Researchers say that if you just imagine yourself exercising, you can increase the strength of even your large muscles. This discovery could help patients who are too weak to exercise to recuperate from a stroke or other injury. It could also help those of us who are reluctant to get off the couch and out to the gym.

Muscles move in response to impulses from nearby motor neurons. The firing of those neurons 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,? says Guang Yue, an exercise physiologist at the Cleveland Clinic Foundation in Ohio.

Yue and his colleagues found that mentally visualizing doing exercise was enough to increase strength in a muscle in the little finger. Then his team tried this with a larger, more frequently used muscle, the bicep. 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 make sure that the volunteers were not unintentionally tensing their biceps, 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. Control subjects who did not do the mental workout showed no improvement in strength.

The researchers are now repeating the experiment with people aged 65 to 80 to see if mental gymnastics works for them.

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