Just as we're getting used to the idea of heart rate zone training in our running, there's a new metric in town: Heart Rate Variability (HRV). But before you start stamping your running shoes in protest at yet another complex new number to grapple with, hear us out, because HRV doesn't have to be complicated.
With the right tools HRV can be simple to monitor and with a little knowledge it's even easier to put into practice to help you train better, whether your goal is a sub-three-hour marathon or your first 5km.
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Here's our guide to using HRV for marginal running gains without the need for a sports science degree.
HRV and wearables
The variety of wearable devices offering heart rate tracking has risen sharply in the past few years. From the chest straps like the Polar H10, to hearables like the Jabra Sport Pulse headphones, via optical sensor-laden GPS watches and fitness trackers, it's never been easier to monitor your BPM during workouts. But how hard your heart works during a session is just the half of it, and more recently we've seen the big hitters, Garmin and Polar, add HRV-powered tests to their higher-end products.
Garmin offers an HRV Stress test by pairing a Garmin HRM chest strap with the Forerunner 935, Fenix range and Forerunner 630 to name a few. Meanwhile owners of the Polar H10 chest strap and Polar V800 can access its Orthostatic test. You'll notice both companies recommend chest straps for monitoring HRV effectively and that's something Jason Moore, founder and CEO of EliteHRV, a smartphone app that monitors HRV trends, also recommends.
"When it comes to HRV tracking you need a very high level of accuracy, so a chest strap such as the Polar H10 is the way to go," says Moore. "The problem with wrist-based solutions is that they can be affected by too much movement of the arm and the sensors have been built to fit into the watch designs rather than specifically for monitoring HRV."
If you don't much like wearing a chest strap ‚Äď and let's face it plenty of us don't ‚Äď Moore's team has been busy working on a wearable solution set for general release in February 2018. CorSense HRV is a finger-worn monitor that's been created specifically to measure HRV, with a design that's a little more friendly than the chest strap.
What is heart rate variability and what should I look out for?
If you want a more detailed explanation of HRV, we cover that in an earlier piece. But distilled into a single mouthful, HRV is basically the measurement of the time interval between heartbeats. The duration between the two peaks in a cardiogram.
Broadly speaking a higher HRV is an indicator of higher levels of fitness while a low HRV can also be a sign of overtraining, stress or incoming illnesses such as colds.
Monitoring your Heart Rate Variability (HRV), therefore, can be used to chart fitness progress over time, helping to track how your body is adapting to your training regime. A vastly simplified way of looking at this, is that if your HRV slowly increases over time, it's a good sign your fitness is improving. Meanwhile any sudden changes in HRV, particularly downward, can indicate poor recovery, heightened stress or the onset of illness. And that's where things get particularly interesting. By tracking HRV over time, you can start to see how well recovered you are from your recent training sessions and alter your training schedule accordingly.
Training with the heart
So HRV is just for serious runners and professional sports people, right? Wrong. HRV is relevant for anyone who wants to train smarter, avoid injury or monitor their general health. Even if you just run for general fitness, HRV can help you identify when and how to run to maximise the benefits of the time you spend training. For example, if your HRV is very low, you're less likely to benefit from high intensity runs and so a slower recovery run might be more suitable at that point.
So if you like the idea of using HRV to make your runs more effective, here's where to start.
Get your baseline HRV during basic training
According to Polar's official advice on its orthostatic test (which includes HRV), "six baseline tests should be conducted over a period of two weeks to determine your personal baseline value. These baseline tests should be made during two typical basic training weeks, not during heavy training weeks. The baseline tests should include tests taken both after training days and after recovery days." Follow this approach to get your personal baseline HRV to compare to subsequent results as you train.
Then test yourself often
If you're going to use HRV successfully you need to test yourself regularly. Leading HRV expert Dr Ville Vesterinen, of Finland's Research Institute for Olympic Sports, recommends 3-4 assessments per week for a reliable view of your current training status. "Given the large day-to-day variation, it is better to use long-term (eg seven-day) trends rather than one single HRV value." It's also good to test on days after a recovery and heavy training.
Same time, same place
To ensure your HRV data can be accurately compared over time, it's crucial to create consistent test conditions that are repeatable. That means, as far as possible, taking readings in the same environment. Things that affect the reading include the time of day, your body position (standing, sitting, lying, doing the downward dog) and your activity before the reading (pre or post workout). Garmin and Polar's test protocols vary (one is standing, one sitting) and we'd recommend following the manufacturer's advice on body position but that you do the test at the same time and location first thing in the morning.
Let an app do the number crunching
To make things simpler, you can use a smartphone app such as EliteHRV along with chest straps including the Polar H10 or H7, Wahoo TICKR Bluetooth or Suunto Smart Belt. The app crunches the data from a two-minute daily test and gives you a simple traffic light 'morning readiness' score, which you can use to adapt your training for that day.
Don't forget, context is king
"HRV is a systemic metric," says Moore. "It can give you a big picture view of your health and tell you that your whole system is stressed, but what HRV doesn't know is the source of that stress. This is where all of the other factors beyond your training come in and what we often find is that for most normal people it's often the factors outside of training that can make the biggest impact to HRV."
As a result HRV is more useful when it's used in context with other information about your lifestyle and how you feel. It's good to keep note of other factors that can influence your HRV score, such as general feelings of tiredness, significantly reduced performance levels, muscle soreness, body weight and even blood pressure. This includes whether you've slept well, had alcohol or any outside stresses that may have an impact. The more information you have, the better judgements you can make about why your HRV might be up or down and how well you're coping with your training schedule.
Volume vs Intensity: Know what type of training programme to follow
Go onto any marathon training forum and it won't be long before you stumble on a thread discussing the age-old debate of volume versus intensity. Questions such as "How many miles do I need to run a week?" or "Can I do fewer miles and more speed work to hit a marathon PB?" are really common. HRV may well have an answer to this.
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In a study, published in the Scandinavian Journal of Medicine and Science in Sportsin 2015, researchers measured the response of recreational runners to either high volume or high intensity training over a 16-week program.
Participants' HRV was assessed before the training period started and was used by researchers to try and understand how individuals responded to different volumes and intensities of exercise.
The study found that the runners who had a higher HRV before the program improved to a greater degree when they trained at high-intensity. Meanwhile those with a lower HRV responded better to a higher training volume rather than training intensity.
The researchers concluded that HRV could be used effectively to personalise endurance training plans for recreational endurance runners, "especially to adjust training volume and intensity, to achieve greater improvements in endurance performance." What does this mean for you? Well, rather than following an off-the-shelf marathon training plan, your next marathon schedule could be more effective if it's built based on your HRV before you start.
Know when to do your high intensity sessions for maximum payback
Recent research by scientists in Brazil and Canada has shown that recreational runners in the couch-to-5k bracket achieved better results when their high intensity training sessions were performed on good HRV days.
The study put two groups through a training programme with the experimental HRV group, basing the timing of higher intensity training solely on HRV, measured every morning. A normal, acceptable HRV meant a higher intensity workout was scheduled for the day. If HRV fell outside the range, low intensity training was performed. The results were impressive.
Peak speed in the HRV group improved 10% compared to the control. Time at peak speed only improved in the HRV runners, whose 5km time also improved by an average of 17.5% compared to 14% in the control.
Use HRV to chart your recovery
Recovery is a must if you want to get the best results from your training. Without adequate recovery subsequent training sessions can either be ineffective, or worse, detrimental to your fitness. While a raised resting heart rate (RHR) is sometimes used as an indicator of fatigue, the natural variations in RHR can make it a bit of a blunt instrument. HRV, on the other hand, offers a far more accurate indication of whether you're stressed or fully recovered and ready to go again.
The loose rule of thumb: when compared to your baseline HRV, a low HRV equals not fully recovered and high HRV means go again. Though there are many levels in between and it's not always true that low means bad and high means good.
Low HRV isn't always bad
There are situations where a temporary run of low HRV readings can be a good sign, such as after an intense workout. Failing to adequately stress the body fails to stimulate adaptation, growth, and improvement. Putting the body under the right kind of stress, for example during a workout, can help bring on that positive adaptation. The important thing is to know when to push and when to back off. "By looking at HRV a runner can make intelligent changes to their schedule," says Moore. "For example, if today is a hard run and tomorrow is a recovery day, and your HRV score has dropped, it can be beneficial to move your rest day to today and do the run the following day when you're in a more recovered state."
Don't take a reading on race day
There are a few good reasons for avoiding readings when race day comes. For a start the context of moving into race mode throws up many factors that can affect HRV, including nerves, the potential change in surroundings, different patterns of behaviour, potential lack of sleep due to the pre-race jitters‚Ä¶ these can all make a difference to your results.
More importantly, on race day there's nothing you can change and the last thing you want is the psychological blow of a 'bad' HRV readout on the morning of your target race. Once you hit that start line, you have to believe what you have is enough.
Keep on running