By the way, I learned something about 'muscle' from the physicist Hans Bethe. The Manhattan Project was full of brilliant people, but they called Bethe 'the battleship' because - while perhaps not a genius like some others - he was determined and unstoppable:
> It is not possible for us to mirror the extraordinary mental faculties of minds like Bethe and Einstein. But we can very much try to emulate their personal qualities which are more accessible if we persevere. In case of Bethe, one of his most important traits was an uncanny ability to sense his own strengths and limitations, to work on problems for which he "possessed an unfair advantage". Bethe knew he was not a genius like Dirac or Heisenberg. Rather, his particular strength was in applying a dazzling array of mathematical techniques and physical insight to concrete problems for which results could be compared with hard numbers from experiment. He could write down the problem and then go straight for the solution; this earned him the nickname "the battleship".
> Another important thing to learn from Bethe was that just like Fermi, he was willing to do whatever it took to get the solution. If it meant tedious calculations filling reams of paper, he would do it. If it meant borrowing mathematical tricks from another field he would do it.
(From a [review](https://blogs.scientificamerican.com/the-curious-wavefunction/book-review-e2809cnuclear-forces-the-making-of-the-physicist-hans-bethee2809d-by-silvan-schweber/) of _Nuclear Forces: The Making of the Physicist Hans Bethe_, by Silvan Schweber.)
It's good to try to cultivate cleverness and 'brilliance', but it's also good to build 'muscle', because a combination of these skills is much more powerful than just one.