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I generally withhold my verdict on a running shoe until the conclusion of a review—not this time. The Merrell Agility Synthesis Flex is an almost perfect hybrid running shoe for road and trail.



There, I’ve said it. Now, for the details.


The shoe

According to Merrell, “This trail runner flexes and protects for agility to excel from road to trail.” It is recommended as a shoe for running on moderate trails.

And the shoe is shown to have six features:

  1. A responsive foam midsole, which comfortably cushions the foot;
  2. A Neoprene tongue for comfort;
  3. Flat, slightly plastic laces that stay tied;
  4. A woven jacquard upper, which is both protective and breathable;
  5. A protective toe cap shielding the toes from rocks and roots; and,
  6. Merrell’s stickiest rubber outsole with skeleton-inspired lugs, which provide aggressive traction on variable terrain.


The Agility Synthesis Flex has an 8mm drop and weighs either 8.4 or 8.5 ounces, depending on the source. The shoe has an EVA insole which fits perfectly.



This is something I’ve rarely said. I find that most insoles in running shoes tend to be either too thick and heavy or too thin and unsupportive. This insole is an exception.

It’s just right in thickness, just right in weight, and supports the feet well; the cushioning is evident. Goldilocks would approve!

The shoe has a true-to-size fit. There’s no need to order a half-size up or down with Merrell models. The upper is cut moderately low over the top of the foot, but not so low as to become bothersome.



The Agility Synthesis Flex comes in one of three bright, interesting and distinctive colorways. I received the Lime Punch upper, which I found to be quite attractive.  

There’s highly reflective detailing on the sides of the shoe, which means you can easily be seen in low-light situations.



True to Merrell’s word, the laces only need to be tied once.  They do not come loose on a run, or even during a full day spent knocking about.

The stack height on the Agility Synthesis Flex feels comparatively low for a trail shoe.  It’s immediately clear that this is not a heavy, bulky, plodding trail shoe

There are three flex grooves on the sole in the forefoot, something appreciated by runners who need flexibility even in trail shoes.



The skeleton-inspired lugs are quite similar to those found on the Merrell Agility Charge Flex.

On the road

The Agility Synthesis Flex feels well cushioned and quite comfortable for walking in.

During an initial run, one will notice that it gives the impression of being low to the ground, like a performance training shoe

The shoe is perfectly neutral. It means there’s no need to alter one’s natural footstrike; Yet, there’s a smidgen of stability, which keeps feet moving straight ahead.

On asphalt and concrete, the shoe feels lively, fast, and lighter than its listed weight.

The great bounce back on asphalt means the Agility Synthesis Flex would be a pleasure to use in races as short as a 5K or as long as a marathon.

The Agility Synthesis Flex feels right at home on a dirt and asphalt trail. Yes, this is a versatile running shoe for light to moderate terrain.

The shoe performed excellently and protectively, thanks to a TrailProtect rock plate, on a trail with large, hard rocks. It’s an exemplary fire road runner—probably my favorite ever on this surface type.

It provides good ground feel on flat sections and perfect balance on uneven sections of the road. Instead of avoiding uneven surface trails, one wants to attack them. 

According to Merrell, the Agility Synthesis Flex has a Hyperlock molded TPU heel counter. This heel counter provides security for multi-directional movement.

Indeed, the stability provided by this shoe is so impressive that it’s almost as if one has gone from two feet to relying on a sophisticated all-wheel-drive system.

I wound up making a note to myself, “This shoe shines on uneven surfaces!”



The 5mm lugs on the Agility Synthesis Flex provide for excellent grip, something well appreciated on uphill and downhill trail runs.

(Knowing that the grip is there builds up confidence and can result in faster training runs.)


I could not find a single issue to complain about with this shoe. 

The verdict

Good-to-great running shoes make you want to get out there—and this is a great shoe. It virtually screams, “Go for a run!”

The Merrell Agility Synthesis Flex is a shoe that can be used for road training and road racing.

It’s a shoe that very much displays its strengths on trails—especially on those calling for quick descents and sharp turns. And with these shoes, there’s no need to avoid uneven surface trails.



The shocker is the price, just $110.00. I would have guessed that this model costs $130.00, which is the price Merrell charges for this shoe’s cousin, the Agility Peak Flex 3.

(The Agility Peak Flex 3 comes with a Vibram sole, which is not needed on the Agility Synthesis Flex.) Thanks, Merrell for undercharging and overproducing.

Set aside the notion of buying one pair of running shoes to use on concrete and asphalt, and a different pair to use on light to moderate trails.

This hybrid shoe will meet the needs of most runners on most surfaces. Enough said.

I highly recommend it!

| Level 4 expert Verified
Joseph Arellano has run in and reviewed running shoes produced by more than 20 different brands. When he finds a "perfect" running shoe, he picks up about six pairs. He believes that most problems can be solved through the purchase of a new pair of running shoes.

Updates to Merrell Agility Synthesis Flex

  • 0% animal products were used in the making of this shoe. With this, Merrell offers a Vegan-Friendly option for those who do not use any animal product in their diet or in the items they are using.
  • The Agility Synthesis Flex has the Merrell Hyperlock technology. This feature is a molded TPU heel film which helps secure the foot in place while the runner is going over the trail. The support it gives is not significant, but it still offers minimal stabilization.
  • The midsole has the FLEXConnect feature – the design on the platform that’s composed of flex grooves that move in dual directions. These grooves allow the midsole to effortlessly adapt to the runner’s movements, making each stride adaptive and easy.
  • The M Select Fresh antimicrobial footbed was added to combat bacteria and moisture from within the shoe. This extra padding keeps the in-shoe environment balanced as it absorbs sweat and eliminates bad odor.
  • On the outsole is the M Select Grip which affords the bottom of the shoe with enough traction to tackle various terrain conditions. Though this outsole was designed for light trail, it gives a decent amount of traction and grip.
  • There’s an extra layer on the midsole that protects the bottom of the foot from sharp rocks or other objects on the trail that can potentially cause injury. It is called TrailProtect.

Size and fit

The Merrell Agility Synthesis Flex is available in regular sizes. Runners will be able to purchase the shoe using their usual size preference. Regarding width, the shoe comes in the standard B-medium and D-medium measurements.


The outsole is made of a durable rubber called M Select Grip. It is wear and tear-resistant, prolonging the life of the footwear, as well as bringing a tough layer of protection underfoot. Aside from its durability, it provides the right amount of grip and traction on both wet and dry surface conditions.

On the forefoot area of the shoe’s bottom, phalange-like lugs are placed strategically to mimic the anatomy of the bones on the foot. The placement makes it easier for the outsole to sink into surfaces, even ones that are uneven. The lugs also make the bottom self-cleaning, releasing tiny debris and dirt on its own. Meanwhile, the heel portion has lugs that are closer together to deliver the most hold on different surfaces.


The midsole is made up of a material called EVA which is short for Ethylene-Vinyl Acetate. This dense foam is excellent when it comes to providing enough underfoot comfort, cushioning and shock absorption. It allows runners to have a responsive ride with enough energy return.

On the sides of the EVA foam, dual-directional flex grooves were designed for better flexibility, making runners feel less restricted when it comes to moving. Also, the grooves help with balance to some degree. When the foot is on uneven terrain, the grooves compress to help even out the level of the shoes.

As an extra layer of protection, a pad called TrailProtect was added. Appropriately named, it basically protects runners from rocks that can easily be stepped on while going over the trail, thereby preventing injury or disruptions in the continuity of running.


The upper of the Merrell Agility Synthesis Flex is made up of mesh. It’s a textile that’s comfortable and breathable, letting air circulate within the shoe and minimizing hot spots. It also hugs the foot snuggly without constricting it. The mesh was formed into an eye-catching, chevron pattern with two tones which allow the shoe to stand out.

To reinforce the heel part of the upper, the Hyperlock component was introduced. It is a TPU (Thermoplastic Polyurethane) film found on both sides of the heel, forming one band. It minimally locks the heel down in place, preventing it from moving too much when the runner is moving.

Also reinforcing the upper is the synthetic toe cap. The tip of the shoe is commonly the part that bumps into different objects on the trail. To prevent injury to the toes, the tip was made more durable with a coat of synthetic material, allowing it to withstand impact forces.

Merrell added their trademark M Select Fresh technology as a removable footbed. It naturally keeps odors at bay by making use of reactive microbes that breakdown foot sweat with natural enzymes. The technology absorbs the sweat and eliminates bacteria thereby maintaining a healthy environment within the shoe.


How Merrell Agility Synthesis Flex ranks compared to all other shoes
Top 12% trail running shoes
All trail running shoes
Top 7% Merrell running shoes
All Merrell running shoes
Top 14% neutral running shoes
All neutral running shoes


The current trend of Merrell Agility Synthesis Flex.
Compare to another shoe:
Jens Jakob Andersen
Jens Jakob Andersen

Jens Jakob is a fan of short distances with a 5K PR at 15:58 minutes. Based on 35 million race results, he's among the fastest 0.2% runners. Jens Jakob previously owned a running store, when he was also a competitive runner. His work is regularly featured in The New York Times, Washington Post, BBC and the likes as well as peer-reviewed journals. Finally, he has been a guest on +30 podcasts on running.