12

Running Shoes and Form

When you wake up in the morning and slip into your running shoes in preparation for a run, you have unknowingly changed your running form in a significant way without even taking your first step out the door. That’s because research reveals that running shoes have a profound effect on form. Compared with sauntering out the door barefooted or in minimal running shoes, traditional running shoes with elevated, cushioned heels steer you toward the following gait patterns.

Pattern A: Impact Transient

In traditional running shoes, a runner maximizes the “impact transient” (an abrupt collision force acting on the leg during the first 50 milliseconds of stance after the foot hits the ground), compared with barefoot running or running in minimal shoes. The magnitude of the impact transient is three times greater in traditional running shoes, compared with unshod running. In other words, your running shoes, thought to protect you from the impact forces of running, can actually increase the impact forces (1) (figure 12.1).

Figure 12.1   Running in conventional shoes is associated with a dramatic impact transient—a powerfully increasing impact force moving up the leg during the first 50 milliseconds of stance.

Pattern B: Heel Strike

In traditional running shoes, the ankle is less plantar-flexed each time the foot hits the ground. This means that a runner is much more likely to make initial contact with the ground with the rear portion of the foot, instead of the front area (in other words, the runner would be running with a rearfoot or heel-strike pattern) (2) (figure 12.2).

Figure 12.2   The modern running shoe tends to steer runners away from (a) a more natural unshod landing pattern on the ground, and toward (b) a landing pattern that features a dorsiflexed ankle and a ground strike on the rear portion of the foot.

Pattern C: Knee Angle

In traditional running shoes, the knee is almost always less flexed at ground contact, meaning that the runner is hitting the ground with a significantly straighter leg, compared with running barefooted or in minimal shoes (3). The “knee angle” (the angle made by the posterior portions of the thigh and calf) for almost all runners wearing traditional, heel-elevated shoes is consistently in the range of 166 to 180 degrees at ground contact (4) (figure 12.3a).

By contrast, knee angles for barefooted runners, athletes wearing minimal running shoes with non-elevated heels, and runners who have carried out the running-form drills outlined in this book, fall within the range of 148 to 158 degrees for middle-distance and distance runners and 158 to 166 degrees for sprinters at ground contact (5) (figure 12.3b).

Landing on the heel with the ankle dorsi-flexed and a straight leg is responsible for the heightened impact transient in thick-heeled, traditional running shoes. If this is difficult to understand, think for a moment of the leg as being an iron pole that strikes the ground at high speed. Contrast that with an elastic appendage that can bend and store energy at its base (the foot), near its bottom (the ankle) and also at its middle (the knee). During the first 50 milliseconds after impact with the ground, which of these two structures would experience the greatest impact force in its top region (the end of the structure farthest from the ground)? It seems obvious that the knee, hip, and thus the spine would take greater, destructive poundings in traditional shoes, compared with running barefooted or the use of minimal shoes.

Figure 12.3   (a) In traditional shoes, the knee angle is larger at impact with the ground, meaning that the leg is straighter. (b) For the barefoot runner or the runner in minimal shoes, the knee angle is less at the moment of impact, meaning that the knee is less straight (the knee is more flexed).

Impact Forces in Heel-Strikers

When a runner lands on the ground with his heel and a straight (or nearly straight) leg, the force of impact is transmitted extremely rapidly, straight through the heel (which cannot store energy by flexing as the ankle does), straight through the unflexed knee and ramrod-straight leg, and then through the hip to the spine and thus all the way to the head. A pronounced heel-first landing sets off a chain reaction of shock transmission throughout the entire body. This chain of events begins with a sledgehammer-like impact on the posterior aspect of the calcaneus (heel bone) and progresses nearly instantaneously through the leg, hip, and upper body. Research reveals that the sledgehammer landings are not effectively moderated by the high, foamy heels of traditional shoes (in fact, the impact transient can be three times greater in such shoes).

Figure 12.4   With a heel strike, impact force is transmitted quickly through the calcaneus and talus into the tibia.

When the heel strikes the ground during a rearfoot-strike, the impact shock force moves upward in the following way:

  1. From the initial hammer strike on the calcaneus, up through the talus via the subtalar joint (figure 12.4)
  2. From the talus into the tibia (shin bone) via the tibio-talar joint
  3. From the tibia directly into the femur (thigh bone) by means of the tibio-femoral joint
  4. From the femur into the pelvis (hip) via the acetabulo-femoral joint
  5. From the pelvis into the vertebral column (spine) via the sacroiliac joint
  6. From the spine into the cranium via the cranio-vertebral joint (figure 12.5)

Figure 12.5   With heel-striking, the impact transient is magnified and impact forces travel quickly up the skeletal chain to the head.

The calcaneus is neither positioned nor structured to store and release energy by elastically flexing and recoiling after impact with the ground. As a result, the shock force of landing is transmitted straight up the body to the head in milliseconds. This brutal scenario occurs about 7,000 times during a simple run of 5 miles (or 8 kilometers); it is a key reason why at least 65 percent of runners (and 90 percent of marathon trainees) experience a running-related injury in any given year (remember that 95 percent of runners are heel-strikers) (6). The fast transmission of impact shock with rearfoot-striking is especially troubling when one considers the relatively poor functional strength of the legs in many runners.

Impact Absorption in Midfoot- and Forefoot-Strikers

Contrast what happens with heel-striking with what takes place during midfoot-striking. When a midfoot-landing occurs, the initial force of impact is taken on by the phalanges, metatarsals, cuneiforms, navicular bone, and cuboid bones—and the muscles and connective tissues in the joints in between them (figure 12.4). This is no small matter. Not counting the talus and calcaneus, there are 24 bones in the foot, along with about 30 joints between them and nearly 100 muscles, tendons, and ligaments. The foot is like a web of shock-absorbing structures, shaped by natural selection to accommodate and enhance running and walking, and it is this force-dissipating network that is bypassed when a runner chooses to crash into the ground with his heel.

The foot’s two key arches (longitudinal and transverse) play an extraordinary assisting role during midfoot-strikes, not only supporting the bones of the foot and their related connective tissues and muscles, but also absorbing impact shocks and balancing the body (figure 12.6). The actions of the foot’s arches and numerous joints explain why the impact force reaching the calcaneus and ankle is greatly reduced after a forefoot- or midfoot-impact, compared with a hammer-like heel strike.

Figure 12.6   The two key arches of the foot mollify, store, and return force and reduce the magnitude of the impact transient.

In addition, when a runner hits the ground with a compliant, plantar-flexed ankle and also with a compliant, flexed knee, much of the impact force is stored productively in the ankle and knee, instead of being passed “up the chain” to the head as a damaging shock force. The word “productively” is used here because the force of impact—which dorsi-flexes the initially plantar-flexed ankle and flexes the knee even further—stretches the muscles, tendons, and ligaments at those joints (effectively storing “elastic energy” in the joints). When these muscles snap back elastically to their unstretched positions, they provide “no-cost” (energy expenditure free) propulsive force which drives a runner forward.

Effects of Thick Heels on Form

With traditional running shoes, your cadence automatically decreases because you take fewer steps per minute as you run, compared with running barefooted or in minimal shoes. This cadence reduction can have a very negative impact on running velocity, which depends entirely on step rate and step length. Traditional shoes tend to decrease step rate by expanding time spent on the ground per step by about .01 seconds. For each 180 steps (expenditure free energy), this can keep a runner on the ground for an extra 1.8 seconds, thus causing running velocity to decline.

Why do traditional running shoes have such a corrupting impact on running form? One of the most glaring problems associated with the traditional running shoe is the presence of extra midsole material in the heel, which elevates the heel above the rest of the foot. In fact, there is about a .5-inch (or 12-millimeter) differential between the heel and forefoot in many conventional running shoes. This additional padding in the heel area is often believed to enhance shock absorption and therefore limit the risk of injury, even though injury rates have not improved since the 1970s when heel-elevated running shoes were introduced (figure 12.7). In fact, many runners actually select shoes on the basis of heel height, believing that heel-elevated shoes, with their excess of foamy material in the rear portions of the shoes, produce softer landings with the ground on each step. Running shoe companies often tout their big-heeled shoes as providing superior “cushioning” for the runner who needs a “softer ride.”

Figure 12.7   Thick-heeled running shoes lead to a number of very bad running-form habits, including heel-striking and landing on the ground with a straight leg and high SAT (and thus lousy “golden ratio”).

A key problem with all of this is that heel-elevated shoes are actually associated with greater impact transients, compared with “zero-drop” shoes (which by definition have no difference between the height of the heel and the front part of the shoe). Runners who wear heel-elevated running shoes almost universally make the first impact with the ground on their heels, with each step as they run. Research has revealed that about 90 to 95 percent of runners clad in conventional running shoes are heel-strikers (7). The result is an impact transient that is three times greater, compared with midfoot-striking while running in minimal shoes or while barefoot.

Footwear Lessons From Kenya

Demonstrating that it is the shoes themselves that produce this effect, individuals who wear zero-drop shoes are much more likely to use a midfoot-strike pattern with the ground, and individuals who run barefoot rarely strike the ground with their heels first.

In countries where young runners tend to run barefoot and only later make a transition to shod running (usually in their mid-teen years), the change in footwear almost always corresponds with an alteration in running form. I have taken photos and videos of numerous Kenyan young people ranging in age from five to 13 and noted a consistent pattern: When these young runners move along barefoot, they strike the ground consistently with a midfoot-landing pattern. Close to 100 percent of the young runners one can observe in the Kenyan countryside are midfoot-strikers (figure 12.8).

By contrast, Kenyan runners in their late teens and older have usually acquired conventional running shoes, and consequently the frequency of midfoot-striking has dropped significantly. I have travelled to Kenya on 25 occasions and have taken many hours of video of Kenyan runners. By my estimates, the frequency of midfoot-striking has dropped from universality (when barefoot) to around 50 to 60 percent in well-trained, shod Kenyan harriers.

Interestingly enough, when the young, usually barefoot Kenyan runners slip on running shoes for the first time, they almost instantly become heel-strikers (figure 12.9).

Figure 12.8   Individuals who run barefooted almost universally rely on a midfoot landing pattern

Figure 12.9   When these same runners slip on running shoes, they instantly become heel-strikers.

A Host of Shoe-Related Form Problems

This pernicious effect of running shoes on foot-strike patterns leads to a number of problems, detailed on the pages that follow.

Increased Impact Transient

As mentioned, the heel-strike induced by cushioned, heel-elevated shoes actually increases the magnitude of the impact transient, instead of lessening it. This appears to be paradoxical because during heel-striking a runner is landing on the “softest,” most-cushioned portion of the shoe. However, the mattress-like landings limit proprioception and thus may not initiate feedback loops in the nervous system which attenuate shock. Feedback loops diminish shock and usually involve greater ankle plantar flexion and knee flexion at impact with the ground, both of which soften the impact blow. In addition, a heel-strike, while it indeed involves landing on the mattress-like heel of the traditional running shoe, nonetheless bypasses the force-dissipating structures in the front of the foot.

Increased Force Rate

Heel-striking, compared with forefoot-striking, increases the rate at which force travels up the legs and through the body. This is because the heel-landings bypass the front areas of the feet and are associated with dorsi-flexed ankles and straight legs that are not flexed at the knee; thus there are no anatomical configurations in place to mollify force as it travels up the leg after impact.

Figure 12.10   Runners who wear traditional, high-heeled shoes tend to have excessively high SATs.

Excess Shank Angle

Heel-striking is usually linked with a straight leg at impact with the ground and, on average, an SAT in excess of 14 degrees (figure 12.10). This leads to the production of extremely high shock transmission and braking forces after contact with the ground is made.

Increased Vertical Forces

A high SAT yields vertical ground reaction forces that reach one times body weight and 1.5 times body weight while the shank angle is still positive (before the shank has achieved a vertical position for the first time during stance). This means that a huge amount of vertical force is funneled into braking—rather than forward movement—since the forces are directed upward and backward, instead of upward and forward.

Although the relationship is never mentioned in popular running publications and books, the moment during gait when vertical impact force reaches 1.5 times body weight is actually a critical form variable and an excellent predictor of overall form. If you can measure a runner’s shank angle at 1.5 times body weight, you can immediately tell if he has good form or not!

For example, take the case of a midfoot-striker who hits the ground with an initial shank angle of six degrees, compared with a heel-striker who touches down with an initial shank angle of 15 degrees. From the moment of touchdown, the ground reaction force increases as the shank and foot move backward in relation to the rest of the body. Eventually, the ground reaction force reaches 1.5 times body weight. At that point, the difference in shank angles is even greater between the midfoot- and heel-striker—much greater than the initial nine degrees. This is because the rate of force development is so much greater when the SAT is high and contact with the ground is achieved via heel-striking.

In fact, in almost every case of heel-striking, the shank angle is positive (the foot is ahead of the body) when a force of 1.5 times body weight is attained. In contrast, in almost every case of midfoot-striking, that 1.5 times body weight is reached at a negative shin angle, with the foot behind the body.

As a runner, when would you want to produce a significant propulsive force against the ground of 1.5 times your body weight: When your shank angle is positive and you are exerting braking, upward and backward forces on the ground, or when your shank angle is negative (with the foot behind the body) and you are exerting propulsive, upward, and forward forces against the ground? The position of the shank at 1.5 times body weight reveals overall running form and the potential to run quickly.

The thick heel of the modern running shoe leads to heel-striking on a nearly straight leg with a large SAT, which then leads to “braking and breaking effects.” With the foot so far ahead of the body, braking forces are maximized. With the landing occurring on the heel, the rate at which impact forces are loaded into the leg increases, and the forces travel directly through the heel—untempered by the foot and ankle—and then through the ankle, shin, knee, upper portion of the leg, hip, spine, and head.

The heel-striking induced by the significantly elevated heel in the modern running shoe also lengthens the duration of the stance phase of gait, compared with forefoot-striking. The extent of this elongation can vary, but an average increase in time spent on the ground with heel-striking is about .01 seconds. This may seem quite small, but bear in mind that for the heel-striking runner taking 180 steps per minute and running one mile in six minutes, she would have 180 × .01 × 6 = 10.8 seconds of wasted time built into the mile. With a midfoot-strike and thus without the extra .01 second per step, she would have the potential to clip off a mile in just 5:49.2 as a result of the change in form, without the need for further arduous training.

To summarize, heel-elevated shoes strongly encourage heel-striking. Heel-striking forces a runner to spend more time on the ground per step; produces more physical damage with each step; and leads to greater braking forces, diminished productive propulsive forces, and slower running speeds.

Other Effects of Shoes on Form

In addition to causing heel-striking, traditional running shoes have other significant effects on form. For example, such shoes tend to increase maximal shank angle (MSA) at still point (the point at which a foot reaches it farthest forward progress during swing, relative to the rest of the body). After MSA is reached, most conventional running shoes decrease reversal of swing (ROS, or the extent and magnitude of the subsequent downward and backward sweep of the foot toward the ground before landing). The lofty MSA and small ROS translate into a very high and sub-optimal SAT of approximately 14 to 20 degrees for the average distance runner—compared with two to six degrees in barefoot runners and individuals running in minimal shoes. This leads to the production of unnecessarily high braking forces during the stance phase of gait. The large MSA at still point is part of a runner’s unwitting preparation for a heel-landing, which is tightly linked with the wearing of heel-elevated shoes.

Overall, traditional running shoes are associated with the following form characteristics:

It’s important to note that transitioning from thick-soled traditional shoes to shoes with more minimal midsoles (also called minimalist shoes) is a good idea for many runners, but it is not without its perils. Changing from battleship-soled shoes to minimal models tends to result in a change from a heel-striking foot-strike pattern to midfoot contact. (Without a huge mattress at the heel end of the shoe, heel-striking suddenly begins to feel very uncomfortable.)

Midfoot-striking is desirable over the long term, of course, but a sudden shift from 30 weekly miles of heel-strike running to 30 miles of midfoot-striking is a recipe for disaster. The metatarsals of the feet and associated connective tissues will not be accustomed to directly absorbing the impact forces of running 90 times per minute per foot (given a cadence of 180)—a task that had been undertaken by the heels in thick-soled shoes. The abruptly elevated workloads placed on the metatarsals can produce foot pain, edema, inflammation in the metatarsals, and even stress fractures.

Furthermore, the Achilles tendon and calf muscles do significantly more work per step when midfoot-striking is in play. This is because the calf complex has to control ankle dorsiflexion after each contact with the ground and is stretched dynamically while contracting, a proven formula for muscle damage, soreness, and tightness. (During heel-striking, the calves don’t have to control dorsiflexion, because the ankle is already dorsiflexed at ground contact and thus undergoes plantar flexion, placing stress on the shins.) This is why runners who shift from traditional shoes to minimal models and then go out for a 10-mile run often awake the following morning with excruciatingly painful calves.

What should be done to prevent the foot and calf damage associated with the shift from big-soled brogans to minimalist running shoes? Obviously, the answer is not to keep running with heel-strikes in the minimalist models. Rather, running volume (mileage) should be reduced drastically during the first few weeks of wearing the minimal shoes. Fitness can be maintained with the use of intense non-injury-producing cross-training (biking, swimming, rowing, etc.). A reasonable adjustment would be to reduce running-training volume to 20 percent of normal during the first week of minimal-shoe usage, with no long runs (greater than five miles) carried out in the minimal models. Successive weeks could then build to 40, 60, 80, and finally 100 percent of the usual training load. Thus it should take about a month before the usual training levels are resumed after the initiation of minimal-shoe usage.

Pronation and Supination

Runners sometimes hear that certain traditional shoes provide excellent control of pronation (inward rolling of the ankle) and supination (outward rolling of the ankle) during the stance phase of gait and thus furnish protection against injury. It is sometimes argued, for example, that such inward or outward actions can produce unusual torquing actions on the knee, increasing the risk of knee discomfort and damage. The implication of this kind of thinking would then be that minimal shoes increase the risk of knee injury, because they do not have the special features of the shoe models that allegedly control pronation and supination and thus keep the knees safe.

Such contentions are absurd. First, there is little evidence to support the notion that specific traditional shoes limit pronation and supination. In fact, because traditional shoes feature a high midsole platform, they are inherently unstable in medial and lateral directions, thus spiking pronation and supination. This is why you should never wear traditional running shoes while playing tennis, for example: If you move aggressively around the court, you would be at high risk of blowing out an ankle or a knee.

Second, there is little convincing scientific evidence to suggest that runners who pronate or supinate to an above-average extent have a higher risk of getting hurt. Rather, science continues to point out the key causes of running injury: 1. heel-striking and thus a lofty VALR (vertical average loading rate), 2. lack of running-specific strength, 3. overtraining, 4. poor recovery between workouts, and 5. a previous injury (about half of running injuries are simply re-occurrences of a previous problem).

Summary

The negative effects of running shoes on form can be changed. The truth is that a runner can run with good form in nearly any kind of shoe—traditional or minimal—and also barefoot. It is simply harder for a runner to do so with traditional, heel-elevated shoes, unless he is fully aware of the nature of good form and relentlessly conducts the proper form drills. Traditional, heel-elevated shoes tend to steer runners toward poor form practices. Instead of promoting stability, protecting against impact forces, and enhancing running economy, they tend to do exactly the opposite.

References

1. D.E. Lieberman et al., “Foot Strike Patterns and Collision Forces in Habitually Barefoot Versus Shod Runners,” Nature, 463 (2009), 531–535.

2. Ibid.

3. Ibid.

4. Walter Reynolds, personal observation from “Good-Form Running” Instructional Sessions in Lansing, Michigan (2012–2017).

5. Ibid.

6. I.S. Davis et al., “Greater Vertical Impact Loading in Female Runners With Medically Diagnosed Injuries: A Prospective Investigation,” British Journal of Sports Medicine, http://bjsm.bmj.com (accessed April 1, 2016).

7. D.E. Lieberman et al., “Variation in Foot Strike Patterns Among Habitually Barefoot and Shod Runners in Kenya,” PLOS ONE, https://doi.org/10.1371/journal.pone.0131354 (2015).