I can't figure it out, and I don't see it in the tutorials.
I thought at first it was to indicate a fatigued player as it seems to happen later in the game, but I see it around my guys with a good amount of stamina.
Also, it took buying a 65" tv to finally realize the blue ring around a selected player's feet indicates their stamina. I've been playing this for months, and now I feel like a dum-dum. It would be cool to increase the contrast of Full Stamina vs. Depleted Stamina colors. The blue vs. slightly less opaque blue gets lost pretty easily.