A twice-weekly trip to the gym may not just give you stronger muscles - it may give you younger muscles as well.
The secret of turning back the clock?
Research on over-65s has shown that regular resistance training appears to reverse signs of ageing in the muscles.
Analysis of muscle tissue showed the molecular machinery powering muscle cells became as active as that in 20-year olds after exercise.
Its authors say the Canadian study, published in the journal PLoS One, shows the benefits of remaining active.
Around 25 healthy adults over the age of 65 were given twice-weekly hour-long training sessions for six months. The results were compared with participants aged 20-35 years.
Before the sessions, which used standard gym equipment and a programme of 30 contractions of each muscle group, the older adults were 59% weaker than the younger adults.
But after the training the older adults were only 38% weaker.
The researchers also took tissue samples to look at changes in the mitochondria, the rod-like "power plants" that sit within every cell and generate energy.
Previous studies have suggested that mitochondrial dysfunction is involved in the loss of muscle mass and function commonly seen in older people but the team wanted to look specifically at the gene activity in the mitochondria.
The results showed the gene expression - the generation of functional proteins by a gene - declined with age.
But exercise resulted in a reversal of the gene expression back to levels similar to those seen in the younger adults.
Dr Simon Melov, who co-led the research at McMaster University Medical Centre in Hamilton, Ontario, said: "We were very surprised by the results of the study.
"We expected to see gene expressions that stayed fairly steady in the older adults.
"The fact that their 'genetic fingerprints' so dramatically reversed course gives credence to the value of exercise, not only as a means of improving health, but of reversing the ageing process itself, which is an additional incentive to exercise as you get older."
Co-author Dr Mark Tarnopolsky added that a further four months of follow-up found most of the older adults were no longer doing formal exercise in a gym, but were doing resistance exercises at home, lifting soup cans or using elastic bands.
"They were still as strong, they still had the same muscle mass."
"This shows that it's never too late to start exercising and that you don't have to spend your life pumping iron in a gym to reap benefits."
Professor Marion McMurdo, head of ageing and health at the University of Dundee, said: "There is an age-related loss of muscle from about 35 onwards.
"It used to be thought this was completely irreversible but what we have begun to understand is that there's a sedentary loss as well."
"We know that people in their 90s can regain lost muscle tissue and lost muscle strength with quite a modest amount of exercise."
"The encouraging thing is it doesn't have to be vigorous and only a little exercise is necessary."