The pills could help people exercise for longer
Scientists are moving closer to developing a pill which could deliver some of the benefits of exercise - even for those who do not move a muscle.
The journal Cell reports US researchers now have two possible pills which appear able to build muscle, increase stamina and even burn fat.
In tests, mice were able to run 44% further - suggesting humans may be able to do the same without prior training.
The concept is controversial because of fears they could be misused in sport.
With this in mind, lead researcher Professor Ronald Evans, from the Howard Hughes Medical Institute, and the Salk Institute in California, has produced a test which will allow the drugs to be detected in the urine and blood of competitors.
He says the drugs could eventually help tackle muscle wasting diseases, or help improve the health benefits of exercise in people at risk of conditions such as diabetes.
The two drugs, labelled AICAR and GW1516, appear to have an effect on a gene involved in the building and regulation of muscle.
This "master gene" - PPAR-delta - has the ability to control the activity of many other genes - so adjusting it could in theory have a widespread effect on the way the body works.
Genetically altering mice to enhance the activity of the gene led to the development of muscle which was much more likely to burn fat than burn sugar.
It also made "marathon mice" who were able to run much further on a treadmill.
The next step was to produce similar effects using a drug rather than a genetic alteration.
The first version, a pill called GW1516, again produced the "fat burning" effect, but no change on exercise performance until the team started training the mice with long treadmill sessions.
At the end of a series of these, the mice given the drug were running 77% longer than those training without its benefits.
The latest drug, AICAR, goes one step further, finding a different way to act on the same muscle cell mechanism.
This time the mice did not need to train - after just four weeks on the drug, they ran 44% further on their treadmills without any prior exercise.
Both versions could one day serve a purpose in humans, said Professor Evans.
"If you like exercise, you like the idea of getting 'more bang for your buck'," he said.
"If you don't like exercise, you love the idea of getting the benefits from a pill," he said.
He said the most obvious potential use was in conditions, such as diabetes, where exercise was a proven benefit.
"Almost no-one gets the recommended 40 minutes to an hour per day of exercise - for these people, if there was a way to mimic exercise, it would make the quality of exercise they do more efficient."
However, Colin Palmer, a professor of pharmacogenetics at the University of Dundee, said that the idea of the drug had proved controversial.
"It's basically a drug that enhances training. The thing that raises eyebrows is the concept of a drug that improves endurance training for sports professionals."