It is a couch potato's dream - just imagining yourself exercising can increase the strength of even your large muscles. The discovery could help patients too weak to exercise to start recuperating from stroke or other injury. And if the technique works in older people, they might use it to help maintain their strength.

Muscles move in response to impulses from nearby motor neurons. The firing of those neurons in turn depends on the strength of electrical impulses sent by the brain.

That suggests you can increase muscle strength solely by sending a larger signal to motor neurons from the brain.

Investigators have already found that mentally visualizing exercise was enough to increase strength in a muscle in the little finger, which it uses to move sideways. Now his team has turned its attention to a larger, more frequently used muscle, the bicep.

Thought experiment

They asked 10 volunteers aged 20 to 35 to imagine flexing one of their biceps as hard as possible in training sessions five times a week. The researchers recorded the electrical brain activity during the sessions. To ensure the volunteers were not unintentionally tensing, they also monitored electrical impulses at the motor neurons of their arm muscles.

Every two weeks, they measured the strength of the volunteers' muscles. The volunteers who thought about exercise showed a 13.5 per cent increase in strength after a few weeks, and maintained that gain for three months after the training stopped. Controls who missed out on the mental workout showed no improvement in strength.

Annual Meeting of Society for Neuroscience conference in San Diego, California November 2001