In this chapter of the story, Santa has finally taken up the exercise habit himself by hitting the treadmill. (If you remember, in part one of this series he was trying to get the reindeer to work the treadmill for him.) The effort is difficult, and he's also taking some verbal flak from Mrs. Claus who points out that it's normally the reindeer who are pulling his weight around (because they pull the sleigh, remember?).
When Mrs. Claus remarks that it's nice to see Santa pulling his own weight, one of the reindeer remarks, "Darn straight!" Those reindeer have a sense of humor, huh? But make no mistake; they love Santa and they're helping him stay focused on his exercise so that he can improve his health and beat diabetes before Christmas.