Everything you need to know about 'illegal' running shoes

super trainers
Rise of the 'illegal' running shoesTrevor Raab

Banned shoes emerged courtside long before they found their way onto a marathon course. Legend has it that nearly 40 years ago, former US basketball player Michael Jordan laced up a pair of red-and-black high-tops in violation of the NBA’s approved uniform colours.

In 2010, the league cracked down again — this time, for reasons besides aesthetics — on a pair of basketball shoes that were shown to increase a player’s vertical-jump height. Today, Nike’s Air Jordan brand is a household name and multibillion-dollar business, and shoemaker Athletic Propulsion Labs (APL) has since released additional banned styles (Superfuture and Concept X) of its performance-boosting kicks.

Though some speculate these bans were mostly crafty marketing, the same can’t be said about the wave of 'illegal' shoes that's flooding the roadways on the feet of runners.

Super shoes have proven a true performance boon to both elites and recreational runners on race day. Now, the tech has trickled down to shoes for daily runs, ushering in super-thick, though still permissible, shoes that have created their own category: super trainers.

What is a super trainer?

Technically, a formal definition doesn’t exist. But we’ll establish some general parameters.

Super trainers share many of the same qualities as super shoes built for race day: a tall stack height, efficient midsole geometry, high-powered super foam, and usually some sort of plate. But these trainers aren’t meant for racing, like a Nike Vaporfly or Saucony Endorphin Elite — they’re designed for everyday mileage.

Super shoes for racing have to balance cushioning and propulsion, but still be lightweight. Super trainers, though, can have beefier constructions that help to extend their life spans. For example, shoe brand On guarantees only four marathons from the Cloudboom Echo 3 — that’s little more than 100 miles. Super trainers can handle more mileage, and should have the typical 300 to 500-mile range of a normal running shoe.

on cloudeclipse
On’s Cloudeclipse super trainer, seen here, can handle more mileage than the brandTrevor Raab

To do that, they may have more outsole rubber for durability. Their uppers are usually softer and thicker than a racing shoe’s, too, with comfortably cushioned ankle collars and tongues. They tie up with sturdier laces and offer a more forgiving fit overall, closer to what you’d expect from a workhorse daily trainer. And, since super trainers aren’t made to toe a starting line, brands can pile on midsole foam in excess of the 40mm limit imposed by World Athletics.

But they’re more than just extremely cushioned running shoes. Super trainers have additional mechanisms and 'super foams' geared toward running efficiency and energy return.

The anatomy of a super trainer

If we break down a super trainer into its key components, we see a recurring theme: a stack of super foam in the ballpark of 40mm (or more), a carefully sculpted midsole shape for propulsion, and a plate that harnesses those pieces and holds it all together.

This is the tech that puts the 'super' in super trainers; it needs to interact with the foot and the rest of the kinetic chain in specific ways to deliver the intended ride and energy return.

Here’s what’s happening inside the midsole under your foot, and how these elements work to boost your stride.

1. Rocker Geometry

When we talk about a rocker sole, we’re describing the curvature of the bottom of the shoe’s sole. Both the heel and the toe curve upward, away from the ground, to give the shoe a more rounded base. The result is a shape like that you’ll find on a rocking chair. This is needed because the shoes are so thick, they don’t bend under the forefoot for a smooth transition during your gait cycle.

Shoe developers can also manipulate the inflection points of the rockers around a runner’s pace and foot strike to potentially manipulate transition time. Usually, you’ll find the most aggressive rocker designs in racing shoes. Still, pronounced rocker soles on super trainers are essential to make transitions comfortable.

adidas adizero prime x 2 strung
The Adidas Adizero Prime X 2 Strung has a pronounced forefoot rocker.Trevor Raab

→ How do rocker soles work?

Rocker soles in shoes function to mimic the natural rockers of the foot to make us feel more efficient. The foot has three functional rockers — the heel (first), ankle (second), and forefoot (third) — that we use while walking and running. The heel bevel in a shoe prepares the foot to contact the ground and smooths out the shoe’s ride when you land. Forefoot rockers assist movement during toe-off, which generally means the toes and ankle joint don’t have to work as hard.

For some runners, a stiff rocker sole can decrease demand on the calves by transitioning effort upward to the knee and hips. A runner’s experience will depend on their specific biomechanics, where the rocker is placed, and how their footstrike contacts it.

2. Super Foam

Polyether Block Amide, or PEBA, is a common foam compound for super trainer midsoles (a popular trade name for the material is Pebax, made by a company called Arkema). It’s the basis for foams like Nike ZoomX, Saucony Pwrrun PB, Puma Nitrofoam, and Asics FF Blast Turbo. Many of those brands leverage the same foam used in their top-tier racing shoes for their super trainer models.

Right now, PEBA-based foams lead the way for energy return. While current research indicates that supercritical EVA and TPU blends are also impressive, PEBA has the edge more often than not. Why? It’s lighter than TPU and far softer and bouncier than supercritical EVA. Some iterations are firmer than others, depending on which brand’s flavour you try.

a person's legs and shoes

→ How does foam work?

Cushioning serves to absorb shock upon landing and provide overall comfort while running, but it can also return energy to the runner and stabilise the shoe. PEBA foams are especially lightweight, so brands can pile it on — both in thickness and in width — without the same weight penalty applied when using EVA or TPU materials. The additional width is just as important as the height. These shoes often need a wide platform of midsole material to remain stable.

Every brand’s PEBA-based foam compresses at a different rate, with varying amounts of responsiveness, so each offers a unique benefit based on the runner. For example, runners who land with lots of force can bottom out and crush a foam that’s too squishy, so they may prefer a firmer platform. A harder midsole can offer more support without any additional stability elements. On the other hand, a softer foam can provide more comfort, and research suggests that it may have a protective benefit for lighter runners.

Keep in mind that dozens of pieces within a shoe, as well as countless runner-specific characteristics, come together alongside a super foam to deliver its specific ride and response. It’s not always possible to parse each element individually.

3. Plates

Not every super trainer has a plate in its midsole, but it’s increasingly common. That’s because — regardless of whether it’s made from TPU, EVA, PEBA, carbon fibre, or a mix — a plate stiffens the midsole and can add support. It may also give the ride some extra pop, but it largely functions to stabilise the shoe. The Adidas Adizero Prime X 2 Strung, which has a heel stack height of 50mm, requires two stiff carbon-infused plates.

Some shoes need less support and utilise pliable materials with thinner rod constructions or forgo full-length plates for ones that extend only halfway along the midsole.

new balance fuelcell supercomp trainer v1
The New Balance FuelCell SuperComp Trainer v2 and v1 (shown above) both use carbon-fibre midsole plates.Trevor Raab

→ How do midsole plates work?

While plates can add propulsion, research suggests that the true hero is the foam itself. A plate mostly serves to 'facilitate motion and add stiffness to the sole to balance out the softer compliant foams,' says Matt Klein, founder of the Doctors of Running website and podcast.

He explains that the effects of stiff-plated shoes are not clear-cut. 'Everything we know [about super trainers] is applied from super shoe and maximal shoe research. These shoes shift work up to the knees and hips. The ankles may do less, but that doesn’t always apply. If the shoe is too stiff, the ankles will actually end up doing more work. But once a midsole gets so thick, it may not need a plate; the amount of material naturally stiffens the ride.'

What are the benefits of super trainers?

Simply put, these shoes are so much fun to run in. They can lend a bounce to your stride that feels like bounding on a trampoline. And since the springy sensation and high energy return can help you move more efficiently, running fast feels easier.

Plus, a review of 63 research studies concluded that these trainers can also help attenuate shock and dampen some of the hard impacts from training. So, runners recover from hard efforts more quickly; they’ll lace up the next day feeling fresher and less sore than if they’d worn regular running shoes. And for those eager to increase mileage, this may allow some runners to increase their weekly workload with less fatigue and fewer aches and pains.

saucony kinvara pro
Trevor Raab

But, the recovery boost from super trainers can make it tempting to tack on too many miles too soon or overcook the pace on easy days. 'Just because you are less sore does not mean you are at a lower risk for injury. You can still get injured with overdoing things,' Klein says.

Our wear-testers report that they’re able to run faster paces with less effort in super trainers, but it’s still important to keep easy jogs relaxed. Double-check your training log and pull back on the reins if you notice spikes in mileage or intensity outside your typical training volume.

Are there any risks to super trainers?

Though they feel great, super trainers can have an impact on our natural gaits and biomechanics.

'One of the injury risks for this shoe type is that it is so different from normal mechanics and footwear. The current evidence from super shoes and super-max stack-height shoes is that we tend to stiffen our legs in taller, softer shoes,' Klein says. 'This is not bad short-term, but some evidence suggests long-term super-max stack-height shoes can actually increase joint loading due to that stiffening.

'I suspect running in them a ton will cause less joint motion, as the body will stiffen up to find stability as shock absorption isn’t needed. This may be great for a recovery run/faster run, but trying to transition back to normal shoes can take time if you only run in these and super racing shoes.'

Klein recommends making the transition slowly to avoid injury and keeping more than one pair of running shoes in your rotation.

'I do suspect that with the workload being shifted up to the knee and especially the hip, there may be an increase in injuries in those areas. The hip still has to work harder being on a softer, less stable surface. So, those with hip instability and strength issues may want to avoid super shoes, at least until these issues have been addressed,' Klein says. New runners should focus on building strength and experience before experimenting with super trainers.

There is little doubt that these shoes have effects at the point where our feet meet the midsoles as well as farther along our kinetic chains. Super trainers shift more of the workload of running to the hamstrings, which means that you won’t engage the calves and lower leg muscles as much. Moving some of that effort to larger muscle groups can be beneficial for some runners, like those who are struggling with calf or Achilles tendon injuries. But those who have a nagging hamstring strain may need to be wary. Whether or not it’s your goal, those upper leg muscles will receive more of a workout.

At the very least, one thing can be said for sure. It’s that all the foam underfoot is going to put a lot of distance between your foot and the ground. This limits the amount of ground feel you receive from the shoe. It can be easier to wipe out if you’re hopping a curb on a night run. And on rain-slicked pavement or technical trails, it can be tough to steer a thick and bulky midsole.

If you’re prone to ankle rolls or need a keen sense of where and how your foot is landing whenever you touch down, a super trainer might not serve you well.

Will I really get disqualified in a race?

It depends. If you’re an elite runner, absolutely. But if you’re going for a BQ attempt, it’s unlikely. Ethiopian runner Derara Hurisa won the 2021 Vienna Marathon in 2:09:22 but was disqualified when race organisers found he’d raced in the Adidas Adizero Prime X, a different shoe than he’d declared on his pre-race form.

Our favourite super trainers

Running shoes aren’t cheap. Maxed-out midsoles with top-end shoe tech only add to the bill. Most super trainers will cost you £200 at least.

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