Coerce means to do something by using force or threats, he wants to force himself into thinking of it but could not achieve it because his muscles were preventing it.
In this case, coerce<span> means the man had to force himself to do something against his own will. At this particular juncture in the story, all four men were exhausted. Even though the men were near land, the correspondent, being the cynic that he was, still believed that there was more yet to endure. His muscles had been overworked to the point where death seemed to be an equally good option to having to swim toward land.</span>
