With summer weather upon us, a lot of people are breaking their flip-flops out from the closet.

My advice is: don’t.

First, if you’re not currently a member of a fraternity, it’s probably not helping your look.

But second, and more importantly, flip-flops are a biomechanical disaster.

Your feet are a beautiful system, designed over thousands of generations of evolution to withstand pounding by forces several times your bodyweight, thousands of times each day. Fundamentally, your foot is a mechanical arch (cf., the arch of your foot), leveraging physics to accept and then dissipate force with ease.

Or, at least, that’s how it’s supposed to work.

To illustrate, try this: lift your big toe. You’ll notice that, when you do, the arch of your foot pulls taught. The same thing happens when you keep your toe planted, but lift your heel – the way your back foot moves on each step as you walk. That pulling taught is called the Windlass Effect, and it allows you to support your weight using the strength of your fascia, tough connective tissue that surrounds your muscles. The Windlass action tightens up the fascia in your arch (the plantar fascia), as well as fascia in your calves and upper legs, like the IT band. And that fascia is super strong. In fact, you could literally hang a car from your IT band.

But if you’re wearing flip-flops, your big toe does something different: it scrunches in on every step, holding your shoe in place. That prevents the Windlass effect, so instead of pounding your strong fascia, you instead mash the tendons and soft-tissue of your feet, the cartilage and meniscus in your knees, etc., none of which were designed for that job.

So, in short, if you want to avoid plantar fasciitis, knee replacements, etc., stop wearing flip-flops. Sure, you can wear them on the beach / at the pool. But if you’re just walking around in warm weather, try something like a Vans slip-on (timelessly surfer chic), a strappy sandal, or anything else that holds on to your foot without your active effort.

You’ll look better. And you’ll make it to fall with a healthy leg up.