When Bill McDonough rides the commuter rail into Boston, a 40-minute trip from Hanson, the 55-year-old often offers his seat to others — even younger passengers.

Super polite? No. He’s part of an anti-sitting movement that’s gaining followers as evidence continues to build that what was once considered a benign pastime — sitting for extended periods — can actually shorten your lifespan, increase the risk of cardiovascular problems and diabetes, and contribute to obesity later in life.