Want to know what the job of the future looks like? Go to the gym.

Phillip Hoskins did, but not to work out. He went to find clients, and to join the ranks of personal trainers, one of the fastest-growing American occupations.

"I knew I didn't want a desk job," said Hoskins, of Louisville, Ky., who became a personal trainer after being let go, after 17 years, from a middle-management position at a car-repair shop in December. "I'm pretty fit for 51 years old, and I knew I could do something with that."

Once stereotyped as the domain of bodybuilders and gym devotees, personal training is drawing the educated and uneducated; the young and old; men and women; the newly graduated, the recently laid-off and the long retired.

From 2001 to 2011, the number of personal trainers grew by 44 percent, to 231,500, while the overall number of workers fell by 1 percent, according to the Labor Department.

It is no wonder that so many Americans are trying to transform a passion for fitness into a new career.

Personal training requires many of the skills and qualities of the new typical middle-class American job: It is a personal service that cannot be automated or sent offshore, that caters to a wealthier client base and that is increasingly subsidized (in this case, by employers and insurance companies).

But as people with such jobs have found, the pay is low. Unlike the clock-in-and-clock-out middle-class jobs of the past, personal service occupations have erratic hours, require entrepreneurial acumen and offer little job security.

"The kind of job where you come in and work 9-5, and where someone tells you what to do all day is becoming scarcer and scarcer," said Erik Brynjolfsson, an economics professor at MIT and co-author of "Race Against the Machine," a book about how automation is changing the job market.

"The kind of job where you have to hustle and hustle and where you're not sure whether you will have enough clients next month, where you have less job security, is becoming much more common."

For personal trainers, the median hourly wage is less than $15. Because they have to find clients and set up their businesses, trainers must be flexible, adapting to client schedules and physical abilities, as well as the availability of exercise machines and accommodating weather.

They must also be able to engage with all sorts of personalities — precisely the skills that help keep these jobs around while others are replaced by algorithms.

The industry is mostly unregulated, with private organizations rather than governments issuing certifications. Once upon a time, some certification organizations required bachelor's degrees and intensive study; now dozens of groups offer ever cheaper and easier certifications to serve the fitness boom.

The fitness industry has been growing steadily in good economies and bad, with U.S. health clubs adding about 10 million members since the recession officially began in 2007, according to the International Health, Racquet & Sportsclub Association.

Facing a sea of options, Hoskins chose an online test that cost $60 by Action, an organization founded in 2008. Action is not accredited by the National Commission for Certifying Agencies, the group the industry uses to vet such certifications, though, and he could not find a local gym that recognized the credential.

He is studying for a more in-depth test from an older group, the American Council on Exercise, and trying to train clients on his own until he can qualify to work for a gym. The study materials and test cost about $500.

In a country with a 35.7 percent obesity rate, potential customers are plentiful, at least in theory.

But personal training, like other personal services, is increasingly freelance. Most trainers drum up their own business. It does not help that trainers must persuade strangers to pay to do something they probably do not like to do: exercise.

"I've got a website, a blog, a Twitter account, a Facebook feed, an email blast, basically any kind of social media I can find to get the word out there," said Mark Spurbeck, 28, of Eagan, Minn., a college graduate with a degree in English and history. He got his certification in May but is still working as a website designer.

"I've been telling my friends: 'Let's work out for free. Don't worry about paying me. My only fee is telling your other friends to hire me,' " he said.

So far he has two paying clients, whom he trains at their homes.

As for Hoskins, he continues trying to find clients while he studies for certification.