On our last Pyrenees-trip, Jan-Willem and I explored the terrain between bm137 and 139. On this steep hill, a more or less direct descent via bm138 seems quite difficult. But a detour (especially between 138 & 139 as I did in 2009) requires extra kilometers on tarmac.
We found out that the descent from 137 to 138 is quite steep and rough although wayfinding is no problem and part of it is a trail. The route between 138 and 139 is more gentle and pleasant.
It’s all shown on this map (from the Sitna-site): click on it to enlarge.