Difference In Volume Of Exercise Versus Intensity Of Exercise On Physiology And Behavior

Cardiovascular exercise is associated with numerous health benefits, including reduced risk of the world’s deadliest diseases such as heart disease, stroke, Type II diabetes, Alzheimer’s disease, and cancer. Exercise has also been shown to improve mood, alleviate stress and symptoms of anxiety and depression, and improve cognitive function while decreasing the risk of dementia in later life.

Questions remain as to how MUCH exercise (volume) and how HARD we have to sweat (intensity) in order to achieve the maximum benefits. Others may be interested in knowing the minimum effort required to see any sort of meaningful improvements in health outcomes.

Most work investigating the effects of exercise in mice provide unlimited access to a running wheel. The problem lies in that mice will naturally run a lot more than your average human when given unlimited access to fitness equipment, resulting in large volumes of exercise that are not clinically relevant. For example, in our study, we found that mice who were given access to a running wheel for the entire time that they are normally awake spent 5-6 of those hours running! We wanted to create exercise paradigms that more closely mimic the amount of time humans would spend engaging in exercise, without forcing the animals to run for a certain time. Studies have been able to control both the volume and intensity of exercise, for example, by putting animals on treadmills; however, this has been shown to be stressful to the animals.

Mice in our study were allowed to exercise 5 days per week, from about 4 to 12 months of age. In humans, this is roughly the equivalent of a person performing regular aerobic exercise from their mid-20s to mid-40s. A unique aspect of our study is this relatively long length of the exercise intervention period. Whereas most studies provide exercise for 4 months or less, ours provided double that, modeling a long-term commitment to cardiovascular fitness. In addition to a sedentary group that did not exercise, mice were split into three exercise groups that provided different “doses” of exercise. These exercise mice were allowed access to running wheels for either 1 hour, 3 hours, or 12 hours per day, and could freely choose the amount and speed at which they ran. By giving mice 1 hour of wheel access, we were able to create a group that exercised for about 45 minutes, while mice given 3 hours of access exercised for about 2 hours – amounts of exercise that are closer to what humans may be capable and willing to perform on a daily basis.

In addition to creating exercise regimens that differed in volume (running distance), we found that these different-length access periods resulted in exercise regimens that differed in running “quality,” or what we have interpreted as measures of intensity. This included the speed at which mice ran, as well as how many breaks in running they took each hour, with greater speed and fewer breaks interpreted as higher intensity exercise. Longer access to a running wheel resulted in mice running greater distances and higher speeds but taking more breaks.

After characterizing our exercise regimens, we were interested in determining what effects they had on physiological, behavioral, and cognitive outcomes. What we found surprised us a bit – in a good way! All exercise doses reduced weight gain during aging – though surprisingly they did so to a similar degree, likely because the mice who were exercising more were also shown to be compensating for exercise-induced caloric deficits by eating more food.

All exercise groups also had similarly increased relative muscle mass and aerobic capacity, as well as decreased levels of the stress hormone corticosterone (the mouse equivalent of cortisol in humans). Additionally, all exercise groups had reduced anxiety, as well as improved motor performance on a task requiring balance and strength. Moreover, exercise improved some measures of cognitive function, even at the lowest dose.

The only measure that seemed to be improved in a truly “dose-dependent” manner (greater volume exercise = greater performance) was working memory – or the ability to remember information over a short period of time (such as remembering the numbers of a phone number someone tells you as you write it down). These findings suggest that a long-term commitment to even relatively small amounts of cardiovascular exercise may have wide-ranging benefits to the body, mood, and cognitive function.