According to The New York Times, fitness is full of numbers meant to help you become faster and stronger. There’s your mile run time, your resting heart rate and measures of strength and flexibility. But perhaps the gold standard is VO2 max.

A handful of years ago, the test — which tracks how much oxygen your body absorbs — was an obscure tool mainly used by elite athletes. Today, it’s touted by fitness professionals and wellness experts as being a useful measure for all exercisers.

But getting an accurate number requires an expensive and exhausting lab evaluation. And estimates provided by wearable devices might not tell you much. So how useful is it to invest time and money in the full work-up, and how important is knowing your VO2 max?

For everyday people who want to be healthy and live a long time, the measurement is “the best piece of empirical information we have on health and longevity,” said Kate Baird, MA, ACSM-CEP, CSCS, exercise physiologist at HSS.  The key, she said, is acting on what the data tells you.

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