The entrepreneurial, fast-growing, middle-market companies on this year's Founders 40 list have taken on a new challenge in the past three years: the always unpredictable public markets. But these companies haven't let the change in their status distract them from their original mission--or their commitment to rapid growth.

All successful startup founders eat, sleep, and breathe their companies' missions--but at Fitbit, James Park and Eric Friedman go a step further. The co-founders have given over so much of their brains to the business of making wearable devices to quantify physical activity, the work seems to control even their basic speech functions. Neither can get through more than a few sentences at a time without committing an inadvertent, sometimes groan-inducing, fitness pun.

Just listen: Fitbit spent the years before its June 2015 IPO "building our muscles to become a public company," says Park. A class-action lawsuit alleging that some Fitbit heart rate monitors don't work as advertised is "something we have to keep a pulse on." Do they do this all the time? Of course, says Friedman: "It's almost like the business is a perfect fit for us." (OK, that one might've been intentional.)

No matter how muscle-building it was, Fitbit's decision to brave the public markets is one that few of its peers are emulating these days. The $30 billion raised by U.S. companies in 169 IPOs last year, according to data from Renaissance Capital, was the smallest haul since 2009, when the financial crisis depressed market activity. The retreat continued into this year; there were no IPOs at all in January, the first such monthlong freeze since September 2011.

Going public, in other words, seems less attractive than it has in years--at least if you find yourself overly distracted by forces outside your control. (Not that raising money in the private markets has been much easier of late.) But the passionate, accomplished entrepreneurs behind the companies on Inc.'s second annual Founders 40 list have learned how to stay focused on growth, while tuning out the investor noise around them.

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