As a younger man, Santiago is involved in an arm-wrestling match with a man who is both younger and bigger than he is. Though he is apparently out-matched, Santiago eventually wins the match.
The match lasts an entire night, but in the end Santiago's will to win overwhelms that of his opponent. This episode relates to the main story of the book as Santiago's endurance and strength of spirit are put to the test by the fish in a similar way.
Additionally, the arm-wrestling episode that led to Santiago's nickname, El Campeon (the champion), shows that Santiago was once seen differently in his community. When he was younger, he was a champion, an example of the virtue's honored by his culture. As an old man he still possesses these virtues, "he retains much of his youthful strength", but his community does not always recognize this fact.
Santiago, indeed, continues to maintain a remarkable strength of will and an ability to endure that separates him from others. His former nickname still applies.
...he is willing to go far beyond the limits of younger, more successful fishermen and to test his physical and mental endurance.