As you might know, we’ve been in the midst of a running shoe revolution for the past few years, one in which shoes have gotten lighter, lower to the ground and more minimally designed. (And personally, I’ve been saying, “Hallelujah” for years.)

One of the most commonly talked about details of the new shoe order is the heel-toe drop—the differential of the height off the ground of the heel and of the forefoot. (In other words, it’s the different in height between where the heel sits on the footbed in a shoe and the forefoot sits on the footbed in the shoe.) Some refer to it as “Heel Drop” or “Delta H” or simply the “H-Delt” of a shoe.

For years, shoes have been built with a heel that sits much higher than the forefoot. (Since the 1980s, the standard differential between the height of the heel and the forefoot has been 12mm, although some have been as high as 15mm to 18mm.) The idea has been that it takes the strain off the Achilles tendon, promotes forward momentum and adds cushioning in the heel.

Now many shoes either have a more moderate slope, roughly between 4mm and 10mm, or they fall into the “zero-drop” (or flat) category. Although a lot of shoes claim to be “zero-drop,” anything in the zero to 3mm range constitutes a fairly flat shoe.

Why the dramatic change in shoe geometry?

Running in shoes with less of a slope engages more of the muscles a runner’s foot and lower leg and allows greater range of motion. Those things can lead to greater dynamic movement and running stability, which ultimately can lead to better form and improved running economy. Running in shoes with a lower heel-toe drop can also help reduce some of the impact and rotational forces that typically lead to overuse injuries. Those are all good things and reasons to make a change.

Renowned running gait analyst and running injury expert Jay Dicharry, director of the SPEED Clinic at the University of Virginia’s Center for Endurance Sport, says running in thin, lightweight shoes with semi-firm cushioning and a low heel-toe drop is ideal because it helps put the body in an optimal running posture, promotes proprioception and increases a runner’s intrinsic stability. Numerous studies back it up.

“Running in a shoe with a minimal drop of about 3-5mm clearly reduces the torque on the body and enhances a runner’s stability,” Dicharry says.

But it takes a while to get there. As a lot of early adopters and zealots have found out, you can’t just jump into a pair of Vibram FiveFingers (or any other brand making shoes with flatter profiles, like Newton, Merrell, Altra or Saucony, among others) and seamlessly continue running high-mileage and fast workouts. It takes time — anywhere from 6 months to two years.

To make the transition, a runner must be committed to building and maintaining strength and flexibility in their feet and lower legs. (And if you’ve been running for years in shoes with a 12mm drop, as most of us have, it means you probably don’t have the dynamic strength and flexibility.) That’s only accomplished through relentless dedication to a variety of drills and stretching exercises, as well as a gradual transition to more minimally designed shoes.

That story has been told quite a bit in the last year or so, but it appears a lot of runners are veering away from a possible move to shoes with flat profiles. Several manufacturers and retailers I’ve talked to in the last two weeks — including several at the Fleet Feet dealer convention in Washington DC in late June — have suggested that the trend is pulling back a little. Runners are coming in and saying they don’t want to deal with the lingering soreness and nagging Achilles strain that often accompanies the partial transition to shoes with a lower heel-toe drop.

Maybe they don’t want to make the commitment to building strength and flexibility. Maybe it’s their biomechanics. Maybe it’s just that they like the soft, cushy shoes they’re used to.

As many shoe experts, coaches, biomechanists have suggested, not everyone is ready to veer to shoes with such flat profiles — and many might not ever be. But that’s OK, too.

“There is a notion that zero-drop is the end goal and that’s what you need to aspire to, but the goal is to allow you to find a shoe that is going to allow you to run and achieve what you want out of running,” Dicharry says. “And that could be a zero-drop shoe or it could be a shoe with a 12 mm drop. If you’ve been running in a shoe that has worked for you for years and you haven’t been injured, then why mess with success?”

Personally, I’m glad shoes have gotten lighter, flatter and lower to the ground. If I run with a shoe with a 12mm heel-toe drop now, it almost always feels inordinately chunky and lacking agility or speed-inspired zip. But at the same time, I can’t run in zero-drop shoes very often (yet I love the new Altra Samson) and I’m not sure I could have when I was 18 or 28 either. My sweet spot seems to be shoes in the 4mm-8mm range—lately some of my favorites have been the Puma Faas 350, Brooks PureConnect, ASICS Gel-Lyte 33, Adidas Adipure Motion, Nike Free 3.0 v4—but it depends too on how far, how fast and on what kind of terrain I’m running.

The bottom line is that it’s a personal situation for everyone. What’s your take on shoes with lower heel-toe drops? Have you tried them out? Or are you still running in the same kinds of shoes you were running in several years ago? (There are no wrong answers if you’re achieving your running goals and staying mostly injury-free.) In the coming months, shoe companies will be releasing new models on both sides of the heel-toe drop spectrum — some in the zero-drop range and some in the 4mm-10mm range.