You’re getting ready for another work out, but today seems different. You’re a little more sluggish. Maybe you’re even feeling sore. So you have to decide: Do you take a beat to rest and recover? Or do you keep pushing yourself?

It’s a tough question that comes up often for elite athletes and high performers. If they rest, they might fall behind their training schedules. But if they push themselves, they could end up overreaching and get seriously injured. That’s why sports medicine has been hard at work looking for biomarkers that accurately measure training readiness. And after years of searching, they found one reliable measure: heart rate variability (HRV).

In this article, we’ll discuss exactly what HRV is and shine a light on another well-established marker of training readiness and recovery: cortisol testing…

What Is HRV?

If you haven’t heard of HRV, it measures just what the name implies: the variation in time between each heartbeat. Think of it this way … your heart never actually beats a steady rhythm. Even when you’re resting on the couch, with your heart pounding an average of, say, 66 beats per minute, you may have .97 seconds in between beats. Another time, it may be 1 second. HRV measures just how much you vary from your average beat.

This information can be insightful about your health. As Harvard Health Publishing puts it: “If a person’s system is in more of a fight-or-flight mode, the variation between subsequent heartbeats is low. If one is in a more relaxed state, the variation between beats is high.”

This implies that those with a high HRV can switch gears quickly. Their system is flexible and working effectively. They may have greater cardiovascular fitness and be more resilient to stress. On the other hand, a low HRV has been linked to issues such as depression, anxiety and an increased risk of cardiovascular disease. It may be a warning sign that something is up.

With this information circling, many companies have promoted wearable devices that measure HRV. It’s become a one-stop-shop for gaining awareness about our health. And it can certainly be helpful. There’s one problem, though...

HRV has also become a popular method for gauging overtraining. The rationale is that as you work out and get healthier, your HRV will rise. But if your HRV decreases suddenly, it could signal that the athlete is pushing themselves too hard.

It makes sense on the face of it. Researchers have noticed a lower HRV in a number of athletes right before competition. The athletes are excited or anxious in these cases, not burnt out. In one study, Andrew Flatt, Ph.D. at Georgia Southern University’s Department of Health Sciences and Kinesiology, collected data from a collegiate sprint swimmer before a conference. The swimmer saw an acute drop on the first day he competed, when he set a personal record.

Flatt went on to explain: “A low HRV score as a result of fatigue also does not necessarily forecast impending reductions in performance.” He worked with three high-level tennis players, who showed improved performance at the end of their training program, despite showing decreases in their HRV.

On top of that, Flatt reports that a higher HRV might not always be a good thing. Several studies show increasing HRV trends in overtrained endurance athletes. In one example, their exercise performance decreased while their HRV scores increased following a three-week overload period, compared to a control group who saw no changes.

And in one 2021 meta-analysis, several institutions around the world gathered the findings of eight HRV studies. In these cases, participants who monitored their HRV saw clear improvements in certain physiological factors (for instance, lactate threshold). But they saw no statistically meaningful improvement in their cycling or running times.

In the end, more research is needed in this space to ensure that athletes are getting the most accurate information possible about their health. So what can help give clarity?

The Science Supports Cortisol

Cortisol is your body’s main stress hormone. Think of it as your body’s natural alarm system. While you produce it daily in order to help regulate your sleep-wake cycle, your metabolism, your blood pressure and more … cortisol levels surge in your bloodstream during periods of stress.

Sometimes this is a good thing. It helps you focus and react quickly to threats. That’s why cortisol levels can rise when you work out. Your body is under stress, and cortisol is meant to help you handle it. But when you train too hard for too long, those cortisol levels can remain elevated, resulting in a number of health complications. That’s why it’s important to monitor it. (You can read more of the research behind cortisol here.)

The science supports this. As our scientific advisory board member Professor Chris McLellan put it: “There’s an abundance of evidence to substantiate cortisol as an indicator of internal and external training load. And that can be incorporated into a training and performance planning model.”

Professor McLellan went on to tell us: "Normally, under stressful situations, HRV decreases and cortisol increases. And there are a number of studies that report this trend in psychologically challenging situations. But most studies have not established significant correlations between HRV and cortisol."

So, if you're looking for a comprehensive method of measuring chronic stress while training, cortisol testing is a good addition. That way, you’re able to monitor your overall health, recovery and preparedness when training.

The other main takeaway is this: Recovery is important. No matter if you’re an elite athlete or just trying to bring your performance to the next level, it’s vital to understand the science of stress. That way, you can make the best decisions about when to push yourself and when to rest. That’s the path to performing better in life.

Before you go, we have one more question for you: How can we get better? Here at Pardigm.com, our goal is to gather the best information to help you understand the science of stress. If you think we missed anything in our research, please let us know. You can reach us here.

Related Research:

1. Heart rate variability: How it might indicate well-being

2. Interpreting HRV Trends in Athletes: High Isn't Always Good and Low Isn't Always Bad

3. Monitoring and adapting endurance training on the basis of heart rate variability monitored by wearable technologies: A systematic review with meta-analysis

4. Association between Cold Face Test-induced vagal inhibition and cortisol response to acute stress

5. Cortisol is significantly correlated with cardiovascular responses during high levels of stress in critical care personnel