Some people will tell you that the detail that distinguishes the human being from the rest of the animal kingdom is the opposable thumb. Others will say that the distinction is the ability to express ourselves in language. With due respect to the science, I think both ideas are wrong. The distinguishing characteristic of the human being is the ability to hold multiple meanings for a single referent in our minds. We perceive in layers, sometimes seeing them all as if looking through transparencies, sometimes considering them individually as we peel them back.
"Asurim" (the plural of "asur") has three meanings. The question is: which one is best applicable in prayer.
The most usual translation is someone who is tied up or imprisoned, that is, shackled and unable to free him- or herself.
The second meaning – don't laugh – is someone whose bowels are bound, that is constipated.
And the third meaning is "prohibited," used most frequently in referring to categories of actions in Jewish law (and, in modern times, law in general).
To affirm that God is "matir asurim" according to the first meaning is to see God as the great liberator. More powerful than chains or a jail cell, God releases those physically (and perhaps emotionally) imprisoned.
To affirm that God is "matir asurim" in the second meaning is, I think, most consistent with the theme of this paragraph of the Amidah. As I mentioned before, sickness was not a casual thing in earlier times. People died from conditions we treat with simple medications – from things that made them fall (noflim) and fail (cholim) and find themselves unable to relieve their urge to evacuate. They were often the harbingers of death. God, we affirm over and over in this brakha, is more powerful than death.
But most intriguing is the third meaning – God "permits the prohibited." So much of what we presume to be permanent gives us stability in our world view, but closes off other potentials. With God's permission, the forbidden become possible. And we who are gifted with the ability to see multiple meanings in words and ideas by dint of being created in God's image must not be afraid to explore the possibilities.
The real meaning of free will is not to choose to do right or wrong, or to elect one fork in the road over another. The real meaning is to consider the many choices that present themselves as part of the whole. God releases the shackles that close us to that potential.