No matter how good your fitness is, it can't stop this part from aging.

Countless people spend hours in the gym trying to keep themselves looking young. But when it comes to slowing down the aging process through fitness, you might as well stay at home on the couch, because fitness can't stop muscle aging.

According to scientists, regular fitness cannot stop our muscles from losing tone and shape, and the findings challenge our long-held belief that inactivity is the main cause of muscle loss in older people. Professor Jamie Timmons from Loughborough University said the research showed the link between muscle ageing and lack of exercise was not strong. He added: "When it comes to ageing muscles, experts explain this as being due to factors such as inactivity. But we see changes in human muscle as we age, both in the UK and the US."

"We didn't observe that physical activity altered age-related biological changes, so exercise may have some good functional effects in some people but not prevent muscle degeneration in others." Professor Timmons said a quarter of people would not grow new muscle tissue even if they exercised vigorously. He also stressed that a "one-size-fits-all" approach would not be effective in preventing muscle aging and degeneration.

The researchers found specific chemical markers or fingerprints of muscle deterioration, and once the chemical signature was determined, scientists were able to see how they would respond to endurance training. They were observing improvements in their patients after exercising, but they soon realized that the volunteers were working hard in the gym but their muscles were not stopping deteriorating. Professor Timmons, who specialises in exercise and health sciences at Loughborough, said: "We found absolutely no link. The main problem is that muscles age and degenerate but 25 per cent of people don't respond to exercise and grow muscle."

Sex and the City star Sarah Jessica Parker, 47, was forced to cut back on her fitness routine after her designer left her with muscular but veiny biceps. Scientists say new research suggests that when it comes to exercising to slow the ageing process, you might as well stay at home on the sofa because it finds fitness has little effect. According to scientists, regular fitness exercise cannot prevent our muscles from losing tone and shape.

He warned that repetitive exercise could lead to more than one in 10 people developing high blood pressure and nine per cent being at higher risk of diabetes. "Our public health strategy is focused on physical activity, and fitness as an approach is not working in many cases," he said.

"For some people, maybe the focus should be more on diet and sleep." These findings are bad news for fitness-obsessed celebrities trying to stay young through fitness.

Although Madonna's fitness method is very scientific and rigorous, the veins in her hands and arms usually reveal her 54 years old. Last year, 51-year-old Meg Ryan was found to have prominent veins on her arm. The tell-tale sign of exercise is prominent veins, which become more noticeable as the body ages. But at 75, Jane Fonda inspired a generation of women in sports and still makes stars look half their age. Even though she looks younger than her actual age, her training won't stop her muscles from aging.