https://playground.babylonjs.com/#9JWAWG#6
Orange lines show the bone direction. However, this direction may not be the parent-child direction you’re looking for.
You may want to check out the post Return to Rest not Behaving as Expected? by @Pryme8, @Evgeni_Popov, @Drigax. We could get the parent-child bone direction by using the same matrix method on the bone meshes rather than the bone itself.