Scientists are racing to develop a muscle drug that could allow people to stay toned without exercising.
The pill would maintain muscle tone without the need for pumping iron
The aim is to prevent muscle weakness and wasting in the sick and elderly and to help make long space flights feasible for humans.
But anti-wasting drugs would inevitably be tempting for athletes, experts told New Scientist magazine.
Teams have been studying the genetic pathways controlling how muscle builds up and is broken down in the body.
Gym in a bottle
Tests on rodents showed that manipulating these pathways can halt muscle wastage from disuse or disease.
At least three research groups have identified some of the genes responsible.
Alfred Goldberg and colleagues at Harvard University in the US and a pharmaceutical company Regeneron have found genes called atrogin1 and muRF1 that are active during muscle wasting.
A team at Purdue University, Indiana, has been looking at a gene called erg1.
Scientists are confident that they will soon have a workable therapy ready for testing in humans.
Such drugs could be given to patients confined to bed for more than a few days or to elderly patients to help keep them mobile.
The drugs might mean that people with broken bones could avoid long and painful physiotherapy sessions to rebuild muscle strength.
Weaning people off respirators would also become easier as doctors could prevent wasting of the diaphragm.
NASA is also interested in the medicines because astronauts lose muscle mass on long missions.
Open to abuse
But while there are valid medical and space applications for anti-wasting drugs, they will inevitably be tempting for athletes, Dr Goldberg said.
Experts warn that the although the drugs would maintain muscle size, they would not provide any of the other health benefits of regular exercise.
Professor Paul Greenhaff at Nottingham University in the UK has been conducting similar research into muscle growth and wasting.
He said: "It's extremely exciting and of enormous potential for clinical intervention.
"But we need to do these experiments in patients now.
"And there is a potential for abuse of these drugs," he added.
He said that the role of exercise had been underplayed.
"Contraction of the muscle itself is important. Exercise itself is the most highly potent stimulus of muscle growth.
"We must also look closer at nutrition as well."
He said dietary intake of carbohydrate and protein is known to promote muscle protein synthesis, he said.
Dr Julia Thomas at the Muscular Dystrophy Campaign said: "In order for this treatment to be truly beneficial it is important that muscle strength is also increased and future trials and research will be key in determining whether this is the case."
She said a pill to prevent muscle loss would unfortunately not be able to change the genetic cause of muscular dystrophy, but might slow down the disease progression.
"It is our hope that if this research becomes a workable drug the concentration of its use will be on serious medical conditions, such as muscular dystrophy, rather than for those desiring a 'gym in a bottle'."