A PT shares 1 exercise you should do to build full-body muscle and stay youthful as you age

At 34, physical therapist Will Harlow works out like someone decades older.

Their goals are very different — his clients are regaining mobility and independence, while Harlow is training to do sports and "look good in a T-shirt."

But the same basic principles of longevity apply whether you're 18 or 80.

In his new book, "Independence for Life," Harlow maps out the four pillars of health that should be baked into anyone's workout routine: strength, mobility, balance, and healthy bones and joints.

Harlow said one exercise in particular checks all the boxes for longevity and performance, whether you're a high achiever looking to gain muscle in less time or a fitness newbie trying to build up your health.

You don't need a gym or much equipment to get started, so don't wait, Harlow says — the younger you start training for longevity, the more muscle and fitness you can bank for the years to come.

"There's no such thing as too early. And the earlier you start, the better of a base you build," Harlow said.

The best all-in-one exercise for longevity

Harlow's top exercise for all-around health and fitness is the goblet squat, a movement that trains strength, stability, and mobility at once.

The exercise involves holding a weight, like a dumbbell or other heavy object, with both hands in front of the chest (as if you're cupping a giant drinking goblet, as the name suggests).

That's it — you don't need a gym or any experience with a barbell, machine, or other equipment, and the risk of injury is low.

"The beautiful thing about the goblet squat is it's highly safe because if you get into trouble, you can just drop the weight," Harlow said.

Goblet squats work the entire body at once, engaging the muscles of the back, arms, and legs as you control the weight. As you squat, the joints of your knees, hips, and ankles are working, too. Plus, positioning the weight in front of your body activates your abs and hips.

Elite athletes and top trainers love goblet squats for building explosive power. For the rest of us, the goblet squat is a simple way to build muscle that's essential for healthy aging, and mimics real-world activities like sitting down and getting up safely.

"It's so vital to keep that muscle on our body," Harlow said. "Strength is just a proxy for independence because if you're weak, you can't open heavy doors, you can't get on and off the toilet unassisted, and you can't get on and off the floor."

The longevity 'sweet spot'

To get the most out of your squat workout, Harlow recommends focusing on a concept called reps in reserve: that's how many more repetitions you could complete before hitting muscle failure, when you can't lift anymore.

Research suggests the specific number of reps is less important than challenging your muscles, working until you could barely manage another rep or two if you absolutely had to.

You want a weight you can lift for at least 10 reps, but not more than 20 reps, for the best mix of strength, muscle-building, and health benefits, according to Harlow.

"That's a nice sweet spot for building muscle. It's also heavy enough that you're going to have an impact on your bone density and you'll improve your mobility as well, but it's not so heavy that we are elevating that risk of injury," he said.

This article is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your qualified physician or healthcare provider.