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Newton Isaac Review: Action/Reaction Goes High Stacked!
By Andrea Myers and Matthew Klein

The Newton Isaac is Newton's first highly cushioned shoe, with a full length Pebax plate, NRG+ (TPE and EVA blend) foam, Hytrel foam strobel, and aggressive rocker geometry. In classic Newton fashion, the upper and laces are made of 100% recycled plastic and the Pebax plate is bio-based. Newton also redesigned the forefoot lugs for the Isaac, utilizing eight split lugs of varying heights that have more flexibility and feel less prominent underfoot.

Newton Isaac
Price: $145 at Newton Running
Weight: 9.2 oz, 261 g (men's size 9), 7.8 oz, 221 g (women's size 8)
Stack Height: 29mm heel / 21mm forefoot
Drop: 8 mm
Classification: Transitional Daily / Performance Trainer


Matt: The Newton Isaac 2023 brings a new vision to Newton's transitional series, featuring both classic and new elements. A flexible but narrow/snug-fitting upper sits up top, providing a high level of breathability for those used to Newton's comfortable upper. A new NRG+ midsole provides a surprisingly cushioned ride while split lugs provide some of the Newton forefoot with a little more flexibility than normal for this company. The Newton Isaac is best for those with narrow feet wanting a solid cushioned shoe for daily training and some uptempo work. 

Andrea: I was excited when the Newtown Isaac showed up at my door, because the Newton Gravity+ is one of my favorite shorter workout shoes, and I was interested to see how the thicker midsole of the Isaac would stack up. The classic Newton forefoot lugs are less noticeable than those of the Gravity+ or Fate 8, and the large heel bevel and forefoot rocker make midfoot landings feel perfectly natural. The upper is similar to the comfortable, breathable mesh found in the Gravity+ and Fate 8. I found the Isaac to be comfortable for easy miles, but felt best at faster paces. This could be a great daily trainer and workout shoes for runners with neutral mechanics who love Newton uppers and lugs.

: ASICS Dynablast 3


Matt: The Newton Isaac fits me slightly short in my normal men's US size 10. The forefoot feels both narrow and tapered, thus I would highly suggest going up a half size in these if you want enough room. For those with narrow feet wanting a close fit, you should be fine. The volume is a weird mix between normal and slightly higher. The upper does stretch and did accommodate to my feet over shorter runs. It is Newton's classic engineered mesh that stretches and sits comfortably against bare skin. I found running sockless did help significantly with the sizing, although the fit was still a little snug. The midfoot is more normal in width and features a tongue with medial and lateral attachments. I had no tongue slippage but did have to tighten down the laces. The tongue is moderately thick and I had no issues with lace bite. The heel is normal width with moderate heel collar cushioning.

The heel counter is decently stiff and I did notice it while running (those with heel sensitivities may want to avoid this shoe). I did have some heel slippage in this shoe which was easily fixed with a lace lock technique. However, any lateral motion was difficult as both my feet easily slid side to side due to a lack of upper reinforcement. As mentioned, running sockless did fix some of the tapered forefoot experience but I want to emphasize that this is the rare Newton that will fit those with narrow feet as long as they run in a straight line.  

Andrea: The Newton Isaac fit slightly short in my usual women's 9.5. I looked back at what size I received in the Gravity+ and Fate 8, and both were size 10. Newton recommends sizing up 1/2 size in the Isaac, and I definitely would choose to do so if I were purchasing the shoe. While I found the length to be short, I found the volume of the upper to be big. It was hard to lock the upper down enough that I did not experience superior translation of my feet while running, especially at faster paces. The upper actually folded over on itself at the eyelet closed to the toes due to how tightly I needed to make the laces. The width of the forefoot provides enough room for mild toe splay and I found this to be comfortable. The midfoot feels wide due to the high volume upper and the rearfoot is normal width. The moderately padded tongue is partially gusseted and stayed comfortably in place while running. The heel counter is fairly rigid and there is a medium amount of padding internally. I found the heel to be comfortable, but I did experience frequent heel slippage due to my difficulty achieving lockdown. I rarely find shoes to have too much volume, particularly in the forefoot. It was surprising to experience this in a Newton shoe because their uppers have been some of the most comfortable I have tested.


Matt: The Newton Isaac is sold as a daily trainer, but functions more like a performance trainer. The full-length NRG+ provides a softer feel while the shoe feels oddly nimble. The foam is responsive and does react well to uptempo paces. The Pebax plate becomes more apparent at faster paces, while at slower places the forefoot feels moderately flexible. At slower paces the lugs are almost not noticeable but seem to pop more with faster efforts. Being split, they are far less intrusive that traditional Newton lugs, so those not used to them may feel something is missing. A solid, slightly lateral posterior heel provides smooth rearfoot landings, adding to the solid rocker shape of the sole. There is an 8mm drop listed, which is exactly what the shoe feels like. It does not have the dangerously low/zero/negative drop feeling most Newton shoes do, so those transitioning from other shoes will not have to worry about Achilles/calf stress as much.

Purpose-wise, I was limited by the narrow-fitting upper. However, even with the cushioning I found the Newton Isaac best for moderate distance runs at best. This is not a shoe I would choose for long runs, but certainly shorter runs where there may be some uptempo work. I have used this shoe for easy runs, a fartlek and a tempo run. I found the shoe best for fartleks as the faster paces engaged the foam and lugs better. The tempo run went decent and I was surprised that this shoe felt much lighter than its listed weight. Easy runs were fairly average and I found myself wanting something a bit more stable when running relaxed. Durability has been fairly normal with average wear after 20 miles. I would expect a normal number of miles out of these given the rubber placement and likely more mileage out of the lugs given there is more to wear being split compared to the full-length traditional ones.

The Newton Isaac performs like a lightweight trainer. It feels lighter than its stated weight and it provides a moderate amount of ground feel. The midsole reminds me most of New Balance's Fresh Foam X, in that it has a light feel, low compliance, and moderate responsiveness. The full-length Pebax plate becomes more noticeable at faster paces. The flexible forefoot and the lugs contribute to its increased ground feel and level of responsiveness. The sharp heel bevel and early forefoot rocker make midfoot landings feel natural, and definitely make the shoe feel less than its stated 8mm drop. At slower paces, I did not feel like I engaged the lugs at all, and the shoe feels more like a firm trainer. Unlike the Gravity+ and Fate 8, I did not feel like the lugs were a speed bump that I had to get over at slower paces. The lugs in the Isaac are different from the Gravity+ in that the Isaac has split lugs that are taller in the middle portion of the shoe and shorter in the areas of the 1st and 5th MTPs. This likely allows the lugs to flex more and makes them a less prominent feature of the forefoot. The 8mm drop also prevents the lugs from being the most prominent feature of the outsole.

The Isaac performs best at faster paces, particularly faster than threshold pace. It felt best to me while doing 20 second strides at 1 mile pace. At this speed, I felt like I was actually properly loading both the plate and the lugs, and that this engagement resulted in the forefoot springing me forward. I again noticed how light the shoe felt on foot - if I didn't see the specs I would have guessed it was below 7oz. The only negative I experienced at both slower and faster paces was the instability of the upper. I could feel my foot translating superiorly, which did not give me a lot of confidence in the shoe. This also prevented me from testing the shoe for longer, fast paced intervals, due to my concerns about foot and ankle stability. If Newton refines the upper to make it fit more securely, this could be a great performance trainer.

A few of my runs in the Isaac were on wet roads, and I had no issues with traction. The lugs and extensive rubber outsole coverage also make the shoe quite comfortable on grass and dirt. I would expect higher than average durability from the shoe based on both the midsole foam and the extensive rubber outsole.


Matt: The Newton Isaac is a neutral shoe without any traditional methods of stability. While there are sidewalls in the medial and lateral heel/midfoot, I found the softer foam to collapse quite a bit. The midfoot also narrows, which combined with the softer foam left me wishing for a more guided forefoot. The heel and forefoot do feature a decent amount of sole flare, but the softer sole seems to negate these. The upper also lacks decent security and I found myself sliding until I lace locked the shoe, which at least took care of the heel. Those sensitive to guidance and stability methods will do quite well in this shoe, while those with stability needs may find themselves limited to short/moderate distances in these. 

The Newton Isaac is a neutral shoe, but it does have significant guidance features. I would not classify it as a stable neutral shoe, because the lugs and flexible forefoot do require some forefoot stability and mobility. The aggressive heel bevel and forefoot rocker make for fast transitions and will encourage those who typically heel strike to land a little further forward. The forefoot rocker and lugs will propel the runner into push off sooner, which may put more stress on the hip, which the runner needs to be biomechanically prepared for. While the rearfoot and forefoot platforms are on the wider side, the midfoot narrow significantly, which could cause problems for habitual rearfoot strikers who may find themselves landing further forward in this shoe. The high volume upper also created some instability for me because I could not prevent superior translation of my foot in the shoe. 

Thoughts as a DPT: The Importance of Toe Box Space in Forefoot Shock Absorption
By Matthew Klein

A key characteristic of Newton shoes is (was) the facilitation of a more forward footstrike. While evidence has suggested that one type of footstrike is not superior to another for every person, this company does provide a unique forefoot stimulus (Anderson et al., 2020). Whether this stimulus actually facilitates a forefoot strike is also debatable, as prior evidence found that runners in minimal shoes had variable footstrikes (Larson, 2014). It certainly increases load through the forefoot, which requires adequate calf and forefoot function for adequate transitions. If a runner loads the forefoot more, it is incredibly important that metatarsals be able to move appropriate for adequate shock absorption.

One method that the forefoot utilizes for this is the mild abduction/spreading of the toes and joints upon loading (often called toe splay). This small movement is important to disperse forces away from sensitive tissues like the nerves and arteries that run between the metatarsal bones. Compression of the boney and neurovascular structures can, with excessive load and inadequate recovery, may lead to such pathologies as Morton's Neuroma, stress fractures, and more. Having a narrow/tapered toe box may prevent this essential function in those with wider or normal feet. While not every runner needs a huge toe box, all runners need adequate room in the forefoot to allow some degree of this motion for normal mechanics. How much room will vary depending on the person but spending too much time in a shoe with not enough may be problematic for some people. 


Anderson, L. M., Bonanno, D. R., Hart, H. F., & Barton, C. J. (2020). What are the benefits and risks associated with changing foot strike pattern during running? A systematic review and meta-analysis of injury, running economy, and biomechanics. Sports Medicine50, 885-917.

Larson, P. (2014). Comparison of foot strike patterns of barefoot and minimally shod runners in a recreational road race. Journal of Sport and Health Science3(2), 137-142.

Neumann, D. A. (2016). Kinesiology of the musculoskeletal system-e-book: foundations for rehabilitation. Elsevier Health Sciences.


Matt: I do appreciate Newton upgrading their foam and continuing to feature a transitional shoe into the rest of their line up. However, I have a few suggestions that may help. The upper is the biggest thing that needs to modified. Newton does tend to fit a little short and changing sizes can be a nightmare when your metatarsals don't line up with the lugs. Given the importance of toe box room for a shoe attempting to facilitate a forefoot strike (which requires adequate function of the metatarsals), I would highly suggest widening the toe box. Especially given the the Newton Isaac has one of the most narrow fronts I have run in for a while.

The second upper suggestion would be to create some structure at least in the midfoot. This does not have to be major, but enough to hold the foot onto the platform a little better. In regards to the sole, I completely understand the addition of NRG+ foam to provide a softer experience. However, as the stack height increases and the foam softens, the entire sole needs to be wider. Narrowing the midfoot AND removing any rubber outsole reinforcement makes it softer and more unstable. I would widen the midfoot AND leave the outsole rubber to help center the foot. This shoe has a great deal of potential and should certainly be kept on the docket. however, it just needs some modifications.

Andrea: This is a great higher stack offering from Newton, and I hope that they will fix the lockdown issue I experienced by decreasing the volume of the upper. If they do so, I could see the Isaac being one of my favorite performance trainers. 


Matt: The Newton Isaac is for those interested in the feel of a Newton shoe with more cushioning and more subtle lugs. The newest version is far lighter than the older Isaacs, now feeling like a lightweight/performance trainer. The upper will work best for those with narrow feet or wanting a close fit, particularly those interested in sockless running. The ride will work best for those wanting a moderate drop with a moderately soft and responsive midsole for uptempo and daily mileage. The maintenance of a moderate cost compared to the high levels seen in other Newton shoes is admirable, although calling this a super cushion shoe is not appropriate. It still features a more moderate stack height and I challenge Newton to get into the >35 range before they start calling it a super cushion shoe. I understand a few things are in the works in this area, so I am quite excited to see where Newton continues to go. 

Andrea: The Newton Isaac is a highly rockered performance trainer that Newton calls their first super cushion shoe. While it does have a higher midsole stack than their other offerings, it is not a soft shoe and retains plenty of ground feel thanks to the flexible forefoot and split forefoot lugs. The rocker geometry is similar to Skechers M Strike geometry, which works nicely for runners who land further forward. The high volume upper will work best for those with high volume forefeet. While I found the shoe to perform best at faster paces, others may enjoy it for easy runs as well. It is exciting to see Newton expand their offerings and I am looking forward to testing v2 of the shoe, particularly if they tighten down the upper.


Fit: B- (Slightly short fit with tapered forefoot. Below average security but has good on-skin comfort)
B+ (Solid ride that can handle easy and uptempo efforts. Runs lighter than listed weight)
Stability: B- [Neutral] (Highly neutral ride with soft sidewalls that do not contribute much and slightly less stable midfoot)
DPT/Footwear Science: B/B+ (Solid weight drop likely thanks to new foam. Splitting lugs good way to improve forefoot transition)
Personal: B- (Shorter fit makes mileage difficult in this shoe, although higher drop nicely offsets calves compared to traditional newtons. I want to like this shoe but the fit makes it difficult for me)
Overall: B/B-
Fit: B- (fits short like all Newton shoes, but upper is very high volume, making it impossible for me to achieve lockdown)
Performance: B/B+ 
(performs best at faster paces due to appropriate loading of plate and lugs, performance primarily limited for me by fit issue)
Stability: B (neutral) (a neutral shoe with aggressive rocker geometry that lends itself to midfoot landings)
DPT/Footwear Science: B+ (I like the way Newton has adapted their lug technology to make this shoe more comfortable at easy paces as compared to the Gravity+ or Fate 8)
Personal: B-/B (I want to love this shoe, but can't get past the fit issue)
Overall: B


Newton Running
Price: $145 at Newton Running

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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.

***Disclaimer: These shoes were provided free of charge in exchange for a review.  We thank the people at Newton Running for sending us pairs.  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 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.

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