Physical activity may not only reduce depression and anxiety but also lead to happiness and contentment, scientists say.

A study by researchers at the University of Michigan in the US suggests the frequency and volume of physical activity are essential factors in the relationship between exercise and happiness.

The researchers reviewed 23 studies on happiness and physical activity.

Fifteen observational studies showed a positive direct or indirect association between happiness and exercise. The eight interventional studies showed inconsistent results.

"Our findings suggest the physical activity frequency and volume are essential factors in the relationship between physical activity and happiness," said Weiyun Chen, associate professor at the University of Michigan.

"More importantly, even a small change of physical activity makes a difference in happiness," said Chen.

The findings, published in the Journal of Happiness Studies, 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.

Researchers also found that compared to inactive people, the odds ratio of being happy was 20, 29 and 52 per cent higher for people who were insufficiently active, sufficiently active, or very active, respectively.