Canadian Physical Activity Guidelines recommend completing at least 150 minutes of “moderate-to-vigorous physical activity” per week, plus muscle-strengthening activities two days per week. There is no recommended upper limit, but should there be one?
“Endurance exercise training is generally considered good for the heart, as moderate to vigorous physical activity reduces all-cause mortality,” says Robert Bentley, an assistant professor at the University of Toronto Faculty of Kinesiology and Physical Education (KPE). “However, some evidence suggests that endurance exercise training completed beyond recommended levels may increase the risk of adverse cardiovascular outcomes.”
Bentley led a study exploring the effect of chronic exercise training and acute exercise on heart rate variability, recently published in the Applied Physiology, Nutrition and Metabolism journal.
Heart rate variability is a measure of the variation in time between each heartbeat. This variation is controlled by the autonomic nervous system (ANS), which automatically regulates our heart rate, blood pressure, breathing and digestion, among other key functions. The ANS can be subdivided into the sympathetic and the parasympathetic nervous systems, also known as the fight-or-flight or the rest-and-digest mechanisms.
The ANS sends signals to the hypothalamus in the brain, which then leads to either stimulation or relaxation of different bodily functions. When you are under constant stress, have poor sleep habits or even after you complete an exercise, this balance (the cardiac sympathovagal balance) may be disrupted, and your fight-or-flight response can shift into overdrive, which is linked to increased all-cause mortality.
“The purpose of our study was to contribute to the conversation surrounding potential detrimental cardiovascular effects of chronic endurance training by exploring heart rate variability throughout a 24-hour non-exercise period in middle-aged endurance athletes and recreationally active individuals,” says Bentley. “We also looked at how an acute, intensive exercise bout, reflective of a typical daily exercise perturbation, influenced these patterns in endurance athletes.”
119 endurance athletes between the ages of 49 and 57 and 32 recreational athletes between the ages of 52 and 60 wore a Holter monitor, which uses electrodes and a recording device to track the heart's rhythm, over 24 hours during which they did not exercise. 51 endurance athletes then underwent 24-hour Holter monitoring following an intense bout of endurance exercise. Power spectral analysis of heart rate variability was completed hourly and averaged to quantify morning, evening, and nocturnal (night) heart rate variability.
Within power spectral analysis, low frequency bands represent a combination of sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) activity, and high frequency bands represent parasympathetic activity. Endurance athletes were shown to have greater very low frequency and low frequency compared to recreational athletes. Endurance athletes also had a greater low frequency to high frequency ratio at night. Following acute exercise in endurance athletes, only nocturnal (night) heart rate variability was assessed, which showed that very low frequency and high frequency decreased, while the ratio of low frequency to high frequency increased.
“What these results suggest is that in endurance athletes, both long-term and acute exercise increase nocturnal sympathovagal (sympathetic and parasympathetic) activity, though this occurs through a disparate mechanism of an increase in low frequency or a decrease in high frequency bands, respectively,” says Bentley. “Interestingly, the power spectral analysis combined with time-domain analysis of heart rate variability, suggests that endurance athletes seem to have their foot on the gas (sympathetic activity) and brake (parasympathetic activity) at the same time.”
Whether or not that’s a good or bad thing, Bentley doesn’t know just yet.
“Further work is required to understand the mechanism underlying reduced nocturnal heart rate variability in middle-aged endurance athletes and the long-term health implications,” says Bentley.