It wasn't long ago the most reliable measure of a good workout was simply how spent you felt. Then came digital pedometers clipped to belt loops of dad jeans. Now it's bands and watches of all sizes: digital trackers break down exertion in units of steps; they upload information to the cloud and store it for later use.

Last month, Fitbit — a company named after its activity-tracking arm band — had its initial public offering for shares of its fitness empire. Harvard Business School historian Nancy Koehn joined Boston Public Radio to talk about the soaring popularity of tracking devices like Fitbit, Jawbone, and Apple Watch.

What's Fitbit's advantage in an increasingly crowded marketplace?

Koehn: They're gonna hit over a billion dollars in revenue this year. [...] They are very different from most high-tech startups which go to the public market. [...] They've emerged out of the pack. [...] There are a number of players in this market, including now the Apple watch, but Fitbit clearly leads the pack.

Even if people buy them, do they actually use them?

They're a little bit like health clubs: we buy them full of verve and resolve, and that dissipates. [We don't know] how many people are actually wearing them, using them. [They're] clearly for the core group of people that are active registered users, which is something around 20 million. [...] Fitbit [sits] at the center of several important trends and concerns that could end up paying off very, very big for this company.

Does the fitness tracker measure exertion even when you don't take steps?

It does. [...] It measures all kinds of activity and converts them into steps.

There are a lot of phone apps that track steps and fitness, too. What advantage does the Fitbit have over apps like that?

[Tracking steps] is all this does, so that's actually very, very useful if you want to be focused on your motion. And all you do is just touch the button. [...] This is actually really simple. You can make it more complicated on your phone if you want.

And what about fitness tracking works for you and so many other people?

[Sometimes] I'm tired. I'd rather have a Heineken. [...] Instead I think, "Gosh, I think I need to log another 2,000 steps, and take the dogs for a walk around the field!" [...] I don't belong to a health club. [...] I spend a lot of time sitting in my messy, big study, [but] I'm walking.

>>Nancy Koehn is a historian at the Harvard Business School. Her latest book is Ernest Shackleton: Exploring Leadership. She appears every Tuesday on Boston Public Radio.