The Most Overlooked Aspect of Athletic Performance…
Stress and anxiety are risk factors for cardiovascular disease, as are a high heart rate and a low heart rate variability.
Heart Rate Variability
Heart rate variability (HRV) is the variation in the time interval between heartbeats, as measured by the variation in the beat-to-beat interval (R-R interval in the picture below).
When most of us think of our heart rate, we generally think of a number like 50 beats per minute. Actually, your heart rate changes from beat to beat. As you breathe in, your heart rate speeds up. When you exhale, it slows down. So, instead of saying that you have a fixed pulse of 50 beats per minute, your heart rate is probably varying between something like 45 and 55.
HRV is a measure of this naturally occurring phenomenon. About 25 years of clinical research done at top institutions such as the Mayo Clinic, Stanford, and Mount Sinai have shown that when HRV levels are high (greater variability), a person experiences lower levels of stress and greater resiliency. When HRV levels are low (less variability), this is a sign of lower resiliency and higher levels of stress.
I know, this probably sounds counterintuitive. Think of a low HRV level like a metronome, with perfect timing and rhythm. It sounds unnatural. Our hearts are meant to pump as needed, not at a constant rate regardless of the circumstances we are under.
Low HRV has been shown to be a predictor of mortality after a myocardial infarction (ie. a heart attack) and some research indicates that it may be associated with other conditions as well, such as congestive heart failure, depression, COPD, and diabetes.
The Nervous System
There are two branches of your nervous system:
- Autonomic nervous system – controls survival functions like breathing, heart rate, blood pressure, organ control, and digestion
- Voluntary nervous system – consciously controlled to perform daily functions like walking and picking things up
Within the autonomic nervous system, there are two sub-systems that we need to look at to better understand the basis of HRV:
- Sympathetic nervous system – increases physiological performance when a stressor is introduced, creates the “fight or flight” response
- Parasympathetic nervous system – “rest and digest”, helps create an internal environment conducive to recovery
These sub-systems are not fighting each other. They are both always active to some degree, but one is usually more dominant at any given time. In general, a high HRV indicates dominance of the parasympathetic system and a low HRV indicates dominance of the sympathetic system. Here is where you can see why a low HRV is bad – the sympathetic system is the one associated with stress, overtraining, and inflammation.
Ideally, an individual would have powerful responses to both sub-systems – stronger sympathetic responses during competition and stronger parasympathetic responses during rest. Indeed, this is what appears to be observed in Olympic-level athletes and members in the Special Forces and Navy SEALs.
The Polyvagal Theory
The vagus nerve innervates a lot of different organs, like your heart, lungs, and digestive tract. Vagal tone is a term used to refer to the responsiveness of our parasympathetic system. One of the most precise ways to measure it is to measure respiration and heart rate. Luckily, you can still get a fairly accurate picture of vagal tone with just a heart rate monitor and skip measuring respiration.
In 1872, Charles Darwin acknowledged the relationship between the heart and the brain:
“…when the heart is affected it reacts on the brain; and the state of the brain again reacts through the [vagus] nerve on the heart; so that under any excitement there will be much mutual action and reaction between these, the two most important organs of the body.”
Our thoughts, emotions, and experiences are tightly connected to the functioning of our nervous system, breathing, and heart rate.
Stress acts on the autonomic nervous system, creating an imbalance. When the autonomic nervous system is in balance, HRV tends to be higher, and when it is out of balance, HRV tends to be lower. This is why HRV may be a good non-invasive biomarker for stress assessment.
Lowered HRV may be due to any number of factors, some of which you have no control over. Things like age or gender. However, we can train ourselves to increase our HRV by focusing on those aspects that we do have some control over, like stress and coping mechanisms.
So many inputs can alter HRV. Hydration status, prior exercise fatigue, stress, performance anxiety, nervousness, stress from daily life, fitness level, endurance vs. static training, skill, body position, heart left ventricle size, temperature, humidity, altitude, mood, hormones, drugs (like beta-blockers), stimulants (like caffeine) can all affect HRV.
According to Selye’s General Adaptation Syndrome, the body goes through 3 distinct stages in response to stress:
- Shock – alarm stage – HRV decreases, increased sympathetic tone, increased stress hormone output, increased adrenal output
- Overreaching – resistance stage – sympathetic tone increases during exercise, parasympathetic tone increases during recovery, HRV increases due to parasympathetic response, slower recovery, stress hormones remain elevated, increased stress hormone output, adrenal resistance
- Chronic Stress – overtraining or exhaustion stage – shut down production of stress hormones, continued adrenal resistance, depressed immune function, slow recovery to muscle damage, increased inflammation, HRV remains elevated, “burnout”, body fails to adapt to chronic stress
- (Supercompensation or Recovery) – deload phase – stress hormones decrease, adrenals regain sensitivity, inflammation decreases, HRV decreases
Ideally, you progress into stage 2 and then deload for about a week before starting over, otherwise it may take month for full recovery if stage 3 is reached.
“Resonant breathing biofeedback” teaches how to recognize and control otherwise involuntary HRV. In one study, biofeedback training decreased depression, anxiety, and stress among a population of manufacturing operators.
Although HRV is the result of the autonomic nervous system and therefore not normally under conscious control, biofeedback training via HRV monitoring can be utilized through techniques like breathing, meditation, and relaxation.
HRV can even be used to predict when you will get sick! The days immediately before feeling sick are often preceded by an increase in HRV, and the weeks during the actual sickness are marked by lowered HRV (and so need for more rest and recovery).
While you may certainly purchase extensive setups, all you really need to get started is an app on your smartphone and a heart rate monitor with a chest strap, such as several of the models sold by Polar.
Exercise and HRV
Regular exercise may decrease cardiovascular mortality and sudden cardiac death. Trained individuals have been shown to have a low resting heart rate (“training bradycardia”) and higher HRV than sedentary individuals.
However, HRV is much lower in overtrained athletes than in healthy athletes. More training is not always better. By monitoring your HRV every morning, you can see when HRV drops significantly and take it as an early warning sign that your autonomic nervous system is beginning to feel overwhelmed. Don’t fret if it drops a little, as long as you take the time needed to recover. After all, training is about both stress and recovery in order to achieve training adaptations. You want to stress the body in order to achieve higher levels of fitness! Just do it intelligently.
Through apps that monitor HRV and analyzing trends over days, weeks, or months, athletes can use this feedback to decide when to push harder in their next workout, or when they may need to go easy or take an extra rest day in order to avoid overtraining and ensure optimal training sessions and successful recovery.
Endurance athletes, especially, seem to be latching on to this new application of decades old science in order to improve performance. Competitive powerlifters are now also getting in on the action.
“Surprisingly, and perhaps counterintuitively, a high HRV indicates a healthy, fit, well-rested heart. Elevating your average HRV values over time is indicative of improved cardiovascular fitness. A low HRV is believed to be an indication of a poorly functioning heart – perhaps an overtrained athlete or a person who is unfit, overstressed, or has developed cardiac disease risk factors. By tracking HRV regularly, one can establish a baseline value and then be altered to excessive stress or insufficient recovery when HRV readings are lower than normal.”
Don’t disregard your own intuition, however. Use both your subjective impression of how you feel as well as more objective measurements like an HRV assessment to make decisions and optimize your health and training.
Eat well + sleep well + cope well → better HRV → better recovery