Symbolism, Imagery, Allegory
As immediate as food and shelter, physical health is at the center of life in the wilderness. Tommo recognizes that most of the Typee are in superior physical condition: "not a single instance of natural deformity was observable in all the throng" (25.3).
This is thrown into a deeper contrast, while he's just so miserably down-and-out himself. Tommo writes: "my chief source of anxiety, and that which poisoned every temporary enjoyment, was the mysterious disease in my leg, which still remained unabated" (16.2).
The injury prevents him from fleeing the valley, and even limits him to the distances Kory-Kory and others will go with him on their backs. The worse his leg is, the more his own agency is restricted. Even vigorous Toby must return, when he sustains an injury from an attacking Happar.
Yes, it's totally common sense that bodily pain would limit your movements. But what else might this motif be pointing to? Well, chew on this: it seems like a super-important moment, when, at the close of the novel, Tommo grabs "a spear which was leaning against the projecting eaves of the house," (11.31) and uses it as a crutch to make it to the beach, to the ship, to freedom, and to home. At last, he's able to overcome his injury and stand—er, limp—on his own two feet. He is no longer beholden to Kory-Kory or anyone else.