Michael Strautmanis had no sooner started his White House job as a deputy assistant to the president when his boss began badgering him. "Am I working out? Am I eating right?'' Strautmanis recalled President Obama asking. "And then he told me that I should use his trainer.'' The very fit president has sent other White House aides to Cornell McClellan, the trainer, a genial master tormenter who is the secret weapon behind Michelle Obama's famously toned arms and who has overseen both Obamas' exercise programs for more than a decade. Strautmanis made one — and only one — appointment with...