The phrase "has been flexing" is correct and usable in written English.
It is typically used to describe an action that has been ongoing. For example, "The company has been flexing its muscles in the industry for years."
Foursquare has been flexing its ability to use its data for the benefit of corporate partners.
But Tony Blair allowed a vote on the Iraq war, and since then parliament has been flexing its muscles even more.
China has been flexing those relatively new regulatory muscles in recent years as it becomes a major consumer of the world's goods.
In addition, China has been flexing its diplomatic muscles, taking a leading role in trying to defuse the North Korean nuclear standoff.
Barack Obama, who has been flexing his regulatory muscles at home, has an eye on his environmental legacy, even if a Republican-controlled Congress will do its best to thwart him.
China has been flexing its military muscles over disputed islands and is trying to build up regional influence with projects such as the Asian Infrastructure Investment Bank, of which America is not a part.
The other segment, which was once the top surface of the continent, has been flexed down and dips at a gentle angle.
For instance, the Himalayas have been thrust onto the crust of the Indian shield, which is underlain by particularly cold, thick lithosphere that has been flexed down by the weight of the high range.
The idea is offensive to many, and so lots of legal muscle has been flexed to attack targets that pirates have already abandoned.
As India has continued to move towards Eurasia, the plate has been flexed down by the weight of the subducting slab, as well as by the mountains resulting in a basin approximately 5 6 km deep (Raiverman et al. 1983).
And now the Eagles/Vikings matchup on Dec 26 has been flexed to prime time.
