From Nick Soleyn, BLOC Editor in Chief

Every culture has its oral-tradition boogiemen. In New Mexico, in a decades-long campaign to keep kids from playing in ditches and acequias. We were sent home with bumper stickers, t-shirts, and flyers with the message—all caps on a yellow background—“DITCHES ARE DEADLY, STAY AWAY!” next to a picture of a scolding old woman. Why an old woman? Because terror has, historically, been considered an effective instructional medium to warn kids against danger until they are old enough to know better, and for generations, parents in Mexico and the Southwest United States have been telling their children, “Stay away from the water, or La Llorona will get you.” La Llorona, the wailing woman, looking for her drowned children, might snatch you up if played in ditches where shallow, fast-moving water can sweep you away and flash floods can turn a perfect skateboarding surface deadly in seconds with no hint of rain overhead. Terror for a purpose.

Stories like these tend to stick around along with old wives’ tales and urban legends. Once, when my son had a bloody nose at sport’s practice, a helpful grandmother nearby handed me a penny to put on his forehead. I obliged, the bleeding stopped, and a lingering home remedy was given some small infusion of new life. Most of them are harmless, some may be useful, but others we have to confront as flat-out wrong or even harmful.

In barbell training, the idea that lifting weights will stunt kids’ growth is one of these baseless boogieman stories that got told somewhere out of well-meaning ignorance and repeated enough that parents continue to wonder whether allowing their children to lift weights will cause them to stop growing. Despite overwhelming evidence to the contrary and numerous resources on the benefits of weight training for kids, we take a deeper dive to answer that question here. Hopefully, in doing so, we can encourage more parents to lift weights with their kids. Barbell training, done properly, is safe and useful for growing kids, and it can be done at home with minimal equipment and lots of patience.

Does Lifting Weights Stunt Growth?

No. And, it is tempting to leave the discussion at that. There is a significant weight of expert evidence supporting this simple negation. Research into kids and strength training has moved on in the past decades, focusing instead on the benefits of strength training for kids and teens. Not only does lifting not stunt growth, but it is also often recommended as part of a healthy lifestyle, along with good nutrition and regular exercise and activity. Yet, despite a definitive answer to this question existing out there on the internet, people are going to continue to ask it. Because, like old wives tales and urban legends, the oral tradition that plays on our fears for our children is obdurate, gaining new life every time a parent who heard somewhere that repeats it or a long-practicing pediatrician refuses to acknowledge that some ideas have been updated in the last 30 years.

We are going to look at this question by first addressing where the original concern seems to come from. To do that, we have to look at growth in general and bone growth specifically.

What is Growth?

There is no standard human height, weight, or body proportions. Though they present in a recognizable range, there is significant variability in what is considered normal or healthy, and there is no such concept as the “ideal” human proportions. Individual variations are too numerous, and far too much depends on both genetic and environmental factors. When doctors measure children’s growth, they look at a host of factors and consider the normal range of growth for the kids’ age, weight, and gender. Growth is one indication of health. A child that is growing normally can be said to be doing some things right, and a child whose growth is stunted or abnormal may benefit from an investigation into possible hampering effects in the child’s environment. So, growth is a signal, and we want that signal to indicate normal, healthy environments.

These measures of growth, however, are whole-body, big-picture measures and do not tell us much of what is happening at a cellular level. From the earliest indications of life, the body starts developing and growing through a process of cell differentiation. Cells divide and organize themselves according to a general human blueprint, collecting together in the right places to become organs and tissues, forming the basic structure. Differentiation occurs when unspecialized cells become specialized. All of the structures in your body came from the same pool of stem cells that were encoded and influenced by other cells to develop in particular ways to perform certain tasks. Some became muscles; some became blood cells, some neurons, and so on. As development continues, cells take on a particular role in the body, allowing for development along a narrower functional path.

Eventually, the cell type gains an identity and develops not so much in terms of function but in terms of size and repair. Once your bones become bones, they form the way bones are meant to form. They will not suddenly sprout ears or change their type. This is growth. Humans grow by increasing the number of existing cells, increasing the amount of non-cellular material around cells, and increasing the size of existing cells.

Original concerns about lifting weights stunting growth come from a basic understanding of how bones grow. If you had heard the line that lifting weights stunts growth, then as soon as you started reading this article, you probably thought about growth plates.

The belief that lifting stunts kids’ growth did not come from observations of actual growth. No one noticed a bunch of stocky, jacked kids hanging around the weight room and started asking whether these mini-Arnolds had been neglectfully unsupervised near the squat rack. There is simply no observational evidence for the claim. Instead, the belief that lifting stunts growth came from an understanding of growth plates and some generous inferences about how lifting might affect them.

Growth plates are sites at the end of a developing skeleton’s long bones where cell differentiation continues until a person reaches maturity.^[1] A child’s growth is cumulative changes in the musculoskeletal system that contributes to their height and body proportions. There are many observable measures of growth: standing height, sitting height, subischial leg-length, chronological and bone ages, annual growth, proportions, and weight. All of these factors are the consequence of micro-growth and cellular level changes.

Children have more cartilage in their joints that develops into hardened bone through ossification. Growth plates (epiphyseal plates) are areas of stacked regions of cell activity. At the far end of the bone is reserve cartilage. Then, moving toward the center of the long bone, comes the proliferating region, where cartilage cells (chondrocytes) multiply in response to growth hormone. Next comes the hypertrophic region where the chondrocytes themselves mature and grow. The chondrocytes undergo a final differentiation when they become hypertrophic, dying and hardening into bone. This final change represents the ultimate specialization of the cell, its final phenotypic change, and is what causes bones to grow in the direction of the growth plates. As long as the plate can produce healthy chondrocytes, bone growth will continue, and it continues in the direction of the growth plate due to the location of the proliferation of cartilage cells. Eventually, the proliferation of new cells slows down relative to ossification, and the growth plate thins until the entire growth plate is replaced by bone. When this happens, the bone stops growing in length.

If growth plates are damaged, the bone may grow abnormally or exhibit stunted growth, which is where the concern over lifting weights comes from. The rationale against lifting looks something like this: Lifting has some non-zero probability of damaging growth plates. Therefore, lifting weights can stunt children’s growth. There are some problems with this argument.

Damaged growth plates are the causal factor in the argument. So, let’s start there. The first question is whether all activities that have a non-zero probability of damaging growth plates is a small number (meaning is lifting weights unique harmful in some way) or a large number?

Growth plates get damaged all the time, most often from youth sports like gymnastics and soccer or from accidents—car crashes, bike crashes, human on human crashes, and the like. Growth plate fractures are bone fractures, and while they may break more easily than hardened bone, the causes are mostly the same. If we decided that all actives that might cause growth plate fractures are to be avoided, then we should either all be living in bubbles, or we should have an epidemic of malformed children. Neither is the case. So, what’s going on.

Growth plate fractures are treatable, like bone fractures. Proper identification and treatment will keep the bone growing in the right way and at the right rate to maintain average, normal growth for the child. As we will talk about later, growth plates are sticklers for maintaining growth rates. While they can be slowed down, they will speed up growth to catch-up when the inhibiting factor is fixed.

So, maybe a non-zero risk of fractures is not the concern. Maybe stunted growth comes from activities that are really dangerous to growth plates. If that’s the case, then once again, singling out lifting weights is inconsistent. Lifting weights is consistently one of the safest physical actives for kids and adults.

So, why did lifting weights get the bad rap of something that will stunt growth in kids? To paraphrase Monty Python and the Holy Grail, because “it looks like one.” Lifting induces compressive, shortening forces on the musculoskeletal system. It looks like it should squash you down and keep you from growing big and tall—like foot binding for the whole body.  Perhaps whoever came up with the stunted growth theory just reasoned that if we temporarily increase the gravity acting on the skeleton, the skeleton would respond to make us shorter. But that is not how skeletons work, and it is not how adaptation works. In fact, adaptation works to return to homeostasis (normality) when presented with an obstacle, often overcompensating by making the body better or stronger than it was before the stress placed on it.

Bones and Compressive Forces

Ignoring kids and growth for a minute, let’s detour to how bones tend to respond to compressive stress. Wolff’s law states that bone in a healthy person will adapt to the loads under which it is placed. The body and its tissues are amazingly adaptable. In strength training, we harness the adaptability (plasticity) of skeletal muscle to make us stronger.^[2] And we can observe a similar in bones to compressive forces. Compressive forces come from gravity and external loading and the constant tension of muscles and tendons pulling on the attachment points on bones. Bigger, stronger muscles tend to provide more tension than smaller muscles. The result of strength training on bones tends to be stronger bones in adults due to the greater compressive forces acting on them constantly. More tension, bigger muscles, and more compressive loading result in thicker (generally healthier) bones, not shorter ones.

That is not to say that lifting weights will produce giants. There is no evidence that resistance training or exercise of any kind has a positive effect on the adult height of developing children.

What Does Affect Growth?

Bone length is the result of activities at the growth plates, and these actives are most affected by hormonal and nutritional factors. Growth plates do not seem to care about compressive forces.

Longitudinal bone growth is mostly dependent on the natural cycle described above, its acceleration, and its cessation as the child enters adulthood. One theory is that the growth plate follows a pre-programmed timetable for growth. When they reach their limits, growth slows, then stops as the growth plate becomes exhausted and, ultimately, fused. However, this timetable, the growth plate’s senescence, can get interrupted, causing catch-up growth. (Ola Nilsson and Jeffrey Baron, 2004)

Catch-up growth describes growth rates above the statistical norm for a child’s age and gender due to earlier inhibiting factors. Factors that stunt growth are studied along with catch-up as part of initiatives to improve children’s health worldwide. The main factors that stunt growth and may lead to later growth disorders in children, adolescents, and teens are malnutrition and malabsorption of nutrients due to disease, endocrine disorders like hypothyroidism, psychosocial stress affecting growth hormone secretion, and disorders in specific organ systems. (Bart Boersma and Jan Maarten Wit, 1997). “On a worldwide basis, the most frequent cause of being small at birth and short in infancy and childhood is protein-energy malnutrition.” (Boersma and Wit) If these conditions can be treated successfully, the child will exhibit a spontaneous catch-up growth spurt. Notably, growth problems are almost never associated with healthy actives such as exercise.

While growth deficiencies are a major concern for the world’s childhood population, the battlefront is focused on nutrition, sanitation, and education for those areas that are at risk. There is no evidence that sports, compressive forces on bones, or that growth plate fractures represent a major issue for normal growth in children.

The Benefits of Lifting

Kids need to be fit and strong, just like adults. If we successfully dispel the myth that lifting is harmful, we should consider what role it might play in a healthy lifestyle for children. One of the best things you can do for kids is to set a good example of a healthy lifestyle. Studies tend to show that children whose parents are active, healthy, flexible, and strong are more likely to be as well. While genetics plays a factor, that factor is far too complicated to attribute to inherited healthfulness. Instead, “nongenetic factors (including lifestyle factors shared among family members such as diet, television viewing time, and so forth) contribute in a major way to the familial resemblance observed in fitness.” (Teran-Garcia 2008) We have many discussions about the health benefits of lifting for adults. If you train, your kids might want to join you. The power of this mimicry should be exploited as much as possible to teach kids what healthy and strong looks like.

We cannot say that lifting weights will make a child grow better. Their bones may become denser, and they may respond favorably in growth. But we can say with certainty that exercise of any kind, including lifting weights, does not stunt a child’s growth. Exercise forms a key part of a healthy lifestyle that includes not just activity, but nutrition and healthy habits are going to benefit children in the long run. If we can gift these things to a child by demonstrating them and letting them join us in the weight room, good things will follow.

[1]. Dimeglio, How Kids Grow (2001) (““The mature musculoskeletal system is the end result of a highly ordered, coordinated sequence of cellular and extracellular matriculates events initiated early in embryonic life and continued to skeletal maturity.”)

[2]. Muscles, when mechanically loaded beyond their normal functioning, will tend to get stronger. In the homeostatic framework, stress must be sufficient to disrupt the system’s happy state of equilibrium in order to signal change and cause muscle growth. Protein synthesis is directed by the mTOR signaling pathway. “The mTOR pathway regulates homeostasis by directly influencing protein synthesis, transcription, autophagy, metabolism, and organelle biogenesis and maintenance.” (The Neurology of mTOR) In the regulation of protein synthesis, mTOR signaling responds to mechanical loading. So, at least part of what makes us stronger is that when we train, the loading and mechanical strain of training signals an increase in muscle protein synthesis. (Mitsunori Miyazaki and Karyn A. Esser, “Cellular mechanisms regulating protein synthesis and skeletal muscle hypertrophy in animals,” (April 2009).) This is not the only signal that causes increased protein synthesis. From the same research article: “We believe that this mechanical pathway likely acts synergistically with changes in amino acid uptake and growth factor availability to contribute to the prolonged activation of mTOR signaling following high resistance contractions/mechanical overload.” (Id.)