This is an excerpt from Running Science by Owen Anderson.
Environmental factors and the physiological variables associated with performance are so complex that there is a tendency for many to take the simplistic view that genes are dominant in determining running success. A facile view is that genes can act as magic bullets that propel athletes with the right genetic compositions to inevitable success. As an example, Scientific American once predicted that performances at the 2012 Olympic Games would depend on the insertion of key genes into the nuclei of athletes' muscle cells. There is a belief that an examination of a runner's genes can yield important information about whether he or she should become a sprinter, a middle-distance athlete, or a marathoner. There is also a common perception that East African runners (primarily from Kenya and Ethiopia) have a monopoly on the genes that code for endurance performance.
Proponents of a dominant role for genes, or nature, in determining running performances point to the relatively recent discovery of more than 100 genes that have an impact on physical capacity. Such findings reinforce the idea that an individual's potential for running performance could be largely determined at birth. A runner with the right configuration of this multitude of genes, for example, might have an inborn talent for running that would always elevate him or her above other athletes with less optimal genetic makeup.
At first glance, such thinking does not seem entirely unreasonable. Research has revealed that an individual's genetic makeup has a significant effect on physical characteristics, including body size and shape. Although there are many exceptions to the rule, the best distance runners tend to be relatively short in stature and light in weight with slim calves, factors that probably have some genetic component. Greater height tends to dampen distance-running performance because of added mass: Bone mass increases exponentially as a function of height, instead of linearly, giving the taller runner relatively more dead weight to move around a 10K or marathon course. In general, enhanced body mass, either in the form of fat or nonpropulsive muscle mass in the upper body, makes endurance runners less economical and less able to sustain high speeds for continuous periods. Scientific studies also have identified many genes that are linked with greater endurance performance.
Somewhat oddly, the East African dominance of distance running is often cited as further evidence that genes are the strongest determinants of endurance performance. An inescapable fact is that the best middle- and long-distance runners in the world are Africans. Over the last five Olympic Games, from 1996 to 2012, male runners of African origin have captured 11 of the 15 possible gold medals in the 1,500 meters, 5K, and marathon competitions, as well as all 10 gold medals awarded in the 10K and 3K steeplechase events. Males of African origin currently hold 11 of the 12 world records recognized by the International Association of Athletics Federation in events ranging from 800 meters to the marathon.
Such African dominance was not present as recently as 20 years ago when European runners ruled supreme at all competitive distances from 800 meters to the marathon. In 1987, 58 of the 120 runners on the all-time top 20 lists of performances in races of 800 meters, 1,500 meters, 5K, 10K, marathon, and steeplechase were European. Just 32 of the 120 best runners of all time were African, and 16 of those 32 were Kenyans. The majority of world-record holders were European.
By 2003 the composition of the lists had changed drastically. There were 67 Kenyans in the top 120 and 102 Africans in all, leaving the entire rest of the world with just 18 slots. The European contribution to the world's-best lists had slipped from 58 runners to only 14.
The Kenyans and other East Africans were sending shock waves through the endurance-running community with their sizzling performances. The issue was not that the European runners had suddenly begun to run slowly; they were running as fast as they always had. The change occurred because the African runners, particularly the Kenyans, were running extraordinarily fast times. An additional startling fact was that the majority of the Kenyans competing on the world stage were Kalenjins, a rather small tribe of about 3 million people.
Observers of this transformation of the running world have been tempted to conclude that Kenyan runners, and especially Kalenjins, have some inborn capacity for long-distance running. In competitive running, nature seems to be winning out over nurture. Kenyans and other East African runners appear to have the right genes for elite performance. There is indirect evidence this might be true.
Testing the Nature-Versus-Nurture Hypotheses
Concluding that genetic differences are the paramount factor underlying endurance-performance success is premature, however. In many cases, further analysis of the actions of specific genes reveals that the effects are not always consistent or that the genes that seem to have the biggest impact on performance are not necessarily monopolized by—or even present in—groups of high-performing endurance runners. Many other possibilities for the determination of performance are apparent. Training, or nurture, is certainly one of those elements; even the biggest advocate of nature over nurture must admit that training plays a large role in determining what the race clock reveals when a runner crosses the finish line. In the East African case, there is considerable evidence that Kenyan training differs dramatically from the training carried out by endurance runners in other parts of the world.
In fact, training is commonly considered to be the most important extrinsic, or environmental, factor affecting performance. Scientists use two techniques in their attempts to disentangle environmental and genetic effects and thus provide answers to the debate over nature versus nurture. One method is to look for evidence of patterns of variation in performance variables (for example, VO2 max or responsiveness to training) in a population. As long as there is variation for a given performance-related trait, estimating the relative contributions of environmental and heritable (genetic) factors to this variation is possible.
This kind of work can be carried out with families. For example, maximal aerobic capacity (VO2 max), a physiological variable linked with exercise capacity, can be studied in large populations containing family groups. If VO2 max varies considerably between families but very little among family members, there is evidence that VO2 max is strongly determined by genetic factors because individuals in the same family tend to have nearly identical VO2 max values and are very similar genetically. If VO2 max varies just as much within families as it does between families, then genetic factors would appear to play a small role in determining VO2 max. The VO2 max of one's father, mother, or sibling is not necessarily closer to one's own maximal aerobic capacity than the VO2 max of the unrelated stranger living across town.
Research using the heritability model has revealed that heritable, or genetic, factors are important but not exclusive determinants of several physiological variables that contribute to success in endurance running. One investigation found that 48 to 74 percent of baseline submaximal aerobic performance—the ability to sustain continuous exercise without previous training—could be attributed to genetic factors. The same inquiry discovered that responsiveness to training—the degree to which aerobic capacity improved as a result of a specific training stimulus—had aheritability of 23 to 57 percent.
An additional scientific study detected a heritability of 38 to 87 percent for maximal aerobic capacity, VO2 max, a traditional measure of running fitness. Another inquiry estimated that the degree to which VO2 max increases in response to exercise has a heritability of about 47 percent, and that anaerobic, or lactate, threshold has a heritability of 55 to 80 percent.
In an important investigation, the performance of mothers, fathers, daughters, and sons on exercise bikes was measured in 86 nuclear families. VO2 max turned out to have a heritability of about 51 percent in these individuals. The other 49 percent of the variation might be accounted for by diet, attitude toward exercise, daily activity pattern, or other factors.
Taken together, these wide-ranging values for heritability tell runners and their coaches that genetics do play a role in performance; after all, heritability does not drop below 23 percent and can be as high as 87 percent. This is hardly a shocking discovery, however, and it contains no practical information for a runner or coach. It is impossible for an individual to tell to what extent his or her performance is based on genes rather than environment, nor would such knowledge have a significant impact on training, which should always be formulated to be the best, most up-to-date, and most scientifically based regardless of underlying genetic constitution.
Note also that heritability studies have trouble truly differentiating between genetic and environmental factors. Family members share not only their genes but also their environments, which undoubtedly include important dietary and psychological factors. Thus, some unknown portion of the genetic variance for the heritability of a trait is probably environmental in nature.
The heritability studies referred to in this chapter suggest that an individual's capability for distance running is determined by both genetic and environmental factors, and the exact proportion of influence is unknown. This is certainly logical, because it is unlikely that a runner's performance characteristics would be completely unmarked by either genetic or environmental elements. However, the heritability research has not been carried out with elite athletes, and it does not answer the basic question of whether Kenyan and other African runners enjoy a genetic superiority that causes the variance due to genetic factors (i.e., the genetic contribution to performance) to be maximized.
Read more from Running Science by Owen Anderson.