Robert Garis wrote in Following Balanchine: "...This time there was a new inflection [regarding Balanchine's choreography] because in Agon I saw for the first time something autobiographical in his choreography. This came fully clear to me only a year later, when the partnering in another new ballet, Episodes, reminded me forcibly of the partnering in the pas de deux in Agon. I was seeing a new kind of collaboration between men and women: an intensely careful, watchful, tender, and grave working together to achieve tense and perilous extensions and balances. From one point of view the Agon pas de deux was just another Balanchine revision of classical pas de deux, but to me it seemed to stem from and reflect, though not quite to imitate, his work with Le Clercq in physical therapy--not that the dance mimics the movements of physical therapy but that in both cases the man and woman seem required by some urgent necessity to move quietly, cautiously, with all the skill and courage they can muster, and in a mood of held-breath crisis.
I think that Garis' theory is quite plausible - Balanchine was as found of expressing mysterious moods and spiritual relationships as he was visualizing musical forms with human movement. It is believable that Balanchine would want to silently examine the 'metaphysics of polio'. But not to dwell on the constraints of the disease, and the negative emotions triggered by disease. And he was too clever an artist to get bogged down in the literal. Balanchine was always primarily interested in the transcendence of restraints - the spiritual aspects of any physical relationship. That might seem strange for a dancer/choreographer, but it adds the necessary dimensionality to his work that makes many of these ballets 'art', and not only steps, and forms.