Then, last June, he was warming up for a race when he hit a squirrel, crashed into a telephone pole and broke his arm so badly he needed surgery.

His reaction to each crash was a variant of mine. He’d taken up cycling about five years ago because he’d injured his hamstring running. “With each wreck I thought, ‘Maybe I should try running again,’ ” he said.

My running friend Claire Brown, a triathlete, crashed a few years ago when she was riding fast on wet roads, getting in one last training ride before a race. Her bike slid on a metal plate in a bridge and she went down, hitting her head and her left hip. She was badly bruised, and even though she broke no bones, she did not feel comfortable riding for the next two years. Even now, she told me, “there are bridges around here I won’t ride on, and I definitely won’t go downhill fast.”

And yet, and yet. Despite how much it hurt, my collarbone fracture was nowhere near as bad as some running injuries. When I got a stress fracture — a hairline break — in a small bone in my foot, I was on crutches for eight weeks. When I finally could run again, my foot hurt because the muscles had atrophied. Running was slow and difficult. I’d lost the rhythm and the stride that make running fun.

With the collarbone fracture, I wore a sling for three weeks but could take it off and ride my bike on my trainer — a device that turns a road bike into a stationary one — and use an elliptical cross-trainer. After four weeks I could run, and running felt good.

George Loewenstein, a professor of economics and psychology at Carnegie Mellon University, says there are several factors that separate running injuries from cycling ones.

Running injuries are often hidden — like a torn hamstring — and tend to heal gradually on their own. Bicycling injuries, he told me, “tend to be more acute and dramatic — often there is blood or even bones sticking out,” and “if it’s a gory image, it tends to deter us.”