We have some pretty awesome ways to measure external training stresses on endurance athletes these days. Cyclist have power meters, runners can use run pace (and algorithms such as graduated run pace that take into account elevation changes), and swimmers can use lap times to give us accurate measures of external workload that takes into account volume and intensity. Strength athletes and workouts can also be tracked by recording weight and reps for workouts to determine total weight lifted for each movement. We can then track external workload over time to get an idea of an athlete’s chronic training level versus their acute training level to get an idea of how rested or fatigued an athlete should be (Training Peaks has perfected this with the Performance Manager Chart). This works pretty well but it doesn’t account for activities or events outside of sport. This is where Heart Rate Variability can be extremely valuable in assessing the acute psychological and physiological stress of an athlete.
Heart rate variability (HRV) is basically the variation between heart rates measured down to milliseconds. Ideally you want your heart rate variability to be somewhat high, meaning that your central nervous system (via. the vagus nerve) is talking very efficiently to your heart and your heart is able to make micro-adjustments relatively quickly. In a well rested athlete the body will make micro-adjustments to heart rate based on breathing patterns as well as other physiological processes. The better your vagus nerve innervates your heart, the stronger your vagal tone which is a direct indicator of the health of your sympathetic (fight or flight response) and parasympathetic (rest and recover) nervous system. The strength and balance of the autonomic nervous system (both sympathetic and parasympathetic) tells us the propensity of an athlete to be over-stressed, or how resilient an athlete will be to training.
The cool thing about heart rate variability is that it incorporates both physical and mental stress. For example, if you have a stressful day at work where you are fighting to meet a deadline, the modern-day “running from a lion,” and then hop on the bike to do a bunch intervals (still running from that lion) then chances are your sympathetic nervous system is going to become fatigued and the parasympathetic nervous system is going to become dominate to help you rest and recover. If you continue down that path without getting rest you will fall into a typical over-training situation. Heart rate will become suppressed during workouts, it will take longer to bring heart rate up during harder efforts, and it will take a lot longer for your heart rate to recover after harder efforts.
Foods and nutrition also play a role in heart rate variability. If you are sensitive to a specific food, it will typically elicit a sympathetic nervous system response. It is a stressor on the body. This is one way that you can test your response to specific foods to find mild food allergies or sensitivities. Going into a fight or flight mode from what you are eating is generally a bad thing. One thing I have noticed with myself and athletes is that oxidized fats, specifically processed seed and vegetable oils cooked at high temperature, can be particularly stressful. That can become a problem for an athlete eating a diet high in french fries and deep-fried Twinkies.
Nutrition can also play a big role in autonomous system balance due to the interaction of the sympathetic nervous system and the adrenal glands. The sympathetic nervous system basically runs on adrenaline and cortisol (stress hormones). When our diet is deficient in the micro-nutrients and minerals that support the adrenal glands, or have a lot of foods that stimulate the sympathetic nervous system (caffeine is a good example) then they become deficient and adrenal fatigue can result.
OK, so how can I measure my HRV?!
There has been a lot of research done on HRV (there are over 16000 studies in the NIH database alone) that has led to the development of several algorithms making it easy for athletes to measure and interpret the results. Those algorithms have been utilized by several opportunistic entrepreneurs to develop some great smart phone apps. One of the best is the Sweetbeat app available on the iPhone. It was developed by a team of psychologists and has over 3 years of research behind it. It is extremely accurate and allows for continually monitoring HRV if you want to get into food sensitivity testing. If you have an Android phone I would recommend using the Omegawave app to measure HRV. The Omegawave team has made it fairly easy for athletes and coaches by distilling the raw data from a 2-minute HRV test into more actionable information. They use HRV data and vagal frequencies to determine an overall readiness number, adaptation reserves, aerobic and anaerobic readiness, and give the athlete heart rate recommendations based on those results. Omegawave is also available on the iPhone.
Both of those apps require the purchase of heart rate straps (Polar H7 strap for Sweetbeats and a proprietary strap plus subscription for Omegawave) but if you want to try monitoring stress response before making the investment in one of these systems you can try out Azumio’s Stress Checker. That app uses your camera on the phone to get a basic HRV measurement. This is significantly less accurate the ECG straps from the other apps, but still good enough to pick up basic trends.
One last note on HRV is that monitoring and tracking your personal results are important to figure out trends, recovery rates, and in making positive changes to increase your resiliency. The goal isn’t necessarily to always have a high HRV. You need to apply stress to the body to stimulate adaptations and improvement. You just don’t want that stress to be chronic without recovery periods, and you want to limit the emotional stress as much as possible since that will take up a portion of your bodies finite ability to adapt to stresses.
In Part 2 I will talk more about how you can dig yourself out of a low HRV hole or recover from an acute over-training situation.