How is it possible that 93-year-old Jon Mendes was able to compete in the New York City marathon on Sunday, but people decades younger couldn't dream of running 26.2 miles? Perhaps the near-centenarian’s "fitness age" hasn’t incereased at the same rate as his senior citizen status indicates. So how do you defy the logic of aging when you’re secretly trapped in a teenager’s body (or vice versa)? Pretty easily, in fact.



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Instead of paying for expensive tests and being hooked up to a machine while running on a treadmill, people can take to the Internet and plug in numbers to determine their fitness age. Using a special calculator created by researchers at the Norwegian University of Science and Technology in Trondheim, you can estimate how your body actually functions physically in relation to how it should work on average according to your age, said the researchers in a paper published in the journal Medicine & Science in Sports & Exercise. The online tool determines your fitness age by measuring V02 max (peak oxygen uptake), which shows how well your body uses oxygen. The study evaluated the V02 max of nearly 5,000 people between 20 and 90 to discover how their real age compared with their athletic endurance.



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For the mathematics behind a fitness age, an algorithm is calculated by plugging in the frequency, duration, and intensity of a typical exercise, along with your age, gender, waistline (in centimeters), and resting pulse. As an example, a 40-year-old man who exercises two to three times a week for less than 30 minutes with little hard breathing and sweating, and has a waistline of 33 inches and a resting pulse of 65, has a fitness age of 36. While a fitness age close to your actual age is usually expected, the number doesn’t paint the whole picture. Dr. J. Shah, founder and medical director of the Amari Clinic in Scarsdale, New York, advises Yahoo Shine that “including other criteria will give you the exact longevity of the person.” By incorporating mental, cardiac, pulmonary and metabolic ages, you can figure out a more accurate physiological age, he says.



If you find yourself in a "Freaky Friday" kind of situation and want to turn back the hands of time, here are some tips from top fitness instructor Kimberly Neumann of Equinox:





Just do it. “One of the top excuses for not working out is just not having enough time. People don’t realize how much fitness they can pack in their day.” She suggests making substitutions: Instead of sipping on a highly caloric latte for a pick-me-up during a midafternoon break, run up and down the stairs at work. “You’d be surprised at the energy boost,” she says.



Up the ante. Increase the intensity of your workout by sweating and pushing yourself a little more. “Frequency and intensity are two variables that you have within your capability to alter.” Don’t think your body can stand one more second on the treadmill? Just remember 93-year-old marathoner Mr. Mendes...



Think of exercise as eye cream. Just as products can turn back the beauty biological clock, fitness can have the same Benjamin Button-like effect. “The reality is if you can consider fitness as an antiaging technique, then maybe you can make it more readily part of your routine.”



Be zen. One of the factors for figuring out fitness age is having a normal resting heart rate — a regular one ranges from 60 to 100 beats per minute. “By meditating, you can do your body some good by lowering stress levels.”



Decrease stress. Take some deep breaths and a bunch of chill pills. “Cortisol will cause you to hold weight around your waistline, and one of the contributing factors for fitness age is waist circumference."



