How The Heart Shows Stress

Heart rate. It’s that staple of training that you use to tell how hard you’re working, since your heart provides a more objective gauge of how hard you’re working than your head does.

But your heart rate can tell you a lot more. The “variability” of your heartbeat – the difference in length of time between each heartbeat – is becoming the foremost indicator of how stressed your system really is, and what that means for your exertion – and your health.

One of the people excited about this insight is Rodney Corn, who has a Master’s degree in biomechanics from California State University Chico and is the co-founder of Personal Training Academy Global (PTA Global), which provides education, mentoring and resources for personal trainers around the world. He explains that the interval between heartbeats reflects autonomic nervous system function, “which reveals what your body’s capable of doing from a stress standpoint,” and whether you need to do more for recovery.


A quick explanation of the autonomic nervous system reveals why: it’s the system that regulates involuntary body functions, such as heartbeat, blood flow, breathing and digestion. But the autonomic nervous system comprises a balance between two subsystems: the sympathetic nervous system, which is in charge of your fight-or-flight response and raises your heart rate in response to exercise or stress; and the parasympathetic nervous system, which tries to conserve resources and brings down your heart rate.

“When someone is at rest, there should be significant variability between heartbeats, meaning a difference of milliseconds between intervals,” says Corn. “That indicates that the system is ready to adapt quickly and speed up at any minute. It’s the same principle behind the loping rumble of a high-performance engine that becomes a regular hum when the driver accelerates. At rest, your heart is poised for action, rather than having to shift from a consistent rhythm.”

He continues, “As we start exercising, which is a stressor, our heart rate starts to go up into the higher heart rate zone, and those beat intervals will start to get much more symmetrical and shorter in duration. Then when we slow down and recover, the interval between beats lengthens and the variability returns.” Corn says that’s a normal, healthy response to exercise, as well as everyday stressors like something that catches your eye and prompts you to turn your head while you’re driving.

What’s not so healthy is what happens under emotional stress, or too much “good” stress – including exercise. “What happens under stress is that even though someone may be at rest physically and their heart rate remains constant at 60 beats per minute, the interval between heartbeats becomes consistent, rather than remaining variable.”

This means the sympathetic nervous system – the one that governs your fight-or-flight response – is in overdrive, and the parasympathetic nervous system can’t bring your system back down to recover. “A constant state of stress without recovery causes cortisol production and a host of inflammatory responses, and those can eventually lead to illness or injury.”


Your stress level, Corn explains, will have an impact on your ability to lose weight, gain strength or get leaner to the degree that you can or would like. “It can be as simple as the stress of working out compounded by the stress of day.”

Most of us can’t simply hear or feel the milliseconds’ difference in heart rate variability (HRV), but if you have an app that tells you or a trainer who understands HRV, you can use the information to help you understand how and whether to modify your training session or take steps to prevent overstressing your system.

There are also some tech-free ways you can gauge your state and pace yourself. Corn suggests some of the “readiness” assessors that PTA Global has identified. Assess your state on a scale of 1 to 3.

Lifestyle criteria: How well-rested do I feel? How well have I eaten (quality, not quantity)? Am I hydrated? (1 is poorest, 3 is best).
Physical criteria: Do I feel aches? Pains? Soreness? Tightness? All of these symptoms are indicators of inflammation. (1 is most extreme, 3 is least extreme).
Mental/emotional criteria: What’s my mood over the last 24 hours – depressed/grumpy or full of vitality? (1 is poorest, 3 is best). A cumulative score of 4 to 6 indicates that you’re likely to be in a stressed state and your activity for that day needs to be adjusted.


What to do if you’re stressed? “Imagine a bottle of water that’s just an inch or two short of being full – and call that your cortisol gauge,” says Corn. “It will fill up if you add stress-generated cortisol, so you can only add so much before it overflows and you get sick or you get joint pain.”

He acknowledges that the prospect of not going to the gym can be stressful for people who find exercise is a good outlet. On those days, he says you can adapt your workout to get the mental and physical benefits without overloading your system:

Corn says that breathing is another significant factor in recovery – and managing your heart rate. “Especially on days when you are adapting your workout, if you’re still breathing heavily and your heart rate is elevated after a set, don’t rush yourself to start the next set. If you’re not somewhat recovered, you’re putting more stress on your system.”He concludes, “When people think about not only muscle imbalance but also heart rate variability and recovery imbalance, they can adjust their workouts to help avoid injury and illness that will get in the way of the results they want.”