Nobody likes taking time off training after getting sick, especially during intense training blocks or leading up to an important event. Missing planned training can severely compromise your chances of completing or reaching your goals. Understanding heart rate variability (HRV) is a useful tool that can predict illness before it happens, so you won’t have to take any precious time off due to illness.
What research tells us about training load and illness
Rest is important, but taking too much time off from training due to illness can impact your performance. Researchers in Finland analyzed the relationship between medal-winning success and days sick in cross-country skiers during the run-up to the 2018 Winter Olympics. They found that medal-winners spent an average of 14 days out per year with colds and other respiratory infections while less successful athletes spent an average of 22 days out sick with similar infections.
Similarly, a study conducted by the Australian Institute of Sport on track and field athletes over five seasons found that illness and injury that prevented or limited training were major factors in determining success. Athletes who completed 80% or more of their planned training were seven times more likely to reach their goals and succeed in events compared to those who did not.
On the other hand, the researchers found that most illnesses were associated with overtraining (cue the need for scheduled rest days). About 50% of illnesses occurred in the two months before competition when training loads and mileage were at their highest.
Why high weekly loads can make you more likely to become sick
One of the key reasons is resilience. As loads are ramped up, the risk of illness increases rapidly in recreational and age-group athletes. Elite athletes, partly due to their genetic makeup and partly by being conditioned to high weekly loads, are more resilient and don’t get sick as easily when loads are ramped up. They actually are more likely to become sick when training loads are reduced and they become deconditioned.
As training loads are ramped up, recovery demands increase. If training loads and recovery demands are not in balance, inflammation becomes chronic, and the immune system gets stressed trying to repair too many parts of the body at once.
How illness affects training
Once symptoms appear, it’s important to reduce training loads so the body has a chance to recover. But how soon can you safely return to lighter training? Many people look at the location and severity of symptoms for guidance; for example, if a cold is only above the neck, it’s a sign that training can be resumed. Some helpful tips can be found in this blog: How to Return to Training After Getting Sick.
In addition to listening to your body, you can also use sub-clinical indications (such as HRV) that can be measured non-invasively as guidance.
What is heart rate variability?
Heart rate variability measures the small changes in time between each heartbeat. It’s a reliable method used for assessing the effects of stress on your body. High HRV is increasingly linked to good health and a high level of fitness, while decreased HRV is linked to stress, fatigue, and even burnout. By knowing when your body is fatigued, you can quickly adjust your workouts to avoid overtraining.
Used in many health and high-performance contexts, HRV measures in athletes most commonly take the form of a short waking measure, where current HRV is compared to a personal baseline that can be used to gauge recovery. If you sync your HRV data to your TrainingPeaks Premium account, you can see this baseline on your metric cards in your TrainingPeaks calendar, allowing you to visualize how your HRV is changing over time. See compatible devices.
Good HRV software encourages the athlete to record wellness metrics and import training loads (like Training Stress Score® from TrainingPeaks) to give a complete picture for the athlete and their coach. The context helps the athlete and their coach understand what training and lifestyle factors are causing the higher recovery demand and how to address them.
How to establish your personal baseline range
To establish your personal baseline range, track metrics like morning HRV and resting heart rate consistently for 14 to 30 days under normal conditions. This initial testing period captures your typical biological variance, creating an accurate statistical benchmark to guide your future training.
- Standardize measurements: Record your data immediately upon waking up to eliminate external variables.
- Filter anomalies: Exclude days with acute illness or travel to keep the baseline accurate.
Using TrainingPeaks to aggregate these daily inputs reveals your true physiological norm, allowing you and your coach to detect deviations.
Pro Tip → Add your HRV to your TrainingPeaks Performance Management Chart (PMC) to track HRV trends.
How HRV can detect illness
HRV is good for detecting illness because it is sensitive to how stressed the body is at that moment. HRV directly taps into the balance between the sympathetic (fight or flight) and the parasympathetic (rest or digest) branches of the nervous system.
Lower HRV indicates that the body is more stressed. Although you might expect that different kinds of stress (e.g. physical, mental, or nutritional) affect your body differently, they affect the nervous system in very similar ways. So if you are mentally or emotionally stressed and you eat poorly (e.g. seeking comfort foods), your ability to handle training stress is much reduced. There is an entire series here on managing Total Load.
The 24-to-48-hour early warning sign
A sudden, unexplained drop in your morning HRV can serve as a critical 24-to-48-hour early warning sign of impending illness or severe overtraining. This autonomic nervous system shift often manifests before you feel physical symptoms, signaling that your body is actively diverting resources to fight off a pathogen or recover from deep systemic stress.
Monitor these subtle biomarkers daily in TrainingPeaks to proactively pivot your schedule. Catching this early window gives you the chance to swap a high-intensity interval session for a rest day, frequently neutralizing the threat before it compromises your health and halts your consistency.
Detecting the start of an illness: a case study
The example below shows a typical HRV chart for an experienced recreational athlete with a low resting heart rate between 50-60 bpm and a high level of HRV of around 100. The emojis show when this athlete started to feel unwell and how their HRV fell rapidly at the start of illness.
Interestingly, their resting heart rate only rose above its normal range on the third day after the illness had begun. In other words, HRV is a more sensitive and faster-acting indicator than heart rate because it taps directly into the nervous system.
On the sixth day after the illness began, the daily HRV started to rise, and the athlete began training again with a light load for the next three days as they felt better.
High training loads combined with insufficient recovery can promote chronic inflammation and weaken the immune system. HRV is a sensitive marker of stress and recovery and can be used to identify when you are starting to get sick so you can reduce training loads and pay extra attention to sleep and nutrition.
HRV can also be used to show when you are on the mend so you can minimize the number of missed training sessions.
Spotting systemic overload to prevent muscle injuries
Spotting systemic overload requires tracking overlapping markers like chronic muscle soreness, poor sleep quality, and elevated resting heart rate. When your training load consistently outpaces your recovery capacity, muscular tissue becomes brittle and prone to acute strains. Monitoring your cumulative Fatigue trends in TrainingPeaks allows you to shed training stress before injury strikes.
Addressing low metrics: Can low HRV cause dizziness?
While low HRV itself does not directly cause dizziness, it signals a deeply fatigued autonomic nervous system that is struggling to regulate blood pressure and stress. This systemic exhaustion can manifest as lightheadedness during sudden physical transitions, alerting you to scale back your current TrainingPeaks workload.
Actionable tactics: What is the best exercise to increase HRV?
The best exercise to increase HRV is low-intensity, conversational Zone 2 cardio, such as steady cycling or running. This aerobic base building stimulates vagal tone, shifting your nervous system into a restorative parasympathetic state. Incorporating these easy sessions balances high-intensity fatigue and lifts your overall baseline resilience.
References
Raysmith B., & Drew, M. (2016, October). Performance success or failure is influenced by weeks lost to injury and illness in elite Australian track and field athletes: A 5-year prospective study. Retrieved from https://pubmed.ncbi.nlm.nih.gov/26839047/
Schwellnus M., et al. (2016, September). How much is too much? (Part 2) International Olympic Committee consensus statement on load in sport and risk of illness. Retrieved from https://pubmed.ncbi.nlm.nih.gov/27535991/
Valtonen, M., et al. (2019, September). Common cold in Team Finland during 2018 Winter Olympic Games (PyeongChang): epidemiology, diagnosis including molecular point-of-care testing (POCT) and treatment. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6818521/
