Carbon Fibre Running Trainers: Science behind the super shoes.
In October 2019, Eliud Kipchoge made history, running the first ever sub 2-hour marathon. A triumph of human performance powered by cutting edge sports science and technology. However, perhaps the most talked about aspect of this remarkable feat was the footwear (mind the pun). A pair of custom Nike running trainers with thick wedge soles, a carbon fibre plate and airpods located the forefoot. The latest version of Nike’s performance enhancing trainers boasting a 4+% improvement in running economy.
Over the last 8 years I have been providing biomechanical advice and assessments for a variety of runners, clinicians and coaches; and carbon fibre running trainers are frequently the hot topic of conversation. With the reported 4% reduction in energy cost of running backed by several scientific studies [1, 2], it’s understandable why running trainers with carbon fibre plates are seen more frequently amongst runners. The use is also creeping up from initially competition, into day to day running.
Recently, British Triathlon asked if I would share some thoughts on the use of carbon fibre soled running shoes. One of the big questions being, whether the more frequent use of carbon fibre shoes for daily training and not just race day is a good or bad thing. So, I thought I’d share those same ideas here, the proposed mechanisms of performance enhancement and the potential draw backs of their “overuse”.
Currently, the majority of research has been conducted using the Nike 4% running trainers, with biomechanical changes observed in several experimental studies. Including increased stride lengths, reduced cadence, increased vertical oscillation, longer stance and flight times as well as greater vertical ground reaction force and force impulse [1-4]. So effectively, the shoes seem to give you more time on the ground, but allow for more force applied into the ground and more time in the air; thus creating longer stride lengths. However, despite the observed changes to running biomechanics, they do not appear to correlate with improvements in running economy [1, 2]. This suggests that the performance enhancing effects of the trainers, may be more likely explained by the energy returning properties of the shoe, rather than a specific change in biomechanics.
Carbon Fibre Plate
Perhaps the greatest discussion has been around the use of a carbon fibre plate inserted into the sole of the shoe. The plate is thought to stiffen the length of the shoe, shifting our centre of pressure to the front of the foot during stance [5, 6]. This results in a greater lever arm between the ground reaction force vector and the ankle joint, without increased bending at the big toe [5, 6]. Consequently, muscle work at the foot is lower, and providing the calf complex is strong enough, greater ankle plantar flexor moments and force output can occur [5, 7, 8]. Put simply, the idea is that the plate acts as a lever to reduce foot muscle work and increase force generation at the ankle as we push off.
Sounds great? However, it’s not that simple! Inserting carbon fibre plates into running shoes is not new. In fact, Adidas and Reebok were both trialling carbon fibre plates in running shoes back in the 90’s. Similarly, carbon fibre plates have long been a feature in sprint spikes. Yet only now are we seeing substantial improvements in distance running performance. If it was just as simple as inserting a carbon plate into a shoe, surely we’d have seen these improvements years ago?! Some researchers have even questioned whether inserting carbon fibre plates into running shoes offers any benefit at all! Suggesting that inserting a carbon fibre plate into a running shoe may contribute less than 1% of the reported 4% reductions in energy cost of running . So where does the improvement come from?
The Curved Plate
Interestingly, the carbon plate in the new wave of running shoes may act in a different method to that first thought. In a recent publication, Benno Nigg and colleagues  proposed the curved shape of the plate may be responsible for the improvements in running economy. Suggesting the curvature may act to rock the foot forwards, while simultaneously applying an upward force (or push) to the heel. In modelling studies by the same group, this rocker effect was proposed to result in up to a 6% improvement in running economy! 
Theoretically the rocker effect of the plate may also act to reduce the demand on the calf musculature during running. This mechanism of effect would fit with the biomechanical findings of Hoogkamer et al . In their study investigating the biomechanical effects of the Nike 4%, the authors reported LOWER ankle joint forces and ankle joint work in the Nike 4% as well as reduced work at the big toe. Therefore, it’s possible that while stiffness of the plate acts to reduce bending of the big toe, the curvature acts to rock the foot forward and push the heel upward. Subsequently reducing both the muscular work at the foot and calf complex.
Another unique feature is both the thickness and the compliance of the foam. In most running shoes the midsole is made from a type of foam called EVA or TPU. Unique to the Nike 4% trainers is the use of Pebax foam. A lightweight, compliant and responsive foam which is reported to return up to 87% of energy . In contrast, the Adidas Boost returns 76% and the Nike Zoom Streak only 65.5% .
The lightweight nature of the foam means more can be fit into the shoe, without adding additional weight. The benefit of this compliant foam is that it may reduce the muscular demand to absorb impact forces, while returning energy stored in the shoe during each stride. Considering impact forces have been linked to injury  this could also offer a protective effect upon injury development; however, this is yet to be studied so don’t just take my word for it!
Historically, adding large amounts of foam to the soles of running shoes has had the negative effect of adding more weight to the shoe. However due to the lightweight nature of the shoe, more can be fit in. Giving the shoes their stiletto-like-stack height. One of the proposed benefits of this is the increase in leg length. Researchers have long proposed that increasing leg length may improve running economy by allowing longer stance times and therefore more time to generate force for the subsequent stride . So, its possible that as well as the plate and the responsive foam, the height of the shoe also provides us with a mechanical advantage.
Summary of Effects
Currently most of the conversation appears to be around the carbon fibre plate in the shoe. However rather than a single shoe feature providing the benefit it appears there’s a combination of factors. Not just the inclusion of a plate, but the shape of the plate, the weight of the shoe and the features of the midsole; the compliance, resilience and thickness. It’s perhaps the interaction between these features which results in the perfect recipe for a long-distance running shoe. A lightweight, responsive shoe, which facilitates energy storage and return as we hit the ground, while simultaneously reducing the muscular demand at the lower limb. Basically, the shoe seems to be the perfect ankle joint!
Which are the best carbon fibre trainers?
Currently it’s difficult to tell. Most of the research has been geared towards the Nike 4% shoes, with little on trainers from other manufacturers. However, many companies appear to have been looking at ways to create their own shoes to rival the Nike 4%. Using curved carbon fibre plates and their own version of the responsive foam seen in the Nike trainers. Therefore, over time it’s likely that shoes offered by other manufacturers may offer similar benefits to rival the Nike 4% shoes.
What are the long-term consequences of the shoe?
Although these shoes are clearly beneficial for performance, recently I’ve seen an increasing number of runners using carbon fibre shoes as a day-to-day running shoe. While I earlier suggested the shoe could potentially reduce injury risk (attenuating impact forces and reducing the demand on the calf complex), there may also be consequences of the increased frequency of use.
Muscles and tendons in the human body respond to their loading environment, adapting to the demands we place upon them. Considering carbon fibre shoes appear to reduce foot and lower limb muscle work, over time our lower limb muscles may adapt to the lower mechanical demand. Potentially becoming weaker with repeated daily use. This has long been an argument amongst minimalist running shoe advocates, suggesting high cushioned footwear could lead to a reduction in muscle size and strength [14, 15]. While I’m certainly not a fan of minimalist running shoes, I do see the potential that chronic under-loading of the foot and lower limb could lead to reduced muscle size and strength in the long term.
The ankle plantar-flexors and foot intrinsic muscles act to store and return energy during running. Therefore, a reduction in muscle size, strength and a loss of tendon stiffness could negatively impact our performance. Interestingly, in one study a 14-week strength training program for the ankle plantar-flexors lowered the energy cost of running by approximately 4% . The same amount as the carbon fibre shoes! Although yet to be investigated, my thought is that under-loading of these muscles through chronic carbon fibre shoe use, could ultimately have a negative impact on our performance. Cancelling out the positive benefits gained by the shoe!
My take is that the new era of running trainers provide substantial performance advantages on race day. However, the sustained use of these trainers beyond competition could ultimately have the opposite effect. Although these are just thoughts, my overall opinion is that we should focus on the 96%er’s before the 4%er’s. Optimising training, strength and conditioning, nutrition, lifestyle. Varying the footwear we train in and saving the carbon fibre shoes for race day.
 W. Hoogkamer, S. Kipp, J.H. Frank, E.M. Farina, G. Luo, R. Kram, A Comparison of the Energetic Cost of Running in Marathon Racing Shoes, Sports medicine 48(4) (2018) 1009-1019.
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