“Bigger is better” is the winning formula for carbon plated running shoes. The higher the stack of soft and springy foam underfoot the better the shoe performs, especially in longer events like the marathon. To ensure a certain degree of fairness, World Athletics set a 40mm stack height limit for legal road racing shoes in 2020 that elites have to stick to.
That limit does not apply to training shoes, though, or indeed to amateur runners, which is why manufacturers have continued to develop bigger, bouncier shoes like the Adidas Adizero Prime X, which launched last year with a 50mm stack.
Adidas built the shoe to serve as a long-distance training shoe for elite runners, and a racing option for amateurs who don’t need to worry about being thrown out of races for using illegal shoes (unlike Ethiopia’s Derara Hurisa, who was disqualified after winning the 2021 Vienna Marathon in the Prime X).
The new Prime X Strung replaces the woven upper on the shoe with the Strung upper that was first seen on the FUTURECRAFT.STRUNG concept shoe two years ago. The upper is made by layering yarns to provide more support where you need it around the heel and midfoot, then a more breathable section in the forefoot. Instead of needing multiple parts, the upper is one piece which is designed to hug the foot like a cocoon.
“Strung should feel closer and has a nicer, more integrated fit [than the previous Prime X upper],” says Simon Lockett, global category director of running footwear at Adidas. “It’s built on the knowledge we’ve gathered over time of foot shapes and foot movement patterns. This essentially should feel at one with the foot.”
Beneath the Strung upper the Prime X has a midsole packed with Adidas’s top tech. The foam used is Lightstrike Pro, a lightweight and springy material that is paired with carbon EnergyRods that run under the forefoot, along with a small carbon plate at the heel of the shoe. Beneath the rods in the forefoot are EnergyBlades, strips of TPU plastic to add even more propulsion to your toe-off than you get from the rods alone.
The result of all that cushioning and propulsive tech is a fast and comfortable shoe that’s versatile enough to use for a variety of runs, but designed for the long haul in particular.
“It’s our fast long run shoe,” says Lockett. “Our elite athletes wear it for long sessions, because it’s nice on your joints and it’s giving them something a little bit extra during those runs as well. Then they would move to an Adios Pro or a Takumi Sen for race day.”
Whether amateur runners will use a shoe that’s illegal by World Athletics standards in mass participation races remains to be seen. Personally, I wouldn’t want to wear a shoe that doesn’t meet the regs on race day, but I’m excited to test the Prime X in training for a Coach review and see if it really is a step up in performance on less-than-40mm shoes like the Adidas Adios Pro 3. There’s certainly an allure to the idea of running in a shoe that’s so good elites aren’t allowed to use it.
The Adidas Adizero Prime X Strung is available now and costs €275. US and UK pricing is still to be confirmed.
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Nick Harris-Fry is a journalist who has been covering health and fitness since 2015. Nick is an avid runner, covering 70-110km a week, which gives him ample opportunity to test a wide range of running shoes and running gear. He is also the chief tester for fitness trackers and running watches, treadmills and exercise bikes, and workout headphones.