Prince Philip was spry and walking without a cane right up until he died at 99 in April. What kept him walking tall? It was probably 5BX, the original functional fitness routine. Standing for Five Basic Exercises, it’s an 11-minute workout created for the Royal Canadian Air Force in 1956.

“[It] is designed to show you how to develop and hold a high level of physical fitness, regardless of where you are located,” reads the introduction booklet to the workout. “The 5BX Plan puts physical fitness within reach of every member of the RCAF.”

Several sources report the Duke of Edinburgh completed the simple regime of equipment-free exercises on a daily basis—the workout even made an appearance in an episode of The Crown, a Netflix series about the reign of Prince Philip’s wife, Queen Elizabeth II.

Canadian Bill Orban developed the routine based on research he conducted at the University of Illinois in the 1950s. By testing oxygen intake he realized duration of exercise was less important to overall fitness than intensity. The RCAF knew its personnel were not fighting-fit and hired Orban to develop an exercise regimen that troops could do anywhere, even in their barracks, and was short enough to do every day.

“To develop physical fitness, exercises must be balanced and planned,” explains the narrator in a 5BX promotional film. “They must be progressive. And, above all, vigorous and regular.”

The Five Basic Exercises include toe touches, situps, back extensions, pushups and running. To make it accessible and encourage continuous intensification, Orban included six charts that prescribed modification of each exercise and progress the number of reps. While the fitness, strength and stamina demands increase, the time spent on each exercise and the total workout length does not. For instance, on Chart One, the situp is done with legs flat and demands lifting the head “just high enough to see your heels.” A ‘D’ score is 3 reps in 1 minute; an ‘A’ is 18. On Chart Six, the hardest, it’s 35 to 50 V-sits in 1 minute.

The 11-minute duration and equipment-free nature of the workout were both controversial at the time, but RCAF testing bore out its effectiveness. Orban went on to develop a women’s-specific version called XBX, which included six moves completed in 12 minutes. And 5BX became popular inside the military and out.

In 1961, the RCAF released the workout to civilians by publishing a book called Royal Canadian Air Force Exercise Plans For Physical Fitness. Over the next 30 years it was translated into 13 languages and sold 23 million copies. Some credit it for kicking off the modern fitness culture and it definitely inspired modern fitness gurus and trends—it sounds like a CrossFit workout.

While Prince Philip may not be 5BXing anymore, his daily workout lives on. Prince Charles and Prince William reportedly practice the routine too.

For access to exclusive gear videos, celebrity interviews, and more, subscribe on YouTube!