As he maintains his hold on the great fish, reminding himself that he must be worthy of the great DiMaggio who performed like a champion despite terrible pain because of bone spurs. He recalls how he defeated a large, strong man in a two-day arm wrestling match in order to bulwark his courage and stamina.
All of his mental fortifications encourage Santiago, but he must worry about the night becoming cold because the sun
...will bake it [his cramping] out well now. It should not cramp on me unless it gets too cold in the night. I wonder what this night will bring.
If his hand cramps, it will become "a traitor" to him and will not perform as he needs it to in order to maintain his hold on the fish. However, despite his mistrust of his hand, Santiago calls up his memory and his confidence to "endure" and bring back the fish.
My hand is only cut a little and the cramp is gone from the other. My legs are all right. Also now I have gained on him in the question of sustenance.
Santiago is an experienced fisherman and a Hemingway code hero who acts honorably amid a losing battle.