I have read both in Coach Sommer's book and as well in other sources over the web, that the back lever is the basic and easiest static hold in comparison to the front lever and the planche. It seems that there is a consensus that the hierarchy is as follows: Back Lever --> Front Lever --> Planche.
While I can see why the Back Lever is easier than the planche, I cannot understand why it is easier than the front lever. The same bodyweight is used in both holds at the same lever. The difference lies only in the working muscle groups: in the front lever it's the shoulder extensors, while in the back lever it's the shoulder flexors. The shoulder extensors are a far stronger group of muscles than the shoulder flexors. Therefore, the logical conclusion will be that the front lever should be the easier hold. So what is it exactly that makes the back lever the easier one?
The only thing that I can come up with, is that the difficuly in the levers lies not in the active muscles of the shoulder joint, but in the core and hip muscles which function as stabilizers. In the back lever these will be the erectors and the hip extensors, compared to the front lever where these will be the abdominal muscles and the hip flexors. Perhaps this is what makes it easier. Any thoughts on the matter?