Brooks Glycerin 21 review

We think the Glycerin 21 is a superb option for:

• Heel strikers in search of a max-cushioned shoe that can handle every distance in marathon training.
• Those who want a daily trainer with an outsole that endures mileage like few others on the market, maintaining impressive durability.
• Fans of the popular Ghost looking for a slightly more cushioned option while retaining the classic Brooks look and feel.

We do not recommend the Glycerin 21 for those living in areas with hot or humid summers—the knit upper, while exceptionally comfortable, offers inadequate ventilation for such conditions. Consider the Hoka Mach 6 instead, which is way better suited for warmer climates.

Additionally, we think the Glycerin 21 is not the best choice for runners who prefer a moderate or low drop shoe, as its high offset primarily benefits heel strikers.

We believe runners would find better footstrike geometry and overall fit in alternatives like the Hoka Clifton 9 or the On Cloudmonster 2, which offer similar cushioning with more suitable mid-drop designs.

The upper of the Glycerin 21 initially appears incredibly comfortable, though it doesn't seem well ventilated, being made from a thick knit. However, we'll see how it performs with our smoke-pumping machine test.

The results were mixed. We rated the Glycerin 21 a 3/5, surprisingly higher than we anticipated for such a thick upper. Still, it's apparent that the shoe runs warm, and we wouldn't recommend it for hot summer runs.

To gain a deeper understanding, we examined the sliced shoe under our light, where it became clear—very little light penetrated through, indicating the significant thickness of the knit. Brooks clearly prioritized comfort above all else.

For an even closer look, we put the knit upper under a microscope. We observed a super-thick material with no obvious ventilation holes, only minor gaps insufficient for adequate airflow.

As we noted earlier, the thick upper does have its benefits, primarily comfort, which we felt during our manual assessment. And we also found incredible padding in the tongue and heel.

As we've outlined in our guide about uppers in running shoes, using knit offers advantages like comfort, but typically, durability is compromised since rougher, synthetic materials like engineered mesh are generally more resistant to damage.

Our findings with the Dremel confirmed this. Scoring just 2/5, we can't deem this a positive outcome, though it aligns with our expectations given the Glycerin's plush upper.

After completing the assessment in the toebox, we moved to the heel padding—an area of significant concern for runners who often experience wear and tear in this spot.

To our delight, the performance here was way better than in the toebox. After letting the Dremel run for 5 seconds, we observed minimal damage, prompting us to award the Glycerin 21 a solid 4/5 rating.

Moving to the outsole, we began by testing the hardness of the Roadtack Rubber, a staple of Brooks known for its durability.

The rubber thoroughly covers each area prone to wear in the heel and forefoot, and although there are some spots of exposed foam, they are not a cause for concern for us.

Our initial assessment seems to confirm its reputation, as the hardness measured at 84.9 HC suggests it can provide substantial mileage for those seeking a shoe that lasts well beyond a few hundred miles. Let's fire up the Dremel one more time!

It turns out it's incredibly durable!

Even after the Dremel worked really hard over the rubber, we only observed a minor indentation of 0.5 mm—truly a desired outcome for a daily trainer.

Any doubts about the outsole's durability were immediately dispelled by our final measurement in this area.

We discovered a thickness of 3.2mm, which actually seems excessive given the rubber's outstanding performance against the Dremel. For the v22, we suggest that Brooks could reduce the thickness by ~1 mm, which would still provide enough durability while making the Glycerin even lighter.

Speaking of weight, the Glycerin 21 achieved an excellent result on our scales, coming in below the 10-oz benchmark—this is the ideal weight range for a maximalist daily running shoe to be considered light enough. And it certainly hits the mark at 9.8 oz or 278g.

In fact, it's the most cushioned Glycerin yet, and it's lighter than its predecessor despite boasting a knit upper, making it a clear win-win in our books!

We recently mentioned that the v21 is the tallest Glycerin yet, but let's confirm that with precise measurements.

In the heel, we measured 37.2 mm, which categorizes the Glycerin as a maximum cushioning daily trainer. However, this makes it less ideal for runners seeking a more ground-connected feel. Those runners might consider the New Balance FuelCell Rebel v4 instead, which offers a closer touch to the terrain.

Moving to the forefoot, we recorded a measurement of 26.6 mm.

While this height is less maximalist compared to the heel and feels more grounded, it still provides ample cushioning under the ball of your foot, suitable for runners of any weight or distance.

The Glycerin series has consistently featured a high difference from heel to toe, and the 21st edition maintains this standard with a 10.6 mm drop.

This positioning makes it an ideal choice for heel strikers seeking luxurious cushioning, yet it also accommodates forefoot or midfoot strikers who prefer a higher-offset running shoe.

After evaluating the midsole, we shifted our focus to the insole, which is often overlooked in many running shoe reviews, yet we consider it a crucial component of comfort. After all, your feet rest directly on it, not on the midsole!

We encountered a fairly standard EVA insole measuring 4.8 mm thick. It's straightforward and functional, offering basic performance without any standout features.

Last year, we discovered that Brooks had upgraded the Glycerin to a supercritical, nitrogen-infused EVA named DNA Loft v3—a significant shift from an average, low-energy-return EVA. This year, the foam remains unchanged and delivers a well-balanced ride that combines a nice touch of bounce with stability under high load.

In our testing, we measured the foam's softness at 20.4 HA, indicating a balanced feel underfoot in terms of softness.

While you shouldn't expect the ultra-plush ride of a Nike Invincible 3, the Glycerin is certainly not firm, which aligns with it being a supercritical, airy midsole.

After placing the Glycerin 21 in the freezer for 20 minutes, we remeasured its softness using the durometer. Disappointingly, we found that it had increased by 36.8%—a significant and clearly noticeable change.

Unfortunately, this is a common drawback with EVA-based midsoles, which tend to lose their softness in colder conditions.

While many maximalist running shoes like the Hoka Cielo X1 are adopting a rockered geometry, the Glycerin maintains a classic shape that delivers a natural running feel.

We believe that it's beneficial for everyone to have alternatives to suit different preferences, right? According to our tests, the Glycerin continues to provide a flexible ride that stands out in a sea of rockered options.

If you're searching for a stable, max-cushioned neutral trainer, the Glycerin is an excellent choice, as it offers a more controlled ride compared to other models in its category.

Although it lacks prominent midsole sidewalls or GuideRails like the Glycerin GTS 21—check this one if you have stability concerns—we discovered that it performs better than ever for neutral runners.

We also found that this improvement stems from mostly from the sole's massive, broad dimensions, coupled with a few clever tweaks that we'll explain in a minute.

We previously mentioned that the Glycerin is a neutral workhorse and that it's not designed as a stability shoe. The primary reason for this classification is its impressive flexibility, which contrasts with the stiff and rigid midsoles of stability-focused shoes like the Hoka Gaviota 5.

This flexibility resulted in a score of only 2/5 in our torsional rigidity test, affirming that the Glycerin truly offers a natural feel and adapts smoothly to the feet with every stride. This characteristic is highly positive, and we're hopeful that the Glycerin will maintain these qualities for many years to come.

One of those subtle yet impactful tweaks we previously mentioned is evident here, and it might not be to everyone's liking. The stiffness of the heel counter has increased from a moderate 3/5 in the v20 to an iron-hard 5/5 in this latest update.

What does this mean? Essentially, the shoe is now even more tailored for heel strikers, as those who land on their rearfoot first will find that it effectively guides the foot in a straight line. However, this increased rigidity can be a double-edged sword in terms of comfort, as such a stiff heel might feel somewhat intrusive and uncomfortable for some runners.

Another trick for enhancing stability stems from the shoe's dimensions.

In the forefoot, we measured 117.2 mm, which is slightly wider than your average daily trainer, yet it feels quite standard to us for a maximalist shoe. So, the key must lie in the heel, right?

Indeed! At 100.0 mm, the heel is impressively broad, offering a substantial landing platform for heel strikers.

Combined with the rigid heel counter and the significant heel-to-toe drop, it's clear to us that Brooks has intentionally designed this shoe to cater specifically to runners who land heavily on their heels.

This test is crucial for a versatile shoe like the Glycerin 21, as a very stiff sole could compromise its comfort and daily utility. To assess this, we conducted our signature 90-degree bend test.

We were pleased with the results—our force gauge recorded just 21.7N, placing it comfortably in the flexible range. This outcome ensures the shoe is well-suited for everyday runs and versatile enough for activities like walking the dog or serving as a go-to travel shoe.

A nitrogen-injected EVA compared to standard EVA provides several advantages, including reduced weight, enhanced energy return, and quicker decompression. However, some often overlook another benefit—the stiffness variation in cold temperatures.

Thanks to the supercritical process, the stiffness of the material after spending 20 minutes in our freezer changed by only 14.5%, which is a favorable outcome.

Our first measurement in the toebox is always taken at the widest part, and the Glycerin measured a spacious 99.3 mm, accommodating most foot sizes comfortably.

It's also important to note that Brooks offers this shoe in wide sizing in select markets, making it a suitable choice for those with broader feet.

As we do with every running shoe, we conducted a second measurement in the toebox. This time, our digital calipers showed a measurement of 78.9 mm—an average result.

One improvement we hoped to see from the Glycerin 20 was a move away from the non-gusseted tongue.

Regrettably, the v21 sticks to the old design, which we consider a significant oversight. We firmly believe the Glycerin deserves a semi-gusseted tongue to enhance its fit and feel.

The tongue features exceptional padding—a main pillow-like foam flanked by a thinner layer. Altogether, we measured it at 11.5mm—ideal for runners who prefer their laces ultra-tight, as this design accommodates that preference without the risk of lace bite.

However, we feel that this might verge on excessive padding. A more streamlined design with just 7 or 8 mm could offer nearly the same level of comfort while also providing weight savings and reducing bulkiness.

It's clear that Brooks designers are not fans of heel tabs—the Glycerin 21 maintains the same heel design as its predecessors by omitting them.

We discovered that the insole isn't attached to the last, allowing for easy swapping. Additionally, the stock insole is quite average, so replacing it with a different option is perfectly feasible.

We were quite disappointed to discover that the Glycerin 21 lacks any reflective elements—a feature we appreciate for safety during low-light runs.