Question: is it the frequencies of sound that help you determine what way the skin is stretched ?

Michel Destrade answered on 8 Nov 2016:

Yes absolutely, you’ve understood the very principle of some of my experiments!

When a guitar string is stretched, it vibrates at a higher frequency than when it is slack. By measuring the frequency we can tell how stretched it is. We’re doing the same with skin (and in the brain too!): launch a sound wave in the skin by tapping it with a small device, and measure its speed. Then I do the maths to figure out how much it’s stretched in this direction or in that direction.

Surgeons like to know in which direction skin is stretched the most, because if they cut along that direction it will lead to a neat scar. Really important when you have to operate on a visible part of the body!