Runners who grew up running barefooted or who switched to running barefooted (groups 2, 3, and 4) are generally forefoot strikers (FFS); they tend to land initially on the balls of their feet while running, after which their heels drop down to make contact with the ground.
Impact forces transmitted through the foot, ankle, and leg immediately after impact with the ground are about three times greater in shod runners using RFS than in barefoot runners using FFS. Some—but not all—previous studies have shown this same relationship, with RFS producing greater impact force during the first portion of stance compared with midfoot strikers (MFS) and FFS. The sudden rise in force with RFS immediately after ground contact is known as the impact transient. The disparity in impact transient between barefoot and shod running represents a foundation for the belief that barefoot running is safer and less injury producing.
During the early stance phase of barefoot FFS running, there is greater knee flexion, greater dorsiflexion at the ankle, and a 74 percent greater drop in the center of mass than with shod RFS running. Vertical compliance is the drop in the runner’s center of mass relative to the vertical force during the impact period of stance, and it is greater in barefoot FFS running than with shod RFS running. Vertical compliance varies as a function of running-surface hardness, and this is why force-loading rates are similar for barefoot FFS runners over a wide array of running surfaces as the runners adjust compliance according to surface.
During barefoot FFS running, the ground-reaction force torques the foot around the ankle, increasing the amount of work carried out by the ankle compared with what occurs in shod RFS running. With shod RFS running, the ankle converts little impact energy into rotational energy. Potentially, this difference could actually spike the rate of ankle-area injuries in the Achilles tendon and calf, for example, for barefoot runners, especially if a runner plunges into barefoot running without adequate preparation.
Deciding to Shift and Doing So Safely
What does all of this mean to endurance runners? Although a shift from shod to barefoot running is attractive for a number of reasons (e.g., improved economy, reduced impact transient), such a change, if carried out over a relatively short period of time, might actually increase the risk of getting hurt.
As mentioned previously, barefoot running increases the work carried out by the ankle joint during gait compared with shod running. A sudden upswing in strain at the ankle induced by a change from shod to barefoot running could actually heighten injury rates in the calf and Achilles tendon. The increase in work carried out at the ankle, reduction in impact forces, and diminishment of work carried out at the knee associated with barefoot running are exactly the same effects observed in runners who adopt the pose running method (see chapter 5). Research suggests that up to 95 percent of Pose newcomers are injured during the first 2 weeks after adopting the technique!25
It is important to remember that most injuries in running are caused by an imbalance between the strain and microdamage experienced by a muscle or connective tissue during training and the tissue’s ability to recover from such stress. This imbalance can occur when training is conducted shod or barefoot. A weak or overly tight hamstring muscle that has been undone by excessive running won’t care if its owner was running barefooted or wearing shoes—it will still feel painful to the owner.
It is certainly true that barefoot and shod running are different from kinematic and kinetic standpoints, and this may have a bearing on injury rates. Shod running, at least shod running in big-heeled modern running shoes, almost automatically means RFS. With RFS, the ankle plantar-flexes immediately after impact as the bottom surface of the foot moves downward to make contact with the pavement. This places the shin muscles under strain immediately after heel impact since they have to control this significant plantar flexion. In contrast, during barefoot (FFS or MFS) running, the ankle immediately dorsiflexes after impact, placing eccentric strain on the Achilles tendon and calf muscles as they attempt to control the dorsiflexion. Thus, it’s possible that shod RFS might be linked with a higher risk of shin injuries, while barefoot FFS and MFS could be connected with a greater rate of Achilles and calf problems. These hypotheses have not been tested, however.
A runner who decides to abandon running shoes and carry out all training barefoot will almost automatically be shifting from RFS to MFS. This will mean that the Achilles tendon and calf muscles will encounter unprecedented pressures that they had not encountered before during the athlete’s running career. So, caution is advised.
Instead of tossing one’s shoes away and immediately running unshod, it seems prudent at first to employ very comfortable, relatively minimal running shoes that permit actual proprioception, protect the bottoms of the feet from rough surfaces, and are conducive to MFS. From a performance standpoint, this overall strategy should eliminate the braking action commonly associated with thick-heeled shoes and RFS, in which the foot tends to land out in front of the center of mass, creating a slowing effect, and thus should upgrade speed and enhance economy. A shift from RFS to MFS will also eliminate the impact transient that might be a cause of running injury; it will heighten the compliance of the leg, fostering the ability to run on surfaces of increased hardness without amplifying the impact forces experienced by the legs. MFS also tends to lead to an increased cadence while running (> 180 steps per minute), which has been associated with faster performances. As a runner becomes used to wearing minimal shoes, he or she can gradually increase the amount of running done barefoot.
A shift from RFS to MFS should be accomplished gradually. Abruptly changing from 40 miles (64 km) per week of RFS to the same volume with MFS is a nearly certain way for a runner to find the Achilles heel in his or her running program.
Orthotics
Orthotics (also called orthoses) are frequently recommended by sports medicine physicians, podiatrists, physical therapists, and even running-shoe clerks for runners with assorted lower-limb pains and injuries. The popular belief is that such devices can correct misalignments and improper movements in productive and functional ways that can decrease the risk of injury.
Scientific research has not been particularly kind to such practices and beliefs. Research reveals that injury rates among endurance runners have not declined over the past 30 years, even though the use of orthotics has increased. While it is true that some clinical studies have linked orthotic use with pain relief and the successful management of running injuries,26, 27 in most investigations that favor orthotics, it is difficult to disentangle the effects produced by the orthotics from those associated with natural recovery from injury over time (i.e., the body’s own healing mechanisms).
Up to 40 percent of runners do not get better after they are fitted with orthotics.28 Orthotics are of little use in runners with high-arched (cavoid) feet.29, 30 and orthotic usage has also been linked with the appearance of new injuries,31 suggesting that using orthotics might simply replace one kind of stress on the musculoskeletal system with another. Such findings intimate that the underlying injury-inducing problem for many runners may be a general lack of functional strength, a deficiency that orthotics would not correct.
An additional problem is that static measurements of the feet are often used to create custom orthotics, which are thought to take into account an individual runner’s unique anatomical and functional characteristics. These static measurements may do a poor job predicting the dynamic behavior of the feet and ankles during running,32, 33 creating a situation in which orthotics might work well when a runner is standing around, but not when that runner is surging toward the finish line of a 5K race. Researchers have also questioned the ability of orthotic makers to properly define normal feet and normal foot function during running; 34, 35 seemingly misaligned feet might be fully functional and not promote injury in certain runners with unique anatomical characteristics. No optimal pattern of motion of the foot and ankle during gait has ever been defined.
The use of orthotics can also have unexpec
ted effects. Identical orthotics can have widely varying effects on movement patterns in different runners, indicating that there can be potentially strong interactions between orthotics and the neuromuscular and anatomical characteristics of specific individuals.36 Orthotics can also produce unanticipated results: In one study, podiatrists fitted 12 runners who had histories of Achilles tendon problems with orthotics that were intended to reduce ankle pronation. Kinematic analysis revealed that the orthotics did change ankle movements during running, but the devices actually tended to increase eversion, or outward rotation, of the ankles, an effect that magnified pronation.37
It would appear from the scientific evidence that orthotics might work most effectively in runners with unchangeable anatomical misalignments (e.g., leg-length discrepancies), problems that cannot be corrected via therapy or vigorous, running-specific strength training. When injuries occur as the result of functional weakness, rather than anatomical defects, it is logical to assume that running-specific strength training would provide a better resolution than the use of orthotics, which might weaken neuromuscular function even further by taking over supportive functions and the control of joint movements, processes that should be regulated by nerves, muscles, and tendons.
Conclusion
The scientific research on running shoes and orthotics shatters many myths and is very liberating to runners. It is important for runners to know that specific running shoes don’t really provide superior cushioning, stability, motion control, or protection from injury compared with using other types of running shoes, and especially compared with running barefoot. Soft shoes don’t cushion the feet and legs better than hard shoes, nor do more expensive shoes ensure greater defense against injury and higher performance—in fact, perhaps the opposite. The life expectancy of a pair of running shoes is probably longer than commonly believed, and no brand of running shoes is better than any other. As a result, runners can buy their shoes based on fit and moderate purchase price instead of feeling that they must go for pricey, high-tech, high-end footwear. In addition, runners can take a close look at doing at least some of their running barefoot, a practice that enhances economy, stride rate, and foot-strike pattern and seems to reduce the impact forces running up the legs.
Part III
Physiological Factors in Running Performance
Chapter 7
Maximal Aerobic Capacity (O2max)
Without oxygen use, there would be no such thing as endurance running. The leg muscles provide the propulsive force required for sustained running, and they depend on oxygen to create the continuous supply of energy needed to complete any distance-running event.
A popular belief is that oxygen provides energy by burning carbohydrates and fats in the leg muscles during running. In reality, oxygen latches onto electrons at the end of a key metabolic pathway inside muscle cells called the electron transport chain. If an adequate supply of oxygen is not available to catch the electrons, the pathway grinds to a halt, energy production slows to a trickle, and running must stop.
Turning Oxygen Into Energy
The electron transport chain creates large quantities of a chemical called ATP (adenosine triphosphate), which provides the direct energy muscles must have to produce force. If oxygen is in abundant supply within the muscles, the reactions in the electron transport pathway can proceed at a high rate, ATP creation can be maximized, and high-quality running can be undertaken and sustained for a significant period of time. If oxygen is in short supply, the reactions in the pathway proceed at a slow rate, ATP generation decreases, and running speed must be reduced. In one sense, setting a personal record in an endurance race is dependent on having an adequate supply of oxygen at the ends of the electron transport chains in the leg muscles.
Oxygen takes a circuitous route to those muscles, passing from the atmosphere through the small air sacs (alveoli) of the lungs into the blood and then through the pulmonary veins to the left side of the heart, where the oxygen-rich blood is pumped through the arteries to the muscles (see figure 7.1). The muscles can then utilize this oxygen to create the energy required for running. Inside muscle cells, half of an oxygen molecule accepts two electrons coming down the electron transport chain and also connects with two hydrogen ions to form a molecule of water; the other half of the oxygen molecule does the same thing. Thus, a runner’s muscles use the oxygen that comes from the atmosphere to create energy and make water. The water is produced by combining hydrogen and oxygen, and it can be used throughout the body to preserve blood volume, intracellular water content, and interstitial fluid.
Figure 7.1 The heart sends oxygen-poor blood to the lungs and receives oxygen rich blood before pumping it out to the body.
Oxygen usage is a function of running velocity: As running speed increases, more muscle cells in the legs become active; muscles need more energy to provide greater propulsive forces, so the muscles consume oxygen at higher rates. In fact, the rate of oxygen consumption advances as a nearly linear function of running velocity1 (see figure 7.2). A typical runner cruising along at a speed of 15 kilometers per hour (about 6:27 per mi) is likely to be consuming oxygen at a rate of about 50 milliliters per kilogram of body weight per minute. At 17.5 kilometers per hour (approximately 5:30 per mi), the consumption rate is often close to 60 milliliters of oxygen per kilogram per minute. If the runner can make it to 20 kilometers per hour (4:50 per mi), oxygen consumption would be close to 70 ml • kg-1 • min-1.
Figure 7.2 The oxygen consumption rate increases in an almost linear function as the intensity increases, but it eventually levels off as it reaches O2max.
Adapted, by permission, from W.D. McArdle, F.I. Katch, and V.L. Katch, 1991, Exercise Physiology: Energy, Nutrition, and Human Performance, 3rd ed. (Philadelphia: Lea & Febiger), 213.
Defining O2max
Obviously, there must be some upper limit on oxygen use; oxygen cannot be transported by the heart and used by the muscles at an infinite rate. The topmost rate of oxygen consumption in an individual runner is called the maximal rate of oxygen consumption, or O2max.
In humans, the variation in O2max is exceptionally large. Because of difficulties getting oxygen through the alveoli in the lungs, an individual with a significant pulmonary disease might have a O2max of just 13 ml • kg-1 • min-1. Due to a lack of cardiac muscle strength, and thus a reduced capacity to send blood to the muscles, a post–heart attack patient might check in with a O2max of 22. A sedentary adult plucked at random from a U.S. street could be at 35, and a relatively sedentary young person would probably be close to 45 ml • kg-1 • min-1. In contrast, a runner with a 5K personal record of 18 minutes might reach a O2max of about 60, an elite endurance runner could easily have a O2max of 75 to 80, and an international-level cross-country skier might attain 85 ml • kg-1 • min-1.2-4 The highest O2max ever recorded for an endurance athlete is 93 ml • kg-1 • min-1 in a Scandinavian cross-country skier, which is seven times higher than the maximal aerobic capacity of the pulmonary disease patient and almost three times greater than the O2max of an average adult.5
The figures cited above are examples of relative O2max, which is always expressed in milliliters of oxygen per kilogram of body weight per minute (ml • kg-1 • min-1). Absolute O2max is expressed in milliliters of oxygen consumed per minute without the body mass factor in the denominator. Compared with relative O2max, absolute O2max is a better indicator of whole-body oxygen consumption and thus energy expenditure, but relative O2max provides more information about potential running ability. A 400-pound (181 kg) man, for example, would have a high absolute O2max because of the massive size of his oxygen-using organs and muscles, but one would not expect him to storm through a 10K race at high speed; his relative O2max would not be high, with that 400-pound (181 kg) number lurking in the denominator of the O2max formula.
Possible Factors Limiting Aerobic Capacity
Since oxygen transport and use depend on many different way stations within the body (e.g., lungs, blood, heart, m
uscles), exercise scientists have asked which portion of the oxygen-transportation and usage system is most limiting. That is, which part creates the upper cap on oxygen utilization and thus sets up a O2max that can’t be exceeded.
Pulmonary ventilation, the moving of air in and out of the lungs, does not appear to be a factor that limits O2max in runners. Even when they are at rest, healthy athletes can move more air into their lungs than they require during extremely intense running. The maximal ventilation rate for a distance runner is about 200 liters (53 gal) of air per minute, but elite athletes, even during highly strenuous running, generally do not require more than 180 liters (47 gal) of air per minute to satisfy their oxygen needs.6
So what factors might limit aerobic capacity? These might be the passage rate of oxygen through the lungs into the blood; the ability of the muscles to use blood-borne, incoming oxygen at high rates; the cardiovascular system’s ability to distribute blood to the muscles; or the nervous system’s capacity to recruit muscle cells during intense exercise. A limitation on neural recruitment would cap the oxygen-usage rate by limiting the number of muscle fibers using oxygen.
Scientific studies reveal that the oxygen content of arterial blood emerging from the heart can fall during high-intensity running, suggesting that the diffusion rate of oxygen across the alveolar walls of the lungs into nearby blood capillaries may be limiting in some cases. This seems to occur only in elite runners who are capable of sustaining high intensities for a significant period. However, it is probably not a relevant limiter for the vast majority of endurance runners.7
Running Science Page 11