What do the CrossFit Games and a snow sport developed by the 18th-century Norwegian military have in common? More than you might think.

Just ask Dave Castro, CrossFit’s co-director of training and its Games director. He remembers watching the sprint biathlon during the 2014 Winter Olympics, about six months before the CrossFit Games.

In the Olympic sprint biathlon, athletes cross-country ski three laps around a roughly 3-kilometer track. Between each lap, they take 10 shots at a target with a .22-caliber rifle. Miss a shot, and the athlete has to ski a 150-meter penalty lap. The first one to finish wins.

“I loved the event,” says Castro, who designs all the challenges for The Reebok CrossFit Games. “I wondered, how can I incorporate that concept into The Games?”

The result: The “Muscleup Biathlon,” a signature challenge at the 2014 Games. In it, competitors were tasked with running 400-meters, followed by a set of anywhere from 18 to 12 unbroken muscleups, an insanely difficult gymnastic exercise where you do a pullup and dip in sequence. Miss a muscleup rep, and competitors had to run a 200-meter penalty lap.

(Hate running but still want to get fit? Check out 10 Exercises That Burn More Calories Than Running)

“I take inspiration for CrossFit Games workouts from everything from sports, to art, to theater,” says Castro.

Another example: After seeing a Formula 1 car race, Castro had Rogue Fitness (the equipment supplier of The Games) design sleek, relatively light sprint sleds that the athletes used in a weight pushing event.

Stray from the norm

Castro begins planning each Games event—and there are usually around 13, spread across four days—more than a year in advance. He posts his ideas to a massive white board, and uses scale models of the equipment to visualize how the event will unfold.

Going in, the athletes know the makeup of only about half of the workouts. The contents of the rest of the events are revealed anywhere from a couple days to just 30 seconds before the starting whistle.

“We want the athletes to be able to perform on demand,” says Castro. “Being truly fit it is the ability to show up and do something without having time to rehearse.”

Those surprise workouts are often, well, surprising. In past Games, athletes have been asked to perform tasks like completing a military-style obstacle course, driving a steel stake into the ground with a sledge hammer, and throwing a softball for distance.

Find the right level of difficulty

CrossFit exploded in popularity in part because its workouts are generally more challenging than those of most other fitness methods.

The Games is the culmination of larger CrossFit competitions—like The Open and Regionals—which whittle down more than 200,000 people to the 43 most elite competitors in the world.

“The focus of the workouts of the Games is to find the fittest person alive,” says Castro. “That’s why the demands of the events are much, much higher.”

(Learn the fitness secret of this year’s top CrossFit competitor in 6 Insanely Fit Guys Reveal One Thing They Do Each Day)

Castro manages to not cross the line into making a Games event “too hard” by spending time in elite CrossFit gyms and working out with the best athletes throughout the year.

“Weight is what usually makes a workout too hard,” he says. “I constantly monitor what kind of numbers the top competitors are putting up.”

And those numbers are continually rising. Castro says it’s been fascinating to see how the top CrossFit athletes have evolved since he designed the workouts for the very first Games in 2007.

“I never thought we’d include 400-pound deadlifts,” he says. “But we recently included a workout that had a 405-pound deadlift for reps. It’s completely possible that we’ll have a 500-pound deadlift in the next five or ten years.”

“Too easy” is a different story. “There’s no such thing as ‘too easy,’” says Castro. “Even if the workout is something like 100 air (bodyweight) squats for time, the athletes will do them incredibly fast. The stakes are so high that anything becomes a great test.”

Strive for entertainment

The events also display what Castro calls “the horserace.” “We design the workouts so you can see the athletes advancing through the event,” he says. “You should be able to get up, get a hot dog, and then come back to your seat and see who’s winning.”

That also allows the competitors to see exactly how they’re doing in the event, which makes them charge harder, according to Castro.

Want a taste of a CrossFit workout that challenges Games competitors and beginners alike? Try Cindy, says Castro. “Yeah, Cindy will humble you really fast.”

See the workout and find out just how hard it is in How Many Calories Does CrossFit Really Burn?