I ran my first grade school cross country in the third grade. What I remember from practice was how much time I spent trying to touch my toes, and how bad I was at it. Years later, that was a reassuring reminder, because as a teen and adult runner, I remained shockingly stiff, despite the hours of stretching I spent each week. In my kindergarten class photo, I can be seen sitting cross-legged with everyone else happily, while I looked clearly uncomfortable with my knees nearly reaching my chin. This was an inherited fitness impairment, not a reflection of my laziness or aversion to stretching.
So it’s no surprise to me that new research has found that of all areas of fitness, flexibility is the one most influenced by genes. Publication year Medicine and Science in Sports and Exerciseuses data from twin pairs to elucidate the respective contributions of genes and environment (which can also be thought of as talent and training) across 15 different fitness tests. Overall, the results support the idea that parental choice is a crucial step on an athletic’s path to stardom, but they also reveal surprising nuances about how nature and nurture interact.
How genetic traits and trainable abilities were tested
An international team of researchers led by Kari Silventoinen of the University of Helsinki tested 198 pairs of twins aged between 6 and 18 on the Portuguese island of Madeira. All twins took 15 different fitness tests, and the results were analysed to reveal the extent to which differences between individuals are determined by genes and environment.
The key point is that 78 of the twin pairs are identical twins, meaning they share exactly the same DNA, while the rest are fraternal twins (or sister twins), who share, on average, half their DNA.When the results of a particular test are more similar in fraternal twin pairs than in identical twin pairs, it indicates there is a genetic influence.
The role of the environment can be split into two components: common environmental factors include the area you grew up in, your socioeconomic status, opportunities to play sports, etc., and unique environmental factors reflect a particular path in life, such as whether you joined a sports team, broke your ankle, or had a good physical education teacher. Surprisingly, in contrast to previous data, the influence of common environmental factors appears to be negligible in the new study, and the analysis focused only on genetic and unique environmental factors.
The 15 fitness tests were taken from two different standardized test batteries. Eurofit included the flamingo test (balancing on one leg), plate tapping (moving your hand back and forth between two plates as quickly as possible to test reaction time and agility), sit and reach (touching your toes from a seated position), standing long jump, hand grips, sit-ups, bent arm hangs (holding the top position of a pull-up for as long as possible), 10 5-meter shuttle runs, and 12 minutes of running/walking. Fitnessgram included separate sit-and-reach exercises on the right and left sides, core lifts (lying on the floor and lifting your abdomen and upper body as high off the ground as possible), curl ups (partial sit-ups), push-ups, and the 20-meter shuttle run (better known as the beep test, where you have to run faster and faster for 20 meters until you can no longer keep up with the beep).
What the results showed
Overall, as you can see from the graph above, genes play a big role in the outcomes, with genetic contributions ranging from a low of 52 percent for the standing long jump to 79 percent for the seated reaching flexibility test. Researchers say this range of heritability is similar to or slightly lower than that for height and BMI in children, but higher than the heritability of personality and other psychological traits in adults.
One of the most interesting questions is how well performance on one test predicts performance on the others. After all, why would the same person take a nine-component fitness test if they perform well on all components? Overall, the correlations between the different tests were “moderate to moderate,” which is far from perfect. In fact, we found very little overlap between the tests (except for the three versions of the Sit-and-Reach test, which were basically measuring the same thing).
The test that correlated the least with the other tests was the trunk lift test, which appears to be separate from skills like, say, how fast you can run or how far you can jump. Conversely, the three exercises that correlated the most with the other tests were the push-ups, standing long jump, and the beep test. If you want a quick and easy assessment of someone’s overall fitness, these three tests are your best bets.
Nuances of birth and upbringing
If I had to guess, I would say that sprinting and explosive power abilities are probably more heritable than aerobic endurance. We’ve all heard the stories of untalented, “bad” runners who, after years of hard work, turn into great marathoners. Less common, at least as far as I know, are slower, less coordinated runners who put in long hours of plyometrics and weighted sled pulling to become champion sprinters. But the results here don’t support that assumption. The standing long jump is probably the best of these tests to measure explosive power, Lowest Genetic contribution.
This may be due in large part to differences between two dimensions of talent: untrained performance level and trainability, an idea discussed by David Epstein in his 2013 book. Sports Genesand it has been discussed at length in the long-running debate about the nature of talent. Given 21st-century life, we can assume that the majority of these twin pairs were untrained, at least in a formal sense. Long jump performance may reflect the explosive properties of muscles and a degree of coordination developed from an active (or inactive) childhood, but it does not reflect an intensive effort at training.
Similarly, the high heritability of the 12-minute result does not reflect the impact of, say, running 100 miles in a week. All these estimates of heritability reflect our starting point, but they don’t tell us much about where we could ultimately get to if we try hard enough. Trainability is also partly determined by genes, but to a large extent by different genes than our untrained performance levels.
One last point worth making: A closer look at the data reveals that flexibility (measured by the sit-and-reach test) and aerobic fitness (measured by a 12-minute run/walk) are most heavily influenced by genetics. But not necessarily the same genes. Someone who is naturally good at one fitness domain can be terrible at another, and vice versa. This isn’t necessarily a bad thing. In fact, Hard evidence People with less flexibility tend to be more efficient runners, and my pathetic natural flexibility turned out to be a superpower.
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