The most secretive body parts are muscles, which hide behind layers of skin and appear only occasionally to help lift cars off trapped kittens. Flex your biceps in the absence of a cat emergency with today's Groupon: for $25, you get a four-week Spartan-X Cardio Fitness program at Blackhawk Fitness (a $55 value).

Blackhawk Fitness's 60-minute Spartan-X Cardio Fitness programs engage the entire corpus with a cardio-and-lean-muscle workout that hastens respiratory activity and wrings out sweat glands. Members perform a series of high-intensity exercises that use such everyday items as sandbags, tires, jump ropes, and treasure chests of Spanish doubloons. Grab hold of a training band to execute muscle-building resistance workouts, or rely on no-frills body-weight exercises to get hearts pumping blood faster than incorrectly installed fire hydrants pump gas. Programs commence monthly, with individual meetings taking place at 7 p.m. on Monday, Wednesday, and Friday evenings.