Actress Cobie Smulders' secret to fit: time on a spin bike.

Actress Cobie Smulders, 34, who stars in the new movie Jack Reacher: Never Go Back, has admitted she's a class junkie. One of her favorites: SoulCycle with N.Y.C.-based senior master instructor Laurie Cole, whose classes ramp it up with HIIT intervals. "These intervals are a highly effective way to raise your heart rate in short bursts, with bits of recovery in between," says Cole. "This promotes muscle building and fat burning."

Try the following ladder routine from Cole—perfect for anyone with access to a stationary bike—to get in supreme spin shape like Smulders. Warm up with 10 to 12 minutes of moderate-paced cycling; cool down with five minutes of moderate-paced cycling.

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