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Lambs at rest hint at origin of growth spurts, pains

It has been evident to parents since time immemorial: Children, during their active growth years, gain stature in spurts, often overnight. But that bit of conventional wisdom has never been documented scientifically — until now. With a bit of help from some resting lambs, a team of biomedical researchers has confirmed that growth — at least in lambs, but very probably in other animals, including humans — does indeed occur when animals are at rest.

That clue may also 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 a study published in a recent (November/December 2004) issue of the Journal of Pediatric Orthopedics.

The study was conducted by surgically implanting tiny sensors in the tibias of lambs. Continuous telemetric measurements showed that at least 90 percent of bone growth in lambs 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.

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, has never 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.

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 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.”

In addition to Noonan, Wilsman and Lampel, authors of the new study include Cornelia Farnum of Cornell University, and Ellen Leiferman and Mark Markel of UW-Madison.

From University of Wisconsin-Madison’s School of Veterinary Medicine




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