Course:KIN366/ConceptLibrary/Bone Density

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Movement Experiences for Children
KIN 366
Instructor: Dr. Shannon S.D. Bredin
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Bone forms the skeletal structure of the human body and contains both organic materials like collagen and inorganic minerals such as calcium and phosphorus (Gilsanz, 1998). Bone Density or Bone Mineral Density (BMD) is the amount of mineral per square centimetre of bone which is sometimes recorded in grams per milliliter. This measure is widely used in clinical practice and serves as a good predictor of bone health, bone loss (osteopenia), and risk for osteoporosis and fractures (U.S. National Library of Medicine, 2011). Attaining the optimal bone density is widely acknowledged as a fundamental health goal during childhood and adolescence when the development of bone mass and balanced skeletal growth is expected to occur (Ausili et al., 2011).

Importance of Bone Density for Movement

The skeletal structure is continually broken down and reformed in a balanced, dynamic process. Bone density can be influenced by many factors which can cause disruptions in the balance of mineral levels and the processes of forming or absorption. When minerals are taken away from the bone faster than they can be replaced, the density decreases and the bone becomes weak and brittle (Ma, Samartzis & Shen, 2008).

Bone density is important in maintaining bone integrity that is essential for all movements. Though increases in bone density may not directly enhance a child’s movement experiences, it is needed to prevent common injuries such as fractures which would limit physical activities. Consequently proper bone density allows participation of activities that can then facilitate development of fundamental movement skills required for proper development (Matkovic, 2007).

It is also critical to increase bone density development in childhood in order to maximize the amount and quality of total bone that is produced by the body. At approximately 20 years of age, the rate of bone formation becomes equal to or less than the rate of bone reabsorption (McDevitt & Ahmed, 2010). This means that amount of bone you have built up to this point is the maximum amount you will have continuing into adulthood. Starting with a solid base of bone health in childhood on can potentially allow a more physically active lifestyle throughout the rest of a person’s life. (Matkovic, 2007)

The current problem in Canada is that children are less physically active and instead are spending a large part of their time participating in more sedentary activities (Janssen & LeBlanc, 2010). This significantly decreases the likelihood that a child is participating in movements that will facilitate bone growth. Consequently there are higher amounts of fractures across their lifespan as well as an increased chance of osteoporosis later in life (Greer & Krebs, 2006). In Canada, 1 in 4 Canadians have evidence of fracture on the vertebral column. In Canada alone, approximately 30,000 hip fractures happen each year, and it was predicted that by the year 2030, the incidence of hip fractures would quadruple (International Osteoporosis Foundation, 2012). In addition, osteoporosis affects approximately 1.4 million Canadians, primarily postmenopausal women and the elderly. Osteoporosis is greatly caused by low bone density, affecting 1 in 4 women and more than 1 in 8 men over the age of 50 years (International Osteoporosis Foundation, 2012).


The measurement of bone density in children is an important way to diagnose loss of bone mineral or osteopenia caused by various disorders. Additionally, this process contributes to the understanding of how bones change overtime and where and when we can target treatments to prevent future diseases such as osteoporosis (Gilsanz, 1998). Although bone density can be quantified through techniques such as ultrasound and qualitative computed tomography (QCT), the dual-energy x-ray absorptiometry (DXA) scan is the most developed and biologically validated method (Calatayud, Borreani, Moya, Colado & Triplett, 2013). The DXA scan is recommended for children as it is a non-surgical, painless examination that measures the relative tissue absorptions of low dose x-rays to determine density (International Society for Clinical Densitometry, 2012). DXA usually measures the BMD specifically at the hip and spine, however for children, it is recommended to measure the total body BMD (TBBMD) which includes the bones of the lower and upper extremities. This is important as TBBMD also takes into considerations estimates for child specific body composition differences such as increased in the bone mass contribution of the skull relative to the subcranial skeleton (Matkovic, 2007). The DXA measures are reported and analyzed as T-score and Z-score:

  • T-scores are contrasted to the ideal or peak bone mineral density of a healthy 30-year-old adult. The difference between the measured BMD and the optimal levels of a healthy young adult are measured in units called standard deviations (SDs). A score of zero infers a BMD that is equal to the ideal bone density of a healthy young adult, a negative score (-1 to -2.5 SD) indicates a greater risk for fractures and a score of -2.5 and lower is indicative of Osteoporosis (National Institute of Arthritis and Musculoskeletal and Skin Diseases, 2012).
  • Z-scores contrasts the bone density results to a reference that are from the same age group, gender and race. This analysis is recommended to use for children, teens, women at their menstrual age, and younger men as oppose to the T-scores (National Institute of Arthritis and Musculoskeletal and Skin Diseases, 2012). According to International Society for Clinical Densitometry (ISCD), a Z-score above -2.0 is normal, however, the diagnosis of osteoporosis in children and younger men should not be solely based on bone density tests.

Bone remodeling throughout childhood can also be estimated indirectly through the presence of certain biochemical markers or biomarkers in blood and urine. Bone formation can be indicated by serum alkaline, phosphatase, etc while bone resorption can be determined by markers such as urinary hydroxyproline (Matkovic, 2007). The use of biomarkers is increasing in popularity as they are a cheaper than the DXA scan and can be easily used to estimate changes in bone formation rates (Calatayud et al., 2013).

Major Determinants of Bone Density During Childhood and Adolescence

Early monitoring of children’s and adolescent’s bone density is essential in order to prevent bone diseases to develop later in life and to reduce the risk for fractures (Gilsanz, 1998). There are various factors that determines and influences bone health in children and adolescents which will be discussed in the succeeding sections.


Bone development and growth occurs steadily throughout infancy and childhood with the rate increasing rapidly at puberty (McDevitt & Ahmed, 2010). Increases in bone width and length results in the mass and volume required for optimal bone architecture (Ausili et al., 2011). This growth continues until approximately 20 years of age when the peak bone mass (PBM) is reached. This period of bone growth from infancy is the best time to incorporate external factors such as proper nutrition and exercise that will increase BMD and bone mass before the PBM is achieved. After this time the overall rate of bone resorption will be greater than bone formation and bone will be loss at various rates as the person ages (McDevitt & Ahmed, 2010). The human body can still go through bone modeling after the PBM is reached. This means that bone can be added in certain areas as well as removed in other areas to have an overall consistent amount of bone mass (Grimston, Willows & Hanley, 1993).


It is thought that genetics has an effect on the development of the skeletal system in early childhood but the extent of this influence is unknown. It is likely that genetics plays a larger role in which receptors are present that may impact the types of nutrients we can absorb from our food. This may indirectly effect bone density as the minerals obtained from a person’s diet is a major determinant of BMD (McDevitt & Ahmed, 2010).


Hormonal environment plays a key role in sex-specific differences in bone conformation (Ausili et. al., 2011). Before puberty, bone density is similar for both sexes. However, increases rates of BMD accumulation for females occurs when menarche begins at around 11-13 years old. This indicates the importance of puberty in bone mass development in girls. In males, higher rates of bone accretion are found between 13-17 years old. BMD increases are delayed in males which may reflect their later and more diffuse time of sexual maturation (Kroger, Kotaniemi, Kroger & Alhava, 1993).

Despite having an earlier start, females tend to have a lower overall total bone mass accumulation as the period of development of BMD is much shorter than for males (Kroger, Kotaniemi, Kroger & Alhava, 1993). In addition, some have suggested that girls tend to be more conscious of their weight and may avoid dairy products rich in calcium as they are perceived to be fattening. This is a misconception as not all dairy products are fattening and actually contain many important nutrients for body function. For example, low fat milk contains equivalents amounts of calcium as high fat milk which is essential for skeletal system development (Greer & Krebs, 2006). This may contribute to the reason why fractures are twice as common for girls as compared to boys (Matkovic, 2007).


The skeletal system contains 99% of the total amount of calcium in the body. The maintenance of sufficient calcium intake throughout childhood and early adolescence is needed to reach an optimal PBM (Greer & Krebs, 2006). The need of calcium through diet is greatest during early adolescence when the majority of bone accretion occurs (Matkovic, 2007). According to recent studies, the amount of calcium intake during the peak bone mass also determines the bone density later in life (Johnston et. al., 1992). During the periods of peak calcium accretion, it is estimated that 40% of the total bone mass for the rest of a lifetime is developed. Recent studies of children older than 8 years old living in the United States found that a large proportion were not meeting the recommended intake of calcium. In addition, children who avoided drinking milk (a major source of calcium) had an increased number of childhood fractures (Greer & Krebs, 2006). Other studies have also noted that greater consumption of calcium supplements (1000 mg calcium / day) or calcium-rich foods (i.e. milk or dairy products) during prepuberty enhances the rate of increase in bone mineral density and decreases the risk for fractures in adulthood (Boot et. al., 2010).

Equally as important as calcium is the intake of vitamin D which facilitates the active absorption of calcium into the body. Without the presence of this vitamin in the body, the calcium cannot be used to increase bone mass and density (Greer & Krebs, 2006). A lack of vitamin D in the diet can also lead to rickets which is a pathological reduction of calcium and phosphate levels leading to losses of mineralized bone. Rickets is more commonly seen in children and would lead to changes in movement patterns if the weak bones cause abnormal bone growth (Gilsanz, 1998). As a result it is has been recommended that infants, children and adolescents take about 5.0μg/day of vitamin D (Greer & Krebs, 2006).

Additionally, some foods have been found that decrease the ability to retain calcium. These include caffeine, alcohol, oxalates, phytates and high levels of sodium. As a child moves from infancy and early childhood into late childhood and adolescence, the intake of milk tends to decrease and is replaced with soft drinks and juices. This is a concern as the calcium intake in children is reduced when it is needed the most for optimal development of bone (Greer & Krebs, 2006).

Physical Activity

Despite the importance of nutrition in maintaining and increasing bone density, there is evidence that physical activity has the largest role in reaching a maximal PBM (Greer & Krebs, 2006). Evidence from current research shows that performing weight-bearing physical activity has favorable outcomes on bone health for all ages. During childhood and adolescent stage, execution of physical activities that produce comparably high-intensity loading forces, such as running and plyometrics, together with high-intensity resistance training provides osteogenetic stimulus (Kohrt et. al., 2004). This is supported by the “Mechanostat” theory. According to this theory there must be a minimum effective strain (MESm) that must be surpassed in order to cause bone modeling to occur. Strain is the amount of deformation of a material as a result of applying a load. This is why weight-bearing or resistance training is recommended as they will likely cause enough strain to initiate the bone modeling process (Grimston, Willows & Hanley, 1993).

BMD can be improved or maintain through weight-bearing exercises or resistance training. Weight-bearing exercises are activities that require movements against gravity to maintain a certain body position. Resistance training is when one moves their body against some sort of resistance other than gravity such as a weight. These exercise often include high impact activities such as jumping or running instead of low impact activities such as walking or swimming (Calatayud et al., 2013). In one study it was found that children who reported that their main exercise was swimming had lower bone density than those who had a weight-bearing exercise as their dominant activity. The weight bearing exercises in this study required external loads on the body that were equal or larger than 3 times the weight of the body (Grimston, Willows & Hanley, 1993).

Evidence of the Benefits of Physical Activity on Children's Bone Health

Research has shown that performing physical activities that are high-impact or weight bearing during early childhood may induce significant gains on bone density that, if maintained, could benefit skeletal integrity in adulthood (Zanker et. al., 2003). In many of the studies described below, the benefits of physical activity on bone health been assessed by measuring the relationship between a set physical activity level or training program and bone density, bone mass and/or the incidence of fractures. It has been shown that bone density is higher in children who are physically active as oppose to other children who are performing less physical activities (Slemenda et. al., 1991). In addition, higher BMD was also recorded in children who perform activities that generate high impact forces such as gymnastics and ballet compared to those who participate in activities with lower impact forces such as walking, or non-weight-bearing activities such as swimming (Courteix et. al., 1998). Due to these findings, studies have concluded that performing jumps and various high-impact activities done rapidly, produces greater increase in BMD than low- to moderate-intensity forces (Kohrt et. al., 2004). Weight lifting showed similar effects in the increased BMD especially in the spine and hip (Morris, 1997) These studies support the idea that physical activity has a large effect on bone density.

It should also be noted that lower levels of weight-bearing physical activity could also influence other factors such as balance and coordination. Without these skills, the risk of falling in an unusual body position may be more likely and thus result in more fractures (Goulding et. al., 2003). Studies also show that children with single fracture displayed a slightly higher levels of physical activity as compared to children with recurrent fractures who were performing lower levels of physical activity. This indicates that physical activity improves bone development and reduces the amount of fractures in children (K. Manias et. al., 2006).

Consequences of Low Bone Density


The prevalence of childhood fractures has been increasing (Matkovic, 2007). According to research, children who experienced one or more fractures have a lower BMD throughout the skeleton as compared to those who did not encounter any fractures (K. Manias et. al., 2006). Most of these fractures occur in a period of time approximately a year before the PBM when there is transient lag in the increase of bone mass relative to longitudinal bone growth. It is during this time of peak height growth before the PBM is reached that the bones are weak and are susceptible to fractures (Matkovic, 2007). Most of these fractures occur at the distal forearm or wrist area. Fractures in these areas may limit participation in sports such as baseball or basketball where the use of the hand and wrist is needed for throwing (McDevitt & Ahmed, 2010).


Osteoporosis is a disease that is caused by parallel losses of both bone matrix and mineral. This loss of bone density causes the bone to become more porous and weak (Gilsanz, 1998). Osteoporosis is diagnosed according to the World Health organization of having a DEXA with a T score of -2.5 (Ma, Samartzis & Shen, 2008). This condition is more common in elderly individuals but has been seen in children. If a child had osteoporosis, their ability to participate in high impact, weight-bearing activities would be limited or removed as their weak bones would have a high susceptibility of fractures (Gilsanz, 1998).

Recommendations for Parents to Maintain Bone Density in Children


It is hard to monitor a child’s calcium levels throughout their life as the measurements often require expensive equipment and analysis. Despite this, parents can still optimize a child’s bone density and development through proper nutrition with the aim of meeting the recommended daily intake of calcium (Greer & Krebs, 2006). The recommended daily intake of calcium is 700mg for children 1-3 years, 1000 mg for children 4-8 years and 1300 mg for children 9-18 years (Health Canada, 2012). Though dairy products such as milk and cheese are commonly thought to have the largest source calcium, there are other calcium containing foods such as: salmon, tofu, broccoli, tomatoes and white and baked beans. There are also some foods that are fortified with calcium such as orange juice, some cereals, oatmeal and soymilk that can be incorporated in a child’s diet (Greer & Krebs, 2006).

As previously mentioned it is recommended that infants, children and adolescents take about 5.0μg/day of vitamin D (Greer & Krebs, 2006). If possible, parents should reduce or avoid caffeine, alcohol, oxalates, phytates and high levels of sodium in a child’s diet. In addition to maintaining recommended intakes, studies show that parents that serve as role models for drinking milk have children that are more likely to drink milk. In this way parents can influence their child’s interest and attitude toward eating or drinking foods with higher sources of calcium (Greer & Krebs, 2006).

Physical Activity

From the research evidence discussed it is recommended that exercise for children and adolescents should include impact activities, such as calisthenics, plyometrics, jumping, and variations of moderate intensity resistance training. Participation in sports that comprise running and jumping such as basketball and soccer are also recommended to increase the BMD and improve bone health. The intensity of the activities are recommended to be high and to be performed at least three times a week at 10-20 minute intervals (Kohrt et. al., 2004). High intensity can generally be monitored through the talk test. In this test moderate intensity is when the person can talk but not sing and high intensity is when the person can only say a couple words or cannot talk during the exercise (Jeans, Foster, Porcari, Gibson & Doberstein, 2011). In terms of resistance training, 1-repetition maximum should only be performed at <60% for its safety purposes (Kohrt et. al., 2004). Incorporating some high impact activities causes exercise-induced gains in bone mass in children that are preserved into adulthood. It is essential to create a good foundation of bone health and physical activity habits at an early age in hopes that it will be maintained and continued throughout adulthood.


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