History lesson: Couch potatoes have weaker bones

If you want stronger bones, get on with some intense physical activity as a relaxed lifestyle weakens bones, a study finds.

Human bones are remarkably plastic and respond surprisingly quickly to change.

Put under stress through physical exertion - such as long-distance walking or running - they gain in strength as the fibres are added or redistributed according to where strains are highest.

The ability of bone to adapt to loading is shown by analysis of the skeletons of modern athletes, whose bones show remarkably rapid adaptation to both the intensity and direction of strains.

"After the emergence of agriculture in Central Europe from around 5,300 BC, the bones of those living in the fertile soils of the Danube river valley became progressively less strong, pointing to a decline in mobility and loading," explained Alison Macintosh, a PhD candidate in Cambridge University's department of archaeology and anthropology.

Macintosh suggests that male mobility among earliest farmers (around 7,300 years ago) was, on average, at a level near that of today's student cross-country runners.

Within just over 3,000 years, average mobility had dropped to the level of those students rated as sedentary, after which the decline slowed.

"My results suggest that, following the transition to agriculture in Central Europe, males were more affected than females by cultural and technological changes that reduced the need for long-distance travel or heavy physical work," Macintosh emphasised.

This also means that as people began to specialise in tasks other than just farming and food production such as metalworking, fewer people were regularly doing tasks that were very strenuous on their legs, the researchers noted.

Macintosh is scheduled to present her results at the annual meeting of the American Association of Physical Anthropologists in Calgary, Alberta April 8-12.