Physical Therapists Using Clinical Analysis To Discuss The Art And Science Behind Running and The Stuff We Put On Our Feet

Post Page Advertisement [Top]

Inov-8 Trailfly Ultra G 300 Max Review
Review by Matt Klein

It has been a long time since an Inov-8 shoe has been on our feet. The Road-X 233 was a favorite of mine back in the day as it was the perfect combination of a 6mm drop with a lightweight trainer vibe. Meanwhile, the Road-X Lite 155 was ahead of its time as a minimal racing flat. Both shoes were out long before I created this website, but they continued to leave an impression. Inov-8 was nice enough to reach out regarding the Trailfly Ultra G 300. Having not run in any of their trail shoes and certainly not any Inov-8 shoes recently, my old perception of closer to the ground Inov-8 shoes was challenged. A maximalist, heavy duty, aggressive trail shoe... that still had vibes that reminded me of the models I loved so much. An interesting twist that is worth talking about.

Specifications (per Inov-8)
Weight: 10.58 oz / 300 g (men's size 9) Not Provided (women's size 8)
Measured Weight Men's size 10: 12.7 oz
Stack Height: 25 mm / 19 mm, 31 mm / 25 mm with insole
Drop: 6 mm
Classification: Maximal Trail/Ultramarathon Shoe


The Inov-8 Trailfly Ultra G 300 is a maximalist trail shoe with a wider forefoot, high stack height and a smooth and bouncy G-Fly midsole (graphene enhanced foam). The 4mm lugs provide great grip on rocks, solid dirt, gravel and there is plenty of cushion for road, trail and transitions between them. The upper is secure, but still provides plenty of room for the toes (although the toe guard puts a bit of pressure initially). A shoe for long miles on the trails, road (although you might wear down the lugs) and a bit of everything in between.

Who This Shoe is For: Those who want a road/trail shoe with a high stack height, smooth riding, wider forefoot and have stable, neutral mechanics

Who This Shoe is NOT For: Those with excessive and/or uncontrolled motion at their midfoot/midtarsal joints due to the Adapter-Flex groove.


The Inov-8 Trailfly Ultra G 300 fits me true to size in my normal men's US size 10. The fit felt a little short at first due to the significant toe guard, but this alleviated after one run. There is quite a bit of volume throughout the length of the shoe. This will fit those with wider feet best and I have to cinch the laces down quite a bit to get a secure fit through the midfoot. The forefoot has a good amount of room, although it feels only slightly roomy at first due to the toe guard. The heel is fairly average in width. There is a flexible heel counter that was not bothersome despite the slightly thinner heel collar cushioning. I did not have to lace lock the shoe as the overlays in the midfoot work very well with the laces to lock the foot on the platform. The upper is fairly tough, with additional overlay along the medial and lateral sides of the foot. Despite aggressive trail and road use I have not seen any wear up here. I would suggest using socks with these and not going sickles as the thin tongue can cause some pressure on the top of the foot. With socks it is fine.


The Inov-8 Trailfly Ultra G 300 has a surprisingly smooth roll and ride. There is a 6mm drop, which is noticeable despite the rocketed ride. There is a significant heel bevel and a solid amount of toe spring. The forefoot has very little flexibility, which is offset by the toe spring and general rocker. The graphene enhanced G-Fly midsole has a surprising amount of bounce to it. Despite weighing 10.58 oz in a men's size 9, The Trailfly Ultra G 300 rides like a much lighter and more nimble shoe. This is particularly evident on road as the ride easily protects against asphalt and feels like a road trainer. The ride is smooth, slightly bouncy and rolls along nicely. While the ride feels lighter, this shoe is still best for easy and long miles. It can pick up the pace a little due to the foam, but isn't quite nimble enough for shorter races. For those doing long mileage on a combination of road and trail, this is a great choice. However, the 4mm lugs will not last long if you run a large amount of road and at 50 miles I have almost burned through the ones at the posterior lateral heel. This has not effect the ride or grip, but know outsole durability is not meant for excessive road use. That being said, over 50% of my miles have been road in this shoe without issue.

On trail, the Trailfly Ultra G 300 has great grip on rocky surfaces, gravel and dirt. It deals with technical terrain very well. The grip is great on harder surfaces and rocks. The Adapter-Flex works particularly well on rocky or technical terrain, particularly with large rocks. The midsole feels protective against rocks and even when bombing down hills. I have not come away with bone bruises. However, this shoe struggles with mud as the Adapter-Flex is too flexible and the lugs do not grip well enough in wet mud. Outside of muddy conditions though, those looking for a long distance, maximalist, rocketed, grippy shoe for trail, road, gravel and rocks, the Inov-8 Trailfly Ultra G 300 is a great choice.


While the stiff forefoot and sole flare provide some stability, the Adapter-Flex groove makes this shoe a very neutral shoe. I am not usually a fan of split midfoot areas. The split midfoot has a flex groove that is perpendicular to the mid-tarsal joints. It feels like this area exacerbates midfoot pronation, particularly on harder surfaces. This feels better on even terrain where that is a good thing. However this makes the shoe unstable with longer miles on smooth trail or road as fatigue sets in. The forefoot is very stable. The wider, deep longitudinal flex grooves combined with the toe spring make for a stable and smooth transition forward up front. The heel is also decently stable thanks to the medial and lateral sole flare. This normally would cause a problem (early contact) but is offset by the solid heel bevel. So while the Inov-8 Trailfly Ultra G 300 has a stable forefoot and heel, the midfoot is fairly unstable and will only be good for those who actually need more motion there. 


I am generally not a fan of flex grooves in the midfoot. I appreciate what Inov-8 is trying to do here, but unfortunately the Adapter-Flex is going to exacerbate motion for a large number of people. The major joints of the midfoot are called the mid tarsal joints or transverse tarsal joints. This includes the calcaneocuboid and talonavicular joints. Like most joints of the foot, the axis (ie how they move) are NOT in the major planes of motion (side to side motion: frontal plane, front to back motion: sagittal plane, etc). They are typically at odd angles and vary between people. These joints do need to move as they are part of shock absorption during the early stance (landing) to mid stance (foot under body) phases of gait. Their primary motions are called inversion and eversion and contribute to pronation and supination like many other joints do (including the subtalar joint just proximal to this).

Image from Physiopedia. The image on the left is the longitudinal axis, 
the right is the oblique axis. This is a complicated series of joints. 

The problem with the flex groove is not that it lines up with the oblique axis of the midtarsal joints. It's the fact that the axis of the midtarsal joints CHANGE depending on foot motion. Depending on how pronated or supinated the foot is when it rolls over this part of the shoe, this flex groove may or may not be optimal. This is the challenge with midfoot flex grooves. They may benefit and facilitate motion in some people and may be problematic in others. 

So those people with really stiff, high arches may benefit from a shoe like this that has a flex grooves that lines up more with that axis. These individuals will be able to enjoy this shoe on trail and road without issue. Those however who tend to have too much motion or not enough control of pronation or supination may run into problems here. Those individuals should stick to only using this shoe on technical terrain so it can help them make use of their mobility to adapt to abnormal terrain. When it comes to pounding on road, the excessive motion associated with the greater shock absorption needed for road may cause some problems.  We frequently talk about the fact that certain shoes will work for certain people and not others. This is definitely an example of that.


Okita, N., Meyers, S. A., Challis, J. H., & Sharkey, N. A. (2014). Midtarsal joint locking: new perspectives on an old paradigm. Journal of Orthopaedic Research32(1), 110-115.

Tweed, J. L., Campbell, J. A., Thompson, R. J., & Curran, M. J. (2008). The function of the midtarsal joint: a review of the literature. 
The Foot18(2), 106-112.

Culture Corner: A Brief History of Graphene
By Contributor Bach Pham

Graphene is a unique, very recent material that is extracted from graphite. Graphite is a naturally occurring crystalline carbon found in metamorphic and igneous rocks. It’s a strange material that’s both soft, yet highly resistant to heat. While graphite is impressive on it’s own, scientists discovered that graphene - a single layer of graphite - is remarkable material. In fact, it’s the strongest material ever recorded being two-to-three hundred times stronger than steel and forty times stronger than diamond. Despite its durability, it also is surprisingly flexible.

Graphene was only recently extracted from graphite in 2004 by physicists Andre Geim and Konstantin Novoselov, who later went on to win a Nobel Peace Prize for their efforts. It directly picked up steam in the tech industry due to its ability to both conduct energy quickly and be long-lasting thanks to its durable nature. From aerospace technology to superconductors to telecommunication, graphene has been seen as the future of tech with companies working on solving how to fully utilize its unique properties. The other appeal of graphene is despite being a very new material to work with, it’s fairly affordable to invest in currently.

This leads us to Inov-8, who introduced the use of graphene in their outsole in 2018. Partnering with University of Manchester scientists, the company worked to blend graphene into their rubber outsole and take advantage of the material’s durable properties with the goal to make the rubber last through all of the technical work that trail runners do on a weekly basis while also providing a more flexible grip in the process. Thus graphene grip, or “G-Grip” was born.

The Ultra 300 Max is their first introduction to using graphene in the foam itself.

Graphene’s use is slowly evolving in footwear. Researchers have started considering the use of graphene within winter running shoes due to durability and relatively lightweight which often is a compromise in winter shoes. Due to its minimal nature, there’s potential to combine graphene with other materials to help shore up the weaknesses of less resilient materials while still reaping benefits. Peng Hong and Xia Xinqiao from the Wuhan University of Science and Technology considered combining graphene with aerogel, which has low resiliency that graphene can help compensate for while still getting the benefit of aerogel’s ability to retain heat for the runner.


Graphene - What Is It?

A decade of graphene research: production, applications and outlook

The rise of graphene

University collaboration reveals world’s first-ever graphene sports shoes

Inov-8 on their use of Graphene

Application of graphene and aerogel in winter running shoes products


Inov-8 has a great shoe on their hands here. However there are some tweaks that need to be made. My major suggestion would be to remove the Adapter-Flex. The best place for good flex grooves are in the forefoot, not the heel and midfoot. Let the shoe be fully rockered, which will improve efficiency for a variety of runners. Instead of an oblique axis, I would have deeper flex grooves that run the length of the shoe like they do in the forefoot. This may predispose the outsole to catching rocks, but will provide more guidance. I also think the toe guard should be toned down a little or reinforced with softer material on the inner side. The overlays on the lateral and medial sides of the foot are great, so please leave those alone. 


The Inov-8 Trailfly Ultra G 300 Max is for those who want a max cushion shoe with extra midfoot flexibility that is true to the mid-tarsal oblique axis. The ride is very smooth, the G-Fly graphene midsole makes the shoe feel lighter and bouncier than its weight and allows for great protection over both road and trail. The lugs are fantastic for gravel and touch rocky terrain, providing security on both up and downhills. A great ultramarathon shoe that will keep your feet secure and allow for some swelling thanks to the higher volume upper. I am impressed the direction Inov-8 is taking as this shoe feels both similar to the older models I know with the 6mm drop and great toe box width. However, the maximalist stack height and rocketed ride are unique new twists in line with the current market. I am excited to see how they continue forward and think many people will enjoy this shoe as an ultramarathon racing shoe for difficult terrain. 


Fit: B+ (Higher volume fit that snugs down nicely in the midfoot. Plenty of toe room, although the toe guard makes the shoe feel short at first.)
Performance: A- (Versatile shoe that can handle aggressive terrain, gravel, trial and road. The lugs wear down with excessive road use, but feels very comfortable and rocketed on all surfaces. Grips very well on climbs and decent) 
Stability: (Very stable heel and forefoot. Adapter-Flex midfoot exacerbates motion at the midfoot, so those with excessive mobility there will need to watch out. ) 
DPT/Footwear Science: (Good for those who need facilitate midfoot mobility thanks to the Adapter-Flex groove lined up with the midtarsal joint axis. Not so good for those who do not need mobility there. Will facilitate shock absorption in some and extra motion in others.) 
Personal:  B+ (I really like this shoe except for the Adapter Flex. As I have excessive midfoot mobility, this tends to exacerbate problems for me and I can't take it as long as I would like.) 
Overall: B+   

Interested in the Inov-8 Trailfly Ultra G 300 Max? Visit Inov-8 here.




Compare Cushioned Trail Shoes
Hoka One One Torrent 2 - A trail racing favorite among Hoka fans. Nimble, but cushioned
On Cloudultra - Firmer, long range cruiser from On
North Face Flight Vectiv - Carbon plated racer meant to barrel down trails
North Face Vectiv Infinite - Ultra stable, cushioned trail running for long distances
Brooks Caldera 5 - Cushioned trail running classic

Recently at Doctors of Running
Hoka Clifton 8 Video Review
The Last RC Elite v1 Review on the Net

Asics Novablast 2 Review - Subtle improvements in heel help provide some minor stability improvements to accommodate more runners who loved version one
Newton Gravity and Motion 10 Review | Podcast
New Balance Rebel 2 Review - The ever popular Rebel returns with a refined midsole and geometry
Science Friday: Hips Don't Lie
Puma Liberate Nitro Review -
Super light offering from Puma. New speed day classic?

Thanks for reading!


Dr. Matthew Klein is a 150 lb male with notable PRs of 14:45 for 5k and 2:32:44 for the full marathon.  He typically runs 70-100 miles per week and trains at a variety of paces from 8min per mile recovery runs to 4:40 per mile 1k repeats.  He prefers firmer and responsive shoes with snug heels and medium to wide toe boxes.  The stability guy of the group, he also prefers a little stability in his footwear. However, as a researcher, clinician and running shoe aficionado, he will run in anything. IG handle @kleinrunsdpt

Editor's Note: As always, the views presented on this website belong to myself or the selected few who contribute to these posts. This website should not and does not serve as a replacement for seeking medical care. If you are currently injured or concerned about an injury, please see your local running physical therapist. If you are in the Los Angeles area, I am currently taking clients for running evaluations.

Doctor of Physical Therapy
Board Certified Orthopedic Clinical Specialist
Fellow of the American Academy of Orthopedic Manual Physical Therapists

Nathan Brown PT DPT MS
Doctor of Physical Therapy 
Masters in Anatomy and Clinical Health Science
Movement Performance Institute Certified in Advanced Functional Biomechanics 

David Salas PT DPT CSCS
Doctor of Physical Therapy
Certified Strength and Conditioning Specialist

Bach Pham MS
Marketing and Social Media Manager
Master of Arts in Cultural Anthropology

***Disclaimer: These shoes were provided free of charge in exchange for a review.  We thank the  people at Inov-8  for sending us a pair.  This in no way affected the honesty of this review. We systematically put each type of shoe through certain runs prior to review. For trainers and performance trainers, we take them on daily runs, workouts, recovery runs and a long run prior to review (often accumulating anywhere from 20-50 miles in the process). For racing flats we ensure that we have completed intervals, a tempo or steady state run run as well as a warm up and cool down in each pair prior to review. This systematic process is to ensure that we have experience with each shoe in a large variety of conditions to provide expansive and thorough reviews for the public and for companies. Our views are based on our extensive history in the footwear industry and years testing and developing footwear. If you are a footwear rep looking for footwear reviews or consultations on development, we are currently looking to partner with companies to assist, discuss and promote footwear models. Partnership will not affect the honesty of our reviews.

Like and Follow Doctors of Running
Facebook: Doctors of Running Youtube Channel: Doctors of Running 

Please feel free to reach out, comment and ask questions!

Bottom Ad [Post Page]

// ]]>