Physical activity has long been known to reduce depression and anxiety, and is commonly prescribed to prevent or cure negative mental health conditions.

However, less is known about the impact of physical activity on positive mental health conditions, such as happiness and contentment.

Weiyun Chen, University of Michigan associate professor in kinesiology, wanted to know if exercise increased positive mental health in the same way it reduced negative mental health. Specifically, researchers examined which aspects of physical activity were associated with happiness, and which populations were likely to benefit from the effects.

To that end, Chen and co-author Zhanjia Zhang, a doctoral student, reviewed 23 studies on happiness and physical activity. The 15 observational studies all showed a positive direct or indirect association between happiness and exercise. The eight interventional studies showed inconsistent results.

The studies included health information from thousands of adults, seniors, adolescents, children, and cancer survivors from several countries. A couple themes emerged.

"Our findings suggest the physical activity frequency and volume are essential factors in the relationship between physical activity and happiness," Chen said. "More importantly, even a small change of physical activity makes a difference in happiness."

Findings suggest a threshold effect for the relationship of happiness and physical activity -- several studies found that happiness levels were the same whether people exercised 150-300 minutes a week, or more than 300 minutes a week.

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Specifics: Active and happy?

The review of observational studies found that compared to inactive people, the odds ratio of being happy was 20, 29 and 52 percent higher for people who were insufficiently active, sufficiently active, or very active, respectively.

Several studies reviewed the relationship between physical activity and happiness in youth. One study found that youth who engaged in physical activity once a week compared to none had 1.4 times the odds of being happy if they were normal weight, and 1.5 times the odds if overweight. Another study found that adolescents who were physically active at least twice a week had significantly higher happiness than those who were active once or less a week. An additional study found that college students who participated in physical activity had 1.3 times the odds of being happy than peers who didn't participate.

Three studies looked at happiness and activity in older adults. One found that exercise was associated with happier adults. Another found that total minutes of exercise per week was positively related to happiness. However, the findings suggest happiness was mediated by health status and/or social functioning.

Three studies looked at special populations. Among ovarian cancer survivors, meeting the 150 minutes of moderate to vigorous physical activity was significantly associated with happiness level. In children and adolescents with cerebral palsy, physical activity predicted happiness level, and among drug abusers, the number of weekly exercise sessions, regardless of intensity, was slightly associated with happiness.

In the intervention studies, physical activity included aerobic, mixed school activity classes, and stretching and balance exercises or 30 to 75 minutes from one to five times a week for 7 weeks to a year. Four of the intervention studies showed a significant difference in change of happiness between intervention group and control group, and three did not.

The review only looked at articles in peer-reviewed journals in English, which could lead to publication bias and overestimating the positive relationship between physical activity and happiness. Because there were limited randomized control trials, the researchers could not establish causation between happiness and physical activity.

More details can be found in thee study, "A systematic review of the relationship between physical activity and happiness," which was published online March 24 by the Journal of Happiness Studies.