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The Rundown: 100 Miles In Merrell’s Agility Synthesis Flex

Merrell's Agility Synthesis Flex is a trail shoe that, true to its name, flexes and grooves—to a point.

THE RUNDOWN: Merrell’s Agility Synthesis Flex is a trail shoe that, true to its name, flexes and grooves—to a point.

Surface: Trail               Stability: Neutral        Stack Height: Medium

Merrell’s Agility Synthesis Flex rides like a trail racing flat—it feels light and responsive—but with more protection and cushion. Part of the brand’s Agility line, which features a midsole designed to promote flexibility with dual-directional grooves, it accelerates easily on soft surfaces. While the style doesn’t handle aggressive or wet terrain efficiently, it’s built to rip on less-than-technical terrain.

The Specs

Weight: 7 oz. (women’s)

Offset: 8 mm

Stack Height: 30 mm/22 mm

Midsole: FLEXconnect EVA

Outsole: M Select GRIP

Upper: Mesh and TPU

Price: $110

Photo Credit: Elizabeth Carey

100 Miles In: The Review

Out of the box, the Merrell Agility Synthesis Flex popped with bright colors and a geometric design. On foot, the upper fit comfortably once I tied up the long, stretchy laces that secure the midfoot with a well-integrated flat-lying eyelet system. The cushioning underfoot is unobtrusive yet squishy; it invokes a feeling of protection, without sacrificing the sensation of lightness.

The outsole features what Merrell calls “M Select Grip” (a.k.a. durable traction) with 5mm lugs. While I would expect lugs of that depth to perform better in slick conditions (more on that later), the tread held up as touted, even after 150+ miles on both soft and hard surfaces. Under the forefoot, a skeleton-esque tread configuration provided average grip. Under the midfoot and heel, fewer lugs line the edges of the shoe, and heel-plants didn’t feel as sure-footed as they do in more bomber models.

The tread proved its worth on mixed-terrain, especially on those pavement-to-park runs urban dwellers squeeze into busy days. It also performed well during soft trail workouts and sub-ultra trail races. The Agility Synthesis Flex excelled on pea gravel, pine needles and hero dirt (as most mixed-terrain trail running shoes should.)

The guts of the shoe is the dual-directional, flex-groove EVA midsole. The goal? Simultaneously provide agility and stability. The latter was as expected for a lightweight style; it was neither very supportive nor rigid. The former was notable, thanks to the midsole groves, which allowed the shoe to flex on undulating terrain. Overall, the EVA midsole provided decent spring—that sought-after feeling of perceived energy return.

In lieu of a rock plate, designers placed a “TrailProtect pad” for protection against gnarly surfaces; it was just enough to protect my foot from trail debris but not enough to keep me from feeling rocks underfoot on a range of trails. After logging miles of easy training runs, workouts, and long runs, the shoes—surprisingly—didn’t stink thanks to an antimicrobial footbed.

On top, a woven jacquard upper of mesh and TPU proved durable. The knit is stiff, and feels like outerwear, perhaps a sturdy shell. With a thin-but-firm toe cap, the upper protected against branches and brambles, but breathed enough to avoid swamp foot. Also, it drained well during Pacific Northwest deluges, thankfully fending off any internal sloshing sensations on rainy days. The foamy neoprene tongue laid flat and stayed put.

The rib-like eyelet design allowed me to cinch up the flat, slightly-elastic laces well for a snug fit. The fabric padded collar didn’t cause any friction, either. Instead of a heel counter, a minimally supportive band of TPU surrounds the heel for a half-hug-like lock.

Merrell also touts this, among other styles, as vegan-friendly. While that’s a nice touch, most running shoes from major brands fit that bill; generally, non-leather styles are made with synthetic, animal-free materials.

Photo Credit: Elizabeth Carey

There’s The Rub

The Merrell Agility Synthesis Flex has a higher-volume, almost-too-roomy toebox. When I pushed the pace and/or trail technicality, this contributed to toe and top-of-foot issues for me.

What’s more, in slick conditions, on long runs, and on technical trails, my feet were left wanting more: more cushion, more traction, a better forefoot fit. That is, of course, by design; Merrell created this style specifically for so-called “moderate” trails. However, during one trail half marathon on predominantly non-technical trails, this pair felt sloppy on downhills and tight turns. While the style handled patches of roots and rocks OK, they felt on the minimal end of the footwear spectrum, leaving my feet a touch sore.

Although the upper and tread held up well, paint along the midsole started peeling after 100 miles—just enough to notice if the shoes weren’t covered in dirt—and other signs of wear and tear (like creasing) appeared on the exterior less than 100 miles in.


Designed for soft surfaces, the Merrell Agility Synthesis Flex feels like a cushioned racing flat built for light to moderate trails. It packs perks (comfortable upper and tongue, a stay-tied lacing system, durable 5mm lugs) into a responsive package that excels when conditions are dry or smooth. Length-wise, it fits true to size.