Longevity clinics are booming. Prestigious hospitals want to enter.

CHICAGO — I walked briskly on a stiff treadmill, wearing a mask that evoked the villainous Bane from “Batman.” Prince hummed backhanded compliments over an iPad speaker to keep me energized, promising that I didn’t have to be rich or cool to be his girlfriend. Every so often, the woman standing next to me would ask me to show the color-coded sheet ranking my state of exertion from 1 (basically, “I could do this all day”) to 10 (“Not only do I hate it, but I’m about to collapse if you don’t slow this thing down”). The goal was to get to 10. Pushing yourself to achieve difficult physical goals doesn’t come naturally. I can’t continueI thought. I continued.

This is the VO2 Max test, which shows how efficiently the body uses oxygen (“V” stands for volume, “O2” for oxygen) and is considered the most accurate medical measure of cardiorespiratory fitness. It was the last in a series of assessments I had undertaken at a brilliant longevity clinic affiliated with Northwestern University – as part of a $5,500 annual package.

The quest to overcome the indignities of aging has long prompted costly and questionable practices, from 1920s doctors who believed in the rejuvenating power of transplanted monkey testicles to Bryan Johnson’s since-abandoned blood transfusions of his teenage son. Today, some of the nation’s most prestigious health systems are jumping into the growing longevity business. They promise a rigorous, scientific approach to helping people preserve their youthful vigor for as long as they can, with assessments that give patients more detailed pictures of their current physical condition and aging trajectory than they would get during a typical doctor’s visit.

Some critics, however, question how to reconcile the clinics’ high costs with the hospitals’ nonprofit status and charitable missions, and warn that in their zeal to attract paying customers, they offer tests that lack sufficient evidence showing that they actually improve people’s health.