Scientists in the US have developed a technique that could massively improve the control amputees have over their prosthetic limbs.

The technique, known as targeted muscle reinnervation (TMR) allows a motorised prosthesis to respond directly to the brain's signals.

The research has been led by Dr Todd Kuiken, a physiatrist at the Rehabilitation Institute of Chicago and professor at Northwestern University. He explains: "The idea is that when you lose your arm, you lose the motors, the muscles, and the structural elements of the bones. But the control information should still be there in the residual nerves."

So Kuiken took the residual nerves that used to be associated with a patient's arms, and connected them up to the chest muscles. Some of the patients said when researchers touched their chests, they could feel a sensation in the limbs they no longer had, a phenomenon similar to the ghost limbs some amputees experience.

Crudely speaking, when the patient thinks about moving the missing limb, the nerves (now reconnected to the chest muscle) are stimulated. This causes the chest muscle to contract, which sends an electrical impulse to the prosthesis.

So far the team reports patients are able to "instruct" their new limb to make four fairly crude movements: open and close hand, and bend and straighten elbow. But new research, published in the Journal of Neurophysiology, has shown that the technique has the potential to do far more.

The researchers placed 79 to 128 electrodes on the chest muscles of a group of five patients. Using an EMG (electromyogram), they tried to identify the distinct signals associated with 16 arm and hand movements, right down to manipulation of missing fingers and thumbs. The team reports an accurate recognition rate of 95 per cent.

The work also demonstrated that the neural pathways associated with a limb remain intact for nine to 15 months after the event that leads to an amputation, the team says.

The researchers are now working with US servicemen who have lost limbs in the line of duty. ®

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