A fresh fitness craze is once again sweeping social media, though the practice borrows from time-tested military training methods, and experts urge caution before strapping in.

On social media, influencers are hyping weighted vests as one of the "best-kept secrets" in fitness, claiming they can help you lose 30 pounds and become "unrecognizable" when used during walks and workouts. TikTok searches for #weightedvest have garnered more than 30 million views.

But the vests are far from new, according to Dr. Mark Kovacs, a sports scientist and longevity expert based in Atlanta. 

"They’ve been used for decades in athletic and military training, but social media has helped reintroduce them to mainstream fitness," he told Fox News Digital.

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"The appeal is simple: You turn everyday walking into a slightly more demanding workout without changing the movement," Kovacs said.

While soldiers have marched with heavy backpacks for more than a century — a practice known as rucking — to build stamina and toughness, weighted vests have also long been used by athletes and firefighters. Now, the gear is available on Amazon for as little as $30 and from Equinox, which also incorporates it into classes, for as much as $375. Kovacs said the sleeker, more comfortable modern vests are likely to fuel a longer-lasting trend.

Part of the appeal is that the vests look "hardcore" but are simple to use, said Sarah Fuhrmann, a certified trainer and functional aging specialist in Michigan. 

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"Just strap on and walk, and fitness ensues," she told Fox News Digital, adding that social media often "makes old tools look new." 

In a recent episode of Andrew Huberman's "Huberman Lab" podcast, Michael Easter, a Las Vegas-based researcher and author of "The Comfort Crisis," argued that humans are built to carry heavy things long distances. 

"Humans are the only mammals that can pick up weight and carry it long distances," Easter said. "For most of human history, we carried everything — food, tools, even babies. Technology has stripped carrying out of our lives, but we were literally born to do it."

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He praised rucking as a "two-in-one workout" that combines cardio through long-distance walking and strength training by loading the skeletal and muscular systems.

Experts caution, however, that more research is needed to fully understand the benefits of weighted vests. 

While studies show that weighted vests can increase calorie burn, cardiovascular demand and even help with certain strength exercises like squats, the science remains inconclusive on long-term outcomes such as bone density. One recent study found that obese adults who wore weighted vests lost about 10% of their body weight but saw no reduction in hip bone loss.

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"This study’s findings are counter to some research and consistent with others," Dr. Colin Haines, a board-certified spine surgeon and director of research at the Virginia Spine Institute, told Health.com. "What’s clear is that the broader body of evidence includes both neutral and positive results."

Research dating back to the 1990s has shown more promising signs of improved bone growth, particularly in postmenopausal women. A 2015 study similarly showed improvements in quadriceps strength and femur bone density.

Women in this stage of life may benefit most from the trend, Kovacs said, while people with joint or back issues should be cautious and consult a medical professional before starting.

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For people interested in trying weighted walks or hikes, experts recommend starting with the lightest option available — about 5 to 10 pounds, or less than 10% of your body weight — and choosing a vest with evenly distributed weight and adjustable straps. Start at 15 to 30 minutes, two to three times a week, they add.

"Focus on posture and slowly add weight over time," Kovacs advised. "Always listen to your body. Discomfort is a sign to back off."

He added that weighted-vest walking should not replace heavier strength training or higher-intensity cardio entirely but can be an "excellent supplement."

"It offers a low-impact way to increase training volume, especially for individuals who may not tolerate running or high-intensity workouts," he added.

Fuhrmann agreed that traditional strength training is the "proven gold standard" for protecting bones and muscle, and noted that weighted vests can intensify traditional exercises like squatting. 

"Weighted vests can add a little spice to your walks, but they’re not a shortcut to fitness," she added. "If they help you get more daily movement in, that's great. But if the choice is between a vest and a solid strength program, choose the weights in the gym every time."