Growth plates are soft zones of cartilage near the ends of bones. When a young animal -- or human -- is standing, walking or running, pressure may compact the plate, inhibiting growth.

Growth Is Not Continuous

That clue also may help scientists peel back some of the mystery of another unproven tenet: that the sudden leg pains experienced by youngsters, frequently at night, are truly growing pains.

"Often, our vision of the growth of children is that it's continuous growth, especially if we just look at annual growth measurements, or the growth charts developed by health agencies, such as the Centers for Disease Control," says Norman Wilsman, of the University of Wisconsin-Madison's School of Veterinary Medicine and the senior author of the study.

By surgically implanting tiny sensors in the tibias of lambs, the researchers obtained continuous telemetric measurements that showed at least 90 percent of their bone growth occurred when the animals were sleeping or otherwise at rest.

"We observed this noncontinuous pattern of growth, but what was really interesting was that the bones were growing only when the animals were lying down, and almost no growth occurs when the lambs are standing or moving around," according to Wilsman, a professor of comparative biosciences.

Growth Spurts Documented

That children grow in spurts over the course of a few days has been documented in recent years by Michelle Lampel, an Emory University anthropologist and one of the new study's authors. But the concept of resting growth, and perhaps a relationship to growing pains, never has been demonstrated.

"This is a study that points out that growth is not a continuum," explains Kenneth Noonan, a UW-Madison Medical School professor of orthopedics and the lead author of the study. "There are growth spurts, which may occur within the daily life of lambs and possibly humans, too."

What may be occurring, says Noonan, is that when an animal is at rest, pressure on the growth plates of long bones, such as the tibia, is eased, permitting the bones to elongate.

"I can't imagine other bones aren't behaving the same way," Wilsman adds.

Pressure May Inhibit Growth

Growth plates are soft zones of cartilage near the ends of bones. When a young animal is standing, walking or running, pressure may compact the plate, inhibiting growth.

"Growth plates may be like springs that, during standing and walking, experience compression and tension," says Wilsman. "When these inhibitory strains are eased -- as when an animal lies down or goes to sleep -- they resume growing. That idea seems to make some sense, especially when one considers that growing pains occur predominantly in the weight-bearing lower extremities."

Humans grow over the course of the first 18 years of life. In lambs, growth is compressed to a few years. Extending the data collected in the new study over a greater number of years helps match it to growth patterns in humans, says Wilsman.

The new study does not provide a definitive link to nocturnal growth and the pain that some children experience, but it does provide new data that may begin to put those phenomena on a scientific footing, Noonan notes.

Growing Pains Can Be Intense

Growing pains are one of the most common problems seen in pediatric orthopedics.

"The pain usually comes at night and may be somewhat relieved during the day," says Noonan. "It can occur several nights in a row and it can be intense. Kids wake up and they are in pain."

Other than to ease the symptomatic pain, there is nothing clinicians can do to prevent or treat growing pains, says Noonan. "It is a normal thing, but this study brings some interesting ideas (about growth spurts and growing pains) together rather tidily."