Heart Rate Variability and Overtraining

Every athlete wants to be faster, stronger, better. How do they improve? Training. Elite and amateur athletes alike carefully calibrate their training program to ensure constant progress. But is there a limit?

One of the most amazing things about the human body is our ability to become stronger as needed. Let’s say you’re asked to carry a large boulder from point A to point B. It’s so heavy that you can barely lift it. When you reach point B, arms quivering, you drop it with a relieved grunt. The next day it may seem even harder to carry the same stone. Your arm and leg muscles are sore. But keep this up daily, and guess what? WIthin a week, the rock won’t seem as heavy. You’ll lift it easily. You’ll walk more quickly. Your muscles have gotten stronger from training. Now you’re ready for a heavier rock.

What if, though, on the second day they handed you a rock double the weight? And then extended the distance you had to carry it? You would falter. Become discouraged. Risk injury. You couldn’t finish the task, and definitely would not become stronger. Overtraining occurs when our activity exceeds our ability to recover from the activity. It can derail even elite athletes. Because the treatment for overtraining is prolonged rest (weeks or months), preventing overtraining should be a goal of any training regime. And measuring heart rate variability can help.


How do we build strength? When we exercise, our muscles contract. This stress and effort cause microtrauma to our muscle tissue. Our bodies repair this damage by fusing together new muscle fibers. As the fibers thicken, our muscles grow. But this repair process doesn’t happen while we exercise. It happens when we rest.

Recovery is a period of rest between intervals of exertion. It is essential to building strength.

Overreaching vs.Overtraining

Athletes are competitive by nature. We push. ‘Pain is just weakness leaving the body’,  we tell ourselves. This mindset, however, puts athletes at risk for overreaching or overtraining.

What’s the difference? It’s one of degree. Overreaching is trying to do too much, too soon. Overtraining is doing too much, too often, without sufficient rest.

Overreaching is sometime classified as ‘functional’ overreaching (healthy striving that eventually leads to increased performance) vs. ‘non-functional overreaching’ (NFO). If an athlete does not respond to rest after 2 to 3 weeks of rest, overtraining syndrome may be diagnosed.

An overtrained athlete finds that no matter how hard he works, his performance plateaus, or diminishes. He feels stale. Unwell. Overtraining is not just a physical problem. It can sap motivation, trigger mood disorders, and leave an athlete unable to continue training at all.

It’s a conundrum. You only get stronger/better/faster by increasing your training load. But without without breaks in training and healthy sleep, the body cannot recover. Rest between training bouts is essential. Sleep allows the body to mend tissue, consolidate learning, and renew energy, and is a valuable balm against overtraining. The quality of sleep can be monitored via SpO2 and blood oxygenation.

Robert Herbst is an 18 time World Champion powerlifter who supervised drug testing at the Rio Olympics. “Overtraining,” says Herbst, “can be hard to predict. Many of us have had days where we are tired from work or exams at school and warmups have felt terrible, yet we have had a great workout or performance. On other days where we have done everything right-had a good night’s sleep, eaten well, relaxed-we have performed poorly.

Overreaching, over time, can result in overtraining. Rest will no longer make you stronger. Exertion will no longer improve your performance. Even your metabolism can be affected.

The Science and the Symptoms

Overtrained athletes are susceptible to infection, especially in the respiratory tract. They have higher markers of oxidative stress. Disturbed sleep. They are fatigued. Often apathetic or depressed. They lose their appetite. They may have signs of systemic inflammation. The most significant symptom is unexplained underperformance.

It’s a complicated assessment. Fatigue, for example, is both a cause of, and a symptom of, overtraining. Mood dysregulation is a symptom of overtraining, but can also be a factor. Pain, poor sleep, respiratory distress can each be a cause, or a symptom, of overtraining. Because of the multi-causal nature, it is important to develop a healthy baseline and monitor biomarkers for deviation before overreaching segues into overtraining.

Though overtraining has been deeply studied, there is no fixed definition and the exact process is not understood, Theories include low muscle glycogen, central fatigue hypotheses, glutamine hypothesis, oxidative stress, hypothalamic hypothesis, cytokine hypothesis.

Heart Rate Variability (HRV)

The autonomic nervous system hypothesis centers on heart rate variability (HRV). HRV is the variation in timing between each heartbeat. Healthier hearts have a wider diversity of spacing between beats. HRV helps measure autonomic efficiency. It declines with aging, but exercise slows the decrease in HRV associated with aging. Physical exercise improves our vagal tone, which is why athletes have lower resting heart rates and high heart rate variability. High HRV is a sign of cardiovascular health and fitness.  

Measuring HRV and identifying patterns help athletes maximize performance but avoid overtraining. Measuring HRV daily can help an athlete identify decreases and adjust training.  In one studya reduced HRV was seen soon after awakening in overtrained athletes, suggesting increased sympathetic tone.”  Research indicates that negative effects of overtraining on the automatic system is reversible. The cure? Rest.

A study of heart rate variability in middle-distance runners found that HRV was a better tool than resting heart rate to evaluate cumulative fatigue. Another study determined that HRV was a reliable marker in differentiating between international and national level soccer players. A 4 year study of elite nordic skiers found that “HRV was significantly lower in fatigued athletes. This research supports HRV as a key tool to optimize individual training profiles.

Avoiding Overtraining

Even 15 years ago researchers struggled with the difficulty of measuring biomarkers to assess overtraining.

Wearable tech has brought the sports lab into the home. Athletes can now measures sleep, blood oxygen saturation, SpO2, respiration, heart rate and heart rate variability with a single device. An analysis of these values, and the trends or patterns, helps athletes modulate training, avoid overtraining, promote recovery (the micro tears which regular exercise engenders) and prevent – or recover, from injury.

Overtraining can be a serious issue that takes months to recover from and can compromise an athlete’s performance.

Optimal training is training designed specifically for an individual athlete, tailored to his or her abilities and aerobic capacity. Research shows that long term changes in HRV (> 4 weeks) are a reliable indicator of physiological adaptation in athletes. Understanding their unique individual HRV fingerprint can help athletes adjust training and maximize performance.

Cardio fitness is so significant that the American Heart Association recommends that it be measured as a ‘fifth vital sign’, alongside blood pressure, pulse rate, temperature, and respiration. Heart rate variability (HRV) has traditionally been used to evaluate cardiovascular and metabolic health. Athletes are now harnessing its power to set training regimes, evaluate progress, and avoid overtraining. Recovery is an essential component of any training regime. Routine measurement of HRV helps athletes stay in the growing zone, and out of the slowing zone.

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