Vibram touts their FiveFingers KSO , rumored to stand for “Keeps S— Out,” as the highest selling men’s VFF product. The FiveFingers product line gives the functionality of barefooting while protecting the foot’s skin. It’s no surprise the KSO model is the best seller, as its additional coverage area makes the shoe viable over more varieties of terrain. Although it may not have been designed with distance running in mind, its extreme minimalism and adherence to an anatomical design make it the go-to product for a runner desiring natural foot function on abrasive surfaces.
Vibram’s rubber sole wears comparably to other shoes on the market made of similar materials. It is made of a harder rubber, but it is only 3.5mm thick, so it flexes easily. It is subject to the same laws of wear and tear, except that a runner’s footstrike is likely different in a VFF as compared to a standard shoe. Somebody who lands on the ball of their toes is likely to wear through a VFF at about the same rate they would a standard shoe of the same sole material. A heel striker may find their heel doesn’t strike as hard in VFFs and that the heel portion of the sole lasts longer. As your car dealer will tell you, mileage may vary. Typical running shoes have 24mm of sole and midsole to wear through. The KSO has about 5.5mm (3.5mm outsole plus 2mm insole). This may became important down the line if you have an affinity for grinding your shoes into dust.
The upper of the KSO is mostly designed to keep things from entering the shoe that would otherwise cause friction inside the shoe between insole and skin. Except for getting a little dirty itself, running in the KSO won’t age the upper. The shoe is also flexible enough that nothing is likely to rip, so the longevity of the shoe really does come down to wear on the sole.
The VFF is either a comfortable shoe or else an uncomfortable sock. The antimicrobial microfiber insole feels smooth and comfortable. The upper is not so comfortable, as its materials can cause chafing if your flexing within the shoe causes a hotspot. The shoe is open enough on the top to dry fairly easily, but sealed well enough on the bottom that moisture will not easily escape elsewhere. This can contribute to chafing.
The toes are one size fits most. It is not very easy to slip into the KSOs, but the feat becomes natural once practiced.
The KSOs have a Velcro strap that “laces” once over to hold the shoe to the ankle. Deciding how tight to make this has a great influence on how freely the shoe flexes, where hot spots may occur, and the degree to which stuff gets kept out of the shoe. You may or may not even want to use the lace. It’s nice to have options, right?
The minimalist argument is that you achieve more balance the closer you come to natural, barefoot running. VFFs promote a midfoot or forefoot strike, as their lack of additional heel padding gives the foot more space to square up with the ground before impact. The thin 5.5mm height of the shoe gives the foot an accurate reading of where the ground is. The foot’s strike, pronation, and roll off occur without the influence of cushion, posting, roll bars, or heel-to-toe differential. These are theoretically the most balanced shoes you can buy.
The shoes may not seem balanced to a runner unaccustomed to barefooting or minimal running. The absence of significant padding causes impact to be more pronounced. This typically leads to midfoot and forefoot striking to dampen impact naturally. A heel striker will either a) make this adaptation and find it taxing on the new muscles/joints/tendons being worked or b) continue attempting to heel strike and suffer great impact forces with no padding. The adaptation necessary to run routinely in VFFs for a rookie may require a transition period appropriate to the individual. Attempting to run in the shoe the same way a runner would move in a standard shoe is not advised.
VFFs grew in popularity as a result of runners seeking more minimal footwear. It took Vibram some time to actually respond to the minimalism craze by creating a shoe specifically for running. While the traditional lineup is known as viable for running, Vibram released the Bikila as, “…our first model designed specifically for a more natural running experience.” This shoe is the ordained successor to the KSO.
“In just over a year, the KSO has become our most popular model for men. Featuring a thin, abrasion-resistant stretch nylon and breathable mesh upper that wraps your entire forefoot to “Keep Stuff Out.” A single hook-and-loop closure helps secure the fit. A non-marking 3.5mm Vibram TC1 performance rubber sole is razor-siped for a sure grip, and a 2mm EVA insole enhances plating protection and comfort.
KSO IS BEST FOR: Running, Light Trekking, Climbing/Bouldering, Running, Fitness, After Sport, Water Sports, Yoga/Pilates & Travel”
I have run barefoot, in KSOs, in racing shoes, and in standard running shoes. I’ve done maybe 97% of my running in standard shoes, 2% in racing shoes, and 1% barefoot or in KSOs. It comes as no surprise to me that my body is least adapted to running barefoot or in the original Vibram lineup. I may have been born to run minimally, but I predominantly wore standard shoes through my impressionable years right on into adulthood.
In the spirit of “what doesn’t kill you makes you stronger,” all of this is a good step towards getting back to natural running form. The KSO gives a runner an opportunity to experience a feel similar to barefoot, but with some added protection. I’ve run barefoot and I prefer it over VFFs, as I don’t have to deal with anything touching my feet except the surface upon which I land. When this is flat grass, it feels soft and effortless.
So, one day, I showed up to my local track to find the field newly aerated. Dried clumps of dirt looking like goose poo littered the field. (Okay, I don’t really know what goose poo looks like, but the first aerated field I saw was in Van Cortland Park and there were like a million geese nearby, so I jumped to a creative conclusion.) The clumps were often pushed down into the grass or else soft and crumbly. I would occasionally step on a hard, defined nugget of dirt. This did not cause a problem if I stomped it with the lateral side of my foot or my heel. I would sometimes land as if the dirt clump were pressing directly into a tendon or nerve and I would have to hold in the urge to yell, “F—!” Wearing VFFs goes a long way towards limiting such pain.
VFFs aid the foot differently than standard running shoes. The midsole generally provides less cushion and the heel is usually closer to the ground. Impact is greater and the body may naturally try to mitigate this by performing soft forefoot and midfoot landings. This requires that the heel drop lower to the ground than it may have grown accustomed to in standard shoes, stetches the calf and Achilles, flexes the foot closer to the shin, and the foot springs forward through the arch rather than via rolling over the midsole. This is how the foot was designed to function, but not all bodies are ready to handle this, because you can’t go indoors to buy a Big Mac unless you’re wearing shoes.
The VFF KSOs weigh in at 5.7oz per shoe. This is on par with the lightest racing shoes available. I had no problem running at high speeds or over obstacles with a pair of KSOs on. The 5.5mm height off the ground, though anatomically curved, does not perform terribly differently from racing flats that sit a few extra millimeters off the ground. What am I getting at? The VFFs are not functionally very different from racing flats. What is different, though, is that only a handful of VFF-like shoes are on the market, whereas numerous companies have strong established lines of racing flats and spikes. It’s not difficult to find the same performance and protection at a better fit or for a better price from any of these other companies. Thus, I can only get behind VFFs as being another available option.
Veteran barefooters will not require much transition, as their muscles are already adapted to a similar running form. Shoe wearers will need to undergo significant physiological adaptations. Most shoe wearers will still want to wear shoes for daily activities, too. Maybe this helps the transition from being too much too soon. Or maybe this just makes the foot function less than optimally in both maximal and minimal footwear. Tough call. Either way a person tries to make the transition, though, it is necessary to limit periods of stress and allowing for the recovery necessary to manifest the adaptation. This is a difficult line to ride when minimizing injury. The transition is arguably more difficult for high mileage or heavy runners, as the repetitive stress to the muscles adds up more quickly, necessitates more recovery, and makes the transition required longer. Low mileage runners may be able to tolerate this without it impacting (pun!) their training; high mileage runners would likely be overstressing their bodies to try and make the transition without significantly reducing training volume. I’m quick to recommend VFFs to veteran barefoot runners seeking added protection, but hesitant to recommend them to lifetime shoe wearers.