London: Here's the perfect excuse to take a break in office -- sitting around for long can make your bum bigger, a new study has claimed.

Researchers have carried out the study and found that the pressure put on areas of the body used for sitting produces up to 50 percent more fat in those parts -- and thus it can give one a big backside.

This can explain why couch potatoes and other sedentary behaviour makes you fat when combined with a lack of exercise. But even those with healthy diet and exercise habits will be affected if they spend long periods sat behind a desk, say the researchers.

They found that preadipocyte cells - the precursors to fat cells - turn into fat cells and produce even more fat when subject to prolonged periods of "mechanical stretching loads" -- the kind of weight we put on our body tissues when people sit or lie down.

By studying MRI images of the muscle tissue of patients paralysed by spinal cord injuries, they noticed that, over time, lines of fat cells were invading major muscles in the body, a daily reported.

This spurred an investigation into how mechanical load -- the amount of force placed on a particular area occupied by cells -- could be encouraging fat tissue to expand.

The researchers stimulated preadipocytes with glucose or insulin to turn them into fat cells. They then placed individual cells in a cell-stretching device, attaching them to a flexible, elastic substrate -- or underlying. The test group of cells were stretched consistently for long periods of time, representing extended periods of sitting or lying down, while a control group of cells was not.

The researchers noted the development of liquid droplets in both the test and control groups. However, after just two weeks of stretching, the test group developed significantly more -- and larger -- droplets.

By the time the cells reached maturity, the group that received mechanical stretching had developed fifty percent more fat than the control group.

Professor Amit Gefen, from Tel Aviv University, said: "Obesity is more than just an imbalance of calories. Cells themselves are also responsive to their mechanical environment. Fat cells produce more triglycerides (the major form of fat stored in the body), and at a faster rate, when exposed to static stretching.

"There are various ways that cells can sense mechanical loading. It appears that long periods of static mechanical loading and stretching, due to the weight of the body when sitting or lying, has an impact on increasing lipid [fat] production."

The findings have been published in the 'American Journal of Physiology -- Cell Physiology'.