I’m not a shoe or biomechanics expert.  What you’ll get here is a layman’s description and review based on my experience of roughly 5 weeks and around 100 miles with the Merrell Agility Synthesis Flex trail running shoe.   


I love the look, materials, and idea behind this shoe.  The name is pretty awesome sounding as well.  The tongue is neoprene and the shoelaces are slightly elastic.  The upper is a wonderful mesh.  The midsole is EVA foam.  The outsole is rubber and has a tread pattern with wide, large lugs.  The toe box is deceptively small and actually has plenty of room.

Agility Synthesis Flex Toe

Agility Synthesis Flex protective toe box coating

I bought these shoes sight unseen off the Internet.  This was the very first time that I have ever done that.  I do believe it will also be the last.  I was leery about doing it, but decided, against my better judgement to go ahead and try it.  

My First Impression

When I first received the Agility Synthesis Flex, I was impressed by how light they were.  The pair weighs in at 580 grams (1.2 pounds).  I tried them on and they hugged my feet perfectly.  Seemed like a very nice fit.  I have a narrow heel and it rode up and down a bit.  This was easily remedied with running the lace down through extra hole at the top of the tongue.  I do not have the lace through that hole in the pictures.  

I intentionally sprayed down the deck of the boat to test the wet traction. My base line for this is the adidas Rockadia.  They have the absolute best wet traction of any shoe I’ve ever owned, including boat shoes. The Agility Synthesis Flex have no wet traction.  They were slick as owl crap on a wet river rock.  Since I was going to be hiking in the Sam Houston National Forest at the end of June, I wasn’t too concerned.  

I spent about 2 weeks walking and running in them to make sure nothing was rubbing or irritating my feet.  All indications were that the shoes were a go for a hike. 

How The Agility Synthesis Flex Held Up During the Hike

My full pack weight was 15.7 kg (34.5 pounds).  Pretty much exactly where I was aiming.  Not ultralight, but light enough.  I weigh around 140 pounds.  With 35 pounds added, I figure 175 is what a ‘normal’ 6’1” man would weigh.  So, I don’t feel that the shoes were overloaded.   

For the first couple of hours hiking, I was sure they were going to perform well.  Then, slowly, arch pain starting creeping up on my left foot.  I kept hiking through it and then eventually, my left knee started hurting.  It felt like it had been twisted severely.  I tried adjusting my gate and posture back to, what I presumed, was ideal.  I figured that as I got fatigued my stride and posture were going south.  I attempted to consciously improve it in the hope that the pain would lessen.  It did not.  The discomfort only increased. 

The large lugs of the outsole gave positive and reassuring traction.  I had no issues in sand, mud, or forest detritus.  There isn’t much rocky terrain in the Sam Houston National Forest, but I can only imagine that the EVA midsole, which is exposed through the outsole at the heel, would not fair well in rocky, craggy terrain.   It is easily punctured and torn. 

Tread pattern of the Agility Synthesis Flex

The tread pattern of the Agility Synthesis Flex

I really liked the mesh upper. It allowed my feet to breath and I never felt as if my feet were too hot.  It also wrapped and hugged my feet well and no occasion did it feel too tight.

Agility Synthesis Flex instep

Agility Synthesis Flex instep

The neoprene tongue was comfortable and is sewn into a layer of light mesh that wraps around the full inside of the shoe.  It keeps the tongue in place and prevents it from sliding off to one side.  This was a very  nice feature.

Agility Synthesis Flex neoprene tongue

Agility Synthesis Flex neoprene tongue

Post Hike

I wanted for these shoes to work me.  When I got back home, I didn’t wear them for a few days.  First, BOTH my feet were now protesting and my left knee was screaming at me when I wore them the morning after.  Second, I figured I would give my feet and knee time to “recover” and then see if there was anything I could do to fix the shoes.  Maybe I just needed some different insoles.  As both my arches were now in discomfort, I compared the feel between a few pairs of shoes.  I decided to trim down the arch on the insoles.  I immediately noticed a difference and it seemed to be positive.  But, after a day of wearing them, the pain simply returned.  I then went and purchased some 3/4 insoles.  At first, they seemed to make a positive difference.  Then the pain manifested again.  I trimmed down the insoles I had purchased and put them, along with the original insoles, in the shoe.  So, I now had TWO insoles in each shoe.  I wore them for a day or so and wound up trimming the arches on all of the insoles again.  I finally put the aftermarket insoles under the original insoles and that seemed to be the ticket.  My arches started feeling better and my knee was getting back to normal.  

Agility Synthesis Flex insole

Agility Synthesis Flex insole

Agility Synthesis Flex insole bottom

Agility Synthesis Flex insole bottom giving a little view of how I cut them.

Two weeks later:  I ran a little over a mile in them as a test.  My left knee became swollen and the pain returned.  

Jodi decided she was going to try them on.  Granted, my foot is much larger than hers and she already has knee problems so this was a very unscientific test.  Her first reaction was, “Oh my God.  These make MY arches hurt.”  

Merrell Agility Synthesis Flex:  Conclusion

This shoe appears to be a road running design that Merrell attempted to adapt to trail running.  They are very light and flexible.  Looking at the sole of the shoe, I can only conclude that there is very little stability and support.  The foam, though it is very “cushiony”, just does not support the foot and leg properly with the other shoe features. It appears that the arch and heel area collapse down through the lugs as one walks.  This seems to make sense given that my arch feels like it’s being stretched in a weird fashion when walking/running/hiking in them. I’ve had other foam sole shoes, such as the Nike Free and a pair of inexpensive Champion running shoes, and experienced no issues.

Agility Synthesis Flex Heel Strike Left

Agility Synthesis Flex heel strike on the bottom left shoe.

I have had only one other problem with a pair of athletic shoes in my life.  They were a pair of Brooks that absolutely bruised the crap out of my ankles.  It was the weirdest thing.  So, my feet and body are pretty neutral and accepting when it comes to shoes.  Given my experience, I certainly cannot recommend  the Merrell Agility Synthesis Flex in any way, unless one is looking to have a nice looking pair of shoes sitting on a shelf as decoration.  One positive note is that my feet never developed a stinky foot odor in these shoes.    

Had I gone and looked at these in a store (they weren’t in any around here) and tried them on, I probably would have passed.  I also cannot recommend ordering shoes for any real purpose sight unseen off the Internet.  But, you probably already knew that was a bad idea.  I just had to learn it for myself.

I switched back to wearing my adidas Rockadia (the ones that shrank) and the knee and arch pains have disappeared.  That cannot be a coincidence.      

Published On: 2018 July 31

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