Ask a runner why he or she runs, and you’re just as likely to hear about the mental health benefits as the physical ones. For many people, exercise makes them feel better and helps them handle the stresses of daily life.
At least, that’s how it feels. But can science confirm this impression?
One theory about how this works is the “cross-stressor adaptation hypothesis.” The basic idea is that the body has a specific set of responses to any perceived stressor—any “emotional, physical or psychological threat that perturbs homeostasis,” as one study puts it.
This “fight-or-flight” response, coordinated by the sympathetic nervous system, triggers stress hormones, increases heart rate and blood pressure, and jacks up your mental alertness. That’s great if you need to run away from a saber-toothed tiger, but if you’re chronically stressed, it becomes a negative, contributing to conditions ranging from heart disease to obesity and depression.
Because exercise acts as a stressor, triggering a form of fight-or-flight response, it essentially acts as “practice” for the stress response system. With repeated challenges, your sympathetic nervous system gets better at not overreacting to stress, and at turning off the response when the stress is gone. (One of the nice things about exercise is that it has a clearly defined end-point.) The “cross-stressor” hypothesis suggests that these benefits transfer to improve your response to other forms of stress.
There have been lots of attempts to find out whether the hypothesis is true, but the results have been mixed. One problem is that most studies have been cross-sectional (i.e., looking at a large group of people at a single point in time). Even if you find that people who exercise more have healthier stress responses, it’s hard to know whether exercise caused the difference, or whether laid-back people are simply more likely to find time to exercise. Another challenge is that it’s hard to reproduce real-life stressors in the laboratory.
A new paper in this month’s European Journal of Applied Physiology, from researchers at the Karlsruhe Institute of Technology in Germany, tackles both of these problems with an interesting study design.
The researchers divided electrical engineering students into two groups at the beginning of an academic semester, and put half of them through a 20-week aerobic exercise training program (running twice a week). They measured the students’ stress response at the beginning of the study, and again at the end of the exercise program—during the highly stressful exam period.
Instead of measuring stress response in the lab, the researchers had the subjects wear a heart-rate monitoring device for 36-hour periods (including an exam). You can get an estimate of the activation of the sympathetic nervous system by looking at subtle variations in the time between heart beats: when you’re stressed, your heart tends to beat more regularly, whereas you have higher heart-rate variability when you’re relaxed.
Sure enough, the students who exercised showed increased heart-rate variability during the exam period compared to the non-exercising students, suggesting that their bodies were better able to cope with stress, and offering some empirical support to the cross-stressor adaptation hypothesis.
This helps to confirm what lots of people already feel about how exercise helps them feel less stressed. It also may have some more subtle implications, the authors point out.
We know that exercise is linked to lower risk of heart disease—but the known mechanisms, like reducing blood pressure, weight, cholesterol, and so on, explain only about 60 per cent of the observed reduction in heart problems. So where does the other 40 per cent come from? It’s possible that the stress-mitigating effects of exercise are one of the “hidden” reasons that exercise helps protect your heart.