VO2 Max and The Reason to Measure Heart Rate Variability on the Apple Watch

Keeping track of VO2 max and Heart Rate Variability rates have become indispensable for people who train since it can provide them with valuable information on their cardiovascular fitness. Because these two metrics are so important in measuring not only athletic performance but general well-being, it’s now difficult to find a fitness device on the market that does not measure them.   

What is VO2 Max?

As we’ve discussed in Frequently Asked Questions (FAQ) About Your VO2 Max, VO2 max is the measurement of the maximum amount of oxygen your muscles consume when they are most active. You can improve your VO2 max by specific training. This metric is expressed as mL/(kg·min) where “V” stands for volume and “O2” for oxygen. For example, elite athletes can have a VO2 max of around 90 mL/(kg·min).

VO2 max is widely considered to be the definitive measurement of aerobic fitness. In fact, it is such a critical metric that the American Heart Association1 recommended that it be considered a clinical vital sign. 

Tracking your VO2 max can give you a good idea of current performance capabilities and also help set achievable goals and proper expectations. While it is good to compare your VO2 max to the average, it is affected by factors such as age, gender or altitude. This is why monitoring your VO2 max trend can give you a better idea if your condition is improving or declining.

What is HRV?

Also called “cycle length variability”, “RR variability” or “heart period variability”, heart rate variability (HRV) is the variation in the time interval between heartbeats. 

HRV is not only an indicator of general health and well-being, it can also be used to predict if someone has a predisposition to heart attack (myocardial infarction). 

The brain controls this variation through the autonomic nervous system (ANS). It has two components: the fight-or-flight mechanism and the relaxation response.2 A number of factors can cause and ANS imbalance. These can be stresses like an unhealthy lifestyle, emotional distress, symptoms of a disease and the like. 

When a person is more relaxed, the variation between beats will show up as high. On the other hand, when the person is in flight-or-fight mode, HRV will be low. This is because a healthy heart does not produce a steady pulse, it beats as needed to react to what you are experiencing. 

HRV should not be confused with heart rate. The former is measured in beats per minute while the later is the number of milliseconds in between beats. 

HRV is a good indicator of of both physical and emotional/mental well-being. More specifically, the trend of your HRV should give you a good idea if your health and wellness is improving and you are adapting to stress.

Relationship between HRV and VO2 max

Most people who monitor their VO2 max and HRV are likely to be into fitness. Such individuals are often highly motivated to train and improve all the metrics they are monitoring, including VO2 max. Because of this, driven athletes tend to overtrain themselves which is not only counterproductive but dangerous as well.

While HRV may not provide information on the state of your fitness or aerobic capacity, it can tell you if your training is starting to take its toll. It will show you if you are overexerting yourself regardless of your VO2 max trend. This makes HRV a good indicator when you should take a rest. Additionally, one study3 even showed that HRV influences the prediction of VO2 max. 

However, you should also take into account any changes that you’ve made when looking at your HRV. Your diet, amount of sleep, medication, time of exercise can easily affect your HRV.      

Measuring your HRV and VO2 max

VO2 max can only be measured properly on a machine that can monitor both heart rate and the volume of both oxygen inhaled and carbon dioxide exhaled. Subjects are monitored while they progress through increasing exercise intensity usually on a treadmill or stationary bike, until your oxygen consumption stabilizes despite the increasing intensity.

While devices like the Apple Watch can now measure VO2 max, more accurately with the help of heart monitoring devices like the Polar H10, it still mainly relies on your heartbeat data along with other data the device logs to extrapolate “predicted” VO2 max. You can see how much more accurate the readings are on Better Heart Rate Monitoring with Apple Watch Using Chest Strap Heart Rate Monitor.

Meanwhile, taking your HRV is relatively simpler. Your Apple Watch should trigger a morning reading everyday, or you can take a reading any time you want. It can take a reading while you are working out, walking, or sitting still/lying down during Breathe sessions. 

The Apple Watch uses SDNN to measure your HRV. According to Wikipedia, SDNN or standard deviation of NN intervals (intervals between heartbeats) is a measure of changes in heart rate due to cycles longer than 5 minutes. 

Other devices that measure HRV and VO2 max

Apple Watch is just one of the countless other wearables that can measure HRV and VO2 max. However, wearables that are worn closer to the heart tend to measure HRV more accurately. This is why most watch-style wearables are designed to connect with heart monitors worn on the chest. However, these chest-worn wearables can connect directly to your phone just as well, Some of these are:

To find these devices and other popular heart rate monitors, you can find them at reasonable prices on Amazon.

Meanwhile, here are some examples of smartwatches that can measure both VO2 max and HRV:

Here are other popular smartwatches out in the market

A device I use for HRV that has also been useful in sleep tracking is the Oura Ring.

Improving your HRV and VO2 max

There are a number of factors that determine your VO2 max. While genetics and age play a big role, anyone’s VO2 max can certainly be improved with constant, proper training. Most fitness websites agree that high-intensity interval training (HIIT) works best for increasing VO2 max. In fact, one study4 even showed that it’s possible to reap the same benefits with a smaller quantity of HIIT.

However, there’s also evidence that seemed to point to the contrary, saying that athletes doing HIIT can hit a plateau where they can no longer increase their VO2 max. In this research5, it was also suggested that low-intensity, high-volume training may bring better results.

Meanwhile, you need to take the opposite approach to improve your HRV. The idea here is to eliminate the things that cause you stress. When your HRV trends drop, you should take it as a cue to rest from training, get better sleep, change your diet, meditate or cut out any mentally straining activities.

  1. Robert Ross , Steven N. Blair, Ross Arena, Timothy S. Church, Jean-Pierre Després, Barry A. Franklin, William L. Haskell, Leonard A. Kaminsky, Benjamin D. Levine, Carl J. Lavie, Jonathan Myers, Josef Niebauer, Robert Sallis, Susumu S. Sawada, Xuemei Sui, and Ulrik Wisløff Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign: A Scientific Statement From the American Heart Association[American Heart Association Journals]
  2. Marcelo Campos, MD. Heart rate variability: A new way to track well-being[Harvard Health Publishing]
  3. Leon, Henry. Zea-Robles, Aura. Botero-Rosas, Daniel. Heart rate variability and body composition as VO2MAX determinants[Researchgate]
  4. Martin J. Gibala,1 and Sean L. McGee2. Metabolic Adaptations to Short-term High-Intensity Interval Training: A Little Pain for a Lot of Gain?[SME]
  5. Alan Couzens. How ‘Trainable’ Is VO2 Max Really? – A Case Study[SimpliFaster]

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