Dutch researchers have developed a virtual "body double" system that could help stroke patients regain movement.
The system displays an image of the person training and the force at which they are using their muscles on a screen.
The Human Body Model is being tested at the Sheba Hospital in Tel Aviv, Israel, reports the New Scientist.
It is the latest in a series of virtual reality physiotherapy treatments.
The user must don a suit which has 47 reflective markers pointing to specific muscles.
They then undertake exercise such as running or walking on a treadmill or pushing weights while infrared lights and cameras are used to track the markers.
Sensors on the floor are also used to measure the force applied to the ground by the user's feet.
This information is then fed into a computer and the results projected onto a screen.
"It allows you to see the muscle groups you are using in real time, and even the forces they are creating, which are usually invisible," says Oshri Even-Zohar, who developed the system.
But he conceded it could not show every muscle at work. "There is no tool in medical science that allows you to measure all the muscle forces in motion."
Last year, a UK physiotherapist developed a treadmill with a screen which showed the user walking through various landscapes.
The images tricked the patient into thinking they were moving relatively slowly, which encouraged them to start walking faster, BBC reported.
A spokesman for the Stroke Association said: "Any innovations which may offer new opportunities to deliver greater recovery for stroke survivors are greatly welcomed.
"But, like all medical treatments, it must be rigorously tested before they can be taken up as standard rehab practice."