For out-of-shape kids, inadequate physical activity—a leading risk factor for mortality across all ages, according to the World Health Organization—can have lasting ramifications for future health. All over the world, researchers are finding significant annual declines in cardiovascular fitness among adolescents. As a whole, today’s youth are spending less time than generations past on stick ball, tag, and other types of free play and exercise, such as walking or bicycling, and more time with computers and smartphones. As a result, after-school activities such as organized sports now provide the majority of physical activity for growing children.

But as the rate of childhood obesity continues to rise and the general aerobic fitness of children and adolescents continues to fall, another trend has emerged: Many of the children that still play sports often do so in excess.  While a great deal of attention has been devoted to the demanding year-round format of competitive youth baseball and soccer,  an increasing number of young athletes are also training for and competing in long-distance endurance events.  These young athletes are doing more than kids’ “fun runs” and 5K’s—they’re running marathons and Olympic-distance triathlons. Notably, the Students Run Los Angeles marathon-training program had more than 16,000 young marathon finishers between 1987 and 2005.

The trend presents an interesting conundrum for pediatricians as they attempt to get kids off of iPads and Xboxes and onto tracks and athletic fields: How much activity is too much? While highly active kids may be improving their cardiovascular health, they also run the risk of overuse injury, which in turn could prevent them from exercising altogether. “Athletes who sustain recurrent overuse injuries may stop participating in sports and recreational activities,” the 2011 National Athletic Training Association’s position statement on pediatric overuse injuries warns, “thus potentially adding to the already increasing number of sedentary individuals and the obesity epidemic.”

Surprisingly, the issue has not been the subject of a great deal of scientific inquiry, and for parents concerned about their young runners, there isn’t a definitive answer. While orthopedic surgeons have warned parents about the dangers of too many baseball throws and soccer kicks, they have been less definitive about running. So when should parents be wary of their children’s running? At present, medical professionals can only speculate about how much running is too much for young athletes.

The unresolved question is whether there are any long-term health consequences to endurance training and racing that would make events like half-marathons and marathons inadvisable for younger competitors. After all, children’s bodies have their own unique needs and considerations when it comes to physical activity.  “Children are not small adults,” cautions a position statement in the Clinical Journal of Sports Medicine titled “Children and Marathoning: How Young Is Too Young?” “Their anatomy and physiology are developing and not fully mature.”

As youth participation in running continues to rise, medical professionals are concerned that injuries will rise along with it. Running by its nature is a repetitive sport, one that involves thousands of collisions with the ground, and the risk of injury at any age is proportional to the number of miles run—even among adult runners, the annual injury rate can be as high as 70 percent. Because the growing areas of children’s bones are vulnerable to injury in the formative years, they may be even less able to withstand the repetitive stress of marathon running or training.

Highlighting the differences between younger and older runners, a recent article in the Journal of Athletic Training revealed that children don’t absorb the impact of running as well as adults.  While the researchers were unable to identify a specific reason for the differences, the health implications are worrisome: Like a car with bad suspension, less absorption equals greater impact to bones, joints, and soft tissue, possibly leaving children more at risk for overuse injury.  

Other studies have established that, compared to adults, children run with different mechanics and have shorter legs in relation to their body size, elements that may contribute to a diminished ability to absorb impact. Dr. Cordelia Carter, a pediatric orthopedic sports surgeon and researcher at Yale University, says, “Kids are still figuring out how to move and their bodies don’t yet have the strength and muscular control of adults.”

The problem with quantifying the risk of injury for marathons and other long-distance events is that the long-term effects are still unknown: No one really knows for sure what happens to kids five, 10 or 20 years after running a marathon. Researchers can only hypothesize that repeated impact to maturing joints can cause serious long-term injuries to delicate joint cartilage or growth plates—conditions that don’t occur in adults.  These theories are based in part on research that has found evidence of growth-plate separation in response to repeated impact and trauma in young gymnasts.  Most of these gymnasts recovered, but in a few of these cases the bones of the forearm were permanently damaged.

Heightening the ambiguity is the fact that not all children mature at the same rate—picture the average high school freshman gym class, in which some boys need to shave and others barely crack five feet. Since physiological age may not equal chronological age, universal recommendations are difficult. Young girls typically mature more quickly and reach full maturity by 14, while boys can still be growing past 18.  “That’s why blanket statements about age don’t work,” Dr. Carter says.

Given this wide variation in maturity at any adolescent age, physicians admit that any guidelines they set for when children and adolescents can safely handle the stresses of training for longer events would be arbitrary.  “But it’s no different than setting arbitrary pitch count limits for Little Leaguers,” says Dr. Lyle Micheli, Director of the Sports Medicine Division at Boston Children’s Hospital, “and establishing 18 as the minimum age for marathons, while not perfect, minimizes the chances of long-term damage in younger competitors.”  The Boston Marathon, among others, has long used 18 as the minimum age for participation.

But is it fair to restrict marathons to those 18 and older when some “kids” may more closely physiologically resemble adults by an earlier age?

Medical professionals’ advice on the subject is divided.  Some, like Dr. Micheli, feel that children have proven to be vulnerable to the repetitive stress injuries that can have long-term negative consequences.  “Until we know more,” Dr. Micheli says, “I don’t advise that runners under the age of 16 participate in any event longer than a 10K.” A review article on children and marathons in the Clinical Journal of Sports Medicine supports Dr. Micheli’s statement, stating, “Training to run in a marathon, which is more than eight times the usual cross-country competitive racing distance, is an inappropriate activity for children and adolescents.”

Conversely, due to the lack of evidence supporting or refuting the safety of kids running marathons, the American Academy of Pediatrics’ statement on overuse injury says, “There is no reason to disallow participation of a young athlete in a properly run marathon as long as the athlete enjoys the activity and is asymptomatic.”

So how do parents and coaches wade through these contradictory recommendations?  “We have to use common sense,” Dr. Carter says. “If running starts to hurt, you need to stop.”  She continues, “But to definitively answer that question, we need to understand more about how kids physiologically respond to distance running.”  Until then, she prescribes cross training and lots of rest to allow joints and bones to recover from the stresses of running.  Above all, she says, pediatricians should urge parents and coaches to keep running and endurance sports fun, improving the chance that kids will embrace activity as a lifelong pursuit.