Gear & Technology, Health & Injuries, Podcast, Science and Physiology

Dustin Joubert, PhD & Dr. Amol Saxena, DPM | EP#383

 March 27, 2023

By  Bernardo Gonçalves


Amol Saxena & Dustin Joubert - That Triathlon Show

We have two interviews in today's episode. First, Dr. Dustin Joubert discusses new research on the effect carbon fibre plated "super shoes" have at slower speeds and how it differs from at faster speeds. Then Dr. Amol Saxena shares a case series of how rapidly transitioning to running in super shoes may increase the risk of navicular bone stress injuries.    

In this episode you'll learn about:

  • The running economy benefits of super shoes at 10 and 12 km/h, and how it compares to the previously studies speeds in the 14-16 km/h range
  • More non-responders to super shoes at slower speeds?
  • How the observed running economy improvements translate to performance gains at marathon times in the 3:30 to 4:15 range
  • Navicular bone stress injuries: causes, symptoms, diagnosis and treatment
  • A case series of how rapid transition to training and racing in super shoes may be linked with navicular bone stress injuries
  • Advice for safely transitioning into running with super shoes

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Dr. Dustin Joubert

Dustin's background

02:53 -

  • I'm an exercise physiologist at St. Edward's University in Austin, Texas. I've been here for a year and was in Stephen F. Austin State. 
  • University where I've done some of my previous work.
  • I'm a long-term runner, always interested in endurance exercise performance and with the new shoes.
  • I will do the Boston Marathon next month and use the original Alpha Fly.

Running economy benefits at slower speeds

04:24 -

  • In previous studies, we tested shoes at 16 km/h, common in other research.  
  • Even the slower speeds, the tests were at 14 km/h, a 3-hour marathon. However, many people run marathons at slower paces than that,
  • Therefore, we started to question the benefits of shoes at slower speeds. And so, we got 16 subjects for this recent study to test at both 10 and 12 km/h.
  • With all the running economy research, you have to test slower than the lactate threshold because you might not reach steady-state oxygen consumption within the five-minute window when you get into the heavy-intensity exercise domain.
  • Our inclusion criteria were people capable of running 23min30s for 5km. So, it was a slower group.
  • We looked and checked we were below 4mmol. 
  • But more importantly, we could see their respiratory exchange ratios below 1, which allowed us to visualise oxygen consumption and reach a steady state in that five-minute window. 

Shoes and Testing protocols

08:00 -

  • Our starting point was the most tested shoe out there, the Nike Vaporfly line. (Vaporfly Next%2)
  • So it made sense to go with one that we knew worked well at faster speeds to investigate at slower speeds. 
  • The control shoe with standard EVA foam, no plate, and similar mass was the ASICS HyperSpeed. 
  • The standard running economy protocol is five-minute stages. 
  • Ideally, you want to test each shoe in duplicate within the same day to limit the variability of your equipment and your person. 
  • To test the shoe at 10 km/h, we would do 4x5min reps at 10 km/h with 5min breaks. We would test the shoes in an ABBA sequence or a BAAB sequence. 
  • Therefore, you would eliminate any sequence effect. 
  • We did all eight trials on the same day, which is excellent for doing the study and limiting the variability that you would see otherwise between days.
  • 08:00 -

    • Our starting point was the most tested shoe out there, the Nike Vaporfly line. (Vaporfly Next%2)
    • So it made sense to go with one that we knew worked well at faster speeds to investigate at slower speeds. 
    • The control shoe with standard EVA foam, no plate, and similar mass was the ASICS HyperSpeed. 
    • The standard running economy protocol is five-minute stages. 
    • Ideally, you want to test each shoe in duplicate within the same day to limit the variability of your equipment and your person. 
    • To test the shoe at 10 km/h, we would do 4x5min reps at 10 km/h with 5min breaks. We would test the shoes in an ABBA sequence or a BAAB sequence. 
    • Therefore, you would eliminate any sequence effect. 
    • We did all eight trials on the same day, which is excellent for doing the study and limiting the variability that you would see otherwise between days.


10:20 -

  • We had seen the Vaporfly brought about 2.7% economic benefit in faster runners, consistent with what you see in the literature.
  • In this study, we only saw a benefit of around 1.5%, so it slightly reduced compared to the faster speeds.
  • At the 10 km/h, a little less than 1% benefit.
  • In the paper, we hypothesise two mechanisms. 
  • My main thought was on how much you're compressing the foam.
  • So there are new, highly compliant, resilient foams made of Piba.
  • Material properties are a significant advancement, but the foam only returns the energy you put into it.
  • If you run at slower speeds, you're putting less energy into the shoe with each step, so you only get back what you put in. 
  • It's also possible that the control shoe with a standard EVA foam may not be as much of a detriment at slower speeds because that foam is good enough at the lower forces put into it.
  • Carbon plates may be more beneficial at faster speeds because of the increase in longitudinal bending stiffness.

Lower running economy with the Carbon plate shoes

13:25 -

  • In our study, where we tested at 16 km/h, there's a range in responses regarding the magnitude of benefits people get in the shoe, but everyone had a beneficial response.
  • At the slowest speed, five of our 16 subjects had a decrement in the economy. 
  • Our average group data shows that there's probably a benefit across this whole group with this shoe at slower speeds. But there's a chance you're one of these people with a negative response. 

Shoe weight

15:01 -

  • We added 13 grams to the VaporFly.
  • In specific laboratory experiments, it makes sense to have the same mass. In this study, I thought it did. 
  • In our previous study, where we were comparing all the different super shoes on the market to one another, it didn't make sense to mass-match things there because you will compete in that shoe as is. 
  • If you run in the Vaporfly and it doesn't feel clumsy and uncomfortable, you may not be one of those non-responders. 

Running economy and heart rate as a proxy for oxygen consumption

18:31 -

  • Heart rate is a little noisier of a signal than a VO2 on a good metabolic cart.
  • Heart rate tends to drift up more across the sequence. 
  • Suppose you look at our data on the super shoe study. Because the variability in the metabolic cards is so low, you can make conclusive 
  • statements or statistical conclusions easier there.
  • The heart rate differences exist across a broad range of shoes, but it's harder to quantify. 

Markers of muscle damage

21:23 -

  • The anecdotal evidence of these shoes is that it causes less muscle damage. 
  • In the long run, athletes are less sore the next day, or the calves aren't as sore after a hard session.
  • There was one conference symposium proceeding abstract from the Nike research group. 
  • In the Portland Marathon, the group who had worn the Vaporfly had fewer blood markers of muscle damage. 
  • Another publication in MSSE was from Matthew Black and Andy Jones's lab at Exeter. 
  • They did biopsy work and all the blood markers. They did laboratory-damaging running exercise. (downhill running protocol)
  • They didn't see any differences, but maybe it was their design. 
  • So we need more research to make a conclusive statement on the extent of muscle damage in these new shoes versus the standard shoe. 
  • Even if the economic benefit is small, you will maintain pace better if you have less muscle damage late in the race.
  • If we can measure the economy late after a long session, those differences become more disparate. 
  • In that Black study, they attempted to do that but caused the same amount of muscle damage. And they did the economy follow-up testing on the damaged muscle 48 hours later, so it wasn't at the end of the two-hour session. 

Takehome message

25:01 -

  • If you're running at slower speeds, you'll still benefit from the Vaporfly. But only some people are going to get a positive benefit. 
  • Translating the economic benefit to a velocity improvement is more of a one-to-one percentage comparison at slower speeds. 
  • If you're looking at the 1.5% economy benefit, it will translate to three minutes marathon time improvement, which is still substantial.

LabRat research

27:29 -

  • Another thought in closing the study: we tested subjects with only two people over 70 kg.
  • Therefore, they might compress the foam less.
  • It would be interesting to see if that hypothesis on the foam and ground reaction force were relevant.
  • We just wrapped up another university project, the benefits of super spikes. 
  • We had the Dragonfly, the Avanti from Adidas, and control spikes.
  • We've got some results on that coming out. 
  • Since we talked last, many case studies have been on that Instagram page. 
  • I've got some other university projects coming up.
  • Most of the research on these shoes has been on a treadmill. We're going to do some road testing with a portable metabolic cart.
  • In our original super shoe study to assess the state of the market, we had differences across brands and models. But the new iterations of shoes probably make that difference lower.
  • Within the next year or so, we'll like to do another assessment of 
  • the market. 

Rapid fire questions

32:07 -
What's your favourite place to train?
My favourite place to train is in Austin, Texas.

What bucket list race or event would you want to do?
I left my triathlon racing, but Kona would always be a bucket list event. I want to run New York.

What would it be if you could acquire an expert level in any skill in the world for yourself in an instant?
I would be an expert biomechanist because I'm a physiologist, not a biomechanist.

Dr Amol Saxena

Amol's background

36:58 -

  • I'm a paediatrician in Palo Alto, California, specialising in sports medicine. 
  • I've been in practice for about a third of a century. 
  • I've treated about 100 Olympians and operated on three Olympic gold medalists.
  • I've published about 150 articles on foot and ankle-related issues. And I'm keenly interested in helping athletes to get back to sports.

The potential risk of bone stress injuries by using carbon fibre-plated shoes

37:58 -

  • I have had an interest in navicular stress fractures.
  • I published an article in 2000 about a navicular stress fracture pattern and came up with a classification that seems correlated with the outcome.
  • Your navicular stress fracture level will predict how long it'll take you to get back to sport or whether you need surgery. (SIXENA classification) Sometimes doctors contact me for advice about treating these injuries.  
  • I had navicular injuries, and it was in the carbon fibre shoes. And they're relatively dramatic or acute.
  • And navicular injuries are slow to get diagnosed. 
  • I had a paper in 2017 on the series of surgery on athletes, and they often take almost nine months to diagnose. 
  • Doctors try to figure out why people have vague ankle or midfoot pain.
  • I shared my conversations with Dr. 1040, who then, in turn, shared his conversations with Dr Hollander, and he's in contact with Tim Honig.
  • We found some similarities in midfoot injuries. 
  • I've seen other midfoot injuries with these shoes that seem atypical. Some other stress fractures in the metatarsals that are slow to heal and in the other midfoot bones that are not so slow to heal, as well as plantar fascia ruptures and posterior tubule tendon ruptures, are noticed in the carbon fibre shoes. 

Navicular injuries

40:41 -

  • The navicular bone is the arch bone. 
  • Your arch is on the high point of your foot, and the highest point is the navicular, just beyond your ankle bone or your talus.
  • It gets a lot of undue stress, sometimes resulting in a bone stress injury.
  • A stress fracture is problematic because if you jump off a bench ten times and then get an MRI, you'll show stress to your knee and midfoot. 
  • With my classification system, we try to classify them to show how bad they +are, which in turn determines the treatment and how long it'll take to get better. 


41:52 -

  • I had one when I was in pediatric school almost 40 years ago. It came on very acutely, and it felt like someone took a chisel just into the top of my foot. 
  • The two cases I provided in the series were very acute.
  • The issue is that some people have quick, rapid, acute pain, and then it reduces.
  • These are essential and potentially career-ending injuries because the bone does not have a good blood supply. (slow healing)
  • As it doesn't have a good blood supply, the bruising and swelling you see with other fractures in the foot don't occur.
  • Many people might brush it off and not care because it is "just" arch or ankle sprain pain. 
  • It is why it often takes eight or nine months to diagnose. So it's essential to know about this because the sports medicine docs must have a high suspicion index. 
  • So if it hurts on top of your foot, we call it the N-spot for navicular. (it hurts to jump up and down on your toes, particularly barefoot)
  • Patients often say they can run 70 miles a week, but it hurts when I do speed work. 

Treatment and rehabilitation process

44:25 -

  • First, you got to understand you've got a severe injury, and it can take a minimum of three months to get better with type I injuries based on the classifications. 
  • Type II is a crack from the top extending into the bone. And then type III is a complete fracture through the bone. 
  • In 2006, we developed a type 0.5, which is not entirely broken but a bone stress injury. 
  • A CT scan is the best diagnostic test scan because the MRI misses stress fractures often. It doesn't show fracture detail. 
  • The treatment for type 0.5 and type one is generally non-surgical, non-weight bearing for three to six weeks.
  • People will only use crutches, and their foot hangs down. If their foot hangs down, they have tension on top of their foot, and bones do not heal with tension on the top of their foot. 
  • They need to be in a boot at 90 degrees and non-weight bearing minimum. 
  • For type one, it will be six weeks, and then you walk in a boot for another four weeks and get back to activity. 
  • For type two, which is the partial fracture and type three, complete fracture, your bone will either not heal, re-break or go on to having arthritis in the joint. 
  • And that was the situation with Paula Ratcliffe.
  • The treatment for those would be surgery and sometimes some bone graft if it's an older injury.
  • We don't have a registry or a data bank for navicular injuries. 
  • If you're low in vitamin D, you're more likely not to heal.
  • The other treatment that's exciting for us is something called shockwave. It's been around for 20 years. Two types of shockwaves exist focused shockwave, which is better for bone, and electromagnetic transduction therapy (EMTT). Those two shockwaves are game-changers.
  • We use that in one of the athletes in the series, and he healed very quickly (run a marathon four weeks later because he had a type 0.5). 
  • So shockwave is definitely an excellent treatment and frequently an adjunctive or addition to surgery. 
  • It usually takes about four and a half months to recover from a type II or III fracture; about 10% of the people can have refraction, and about 10% need the screw removed. 
  • Most of the time, we leave the screw, and I've had several elite athletes, well-known triathletes, with screws in their feet. 

The connection between carbon fibre shoes and navicular stress injuries

49:06 -

  • The carbon fibre plate bends very little, and it's curved. 
  • If you look at the picture in our article, there's an x-ray of a side view of an athlete and where the plate lies concerning his bones. 
  • Your bones line up to have hinges. There are 33 joints and 28 bones. 
  • So your bones and feet will bend where they line up, and the carbon fibre plate won't. 
  • If your carbon fibre shoe doesn't bend, your foot has to bend and may bend in a place where it doesn't have an axis. 
  • We get stresses if the axis is different from where the fulcrum is on the shoe.
  • The transition time from heel or midfoot to toe is shorter and more rapid. And it may be too rapid for your soft tissue to adjust.
  • And that's why some people tear their plantar fascia or get plantar fasciitis, or some people's bone gets stressed. 
  • It can be in the midfoot, and the back of the metatarsals on the instep to the middle bones and then the navicular is the weak bone. 
  • So that's why we saw more injuries there. 

Types of injuries and cases in the case series

51:20 - 

  • A couple of the patients had them acutely. 
  • They were high-level elite athletes that don't normally train in carbon fibre footwear. One needed surgery and one did not. 
  • Three youth athletes did steeplechase events where you land on the ball of your foot, which puts more impact into the navicular. 
  • If you did not have surgery with a type three under age 19, you're statistically more likely to fracture, get arthritis or give up your sport.
  • It's a significant injury, and parents sometimes freak out if you tell them their 16 or 17-year-old needs surgery. 
  • Arthritis is a tricky thing to deal with in the foot. There's no joint replacement in that joint. I've done an Arthur diastasis several times. 
  • You stretch out the joint to try to let soft tissue fill in. 
  • You need to ensure that your foot has enough mobility to bend where the shoe is, forcing your foot to bend. 

Start using carbon plate shoes

53:41 -

  • Don't use them for the first time in a race. 
  • There were some instances of that in our Olympic marathon trials in 2020 in the US. You want to do some gradual transitioning.
  • A helpful device, Blackboard-training, is a hinge-type device that can change and work your foot in multiple planes. 
  • It helps mobilise and work your foot in multiple axes to make it more robust and mobile. 
  • I would consider getting that type of device or foot mobilisation device. You can start with those ankle balls that bend your foot as you adopt carbon fibre shoes. 
  • It's a game-changer for athletes and dancers. 

Additional thoughts about navicular injuries

56:37 -

  • It's a complicated issue with shoe companies needing to sell shoes and the public wanting to run faster. 
  • Doctors need to know about footwear, particularly podiatrists and other sports medicine doctors. 
  • We wrote a letter to the British Journal of Sports Medicine editor. It takes a long time to get these things published and collected.
  • It takes time, and the motivation for awareness in this aspect may conflict. 
  • Before running in carbon fibre footwear, start strengthening and mobilising your foot. Do it gradually. 
  • You need to do shorter runs before more extended runs.
  • The top of the foot or instep pain can be a slow-healing stress fracture.
  • If you have throbbing and aching, it hurts to go up on your toes and worsens as you run. You need to push for a proper examination.
  • Remember that diagnosing the average navicular stress fracture in the medical community takes nine months. 
  • You may need surgery for the navicular injuries. 

Rapid fire questions

1:00:14 -

What's your favourite book or resource related to endurance sports?

Jack Daniel's distance running formula from 1998. 

There's a new book by Mark Coogan. And Brian Fulham, another sports podiatrist, has a good book on foot and ankle injuries, knowing your foot and ankle. 

The Triathlete's Training Bible

What's an important habit you have benefited from athletically, professionally, or personally? 

Preparation is the key to relaxation. So you need to be prepared and anticipate. You need to learn how to manage time.

Multitasking is an important skill, and I think in athletics, in your profession, you need to learn how to juggle. 

Who's somebody that you look up to or that has inspired you? 

Mahatma Gandhi: be the change. 

Martin Luther King has a lot of great sayings as well. I'm doing a master's in public health through the Dartmouth Institute, Dartmouth College. 

It's essential to keep learning and look at self-analysis and your outcomes.


Bernardo Gonçalves

Bernardo is a Portuguese elite cyclist and co-founder of SpeedEdge Performance, a company focused on optimising cycling and triathlon performance. He writes the shownotes for That Triathlon Show, and also produces social media content for each new episode.

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