To obtain substantial health benefits you need to engage in moderate-intensity physical activity for at least 150 minutes a week. Do not forget: 150 minutes is the lower threshold; longer durations are associated with higher benefits.
A large body of evidence confirms the numerous benefits of regular physical activity in improving health and fitness and reducing the risk of many chronic diseases, but how much of it is necessary to be healthy?
The Physical Activity Guidelines of the USA (2008) indicate that 150–300 min/week (from 2 hours and 30 minutes to 5 hours) of moderate-intensity aerobic physical activity provides substantial health benefits for the general adult population. Equivalent benefits may be achieved by 75–150 minutes per week of vigorous activity or by a combination of moderate and vigorous activity. Aerobic activity should be performed in single sessions of 30 minutes or in several episodes of at least 10 minutes during the day, and preferably, it should be spread throughout the week. Aerobic activities are very important because they are associated with the broadest range of health benefits, especially in coronary heart disease, which is one of the main conditions whose incidence can be reduced by regular physical activity (Powell et al., 2011). Some researchers have observed that 30 minutes of aerobic exercise is as beneficial as sessions of 60 minutes or more in reducing the risk of coronary hearth disease, if the weekly totals are equivalent (Lee et al., 2000). http://www.ncbi.nlm.nih.gov/pubmed/10961961
However, it seems that there is no lower threshold for benefits. Reductions in the risk of mortality appear to begin with the first increase in activity beyond baseline. We know that sedentary time is associated with obesity, abnormal glucose metabolism and metabolic syndromes. A recent study examined the effects of breaks in objectively measured sedentary time. Independently on total sedentary time and on moderate-to-vigorous intensity activity time, increased breaks in sedentary time were positively associated with waist circumference, BMI, triglycerides, and 2-h plasma glucose (Healy et al., 2008). This study shows the importance of avoiding prolonged uninterrupted periods of sedentary time and also proves the importance of short periods of movement to improve the health. http://www.ncbi.nlm.nih.gov/pubmed/18252901
A recent review compared the cardio-protective benefits of vigorous and moderate aerobic activity. It found that vigorous intensity exercise (typically ≥ 6 METs) increased aerobic fitness more effectively than moderate intensity exercise (Swain et al., 2006). http://www.ncbi.nlm.nih.gov/pubmed/16377300
So, while it is true that vigorous exercise is more beneficial to one’s health than low-intensity exercise, it is also true that light activities are beneficial. Indeed, several studies report that light activities produce not only substantial muscular improvements, but also healthful metabolic change (Powell et al., 2011). http://www.annualreviews.org/doi/pdf/10.1146/annurev-publhealth-031210-101151
It is not very important what type of physical activity is performed. Sports, planned exercise, and household or yard work are all beneficial. The key factor is total energy expenditure; if that is constant, improvements in fitness and health will be comparable (Blair et al.,1992). http://www.ncbi.nlm.nih.gov/pubmed/1599603
The ideal intensity of activity that provides substantial health benefits suggested by the Guidelines is of 500–1000 MET-min/week (metabolic equivalents).
Besides aerobic training, the Guidelines suggest that adults should do muscle-strengthening activities of moderate or high intensity and which involve all major muscle groups on 2 or more days a week.
Older adults should also follow these Guidelines, if their abilities and conditions allow it. Furthermore, they should do exercises that maintain or improve balance if they are at risk of falling.
We must not forget that physical activity is essential in our life for improving health and preventing many diseases. The scientific evidence shows that every occasion of movement can generate healthy effects. The Guidelines indicate that some activity is better than none, and that more than the suggested volume of activity does provide additional benefits.