Guide: Flexible vs. Stiff Running Shoes

Posted on 29 August, 2021 by Jovana Subic

Stiff running shoes

Why are stiff running shoes getting the spotlight? Why do runners flex and twist shoes in running stores? How is stiffness related to your running performance? 

20+ hours of research is summarized in this guide - covering both scientific studies and authoritative articles. 

Choose flexible running shoes if you are looking for comfort and shoes for long runs.

Choose stiff running shoes if you want to improve your running performance and do fast runs or if you’re looking for stability features for overpronation. 

Manual testing of flexibility in running shoes

As always, a disclaimer: individual approach matters a lot, especially when talking about performance improvement. Even this study suggests it when it comes to finding optimal bending stiffness for a runner. There are a lot of things that influence responsiveness and stiffness is only one of them. It has to play perfectly with other pieces of the puzzle to deliver the promised running performance improvement. 

Stiffer shoes improve running performance 

What stiff shoes are mostly about is responsiveness: you can raise the stiffness of the shoes to improve speed and economy, but only up to a point. After that, performance drops. However, that point isn’t fully understood. 

Running in a highly responsive and stiff shoe (Vaporfly Next% 2)

Even though the brands are working hard on it, it still can’t be fully dissected because no runner’s body is the same: and that’s why the individual approach is important. It’s also why carbon-plated premium shoes might work better for elite runners than for recreational (slower, heavier) runners. 

This is an illustrative example of what happens to running performance when increasing the stiffness of the shoes (as explained in this study). Here, we’re talking about performance-oriented stiffer shoes and not shoes that are stiffer due to their stability features.

This study has shown that stiffness is beneficial if it does not disturb the natural joint flexion).

3 steps to go stiffer

When looking at the flexibility of the shoes: most of them will work for you. However, if you want to get into the stiffness game, follow these steps: 

  1. Start with flexible running shoes. Pay attention to your gait and overall movement. 
  2. Buy a bit stiffer shoe. Try and notice whether your gait changed. If it has, it is too stiff for you. It if hasn’t, you are good to go. Also, listen to your landing: the quieter it is, the better the shoe for you. How loud your landings are can depend on the flexibility of the shoe and whether it works for you.
  3. You can go stiffer as long as your legs bend and push off the same as in less stiff shoes. If you rush it or keep running in stiff shoes that change your gait, you might get injured. 

If you’re unsure, use video to analyze your potential stride changes: record yourself running in both pairs of shoes (flexible and stiff) and compare.

Going stiffer in running shoes

It’s also important to note that the stiffness of your forefoot is dominant: the total stiffness during running is dominated by the stiffness of your forefoot and not by that of the shoe. Forefoot stiffness and midsole bending stiffness act together during push-off, but the shoe stiffness is generally much lower than the mean human forefoot stiffness (source). 

2 reasons carbon plates might not work for you 

High-priced carbon-plated shoes are created for elite runners with the intention to improve their running economy and performance. 

Carbon fiber plate in running shoes

Here are 2 reasons why this matters:

  1. While foams allow for great tolerance levels, carbon plates and plate stiffness do not. For them to work (great), researchers at Nike focused on a group of runners with similar characteristics (weight, contact time, pace, limb stiffness). This is why Vaporfly 4% might not work for you if you’re slower than elite runners or, simply, weigh more. 
  2. The so-called spring or pop in running shoes - the most wanted feature that promises responsiveness and better performance, is specially tuned for elite runners who have a certain stride. If you strike the land in a different way, have stronger or weaker impact forces, different ground contact times… you simply won’t get the same propulsion as elite runners. 

Cutting a carbon-plated shoe in half in our lab: 

We've covered this topic in great detail in our guide: Carbon-Plated Shoes Cut In Half, Explained and Lab-Tested.

2 types of stiff running shoes 

Stiff running shoes are usually split into two groups based on the purpose of their stiffness: 

  1. Stiff running shoes for stability and protection. They don’t have carbon plates. These shoes are stiff due to their stability features for mild or severe overpronation (we’ve covered pronation in great detail in this guide).
  2. Stiff shoes for performance use stiff foams, combinations of foams, or (carbon/nylon) plates that stiffen them up. 

Stiff for stability and stiff for performance

Above we can see Hoka One One Gaviota 3 (top) with stability elements inserted for severe overpronation and Asics MetaSpeed Sky (bottom) that is tuned for race days.

1st purpose of stiff shoes: stability & protection

This is a list of features of running shoes that are stiff because they offer stability and protection: 

  • Stability features help with overpronation (as shown here, here and here).
  • More expensive than neutral shoes (with no stability features), cheaper than carbon-plated shoes
  • Less responsive and usually made for daily runs (not races). 
  • Usually made for heel strikers
  • They can have a dual-density midsole, guide rails, and midfoot supportive elements to decrease overpronation.

This is how heel focused on stability (for overpronation) looks like: 

Heel area in stability shoes

2nd purpose of stiff shoes: performance

This is a list of features of running shoes without carbon plates that are stiff because they are tuned for performance: 

  • Much more responsive than flexible shoes. Less responsive than plated shoes. 
  • Offer a stiff ride. 
  • Only a small number of models are stiff and without plates.

This is a list of features of running shoes with carbon plates that are stiff because they are tuned for performance: 

  • They improve the running economy (shown here and here).
  • They reduce the amount of energy lost at the metatarsophalangeal joint (as explained here). 
  • Take some getting used to, especially for walking. 
  • Best results if forefoot striking in them.
  • Rocker geometry. 
  • Very responsive, feel like trampolines. 
  • Made for elite runners and are usually premium priced.

Features of carbon fiber plated running shoes

To learn more about these, read our in-depth guide on carbon-fiber plated shoes.

They cause significantly more positive work and less negative work at the metatarsophalangeal joint and less positive work at the knee joint.

If you're looking for stiff running shoes, keep in mind that in the RunRepeat database we differentiate:

DIY test for flexibility and stiffness 

Manually testing the shoes to check their stiffness consists of 2 steps: 

  1. Bending the shoes (longitudinal stiffness) - place the heel of the shoe in one hand and the tip of the forefoot in another hand. Try to bring them together. 
  2. Twisting the shoes (torsional stiffness). Grab opposite ends of the shoe and twist the shoe. 

This is how these 2 shoes compare in our lab (Nike ZoomX Invincible Run is easy to bend and twist - it has the 2/5 rating for stiffness, while Puma Deviate Nitro is really difficult to bend and twist - it has the highest rating of 5/5 which means it’s the stiffest shoe). 

While some say that the shoe’s flexibility might not be assessed well only by playing around with it in your hands or reading lab numbers because it performs differently once worn and run in, for non-researchers this is really a good way of determining the level of flexibility. 

Flexibility test in our lab 

Next to subjective testing done by hand (twisting and bending) where we give 1-5 ratings, we also do a flexibility test by clamping the forefoot of the shoe and then pushing the heel. 

On the left, it’s our most flexible shoe (Saucony Kinvara 12) and we measured 18.3N when bending it. On the right, we can see our least flexible shoe tested so far. It’s Saucony Endorphin Pro 2 and it needs 94.3N to be bent.

The average force needed to flex shoes in our lab is 36.7N. You can look at all our lab-tested shoes here and read about all our lab tests on our methodology page.

Flexibility defines shoe purpose: lab results 

We used our lab results of the force needed to flex the shoe and looked at what the shoe was for: comfort (daily trainers) or performance (tempo shoes and race shoes). Here are the top and bottom 5 examples: 

Shoe model Flexibility [N] Purpose
Top 5 most flexible running shoes
New Balance FuelCell Rebel v2 14.9 Speed training
Saucony Kinvara 12 18.3 Daily running, speed training
Brooks Launch 8 18.7 Daily running, speed training
Brooks Ghost 14 19.2 Daily running
Brooks Revel 5 19.9 Daily running, speed training
Top 5 stiffest running shoes
Puma Deviate Nitro 62.8 Competition, daily running
Asics MetaSpeed Sky 62.9 Competition
New Balance FuelCell RC Elite 68.5 Competition, speed training
Nike ZoomX Vaporfly Next% 2 83.7 Competition
Saucony Endorphin Pro 2 94.3 Competition

The numbers confirmed the theory that flexible shoes are usually more comfortable and used as daily trainers, recovery shoes and shoes for long slow distances. Stiffer shoes up the ante when it comes to performance so they are used for tempo and race days. 

Flexibility changes with cold weather 

In our lab, we don’t only test flexibility at room temperature, but after the shoe has spent 20min in the freezer.

Running shoes in a freezer

We’ve started doing this once we realized running shoe experts around the world report different experiences depending on the temperature. The most curious case was Bondi 7 where we read everything from “extremely stiff” to “really flexible”. 

We measure the difference in flexibility at those 2 temperatures. The average change is 41.3%. 

3 best shoes for cold weather

Top 3 shoes tested in our lab that are best for cold weather (you probably won’t notice the difference room temperature vs cold weather): 

  1. Saucony Ride 14 (6.5% difference)
  2. Nike ZoomX Vaporfly Next% 2 (6.6% difference)
  3. Asics Magic Speed (7.9% difference). 

Best shoes for cold weather

3 worst shoes for cold weather

Using the same test: measuring flexibility at room temperature and after the shoe has spent 20 minutes in the freezer, we’ve ranked the shoes based on the biggest difference that happened between the two conditions. 

Worst 3 shoes to choose from when it comes to running in cold weather (they will stiffen up a lot): 

  1. Saucony Kinvara 12 (130% difference)
  2. Brooks Launch 8 (102.5% difference)
  3. Salomon Sense Ride 4 (96.6% difference). 

Worst shoes for running on cold days

Stiffness vs. hardness & Flexibility vs. softness

In order to make the distinction between flexibility and softness (which are often confused), we will explain both:

  • Midsole stiffness or flexibility explains how much you can bend and twist the shoe. Runners usually describe these using phrases like “stiff ride”, “really flexible”, “don’t adapt to the terrain”. 
  • Midsole hardness or softness explain how comfortable the shoes are in therms of cushioning: do your feet sink in, is the experience pillowy like, etc. Runners describe cushioning using phrases like “running on clouds”, “walking on pillows”, “soft”, “squishy”, “dense”. 

This is why there are shoes that are soft yet stiff (the most popular example is carbon-plated running shoes featuring soft foams that are stiffened up by using the plates which help with the balance a lot). There aren’t many very flexible shoes that are plush (only 6) or very stiff and firm shoes (only 1). The majority is in-between. 

4 features of flexible running shoes 

Flexible running shoes are made for comfort. This feature was highlighted when the minimalist trend hit the scene - one of the main characteristics of minimalist and barefoot running shoes is being able to flex them and bend them. Some, even up to a point where you can curl them up completely due to no stability features inserted. 

Below: Merrell Vapor Glove, a minimalist shoe that’s extremely flexible (would rank 1/5 in our lab).

Flexible running shoe

Flexible running shoes are in general characterized by:  

  1. Being made for comfort. 
  2. Having no stability features. They are neutral and recommended for runners who don’t overpronate. 
  3. Having a better grip than stiff shoes because they can adapt to the terrain better. This is especially important in trail running. 
  4. Being lightweight. 

Up to recently, flexibility meant comfort. However, new technologies are making it possible for stiff shoes to be comfortable as well, even for them to have plush cushioning. These technologies are also allowing stiff shoes to be lightweight, a feature that used to belong to flexible running shoes mostly. 

In our database, you can look for flexible running shoes, very flexible running shoes and moderate running shoes.

Author
Jovana Subic
Jovana Subic

Whether it's a vertical kilometre or an ultra, climbing in the Alps or exploring local mountains, Jovana uses every opportunity to trade walls and concrete for forests and trails. She logs at least 10h/week on trails, with no off-season, and 4x more on research on running and running shoes. With a background in physics and engineering management, she prefers her running spiced with data.