It took just three days of working full time at an Amazon “fulfillment center” outside of Louisville, Kentucky, for Emily Guendelsberger’s body to break down.

She’d been warned by her supervisors that it would be physically demanding. She’d be on her feet for 12-hour shifts, walking a total of 15 to 20 miles through a 25-acre warehouse — as long as seven New York blocks — looking for merchandise to fulfill online orders.

One Amazon training video included a testimonial from an employee who claimed she’d lost 20 pounds from all the walking, “posing it as a benefit,” says Guendelsberger.

She expected to be tired as an “Amazonian” — the official name for full-time employees — particularly as she’d joined the company in November 2015, just before the Christmas season. But this was a whole other level of pain.

“It feels like I’ve been hit by a garbage truck,” she writes of the experience in her new book, “On the Clock: What Low-Wage Work Did to Me and How It Drives America Insane” (Little, Brown), out Tuesday.

The book documents her experiences over two years, between 2015 and 2017, taking on service-industry jobs not just at Amazon, but also Convergys, a customer service call center in Hickory, North Carolina, and a McDonald’s franchise in downtown San Francisco.

Her goal wasn’t just to report on what she saw, but to “get an idea of what the modern experience of low-wage work feels like.”

Guendelsberger, 35, only decided to join the blue-collar workforce after losing her job as a senior staff writer at the (now defunct) Philadelphia City Paper. It was part necessity — she needed an income — and part curiosity.

Other than a few service jobs in her teens and early 20s, she’d never held employment that didn’t involve sitting at a desk. What Guendelsberger learned, she writes, is that she’s “embarrassingly unprepared for what ‘normal’ means outside the white-collar world, and I’ve grossly misjudged what $10.50 an hour is worth to a lot of people.”

Her biggest surprise, she tells The Post, is not just how much abuse her co-workers were willing to endure, but how they remained optimistic and grateful despite often staggeringly brutal conditions.

When Guendelsberger hit her pain threshold at Amazon and ran out of the Advil she’d been popping like candy, she sought out one of the company-supplied medicine vending machines “stocked with single-dose foil packets of pills.”

With the swipe of her ID badge, the pain could go away for at least a little while. But when the vending machine didn’t recognize her badge, a female co-worker (Guendelsberger never learned her name) offered to help.

“Let me guess, it’s your first week,” the woman said, with pity in her Kentucky drawl.

After helping Guendelsberger get pills and warning her about building a tolerance — the co-worker claimed she needed at least four pain meds just to get through the day — she assured Guendelsberger, “It gets easier. It really does.”

But Guendelsberger found no evidence that this was the case.

The work in factories and minimum-wage facilities hasn’t exactly got harder in recent decades, Guendelsberger says. It’s that the jobs have become unreasonably more stressful, mostly due to advanced monitoring technology that meticulously tracks every second of every day for many employees.

The reason, weirdly enough, is that their productivity is being compared to robots.

Because of automation, human workers increasingly have to compete with computers and algorithms, Guendelsberger writes. But robots are still lacking when it comes to fine motor control and empathy. So many industries want a workforce that can “think, talk, feel and pick stuff up like humans — but with as few needs outside of work as robots.”

These so-called “cyborg jobs” demand that low-wage laborers “crush those unuseful human parts of themselves down to atomic size.” And this type of employment is becoming increasingly common, with Oxford University estimating in 2013 that cyborg jobs could account for 47 percent of the US workforce.

At Convergys, Guendelsberger was “lectured about how using the bathroom too often is the same thing as stealing from the company.” Every bathroom visit was clocked from the moment she left her cubicle, and a daily report of her bathroom time was sent to a supervisor for approval.

Amazon workers carry around a scan gun, similar to what you might see at a grocery-store checkout, with an LCD screen listing tasks and a timer counting down exactly how many seconds remain to complete each one, according to the book.

“It also tracks your location by GPS — and you take it everywhere with you, even the bathroom,” writes Guendelsberger. “Failure to stay ahead of the countdown was grounds for termination.”

At fast-food franchises like McDonald’s, employees are often pushed to work at such dizzying speeds — “like a Benny Hill video on fast forward”— that injuries are inevitable, Guendelsberger explains.

Brittney Berry, who worked at a McDonald’s location in Chicago, told Guendelsberger that while trying to keep up with the pace, she slipped on a wet floor and severely burned her forearm on a grill to the point of nerve damage. “The managers told me to put mustard on it,” Berry told Guendelsberger.

(The Post reached out to Amazon, Convergys and McDonald’s for comment on Guendelsberger’s claims but did not hear back from the last two of press time. An Amazon spokesperson responded: “For someone who only worked at Amazon for approximately 11 days, Emily Guendelsberger’s statements are not an accurate portrayal of working in our buildings. We are proud of our safe workplaces and her allegations are demeaning to our passionate employees, whose pride and commitment are what make the Amazon customer experience great.”)

Data on the emotional state of modern workers is, at best, confusing.

On the one hand, engagement seems to be up. According to a Gallup poll from last year, it’s at an 18-year high, with 34 percent of American workers claiming they’re enthusiastic about and committed to their jobs.

But that conflicts with a recent Workplace Democracy Association/Zogby Interactive survey, in which 25 percent of US workers compare their workplace to a dictatorship.

The message seems to be this: Workers have never been more committed to their jobs while at the same time recognizing that work today is more punishing than ever.

‘It’s become so normalized to be treated like garbage at work and clamp down on your self-respect and dignity’

The workers that Guendelsberger met exemplified these conflicting traits. They described Amazon as an “existential s–thole” but also “accepted that this was just the way things were. They knew they weren’t being treated right, but they tried to look on the bright side.”

She met women like Akasha, Blair and Hailey — Amazon employees determined to see the positive in their working conditions. (Some but not all of the names were changed to protect their identities.)

“I felt like someone was always watching in case I screwed up,” Guendelsberger writes. “They felt like someone’s taking note of the good work they do.”

Blair, a young working mom, was especially determined to see how far she could push herself during the randomly announced “Power Hours.” This special incentive challenged workers to fulfill 100 orders in just an hour, with the reward of “a dollar coupon for some — but not all — of the vending machines in the building,” writes Guendelsberger.

“I’m mainly doing it for the thrill of the hunt,” Blair told her. “I want to know if I can win; I want to know I can conquer. And I want to be noticed, hopefully, by management.”

Blair’s belief that people who work the hardest and prove their potential will rise to the top “is an idea that’s deep in the American psyche,” Guendelsberger says. “Many of them believe they deserve it because if they’d just been better and worked harder, they’d be rewarded.”

It’s a grim reality that most workers have just learned to live with. “They don’t have an expectation of being treated like human beings,” Guendelsberger says. “It’s become so normalized to be treated like garbage at work and clamp down on your self-respect and dignity.”

In each job, she learned how to get “harder and more pragmatic, like my co-workers. Like a robot.”

Guendelsberger believes that change is not only coming to the workplace but is also inevitable.

The constant hustle and stress of the modern economy are making people crazy, she says.

“It’s making us sick and terrified and cruel and hopeless.”

Human beings aren’t robots, she says.

“They need to go to the bathroom, take sick days, take Mom to a doctor’s appointment, attend funerals. Stay up until 4 with the baby.”

But any meaningful shift in what’s considered normal work conditions has to start at the bottom, with the undervalued workers who’ve let themselves believe that “the things that make humans less efficient than robots are weaknesses — moral failings.”

Guendelsberger has faith that many of the people she met during her brief two years could someday break through and demand more from their employers. But she won’t be among them.

“Oh, God no,” Guendelsberger says when asked if she’ll ever engage in minimum-wage employment again. “I’m not that strong. I’m going to stick with writing.”

How America works

80%: of US workers feel stressed on the job

46%: claim their stress is caused by “workload”

75%: believe there’s more job stress than a generation ago

1 million: workers stay home every day because of stress

$125 to $190 billion: spent annually treating job burnout-related ailments

42%: claim verbal abuse is common at their workplace

34%: of workers can’t sleep because of work stress

1 in 4: have been driven to tears by workplace stressSources: American Institute of Stress, Gallup, Korn Ferry, Harvard Business Review