Will These Pace-Tuned Kicks Change How We Buy Running Shoes?

This article originally appeared on Outside

Runners need more help than ever wading through the bewildering array of footwear choices and finding the right shoes for them. Gone are the convenient categories of yore: neutral, stability, and motion control--and good riddance, as science has shown they don't do what they had promised. But what we're left with is vague advice: find shoes that feel right for you. That's sound, but it doesn't give any guidance on where to start the search.

Ideally, every runner would have the chance to run in every shoe to find those few that give their feet wings and fill their hearts with joy. But that's impossible for most of us. The reality is more like dating: we pick one that looks nice or comes with a recommendation from a friend and hope we're compatible. If the relationship falls flat, we try a different one.

A new shoe company called Vimazi is trying to change that. The company's proposition? We should choose our shoes based on how fast we run. The brand's seven models (all $175) are sorted by the pace you would typically use them for, ranging from the Z20 (tuned for a 4:30 to 5:30 minute per mile pace) to the Z80 (tuned for running 12:00 to 15:00 minutes per mile). Vimazi's founders, Scott Tucker and John Zilly, claim their shoes are "scientifically engineered for maximum performance within a pace zone."

Defining a Pace-Optimized Shoe

What does it mean to optimize a shoe for a certain pace? Don't ask Tucker and Zilly that unless you've set aside several hours for the answer. After three long conversations, complete with whiteboard diagrams, a deep-dive into physics, calculus, and a bit of Einstein's special theory of relativity (I kid you not), I understand their thinking enough to roughly explain their strategy.

At the core, Vimazi optimizes each shoe for different paces by tuning the relative softness of the midsole foam in the heel and forefoot. These densities are strategically calculated and strictly controlled (they introduced new measuring protocols at the Asian factory to get the fine tuning required between each model).

Tucker, who has degrees in physics and atmospheric sciences as well as decades of experience in the shoe industry, claims he can calculate the exact forces and timing of impact for each pace range. Using these calculations, Vimazi tunes the density of the heel to compress as smoothly and maximally as possible, while adding a forefoot "fast-pod" insert that is slightly firmer, to support the greater forces of propulsion and minimize energy loss at push-off. With more speed comes greater forces, so models designed for faster runners are tuned firmer than those for slower ones.

It's important to note that within their definition of "optimal," Vimazi founders' first assumption is that running shoes exist to provide cushioning. "We started with the belief that cushioning is a good thing," says Tucker. "Having that belief, we want to dissipate energy in a smooth way so that the peak force at any instant is less--plus make it so the shoe doesn't feel mushy. What we have done is applied engineering to the problem to solve the smoothing out of the deceleration on impact, as opposed to ballparking it, which is what is done otherwise."

Complicated Calculations

It's not, however, quite that simple. Calculating the forces of a running stride requires knowing more variables than simply pace, said the two biomechanics experts I talked with about the pace-tuning concept. Not everyone agreed on exactly which variables are necessary to know the intensity, timing, and location of stride forces, but all--including Tucker--concur that you at least need to know weight, cadence, and stride length. Vimazi's sort is based solely on running speed, so none of these variables are known, requiring Tucker and Zilly to make some generalizations about runners at different paces.

Since the runners' weight is unknown, Tucker says, "We made the assumption that runners in the Z20 are lighter than the runners in Z70s. So the Z20 fast-pod is not as much firmer as you may have expected because we adjust for weight." If, however, you're a larger runner who cranks out fast miles, or a featherweight who runs slowly, this will change the force calculations and may make a shoe supposedly tuned to your pace less than optimal.

Then there's cadence. Cadence and stride length combine to create speed, but those who take more frequent steps put less force into the ground with each footfall. Not knowing the runner's cadence, Tucker says that they used 180 steps per minute for all runners at all paces, thus assuming greater stride lengths, and forces, for those going faster.

That's a problem, says Bryan Heiderscheit, co-director of the University of Wisconsin's Neuromuscular Biomechanics Lab, because cadence not only often varies for runners going the same speed, most runners also change both their cadence and their stride length when speeding up. "Obviously step rate changes with speed," Heiderscheit says. "It is highly, highly associated with speed."

Geoff Burns, physiologist at the U.S. Olympic Training Center in Colorado Springs and biomechanics researcher, concurs with Heiderscheit that cadence can vary significantly for runners moving at the same pace, and that step rate is correlated with a runner's speed. Burns adds that runners also differ in ground contact time and flight time at the same pace--which changes the timing and amount of peak forces as their feet interact with the shoe.

Tucker acknowledges that if your cadence varies significantly from 180 steps per minute that your forces will also vary from his calculations, and might make a shoe they've designed for a different pace more optimal. I, for example, tend to run in the high 160's at a nine-minute-per-mile pace, thus putting more force into the ground with each step than a runner who strides at 180 steps per minute at this pace. As a result, the Z50 (supposedly tuned to 7:15-8:45 pace) feels better than the Z60 (geared for an 8:30-10:30 pace). This doesn't surprise Tucker, who would consider me an outlier.

Beyond concerns about cadence and weight assumptions, Burns raises even larger concerns about the Vimazi calculations. While he admits you can estimate the overall peak force of a runner's stride at mid-stance over the whole foot, he says that even if you had all the variables you couldn't calculate the specific forces at one point in the shoe because of individual variability in how people move through the stride.

"Force compression through a foot strike is very much like a fingerprint," Burns says. "When you watch compression through a shoe, it is different for every person." Trying to tune the heel and toe of one shoe for all runners moving at a given pace is impossible, he says.

Burns also questions the Vimazi assumption that making the foam firmer is more beneficial for runners at faster speeds. This may have been true for older foams, he says, but today's bouncy, highly-resilient foams have proven to be effective at fast speeds even when they are very soft and compressive.

Vimazi uses a traditional EVA-based foam, primarily because its density can be tuned more accurately. But, Tucker says, "Not using Pebax or other foam didn't mean that the result we got was worse--we get the same performance in our design, in terms of cushioning and efficiency at propulsion, regardless of polymer." Tucker allows that different foams have a different feel, which may affect how you interact with the shoe, but he believes the foam is irrelevant when it comes to performance. "It's density that matters, not the polymer," Tucker maintains. Burns strongly objects. "That makes sense only if we lived in a world without any other polymer," he says.

Putting Them Through Their Paces

Equations, assumptions, and beliefs aside, what matters is how the shoes perform.

I tested five of the seven models--all but the Z30 (5:15-6:45 pace) and Z80 (12:00-14:00 pace)--on daily runs of three to six miles, plus all together in one session where I ran a half mile in each shoe back to back, hitting a range of speeds from a jog to a sprint, noting at what speed they felt best for my stride and how they felt at their optimized speed.

Overall, the faster-tuned models felt better at faster paces and slower-tuned ones rode smoother when running slower--and each felt best roughly in the pace ranges they are advertised for. I wasn't sure, however, whether that was due to the underfoot tuning of the foam or the differences in geometry and upper construction as I moved up the array of models.

As the intended pace range gets slower, Vimazi gradually increases the stack height (from 25 to 30 millimeters), the drop (from four to six millimeters), the platform width, heel counter structure, and stiffness of the upper. The reason, says Zilly, is that slower runners, "tend to run fewer miles and have less fitness physiologically--so we make a wider platform for them, and add stability, which adds weight. We make the call that that is something a large percentage of those runners like in their shoes." It's a common tactic, but one that may or may not hold true if you are, for example, a slower runner who likes a more minimalist construction.

For me, the Z40 and Z50's less-structured, comfortable uppers made me gravitate toward them regardless of the ride. They're made of a smooth, stretchy mesh with small lateral overlaid stripes and a flexible, molded heel. In the Z60 and Z70 the overlays are thicker, making the upper stiffer, and the heel is wrapped with a rigid plastic external counter--all of which reduced the comfort and natural movement for my foot. On the other end, the Z20's upper is a thin, non-stretchy mesh that wraps the foot tightly. It felt appropriate for the high-speed turnover and powerful strides that the shoe is made to support, but made me feel squeezed onto a too-narrow platform.

The Z50 also felt best underfoot when running my usual easy pace of 9:00-9:30 minutes/mile, even though the Vimazi optimization says that pace means I should fall squarely in the Z60's range of 8:30-10:30 minutes/mile, not the Z50's 7:15-8:45. On my feet, the Z50 rolled nicely through the stride and delivered a combination of cushioned comfort and ground feel that felt natural and enjoyable. In contrast, the Z60 felt mushy to me and lacked the responsive ground feel; it felt like I was struggling to get into the air with each stride.

This can be explained by my lower cadence at this pace than assumed by the Vimazi calculations, as discussed above, and, I'm guessing, may also be because I'm a lighter weight than assumed for my pace (Vimazi didn't reveal their weight ranges, but I'm slower these days mostly because I'm older, not heavier). Interestingly, the Z50s still felt best to me when I dropped the pace to low 7's and even down into the 6's--again likely explained by my cadence, which goes up 20 or more steps per minute at those paces. The faster-tuned Vimazi models also turned over nicely at quicker paces, but, for my stride, felt firmer and less smooth than the Z50s.

The Bottom Line

Overall, all of the Vimazis have a soft, but not squishy heel, and a slightly firmer forefoot that still has some impact-smoothing cushioning (think a Brooks Ghost with a slightly more responsive forefoot). Runners who like a more traditional underfoot feel of smooth, non-bouncy damping will appreciate the Vimazi ride--some models more than others.

Which model will feel best to you seems to depend on your weight, cadence, stride mechanics, and preferences in cushioning, geometry, structure, and fit--as much as it does your pace. Vimazis are worth checking out for their unique ride, and their pace ranges provide a place to start your exploration, but it's likely you still will have to try on a variety of models and assess what feels best for you, just like the rest of the shoes on the market.

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