First, it was the four-minute mile. Who could possibly run a mile in under four minutes? That record was broken by Sir Roger Bannister in 1954. And this month another record was broken – the 2-hour marathon. In this extraordinary race held this month in Vienna, Austria, both the men’s and women’s 26.2-mile records were broken. Although this was a non-sanctioned race, the feat by Eliud Kipchoge (1:59:40) and Brigid Kosegei (2:14:04) shattered previous race times. How did these two athletes achieve these record times? Some say it’s all in the shoes. (image of Eliud Kipchoge from telegraph.co.uk)
In the Vienna race, both Kipchoge and Kosegi wore a new and controversial shoe called the Nike Vaporfly 4%. The Vaporfly 4% is at the heart of controversy within the International Association of Athletics Federations (IAAF). Currently, the only regulations regarding shoes as defined by the IAAF is that the shoes may not confer an ‘unfair advantage’ and must be ‘reasonably available’ to all. Nike Vaporfly 4% shoes are available for $250 on the open retail market.
In a recent article in The New York Times, 1968 Boston Marathon winner Ambry Burfoot describes how the Nike Vaporfly 4% controversy centers on a new arch filler known as Pebax. The use of Pebax as an arch filler/sole and the thickness of the arch filler in the Vaporfly is more than we see in most running shoes. The controversy centers on how Pebax will act as a spring, making runners faster.
Another controversial aspect of the Vaporfly 4% is the inclusion of a carbon fiber spring plate within the shoe. The Vaporfly 4% is the first running shoe to employ the use of a carbon fiber spring plate. Spring plates provide shank rigidity.
New science and products like Pebax are sexy, but sometimes old science is what really gets the job done. What I see as the most important attributes of the Vaporfly 4% have nothing to do with making runners faster. What’s really at the heart of winning a marathon in a pair of Vaporfly 4% is conservation of energy. It’s simply easier to run distances in a pair of Nike Vaporfly 4%. Why is it easier to run in these shoes? Let’s take a closer look at lower extremity muscle function and an old shoe modification called a rocker.
Sprinting and long-distance running are biomechanically very different athletic events. In Roger Bannister’s case, we wanted a short, unsustained burst of energy. If we think of the leg, ankle, and foot as a lever arm, we want that lever to deliver spring-like forward motion in sprinting. To do so, we want the heel low to the ground, tightening the leg/ankle/foot lever for fast, but unsustained forward motion.
Distance running is biomechanically different in that we want to be able to sustain the run. Sustained running requires conservation of energy. To study the biomechanics of long-distance running, you need to understand the biomechanics of walking. In walking, the muscles of the lower leg act to inhibit forward motion of the tibia over the foot. As your center of gravity moves forward, the calf and associated lower leg muscles provide what’s called an eccentric or negative muscle contraction, slowing forward motion of the body over the foot. Once the center of gravity is far enough forward, the heel lifts from the ground, weight rolls to the forefoot and lastly, the toe-off phase of gait occurs as the foot is no longer in contact with the ground. Surprisingly, in walking, this toe-off phase of gait has no active push-off.
Eccentric muscle contraction describes the lengthening of a muscle as it provides force. This is very different from a concentric muscle contraction where force is derived from a muscle while it shortens. The primary distinction between these two types of muscle contraction is energy conservation. In the sprinter, concentric muscle contraction (as the muscle shortens) uses significant energy for short distances. Eccentric contraction of the muscle (force while lengthening), the predominant muscle function used in long-distance running, uses far less energy and subsequently conserves energy.
Shoe modification can also act to conserve energy in long-distance running. Shoe rockers are modifications that are made to the sole of the shoe. Rocker mechanics were first described by Sunderland. (1) Sunderland described a series of three rockers that take place during normal walking. These rockers include the heel, ankle, and forefoot. Rockers are used to decrease resistance to motion and provide conservation of energy. Rockers have been used in the shoe industry for years in the treatment of orthopedic deformities and in diabetic foot care.
In addition to rocker mechanics, the Vaporfly 4% also uses a carbon fiber spring plate to improve the rigidity of the arch and supplement the rocker mechanics of the shoe. Rigidity of the arch improves translation (movement in a single body plane) which also contributes to conservation of energy. I have yet to dissect a pair of Vaporfly shoes, but I’d have to assume that Nike also employs the carbon spring plate to supplement the forefoot rocker mechanics.
For a further understanding of the principals of walking vs. running, rocker mechanics and energy conservation while walking be sure to read my 2009 publication entitled, The CT Band, CT Band Biomechanics and CT Band Syndrome.
Compare these two images; on the left is an image of Sir Roger Barrister with the running flats used to break the four-minute mile. These shoes were spiked for grip and had no heel. A lower heel essentially makes the leg, ankle, and foot into a spring – rigid and built for fast but unsustained periods of running. On the right is a profile of a Nike Vaporfly 4%. Notice on the sole how the heel is rounded? This is a posterior rocker sole. And on the forefoot, notice again how the sole is rounded? This is a forefoot rocker sole. Both rearfoot and forefoot rockers are built into the shoe to conserve energy. The thicker than normal central arch filler (Pebax) is used to create comfort, and to some degree, spring.
What I believe Nike has done with their research is to focus on building a running shoe based on the principals of walking and not the principals of running. The Nike Vaporfly 4% is specifically designed to conserve energy through the use of rockers and spring plates. As a podiatrist and pedorthist who has studied lower extremity biomechanics for 35 years, I think Nike’s strategy is brilliant. They’ve designed a running shoe for sustained periods of running and not simply for speed.
As we head towards the 2020 summer Olympics in Tokyo, I’m sure the conversation about the virtues of the Nike Vaporfly 4% will heat up. In the New York Times article, Geoffrey Burns, a marathoner and doctoral candidate in biomechanics stated that the midfoot height of the shoe should be limited to a specific height or “we might end up with footwear that we don’t even recognize as shoes.” My opinion is that the problem will become self-limiting. Adding science to shoes, such as the addition of rockers or spring plates does not add weight or increase the size of the shoe. The addition of materials like Pebax will be self-limiting in terms of size and weight. I suggest the IAAF keep the rules as they are making shoes ‘reasonably available’ and without ‘unfair advantage’.
- Sutherland DH, Cooper L, Daniel D. The role of ankle plantar flexors in normal walking. J Bone Joint Surg 1980; 42-A: 354-363