According to fitness experts across the board, effective, healthy fitness is a lot like life—it should be dynamic, fluid and it should keep you on your toes. “The key is variety,” says Noel Davis of Miami-based E.A.T Fitness. “You always want to surprise the body. Think of how the brain trained when you were a child: you learn English, math, social studies and so many other subjects; so the muscles are the same way—they require a thorough and well-rounded education.” Circuit training, which is a one-stop exercise session that combines cardiovascular training with toning and resistance, does just that. By tapping into the idea of “switching it up,” you not only call upon all the muscles of your body to rise up and perform, but you naturally avoid the pitfall of routine, which does nothing but keep you static and precludes any actual progress.