Turns out it's a physical activity...

Intense thinking and concentration burn more calories than most people realize. Getting really into a mental exercise can release adrenaline and elevate heart rate. If you wear a smart watch or FitBit and spend a day behind the wheel of your car doing any sort of involved driving, you'll often be surprised at the number of calories it says you've burned.

So imagine what it's like for really intense dudes who are doing olympic-level thinking...

In recent years, the proliferation of livestreams, fit trackers and other tools have created almost a game within a game at modern chess tournaments. Rubbernecking audiences watch for clues of mental cracking and physical distress in the quirky, contemplative figures bowed over the boards. At the 2018 Isle of Man International tournament, fitness metrics projected on a large screen revealed that grandmaster Mikhail Antipov torched 560 calories sitting stock still for two hours. By way of comparison, the average person will burn just 100 calories running a mile on a treadmill.