There is a reason why people feel they have to exercise harder as they age to get the same results, scientists say.
Sometimes working out seems particularly hard work
A US team found a system that boosts muscles fails with age, leading to the need for increased efforts, the paper in Cell Metabolism reported.
The finding could also help foster an understanding of type 2 diabetes, which is linked to ageing, the study said.
But a UK expert said there was little point in people exercising harder, as muscle function could not be regained.
It is known that even the fittest 70-year-old has more fat in their muscles and livers than a 20-year-old.
These fat cells have been linked to the rise in insulin resistance - where the hormone is not processed properly - and type 2 diabetes, which is connected to ageing.
The team from the Howard Hughes Medical School at Yale University School of Medicine, compared the skeletal muscle of three-month-old rats and two-year-olds.
They found that a process called AMP-activated protein kinase (AMPK) slowed down in the older animals.
AMPK's role in skeletal muscle is to stimulate the body to burn off fat and to fuel cells, via the production of mitochondria - cells' power sources.
The skeletal muscles of marathon runners has been found to have a much greater mitochondrial content and a greater capacity to burn fat, which the scientists say is probably linked to high levels of AMPK activity.
The animals were exposed to a chemicals which stimulates AMPK and were also fed more food, which also stimulates the process.
They found that the older rats had lower AMPK activity than the younger animals.
In addition, the muscle of young rats who did more exercise had double the normal AMPK activity - up to 38% - while in older rats, this effect was "severely blunted."
Quality of life
Dr Gerald Shulman, who led the research, said: "The message is that, with ageing, the AMPK pathway has reduced activity."
He said this meant a person would have to work harder when trying to maintain the same benefits from exercise as they did when they were young.
"AMPK activity in our skeletal muscle is probably a good thing because it activation stimulates glucose uptake, increases fat oxidation [fat burning], and promotes mitochondrial biogenesis [production]," he said.
Dr Shulman added that the reduction could be a contributing factor to the development of insulin resistance and Type 2 diabetes because of its effect on fat burning and mitochondria production.
Dr Anne McArdle, an ageing specialist at the University of Liverpool, said: "Loss of skeletal muscle mass and function as we age is a major problem which has a significant effect on quality of life of older people.
"Studies which examine the mechanisms by which muscle function is lost are crucial to the development of interventions aimed at maintenance of muscle mass and function in later life."
Dr McArdle said this study supported other work which had suggested the response of skeletal muscles and other tissues was affected by stresses, including exercise.
But she added: "The data suggest that the ability to increase AMPK activity is completely abolished and so there is little evidence to suggest that 'working harder' would overcome these deficiencies."