Disclosure: This article discusses research and product information provided by Brooks. This is not a hands-on review of the Brooks Glycerin Flex. This post may contain affiliate links. If you purchase through my links, I may earn a commission at no extra cost to you.

Quick Answer: What Is Brooks FlexZone?

Brooks FlexZone is a new sole design created to combine the cushioning of a highly cushioned running shoe with greater natural foot flexibility. Used in the Brooks Glycerin Flex, the design combines an asymmetric groove through the midfoot with forefoot flex grooves intended to let the foot move more freely from landing through toe-off.

But Brooks’ own research also reveals something interesting: more flexibility doesn’t necessarily mean every runner will experience a smoother ride.

Here’s how the technology works, what Brooks found when it tested it, and what runners should know before drawing conclusions from the research.

Why Did Brooks Develop FlexZone?

According to the Brooks Run Research Lab, runners were asking for something that sounds fairly straightforward: a highly cushioned running shoe that still allowed their feet to flex naturally.

The challenge is that those two characteristics can sometimes work against each other.

Brooks explains that highly cushioned trainers can restrict some of the natural movement of the foot and ankle during push-off. The company identified an opportunity to retain the cushioning runners wanted while allowing more flexibility through the running gait.

That led to FlexZone.

Cushioning is only one part of shoe construction. My Athletic Shoe Materials guide explains how foams, rubber, and other components affect comfort and performance.

How Does Brooks FlexZone Work?

FlexZone isn’t simply softer foam. Brooks changed the physical structure of the sole.

The system has two primary components:

• Asymmetric midfoot FlexZone: A groove through the middle of the shoe designed to allow targeted twisting movement.
• Forefoot flex grooves: Grooves positioned around the metatarsal area to allow the forefoot to bend and articulate as the runner moves toward toe-off.

According to Brooks, the asymmetric midfoot groove is designed to interact with movement around the subtalar and Lisfranc joints. It closes during mid-stance when support is needed and opens as the heel lifts to provide additional flexibility during push-off.

In simpler terms, Brooks isn’t trying to make the entire shoe extremely flexible. It’s trying to make the shoe flex in particular places and at particular points in your stride.

FlexZone at a Glance

These are design claims from Brooks rather than observations from my own testing.

How Did Brooks Test FlexZone?

This is where the information Brooks provided gets more interesting.

Rather than relying solely on runners saying whether the shoe “felt flexible,” Brooks compared the Glycerin Flex with the traditional Glycerin 23.

Runners who preferred a cushioned and flexible experience ran in both shoes on a force-measuring treadmill. Brooks also used biomechanical motion capture to measure lower-leg, foot and ankle movement.

After running in each shoe, participants answered questions about how they perceived the transition experience.

Brooks was essentially investigating two questions:

1. Does FlexZone allow more of the intended foot movement without compromising support?
2. Does that additional movement make the transition feel smoother or more flexible to runners?

What Did the Brooks Study Find?

According to Brooks, FlexZone achieved its targeted movement goals.

During push-off, runners wearing the Glycerin Flex demonstrated greater frontal-plane range of motion at the ankle and metatarsal joint than when wearing the Glycerin 23.

In everyday language, Brooks observed more of the side-to-side and twisting foot-and-ankle movement the FlexZone design was intended to permit.

The runners noticed a difference as well.

Among the study participants—who were specifically selected because they preferred a cushioned and flexible running experience—the Glycerin Flex was preferred for comfort because of its perceived flexibility and more fluid heel-to-toe transition.

That distinction about who participated in the study is important.

The Most Interesting Finding: More Flex Doesn’t Automatically Mean Smoother

This was the part of the Brooks research that caught my attention.

More flexibility alone did not automatically make running feel smoother for everyone.

Brooks found that runners’ landing orientation appeared to affect how much difference they perceived.

Runners who landed somewhat farther back on the heel and less toward its outside edge experienced the greatest difference in smoothness. Brooks believes that landing behind the FlexZone and then transitioning across it allows the technology to play a greater role during the stride.

That’s an important point because running-shoe technologies don’t operate independently of the person wearing them.

Two people can wear the same shoe and experience it differently.

What Does the Research Not Tell Us?

This is equally important.

The research supplied by Brooks is an internal company study, not evidence that every runner will experience the same results.

Brooks specifically notes that this was a small study and that participants were selected because they preferred a cushioned-and-flexible running experience.

The company describes the findings as early product-research evidence rather than a guarantee of individual results.

Brooks also says it wants to investigate further how landing orientation affects the way a runner engages the FlexZone during different stages of stance.

So the research supports a fairly specific conclusion:

FlexZone changed the measured movement of the foot and ankle in the direction Brooks intended, and the selected runners perceived greater flexibility.

It does not establish that the Glycerin Flex will feel better or smoother for every runner.

Glycerin Flex vs Glycerin 23: What’s the Difference?

Based specifically on the Brooks research provided for this article, the fundamental distinction is flexibility.

I haven’t personally tested either shoe for this comparison, so this shouldn’t be interpreted as a recommendation of one over the other.

Instead, the research suggests that the Glycerin Flex may be particularly interesting for runners who like substantial cushioning but dislike feeling as though a highly cushioned shoe is controlling too much of their foot movement.

Who Might Find the Glycerin Flex Interesting?

Based on Brooks’ stated design goals and research, rather than my own wear testing, I’d put the Glycerin Flex on the radar of runners who:

• Like highly cushioned running shoes.
• Also prefer a more flexible feeling underfoot.
• Find some highly cushioned shoes too rigid through transition.
• Want to experiment with a different approach to combining cushioning and natural foot movement.

That doesn’t mean those runners will necessarily prefer it.

Brooks itself acknowledges that anatomy, training history, pace, terrain, footwear preferences and other individual factors can influence a runner’s experience.

Why FlexZone Is Interesting Beyond One Brooks Shoe

What interests me most about FlexZone isn’t necessarily whether the Glycerin Flex turns out to be a great running shoe.

It’s the design question behind it:

How do you provide more cushioning without making a shoe feel less natural?

Running shoes have increasingly incorporated substantial amounts of cushioning, but adding more material underneath the foot creates engineering tradeoffs. For a closer look at how cushioning, shoe geometry, and other design features work together, see my Sneaker Science guide to athletic shoe engineering.

Brooks’ approach with FlexZone is to strategically change where the shoe can move, rather than simply reducing cushioning.

Whether runners ultimately prefer that approach will require considerably more real-world experience and research.

But it’s an interesting example of how running-shoe design is increasingly being informed by both biomechanics and the experience runners say they want.

Frequently Asked Questions About Brooks FlexZone

What is Brooks FlexZone technology?

FlexZone is an asymmetric midfoot groove combined with forefoot flex grooves designed to allow targeted foot movement while retaining a highly cushioned running experience.

What shoe uses Brooks FlexZone?

The research material provided by Brooks discusses FlexZone in the Brooks Glycerin Flex.

Is the Brooks Glycerin Flex more flexible than the Glycerin 23?

In Brooks’ internal study, the Glycerin Flex produced increased frontal-plane range of motion at the ankle and metatarsal joint during push-off compared with the Glycerin 23.

Does FlexZone make running feel smoother?

Not necessarily for everyone. Brooks found that the perceived smoothness of the transition appeared to depend partly on how the runner’s foot landed.

Has Sole of Athletes tested the Brooks Glycerin Flex?

No. This article explains the technology and research information supplied by Brooks rather than providing a hands-on shoe review.

Want to explore more running footwear? Visit my Running Shoes Guide for practical advice, shoe comparisons, and firsthand reviews.

Final Thoughts

Brooks FlexZone takes an interesting approach to a familiar running-shoe tradeoff: how do you provide substantial cushioning while still allowing the foot to move naturally?

The Brooks research suggests that the Glycerin Flex produced more of the targeted foot-and-ankle movement during push-off than the Glycerin 23, and runners selected for their preference for flexible, cushioned shoes noticed the difference.

But the study also raises another interesting question: the way a shoe is engineered is only part of the equation. How an individual runner moves may determine how much benefit they actually feel.

That’s something that would be good to experience firsthand.