I’m going to start this review with a bold statement: the Nike Free 4.0 Flyknit is the best Nike Free shoe I have ever worn.
Writing that opening line is a big deal for me because the original Nike Free 3.0 remains one of my favorite shoes of all time. That shoe was lightweight, ultra-flexible, ran silent, and had a sock-like upper that was about as near perfection as a minimal shoe upper can get. My only complaint about the 3.0 v1 was that it was a bit narrow. The new 4.0 Flyknit matches the original 3.0 on all of the positives, and improves on it by having a much wider toebox. It’s an amazingly good shoe.
Interestingly, I almost passed on trying the Free 4.0 Flyknit. Several weeks ago I purchased the new 2014 model of the Nike Free 5.0. I hated it. It had a tight band around the forefoot that dug into my skin, and quite honestly it felt and looked a bit cheaply made. I wasn’t impressed and it went back to the store unworn. However, I’d heard from some fellow shoe geeks that the 4.0 was going to be the big winner among the 2014 Free models, so I spent the $100+ to buy a pair (MSRP is $120). This might be one of the few times where I feel that the relatively high cost might be justifiable – I like the shoe that much.
So what is it that makes the Free 4.0 Flyknit so special? Well, pretty much the entire package. The shoe simply feels like an extension of my foot. It gives me everything I need, and almost nothing that I don’t, and that is exactly what I want in a shoe.
Shoe Stats
Weight: 7.4 oz in men’s size 9; 6.3 oz in women’s size 8 (per Running Warehouse)
Stack Height: 20mm heel, 14mm forefoot (per Running Warehouse)
Sizing: I went a half size up as I typically do in Nike shoes, might have been unnecessary
Upper Construction
The Flyknit upper of the Free 4.0 is minimally structured. No overlays, no heel counter, just a stretchy woven mesh with a few Flywire bands on either side of the lace rows to lock the foot down. It feels like a sock, and it hugs my foot in all of the right ways. Not too tight, not too loose. Just perfect. The heel and midfoot are snugged in tightly, and the forefoot opens up so that the tight weave does not constrict or squeeze the toes. The entire upper flexes and moves with the foot in a way I have rarely experienced in a running shoe.
The upper mesh is open in the midfoot and forefoot to provide for some air flow. Unlike the new Free 3.0 Flyknit, which I have heard fits fairly tightly, the 4.0 has a traditional tongue, and I think this allows for better customization of fit. I’ve run sockless in the shoes a few times and have had a bit of abrasion on one run near the midfoot/arch (maybe from one of the Flywire bands?), but this has been inconsistent. May have just been a hot day with more sweat leading to chafe. No issues at all while wearing socks (side note – these shoes seem to hold a stink when you use them sockless).
Sole Construction
Like the upper, the sole of the Free 4.0 Flyknit is super flexible and moves really well with the foot. The sole is typical of other Nike Free shoes in having individual pods separated by deep grooves to maximize flexibility. The drawback of the grooved sole is that the grooves do tend to collect rocks and pebbles, but this has never really bothered me in any of the Frees and they are easy enough to get out by taking the shoe off and flexing the sole around after a run.
The majority of the sole is exposed midsole cushion, so you can barely hear your footfalls while running in them – I love a silent shoe! The tradeoff to this design is that there is minimal outsole coverage – rubber pods are only present at the back outer heel and under the big toe. As such, sole durability is something to keep an eye on if you tend to be a scuffer. Interestingly, my wear pattern only seems to be from the anteriormost heel pod forward through the midfoot – much more of a midfoot landing wear pattern than I tend to observe in most other shoes that I run in.
You can see that after 40 miles I’ve ground down the protruding portions of the white pods directly above and to the right the 4.0 in the image below:
I’m wondering if the slightly rounded heel, curved sole, and flexibility of the shoe have something to do with this more anterior wear pattern?
Performance
I’ve honestly found it really hard to not pull these shoes out for most of my recent runs (though the Saucony Kinvara 5 is giving them a run for the money as current favorites). I’ve run just over 40 miles in them so far, with a max long run of 14 miles. I’ve run from easy pace down to 5K pace. They’ve worked well for just about everything. I’d have no hesitation in taking them beyond 14 miles in a single run, I might even consider them for a marathon. They are a bit too flexible for my taste for speed or a 5K, and the sole grooves would probably collect too much debris for off-road use, but they are versatile enough to handle just about anything else I could throw at them.
I’m really hard-pressed to come up with anything negative about this shoe – I think the only issue I’ve had was with the fact that the size tag is stitched to the underside of the insole and I can feel the stitching under my heel. It wasn’t bad with socks, but it was noticeable when I went barefoot in them. It might be possible to cut the tag and remove the stitching, but I was lazy and just swapped the insoles out for an identical one from an older pair of Free 5.0s.
In addition to running in them, I’ve also found the Free 4.0 Flyknit to be a fantastic casual shoe. I bought a pair in black since I like to have a few conservative looking shoes for casual wear, and I’m typically sockless in them while walking around. They are amazingly comfortable.
Conclusion
The Nike Free 4.0 Flyknit is a fantastic shoe, easily one of my favorites so far this year. If you want something with a minimal upper, a flexible sole, and a roomy toebox these should be at the top of your list of options to try.
Purchasing Options
The Nike Free 4.0 Flyknit is available in a variety of colors at Running Warehouse and Road Runner Sports. Outside of the US they can be purchased at Wiggle.co.uk. Purchases made via these retailer links help to support this site – thanks!
You might also like:
Nike Free 5.0 2015 Review: Yes, You Can Run in Them!
Nike Free 3.0 v5 Review: Redemption For One Of My Favorite Shoe Lineages!
Nike Free 5.0+ Running Shoe Review
Nike Free 3.0 Flyknit 2015 Review: Flexible Sole, Sock-Like Upper, and Solid Cushioning in a Lightweight Package
Nike Free Flyknit Hyperfeel: The Shoe Giant Goes Ultraminimal?
About Peter Larson
I really like the feel of the Free as well, but I also find them a little too flexible for some workouts. My solution has been the Terra Kiger. I am using new ones for trails and after they sole is a bit worn, 200 miles or so, I move them to road use for the second half of their life. I then buy a new pair for trail use. They have the comfort of the Free with a slightly more substantial sole.
I really want to try the Kigers!
I think you would really like the Kigers Pete. I’ve run in them for 20 miles or so and, while a little soft for me (probably a good thing for you since I like firm shoes), they have a great upper and nice overall feel for smooth trails and road. Nike has definitely got my attention with the Kiger, hopefully they can continue to refine and improve it.
Hey Pete,
Quick question. I used to run in Kinvara’s in v1-3, then switched exclusively to Virrata’s and A5 for racing. I’m kind of itching to try the new Kinvara 5 – if I’m in zero drop’s right now, do you have any inclination on if you’d rather go from a Virrata back to the Kinvara 5 or these new flyknits? Hope that makes sense.
Thanks.
Both are great shoes so tough call. The Kinvara is a bit cushier underfoot, more similar in feel to the Virrata. The Free is by no means a firm shoe, but the sole has a different feel to it – it’s tough to describe. I think the Kinvara will feel more familiar to you since it is a more traditionally styled shoe.
Thanks as always for the input. Might have to try both :-)
I just bought the Nike Free 3.0 and absolutely love them. I agree that the fit is tight, but I don’t notice it at all when I’m running. The one thing I noticed with the specs is that the 3.0 actually has more forefoot cushioning (I think it’s 17mm) than the 4.0 and 5.0. This is the main reason I bought the 3.0 over the others (being a mid/forfoot striker). The sole does tend to gather small pebbles, but I’ve tried them on trails and love the flexibility of them. It’s going to be hard going back to normal shoes after these!
Hey Arlina,
Thank you for your interesting comment. I was wondering if you noticed any difficulties with the heel really low cut?
I tried some 3.0 on a threadmill in nike town but not quite sure yet if it’s a big difference compared to the v5.
thanks
Have you tried the Free 3.0 2014 yet? Any input on the comparison between the two?
No, not yet. Have heard mixed things, and the price scares me a bit.
in any way similar or comparable to pearl izumi N0?
No, totally different shoe. Much more flexible, stretchier upper, wider toebox. The N0 is basically a racing flat.
so you’d call this an everyday trainer?
Depends on your adaptation to this kind of shoe. I could foresee myself using it as an everyday trainer.
I ordered these the other day. When they came in they seem a little long even though ordered the same size I always do in Nike.
I haven’t ran in them yet. Has this been an issue or do they fit fine once you start running in them?
I thought the same at first, almost sent them back for a size smaller, but wound up keeping them. Have not had a problem so glad I kept the 10.5s.
I thought the opposite. I’m usually a 10/10.5 but decided on the 10.5 from the fitting chart and they looked like they’d be nice and snug out of the box. But they just form fitted to my foot with minimal stretch and have been the comfiest shoes I’ve even run in. I’d recomend getting a snug fit as a slightly loose fit may lead to your foot slipping back in the heal (as I found out trying on the 11’s).
Hi Pete,
While I really like the feel of the 4.0s as well, i experienced top of the foot pain on my left foot a day after a 7km run in the shoes. This may have been likely due to the tight midfoot. I’ve removed the insole, and found the shoes to be far more spacious and comfortable for my feet, al beit while walking. Have not tried running yet. Do you recommend that i run in the shoes without an insole, something which im really keen on doing?
I felt that once after wearing them all day casually. I think I had the lacing a bit too tight and the tongue has no padding. I have removed the insoles in other shoes and as long as there s no abrasion from stitching see no reason why it would be a problem. You could also try swapping in a thinner insole from another shoe if you have one available.
This is my favorite “Free” model to date. Exchanged the 5.0’s for the 4.0’s after the 5’s tore my little toe apart. Great shoe. Best flyknit she IMO.
*yet* not she. Nice job autocorrect
Hi Pete. Great coverage as always. How did you like Free 4.0 in comparison to the Kinvara 5? I unfortunately don’t like the Kinvara 5s, for me, they are significantly wider in the forefoot and midfoot and don’t have the racing shoe fit like the 3s or 4s. I found my feet sliding some what and the shoe just felt more boxy. I liked the Kinvara 3s the best; not as narrow as the 4s.
The Frees are going to be much snugger through the heel and midfoot, and though the forefoot is fairly wide it feels lower volume top to bottom than the Kinvara. It might be a better match for you.
did you find you have similar foot strikes between the kinvara 5 and nike 4?
I think pretty similar, but wear is a bit further forward on the Frees.
These do look really nice. I’ve only run in the 3.0v2 and wasn’t crazy about them but the looks of the new sole and the flyknit upper look really nice. Might need to add them to my list to try.
Pete,
I did the exact same thing…bought the 5.0s after being thrilled with the 3.0s because I was in need and didn’t want to create my own 4.0s at the time….but when I saw a few weeks later that the 4.0a were released, I ordered a pair immediately. all I can say is wow. I absolutely love them. truly an extension of your feet and like nothing I felt before. one of the best low profile shoe I’ve ever worn. I purchased the electric yellow and dark gray. I wear them with ultra thin Nike and feetures no show tab socks. no slip ever, just comfort. much different than he 5.0s that don’t give you the feel of the road like 4.0s. o do have one chief complaint. I’ve had them for 3 weeks, been through at least 13 workouts + 25+ miles. they are covered in dirt. absolutey covered…the knit seems to attract dirt like bees to honey. its unreal. if u don’t know how to wash your frees…tie them off in pillow case and wash with detergent in short cold wash. hopefully the knitting doesn’t come apart. dont dry them!
I often just stick a hose inside the shoe and let the water bubble through the upper. Does a great job cleaning them out then let them dry in the sun.
The hexagon shaped outsoles intrigues me. Does these make the shoes better compared to the previous outsoles?
Tough to say, it may flex a bit better in multiple directions, but flexibility is not an issue I have had in the past with Nike Frees.
Hi Pete. I’m in Kinvara 2’s right now (I stocked up!) and I like them so I was looking at the Kinvara 4 or 5’s (I have narrow feet so a discounted 4 might still work for me). Anyways, two part question: How does the 4.0 Flyknit feel vs a Kinvara for sole flexibility when running? It looks like (cuz of the pods) it would be more flexible, which I think I might like. And 2nd, how would you compare/contrast the 4.0 Flyknit and the 3.0 v5? How would I pick one or the other from your experiences with them? Thanks!
The Free is far more flexible than the Kinvara. And if you have a narrow foot, the 3.0 v5 might be better, and you may be able to find them discounted.
I just tried the new free 3.0 and 4.0. I run in go bionics 2 which I like. I wear size 11 in go bionics and 11 in 3.0 and 4.0. I initially liked how 4.0 felt on my feet. It had lots of room and felt comfortable. However when I took it for a run, for me I felt way too much on the heels and I felt that these could give me problems with my calves in the long term. To me the shoes didn’t quite flow well with how I run. I felt little clunky with them. Then I tried then 3.0 and I love it. I was afraid of the tightness as I felt the shoes to feel a little tight. I run sockless and when I started running, it didn’t feel tight at all. The shoes felt excellent and felt smooth as I was running. If you haven’t tried 3.0, you are missing out. Tomorrow I’m going to try running without the insoles and see if that’s even better or not.
Hearing that a lot about the 3.0, may need to try them when the price drops!
After couple more runs in, 3.0’s are the best shoes I have ever run in. I thought Go Bionics 2 are the best shoes out there, but Free 3.0 beats it. I got the size where basically there’s not much of a toe room, and these shoes really feel like part of you. It’s like what you would hope Vibrams to feel like but they obviously fall short with stiff toe pockets. I recommend running without the insoles and they haven’t given me any blisters running sockless.
Peter….I have the flyknit lunar 2’s, and although I generally like them for groomed dirt trails and asphalt bike trails, I get this weird sensation when I make turns that I am sliding off the side of the shoe. I recognize that this would make a horrible technical shoe for real trails, but the shortfall of the flyknit being and advanced form of sock, is these occasions when your foot kind of moves about in it. Have you had a similar experience with the free 4.0?
Tom,
I haven’t run in a pair of flyknits yet, but did try on both the Free 4.0 and 3.0 flyknits. My guess as far as the reason you might feel like you are sliding off the side of the shoe is that it seemed to me, because of the sock-like nature of the flyknit upper, you would want to size them snug and form fitting (like a sock) rather than like a more traditional shoe…the flyknit has some inherent stretch to it and gives, much like a sock. Most trail shoes that fit securely for technical terrain, don’t have a stretchy upper…this can reduce comfort if the last shape of the shoe doesn’t match your foot well, but is necessary for security IMO.
No, but have not taken them off road or any place that required cutting or quick turns. As David said I think it’s a limitation of a Flyknit type upper – just not snug and supportive enough for trail work.
Hi, can I ask how the sizing of the shoe is like? I’m considering to get a size US 8.5 with some wiggle room but I’m not sure if it’ll fit right.
I have a foot that is 25.5 cm (26cm with socks worn) and have a Vans Shoe size US 8.5 that fit well with little room left.
What is your suggestion? Thanks
I have half size larger in the Frees than in most other shoes, but they are quite roomy and I might have been fine with a 10.
Hi, I’ve bought Nike free 5,0 and also can’t decide which size is better 10 or 10,5. Did someone have the same problem? I can return one pair.
You should have about a thumb’s width between your longest tow and the tip of the shoe.
I just bought these shoes. They felt great but when I started running with them around my neighborhood sidewalks and streets, I kept feeling like rocks were getting stuck in between the creases as they opened and closed with the flexing of my foot. I took them off and was considering returning them but I might not be able to since I wore it already. :(. I was wondering if you found that to be a continuous problem of the shoe while running outside with it over time?
Common problem with Nike Free. Doesn’t bother me too much, but I occasionally get a big one stuck in the sole and have to pull it out mid-run.
Love your reviews, always helpful. I have not run in Nikes since 1984, they never feel right to me. Tried the new Flyknit Free 3.0….crazy how good they feel to me. My all-time favorite. Only the pebble collecting is an annoyance. Try them!
Very tempted!
I currently have nike free 4.0 shoes and have loved them so far, but I am due for a new pair. These new flyknit 4.0 shoes look and sound amazing, but my only concern was that they wouldn’t have enough support for trail running because of the sock-like feel? Let me know what you think!
I wouldn’t recommend for the trail. Between rocks in the sole and the lack of a supportive upper I’d envision a lot of sliding around on the sole around turns.
I have been using Nike Free 3.0 v4, v5 and now the Flyknit. I have to say that the toe box is so small in the Flyknit version that I just bought 3 pair of the v5 before I can’t find them any more since I liked those so much. Maybe I could have gone up another half size, but thought they might be too long. I hope that they make the toe box bigger in the next iteration. However, I guess I should have looked at the 4.0 to see if the toe box is better. Thanks for the review!
Greg,
I’ve tried on both the Flyknit 3.0 and 4.0 and the 4.0s do indeed have a little wider toebox. Also, I think the snugger fit is intentional in the 3.0 because the Flyknit material acts more like a sock than a traditional shoe upper and has some stretch to it, but it is a different feeling for sure and one that not everyone will care for.
Peter, if I’m a 11 in the GoBionic Prana with a comfortable fit and an 11 in the Go Run 2 with a slightly snug fit through the instep, what would you recommend on the 4.0 Flyknit? Shoeftr tells me a 12, but I’m thinking that’s way too big. Thanks for your reviews, as always.
I have a half size bigger in the 4.0 than in the GoBionic or GR2, but there is quite a bit of room for me. I could probably even manage the same size in those, but I like a bit of extra room. If anything, only go up a half size.
how would you say this shoe compares to the skechers go bionic? if you could have only one flexible shoe in your lineup, would this be the one?
I’m sure Pete can elaborate more. I’ve tried on the Free Flyknit 4.0 (but not run in it) and I have the GoBionic 2. The Bionic is a bit more minimal than the 4.0 so hard to say, especially for a shoe geek like myself, which would be the only one in my lineup :). The 4.0s would be more versatile because there is more shoe there, but it just depends on how particular you are about drop, weight and overall stack height (i.e. if you want zero drop, then the Bionic would be the one to keep).
Pretty similar shoes, Bionic may be slightly more minimal. On price I’d go with the Bionic I think.
Peter, BTW since we are discussing the 3.0 a bit here, I notice there is already an “update” to this year’s model. The colors have changed and the original ones (I bought them in May I believe) are already being discounted. I can’t figure out what has changed with them; they look the same other than the colors. Do you know the difference? Thanks.
Pretty typical to have color updates several months after a shoe release. They usually discontinue old colors and add in new ones, but the shoes stay the same.
What do you think of the 4.0 or 3.0 as a half or marathon race shoe?
I could use the 4.0 for a half I think, would probably opt for a bit more cushion for a full. Probably depends on the type of shoes you are currently adapted to.
Hi there!
Do you have any opinions on the Flyknit + model?
I recently bought the 4.0 and I love them. But, the flyknit +
looks like it has a thicker sole for more rugged terrain, almost like a hybrid of my once beloved free run 2 and the flyknit 4.0. I live in a rural area and can’t try shoes on before I order them, so I’m torn between just getting another pair of the 4.0s or trying the flyknit+.
Thanks for your input!
My 4.0’s should arrive in a few days, and I was wondering about the best way to wear them. When you say they hold a stink without socks, do you mean they will start to smell, and stay that way, when you are barefoot? Or more along the lines of “hold back” a stink? Thanks!
That they will start to smell if worn barefoot most of the time.
I purchased these shoes yesterday because of the feel. I was told that there were not a good purchase for someone over 40 with a back issue (bulging disc). I’m a beginning runner who likes to jog for about an hour on a treadmill. I was told the barefoot feel is not good for me. Any idea if these shoe may not be right for me? The store clerk said it is great for flat feet.
Really the only way to know is to try, not sure why they would say it is bad for you.
I was told it doesn’t have enough support for someone with back issues
Any idea how the arch support in these is? I’m a high school middle-distance runner so generally my mileage is somewhere between 15-30 miles a week during track season. Most of the mileage comes from high-speed, low distance workouts (200m, 400m, 600m, etc) Anyway, right now I have the Nike Free 5.0+ and am considering these as my replacement shoes. I’m guessing the arch support isn’t great, given this is closer to a bare-foot shoe. If anyone who has high arches has tried these, I’d love some feedback :)
Not a huge amount of arch support, but you could always add in an insole that has some.
How different are the Nike Free 4.0 from the Nike Free 4.0 Flyknits?
Good question, I didn’t even know the non-Flyknit version of the 4.0 existed until yesterday. So I don’t know how they differ, but the non-Flyknit is cheaper.
Hey, Peter! Do you think the Nike Free 4.0 Flyknits 2015 version are good for activities other than running? Are they flexible enough with just the right comfort for doing gym workouts?
I wear the 4.0’s casually all the time. I don’t do much in the gym so can’t really comment on that usage, but they are one of my favorite shoes for non-running use.
Is the toebox wider on the Flyknit 4.0 or the Kinvaras? Thanks!
Just got them in a Black Friday sale for -60%.
I never ran the Kinvaras, but been trying the 5s, and 6s in stores regularly, and the narrow forefoot has always been the reason why I didn’t get them in the end
I have little wider than regular feet, and the Flyknit 4 are considerably wider than the Kinvaras. In fact, width hasn’t been an issue at all when I tried different sizes from 44-45. The fit is, like Pete addressed, very un-Nike-y, really spacious: Great heel, and midfoot lock, and a generous forefoot.
Thanks for your review Pete. When I saw them in the store, I fired up your blog, and quickly read through your review. At -60% the purchase was a no-brainer from then on. Only issue was the size. I have 27cm feet (42.5), and wear 43-43.5 in street shoes, and 43-45 with running shoes. The Flyknits are really long, and it took me 2hrs. to decide between 44.5 and 45. Went with the US10.5 (44.5) in the end, as I am planning on running them sockless from time to time, and want to swap the insole to something thinner after I adapted to the ride. Been wearing them casually and I’m glad I went with the 10.5 instead of the 11.
Take care!
As a bonus I have found the 4.0 Flyknit upper to be incredibly durable. Holding up great with daily wear at work and the occasional run. Thanks for checking in!
I like my running shoes flashy which is why I went with Bright Crimson/Black (2015 model). So they’re a no-go for daily wear at work.
After having run in them for ~50km now (5-8k sessions), I noticed that even with socks the laces in the most upper eyelets cause red pressure marks on my feet. I guess it’s because of the narrow, and unpadded tongue. There are no issues when lacing the shoes more loosely but in doing so they’re only good for very easy miles, as the ride gets very unstable then.
I tried to move the tongue over the area to have some fabric between my foot, and the lace/flyknit structure. Alas it was to no avail, as the tongue will center itself back during running.
I meant Flywire, not Flyknit. On proper inspection it appears to be not the laces but the Flywire that runs inside the Flynit upper that “digs” into my feet.
Same issue for me, only complaint I have about the 4.0 is pressure on the top of my foot from the laces/flywire.
On the 4.0
Hav8 months old great shoese been using the nike frees 4.0 for work jumping in and out of truck and also for running they might wear down but no quicker than other running shoes and they get better the more they wear down the material looks like new with no tears and coming up to
Hi Peter,
This was the article I was hoping to find. I’m not so much a real runner but I walk run. I have been loving these Nike 4.0 ‘s so much and I walk ran some 5 ks in them but now I’m training for the Gate River Run 15 k.
I’m scared and daunted.
I started searching around the net for information that wasn’t too technical, to see if these shoes can make it.Loved everything you said and even experienced a couple of the same things you mentioned (tag).
Anyway, thanks.
Thanks, and good luck!
The best thing about Nike Flyknit Rosherun Shoes is the comfort and fit. However i was pretty shocked to find the soles are poorly designed manufactured from foam rubber molding with no solid rubber grip.. hence the the soles ware easily and become slippy in wet surfaces. sadly i wouldn’t by them again.
Hello
I read your article and I feel pretty convinced about the shoe (except for the picking up stone bit but I’ll get over it) however I have one really major concern
That is how it will perform at everyday gym work I’m worried that especially weighted clay raises might just tear or break the soles. Please let me know on your verdict. Thanks.
I have not worn these to the gym, but they are my daily shoe at work and after almost 6 months of pretty continuous wear the soles still look pretty good, though one of the seam on the upper back near the heel has ripped a bit. But not enough that I can’t still wear them.
Cannot comment on the wear from own experience, a friend uses them in the gym daily, and works out pretty heavily. He says they hold up very well.
Regarding the picking up of Stones I’m a bit puzzled. I read about that in every Free review but have never had a stone in my sole once, despite running about 40k a week in the Frees on gravel, and gras.
Hey Peter!
I am not sure whether this page is still active or not but if it is, great! I bought the same pair of Nike Fre 4.0 in 2015 and have worn them ever since. BEST PAIR OF SHOES I’ve worn so far: durable, comfortable and overall stylish. However, by now they are well worn out and I am looking for an equally good replacement. Do you have any recommendations that come close in 2020? This year’s version looks like a crossover of a croc and diving shoe.