In an era when athletes are inundated with data from power meters and wearable sensors, it’s important to be selective about the data you pay attention to. Not all data is equally valuable, nor are all sensors accurate and consistent enough to provide data worth listening to. Of the options available, heart rate variability for cyclists (HRV) appears to be one of the most useful for gauging fatigue. Dr. Marco Altini, PhD, a leading HRV researcher and creator of the HRV4Training app, recently presented at a CTS Coaching Education seminar. Here are some of the practical takeaways for cyclists.
What is Heart Rate Variability (HRV)?
When your heart beats 60 times per minute, those 60 heartbeats are not evenly distributed at a rate of one per second. There are minute variations in the timing between beats. This is caused by a constant balancing act between your sympathetic and parasympathetic nervous systems (SNS and PNS, respectively). The SNS is your ‘fight or flight’ system and the PNS acts to maintain homeostasis. When you are rested or not stressed, the PNS can respond to minor stressors quickly. This leads to greater variability between heartbeats. When PNS activity is suppressed by fatigue or stress, your heartbeat is less responsive to changing stimuli. As a result, HRV goes down.
The problem is, the factors affecting heart rate variability for cyclists are more complex to conclude, “Higher HRV is always good, lower HRV is always bad.” How and when you measure HRV matter, and the context surrounding the measurements must be considered as well.
HRV should be measured either first thing in the morning or continuously throughout the night as you sleep. Morning measurements are typically taken manually by either using a smartphone camera, as with the HRV4Training app, or a heart rate monitor capable of capturing HRV. Overnight measurements are taken by wearable sensors. Preferred sensors, according to Altini, include Whoop, Oura, and Garmin. The data is not perfect, but it’s closer to electrocardiogram reference data than an Apple Watch, which is not recommended.
Why measure HRV instead of resting heart rate? HRV is more sensitive, meaning factors that might cause minor or imperceptible changes in resting heart rate cause a more noticeable change in HRV. This may be important for aging athletes who experience less of a change in resting heart rate due to fatigue. Alcohol consumption also disrupts HRV to a greater extent than resting heart rate.
“Normal” HRV Values
An athlete’s absolute HRV values are highly individual. As a result, there aren’t ‘normal’ values for HRV. Rather, what’s valuable is an athlete’s acute HRV relative to his or her long-term data trend. To make reasonable decisions based on your HRV, you’ll need to commit to recording daily measurements for at least two months.
When to measure HRV
Some of the most interesting takeaways from Dr. Altini’s presentation involved caveats to the standard guidance to measure HRV in the morning or continuously overnight. First, the recommended protocol for morning measurements is: wake up, use the bathroom and return to your bed, then measure your HRV for 1-5 minutes while sitting up. Why does Altini recommend sitting up instead of prone? HRV measures how well your PNS responds to stressors and return to homeostasis. Sitting up creates a small orthostatic challenge for your PNS to handle.
If you use a wearable device to measure HRV overnight, ideally the device should measure continuously or very frequently. HRV changes with sleep cycles. With continuous measuring, the variations even out. But if you take infrequent samples throughout the night, it is difficult to use the information.
Exceptions and Caveats
Here’s where things get interesting. What if you have a heart arrythmia? According to Altini, arrythmias introduce noise into your data and artificially elevate HRV. If you have frequent arrythmias during the night, your overnight data may not be usable. People with arrythmias may be better off measuring HRV in the morning. By doing so, they can choose to take the measurement when they are not experiencing arrythmia.
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If your wakeup routine is unpredictable, as may be the case for first responders (i.e. firefighters) or parents of small children, an overnight measurement may provide more consistent HRV data. On the other hand, if you are a shift worker and have variable sleep and wake times, take your measurement when you wake up. This at least creates consistent conditions, taking the measurement after a known period of sleep.
What to do with HRV Data
Measuring HRV consistently over a long period of time helps identify trends in the data. This is important for establishing your normative range, so you can tell when your values deviate significantly from your own normal.
One of Dr. Altini’s most important recommendations was to consider “stable HRV” to be good, rather than “high HRV”. If you are recovering well from workout and lifestyle stress and adapting to your training, your HRV values will stay relatively stable day to day, or on a 2- to 3-day rolling basis. When there’s a large deviation from normal, either a spike or a drop, take note. And always consider HRV along with subjective feedback about how you feel.
If HRV has any predictive value, it’s that a drop in HRV can indicate you’re likely to perform poorly. Yet, the inverse doesn’t appear to be true. A sudden spike in HRV doesn’t indicate you are more likely to win. If your HRV drops, your subjective feedback is that you feel like crud, and your future training includes hard workouts, consider swapping for an easier workout or resting more. In competitions, however, athletes often defy low HRV values – perhaps caused by travel or pre-race jitters – to win. Perhaps take this as a sign to keep heart rate variability for cyclists in perspective. Many coaches hide HRV from athletes before competitions to avoid doubt and negative thoughts.
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