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Stories: Harnessing the murmur of muscles

'Muscles emit a small, low-frequency vibration but it's never been used before for prosthesis control'

Our muscles murmur as they contract. Now, researchers at Bloorview will harness those sounds to revolutionize the way prosthetic limbs are controlled.

Muscles emit a small, low-frequency vibration that can be heard with the human ear, but it’s never been used before for prosthesis control,” says Tom Chau, co-ordinator of intelligent systems research and project leader.

The plan is to design and embed tiny hybrid sensors - which measure sound and vibrations - in below-elbow prostheses. That will allow researchers to develop a silicone device that rolls on comfortably like a sock, and is more functional.

Until now, prostheses have been powered by measuring electrical signals produced by the muscles, then converting those signals into movements. The sensors that record these signals must be mounted in a hard plastic socket just below the elbow, so they can be secured on the forearm muscles. But this limits function and comfort. Because the elbow fits into the hard socket, the user can’t naturally rotate his or her arm. And because the device is suspended from the elbow, all of its weight is concentrated there, creating pressure that can be uncomfortable.

By using muscle sounds, Tom’s team plans to change the location of the sensors, so eliminating these problems. Because muscle sounds travel down the limb, they can be measured away from the contracting muscles. This will allow Tom and his colleagues to place sensors close to the end of a person’s stump in a soft, sock-like device that rolls on. The elbow will be covered by a soft silicone - instead of a rigid plastic - “so users will retain the natural rotation of their forearm,” Tom says. And because the device will wrap around the limb - instead of hanging from the elbow - “its weight will be better distributed, making it more comfortable.”

Currently, Tom’s team is designing two components of the sensor that will pick up the small rumblings emitted by muscles. One measures sound and the other vibrations. Both are “about one-quarter the size of a baby nail,” Tom notes.

These sensors will convert muscle sounds into electrical signals that will be sent to a tiny microcontroller that will activate various movements, based on control strategies developed for each user.

An ongoing challenge with powered prostheses has been how to distinguish between contractions a person intentionally makes to activate the prosthesis and background “noise.” This can be created by sensors moving against skin during unintentional movements - such as when people swing their arms when walking - or may simply be a result of noise in the person’s environment.

The research team - which includes Steve Naumann, director of rehab engineering, graduate student Jorge Silva and prosthetist Winfried Heim - hopes to tackle this in two ways. First, by having the sensor embedded in silicone - as opposed to placed in a slot in a plastic, hard-socket device - the silicone will “act as a filter for high-frequency environmental sounds,” Tom says.

Second, the design of the new sensor will address the issue of background noise from unintentional movements. The microphone component, placed closest to the skin in the device, will pick up sounds from both intentional and unintentional contractions of the muscles. The vibration component, however, will be placed farther away from the skin and with a thick layer of silicone inbetween, allowing it to detect only unintentional sounds. The signals will then be sent to a microcontroller that will weed out unintentional sounds before powering the prosthesis.

The team is looking at how many sensors might be required in one device, and whether information can be combined from a number of them. They’re also analyzing signals from muscle sounds to determine if there are characteristic ones associated with different contractions.

The first phase of the project looks at developing the sensors and the second involves embedding them in soft sockets. A prototype is expected later this year.

To be connected with expert sources, contact:

Louise Kinross, Manager, Communications
Tel: 416-424-3866
Pager: 416-589-8826
E-mail: media at bloorview dot ca

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