) and blue are Intensity Factor (IF). So a TSB score of 0 represents being rested. -30 is pretty tired, + 20 is very rested. You can go into the chart and adjust the ATL constant and see how TSB relates to how you feel. Change the 7 to 4 and then 10 and see what happens to the TSB. You may need to adjust the ATL constant a few times through the year as fitness changes and fatigue accumulates. Of note is 4/6 with a high IF (blue dot) and see how HRV (green) drops the following day. Same on 4/12 (Sea Otter Circuit Race). 4/20 was Copperopolis Road Race (high IF and TSS). I think you can see that HRV follows TSB and I use this to adjust ATL constants to better reflect individual fatigue and recovery.

The above image is from the WHOOP app. Blue is my daily strain and the yellow, red and green dots are my recovery. You may note many yellows as I am training and trying to accumulate a manageable amount of stress in this time frame. You can also see the blue shaded area April 14-22. I had the strap placed too close to my wrist and my daily strain scores were inaccurately elevated. But you can see that my strain correlates with my HRV. Of note, is that recovery scores are independent of strain scores, so there is no bias with the WHOOP algorithm. We see a low recovery on 4/25 after a very hard training ride. I did not fuel well with carbs after this ride - I have noted poor recovery when implementing low carb periods, so maybe it is not for my body type - another trend to look at to help guide if you should consider periodized carb training.

The above image shows my poor recovery after a flight and then I decided to tempt fate with a long ride that ended up being hard when I ran into a friend. I felt quite good on the bike as far as my legs, but maybe dehydration or stress of travel was too much. If I am honest, I did feel a bit jet lagged and wanted to get a longer ride in with some upcoming road races on the calendar. Regardless, this ride ended up with an injury and missing my only early season target race. I'll ride easy when I see this next time!

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