German to English Medical Medical (general)

This abbreviation comes from a report about an examination to measure arterial pressures in the lower limb in a patient with PAOD.

There are four graphs from the "Oszillogramm".

These are labelled 19. FSL re, 19. FSL li
41. GZ re, 41. GZ li


The numbers aren't hugely clear, but I am not too worried about them
Re, li are obviously right and left, and GZ = big toe

But what is the "FSL"

What other point in the leg are they getting a measurement from?