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Moira Petit
Kinesiology
PHOTO BY REBECCA NORAN
Cross-sectional images of a human lower leg produced using peripheral
quantitative computed tomography (pQCT).
Images show tibia and fibula bones (white), muscle (purple), and
fat/skin (blue). Top image taken below the knee at the middle of
the lower leg; bottom image taken near the ankle.
Bottom image demonstrates how pQCT scans can distinguish between
compact/cortical (white) and spongy/trabecular (orange) bone material
to quantitate bone strength.
IMAGES COURTESY OF MOIRA PETIT |
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Today's children have grown
up in a remote-control world, never knowing the days of having to
get up off the couch to change channels on the TV. In an era when
the term "couch potato" is now part of our lexicon, health care
professionals have observed a skyrocketing increase in levels of
childhood obesity. This increase has been attributed in large part
to physical inactivity. Physical inactivity not only leads to obesity
and poor cardiovascular health, it also has negative effects on
bone health. Moira Petit, assistant professor of kinesiology, studies
the role of physical activity for optimizing bone health during
growth, particularly during adolescence. Petit's work is helping
to redefine thinking about what constitutes bone health and to find
better ways to build bone strength.
Measuring bone
Traditionally, clinicians measured bone health by considering bone
mineral density in an adult population. Loss of bone density in
this population was blamed on poor nutrition and hormonal changes.
Methods for measuring density were designed for assessing non-growing
bone in two dimensions and did not adapt well to studies in children.
More recently, engineers, anthropologists, and exercise physiologists
have begun to take a functional approach to study bone. Because
bones function to support a mechanical load (a force exerted by
body weight, muscle, growth, or activity), these scientists have
examined the biomechanics of the long bones over a long period of
human history.
Such studies have been aided by the advent of new techniques. One
such technique, peripheral quantitative computed tomography (pQCT;
see images on the left), allows researchers to go beyond measuring
just bone mass or density to be able to estimate the actual strength
of bone. "We can now look more closely at how bone is distributed
and shaped in a cross-section," says Petit. "These new three-dimensional
methods look at aspects of bone shape and size, and are much better
for examining growing bone. In addition, we can look at the cortical
and trabecular bone compartments separately using pQCT." Given these
developments, bone health and strength are now defined in a functional
way, based on how its shape, size, and distribution of mass relate
to its functional requirements for loading (especially muscle).
Bone development in children
Bone is constantly formed and resorbed throughout life in a generally
balanced scheme. However, in a three- to four-year window during
puberty, bone formation is accelerated. In that period, as much
bone material is deposited as will be lost during a person's entire
adult life. During these pivotal years of bone development, physical
activity is important for optimizing bone health, as it has been
shown to reduce the incidence of fractures later in life.
Before coming to the University of Minnesota this year, Petit participated
in the Penn State Young Women's Health Study, a 10-year longitudinal
study of over 100 young women from ages 12 to 22. That study included
assessments of bone, calcium intake, physical activity, muscle mass,
and hormones. The study demonstrated that lean muscle mass, which
exerts the greatest load on bones, was the only predictive factor
of adult bone mass and strength. According to Petit, "All the calcium
in the world won't help [your bones] if you don't do any loading."
Petit's other studies in school children also support the importance
of mechanical loading in promoting bone health. Activities such
as jumping and gymnastics exert the kind of high impact force that
develops bone. Petit found that having school children do just one
minute of jumping (10-20 jumps) three times a day, three to five
times a week caused the children to gain more bone mass. The downsides
of such an activity is that it is not sustained enough to improve
cardiovascular health or to promote weight loss.
Because it is difficult to motivate children to participate in the
type of cardiovascular activities that adults engage in (running,
cycling, aerobics), new strategies must be developed. Petit thinks
she has found a way to motivate kids: the video arcade dance game
Dance Dance Revolution. In Dance Dance Revolution, a participant
responds to a series of directional arrows displayed on a video
or TV screen to perform choreographed dance steps or hops synchronized
to music. Song tempo and degree of difficulty increase as the player
successfully progresses in the game. Because of the game's popularity
and its cardiovascular exercise and jumping (bone-building) components,
it could represent an appealing model for reducing physical inactivity
in children.
Petit has recently requested funds for a pilot study in which children
ages 8-11 would use Dance Dance Revolution for 16 weeks both in
after-school programs and at home. At both the beginning and the
end of the study, participants would be assessed at the University
of Minnesota's Laboratory of Physical Hygiene and Exercise Science
for body composition, height, weight, bone strength and mass (using
pQCT; see images on the left), and cardiovascular fitness.
The proposed project includes co-investigators in the Department
of Medicine (Steve Stovitz and Kris Ensrud) and the School of Public
Health's Division of Epidemiology and Community Health (Mary Story
and Diane Neumark-Sztainer). Also collaborating on the project is
Brock Dubbels, a teacher in the Minneapolis schools and a U of M
education Ph.D. candidate whose interests include the effectiveness
of video games as educational tools. The pilot program would serve
both to gather preliminary data in support of an application for
multi-year funding and as a feasibility study. If the response from
the schools is any indication, Petit may have a hit on her hands.
"We had such an overwhelmingly positive response from the after-school
programs in Minneapolis, we had to stop recruiting," she notes.
Adult bones
Although Petit's work focuses on building bones during adolescence,
her work also has implications for bone health in adults. As of
now, many unanswered questions remain to be explored."Adults lose
bone with weight loss, but we don't know what happens in children.
We also don't know if all of [the bone loss] can be replaced with
training," says Petit. Despite all the unknowns, mechanical loading
does appears to be the key to building bone. "People on bed rest
or in wheelchairs, when given hormones associated with bone development,
such as estrogen and parathyroid hormone, had no new bone formed
unless they also had some mechanical loading of their bones," says
Petit. Interestingly, that load need not be great to be effective.
"Wheelchair-bound people probably generate a sufficient load through
muscle spasticity to build some new bone," notes Petit.
Knowing what she knows about mechanical loading, will Petit be embarking
on a Dance Dance Revolution program to build her own bones? The
chance seems unlikely. "I don't even own a TV," she says with a
smile.
Moira Petit's faculty profile: education.umn.edu/Kin/faculty/mpetit.html
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