Physical Activity Interactions with Bone Accrual in Children and Adolescents

stress-strain intervention and intervention studies, training, exercise, resistance training, physical education and physical education training, physical activity activity. bone bone bones, bone strength, bone and development, bone turnover, keywords osteoporosis, prevention and treatment and


Osteoporosis and peak bone mass
Osteoporosis is a skeletal disease characterized by low bone mass and the deterioration of the micro architecture of bone tissue resulting in bone fragility and susceptibility to fractures (Gordon, 2003). According to the World Health Organization, osteoporosis is estimated to affect approximately 200 million women worldwide (Kanis, 2007) with the burden of osteoporosis being felt both personally and economically. Although the prevalence of fractures is higher is women, the mortality rate related to fragility fractures is higher in men (Center et al. 1999;Hasserius et al., 2003). Moreover, the annual cost of treating fractures in the United States is projected to increase to $25 billion in 2025 from $17 billion in 2005 (Burge et al., 2005). Achieving peak bone mass (PBM) during adolescence and the subsequent rate of bone loss are major determinants of bone mass later in life (Hansen et al., 1991). The amount of bone mass achieved early in life has been shown to predict the level of bone mass and the incidence of fracture later in life suggesting that a primary risk factor for the development of osteoporosis is the inability to attain high PBM (Hansen et al., 1991;Heaney et al., 2000). PBM is generally defined as the highest level of bone mass achieved as a result of normal growth and seems to be established, for most sites of the skeleton, by late adolescence (Matkovic et al., 1994). Previous studies (Bonjour et al., 1991;Bailey et al., 1996) have demonstrated the period between 9-20 years of age to be critical in building peak bone mass as 90% of total body bone mineral content (BMC) is accrued by the age of 16 (Elgan et al., 2003;, with the remaining 5-10% of total body bone mass achieved in the third decade (Cadogan et al, 1998). In fact, the most rapid bone mineral accumulation occurs approximately 1 year after the age of peak linear growth (Bailey et al., 1996); around the time of menarche for females (Cadogan et al., 1998). With considerable increases in bone mass occurring during puberty, maximizing PBM during this time is often advocated as the best way to delay age-related bone loss and prevent osteoporotic fractures (Fulkerson et al., 2004;Molgaard et al., 1999;Valimaki et al., 1994). It appears, therefore, as though there is a critical period, a 'window of opportunity , in which we can influence the amount of bone mass we attain. However, bone development is the product of complex interactions between genetic and environmental factors including diet, hormonal influences, and mechanical stimuli (Gordon, 2003;Steelman & Zeitler, 2001). Permanent deficits in PBM are the result of any process that 381 2008) and controls. Therefore, research investigating the relationship regarding bone markers and different PA types is limited and ambiguous, but even more so in children and adolescents, making it difficult to ascertain the effect of sport on bone. The examination of biochemical measurements of bone turnover, in addition to static measures of bone, is advantageous in the study of skeletal metabolism and growth as they provide an understanding of the dynamic course of bone remodelling. To date, the use of biochemical marks of bone turnover in PA interventions on bone in youth has been extremely limited. Difficulties in comparing and assessing the benefits of PA on bone during growth reflect the varying methodologies used between studies. PA interventions aimed at improving bone health in youth have been subject to limited maturational comparisons as the majority of interventions have been conducted in one distinct pubertal group. Furthermore, the types of PA interventions that have been applied have varied greatly between studies. Discrepancies in results are due in part to the varying bone assessment techniques that are used across cross-sectional and intervention studies. Many of the aforementioned studies measured improvements in BMD using dual-energy x-ray absorptiometry (DXA). The use of DXA to interpret and evaluate BMD in the growing years can be difficult as there are considerable changes to the size and shape of bone (Bailey et al., 1996;Gordon, 2003;Schoenau et al., 2004), making comparisons between youth problematic. Furthermore, the measurements provided by DXA fail to account for the architecture, organization of tissues, mechanical properties and other factors known to impart bone strength. In addition, the bone assessment techniques used in majority of these studies have provided a static rather than dynamic picture of bone, which could in fact allow for more comparisons across studies. Evidence supporting the role of PA on bone health has been accumulated from a wide range of studies investigating different activity methods using athletes, non-athletes and inactive individuals. Although these studies contribute to the literature they do not provide us with causality that PA does impart benefits to bone health. In response, there has been an increase in the number of intervention studies conducted, particularly in the school setting. PA interventions in schools are in many ways ideal places to intervene as they allow for a large population of children and adolescents to be targeted in a somewhat controlled environment, regardless of socioeconomic status, in a location where youth already spend a majority of their day during their most skeletally responsive years ).

Methods
Therefore, the primary objective of this chapter is to conduct a systematic review on the effectiveness of exercise and PA interventions to improve bone accrual in children and adolescents. Key finding from controlled intervention trials using various techniques to assess bone mineral density, content and strength changes will be discussed and be grouped according to maturity status. This will hopefully help to shed light on the best time during growth and development to influence bone health and to ascertain if there is indeed a window of opportunity for bone response. We will also discuss and compare the different types of interventions used to affect changes in bone properties in youth, to determine if there is a modality that is best suited to improving bone development and to what degree these interventions influence changes in bone. Furthermore, we will address the characteristics of loading that have been shown to be best associated with particular structural improvements as interventions can be designed to impart mechanical loading on bone by jumping or by resistance training where the weight-bearing load on bone is applied through muscle. As majority of interventions measure only static properties of bone, this chapter will also be used to discuss bone remodelling parameters influenced by such exercise interventions. To our knowledge there has not been any studies examining the effects of PA interventions on bone remodelling.

Eligibility criteria and search strategy
The aim of the literature search was to find all available randomized control trials and controlled studies that examined the effects of any type of exercise or PA intervention trial on bone status in healthy (non-clinical, non-athletes) children and adolescents between 6 and 17 years of age. For this review we included all types of bone parameters from various bone assessment techniques (DXA, pQCT, QUS etc.) to be used as primary outcome measures as long as there were at least two measurement time points. Primary outcome measures included areal bone mineral density (aBMD), volumetric bone mineral density (vBMD), bone mineral content (BMC), bone area (BA), cortical thickness, bone strength index (BSI), stress-strain index (SSI), maximal moment of inertia (I mas ), section modulus (SM), speed of sound (SOS), broadband ultrasound attenuation (BUA), and markers of bone metabolism. A computerised search of the MEDLINE and PubMed databases was performed on articles up till 2011 using a comprehensive combination of keywords to describe exercise, bone and participant parameters. The keywords used to describe exercise included: intervention and intervention studies, training, exercise, resistance training, physical education and physical education training, physical activity and motor activity. Bone parameter keywords included: bone mineral, bone density, bone and bones, bone strength, bone accrual and development, bone turnover, resorption, modelling and metabolism. For the participants, keywords such as children, adolescents, boys and girls were used. A total of 2728 were found, their titles and abstracts reviewed to determine if they met the inclusion criteria. Papers from all journals were considered and retrieved electronically or by interlibrary loan. After screening the articles a total of 35 studies met the criteria and were used for the current review. Studies were grouped according to the maturity status of their participants based on Tanner Staging of development (Tanner, 1962). Participants were grouped as either prepubertal (Tanner 1), early pubertal (Tanner 2 and 3), and pubertal (Tanner 4 and 5) to maintain consistency with other literature review groupings. Studies in which authors provided results for more than one maturity group were divided into two parts (A and B). Table 1 represents the numerical breakdown of all the intervention studies reviewed into particular categories based on the type of intervention that was used, the method in which bone parameters were assessed, the maturity and sex of the population measured. Studies were included more than once if more than one measurement technique was used and if results were separated by sex or maturity group. Table 2 is a detailed summary of the design and outcomes of all the PA intervention studies reviewed, and are grouped according to the participants' maturity status. The results presented in Table 2  with together being studies that averaged data for more than one maturity group. Boys + girls reflect studies that did not separate results by sex. PE: physical education; WBPA: weight-bearing physical activity; SXA: single energy x-ray absorptiometry; DXA: dual energy x-ray absorptiometry; DPA: dual photon absorptiometry; pQCT; peripheral QCT; HSA: hip structural analysis; QUS: quantitative ultrasound. Majority of the intervention studies were school based with 23 of the studies being conducted as part of a regular physical education class and 5 at some point within the school day. Approximately half (51%) of the studies utilized specific jumping interventions that relied on ground reaction forces in order to elicit a positive response on bone. Fourteen studies consisted of general weight bearing types of activities such as running, volleyball, aerobics etc., with only 3 studies specifically using resistance training with free or machine assisted weights. Significant increases in primary bone outcomes were found in 16 jumping interventions, 14 WBPA interventions, and 1 resistance training study. This translated into 79.5% of physical activity interventions positively influencing some form of bone strength parameter in children and adolescents. Furthermore, 5 studies also included calcium interventions which demonstrated benefits to bone in addition to physical activity. Of the 35 studies reviewed 24 presented results separately for girls, 12 for boys, with 7 studies presenting data for boys and girls together. Moreover, 16 studies conducted interventions in prepubertal and early pubertal children. The smallest number of studies was performed in pubertal youth with a total of 7. All the pubertal interventions were completed on a population of girls, with 1 study (Weeks et al., 2008) including boys in their sample. Based on pubertal groups, an even number of boys and girls were represented in the results of prepubertal youth with 8 studies separately reporting results for boy and girls and 2 grouping results together. In early pubertal children, a larger number of studies were conducted on and included girls. Ten studies reported results separately for girls, 3 for boys and 5 did not distinguish results between genders. DXA was the measurement technique predominantly used (94%) to assess bone, followed by pQCT (14%) and then QUS (8.5%). In total, 5 studies used more than one technique to determine changes in bone and these were all done in conjunction with DXA measurements. Four studies using DXA also performed hip structural analysis (HSA), which is a new application for DXA allowing for the estimation of geometric contributions to bone strength in the proximal femur and may potentially provide a better representation of bone strength (Bonnick, 2007). It is surprising that such a large percentage of studies utilized DXA given the known methodological issues with assessing changes in bone during growth. Until recently, we had thought no intervention studies had used biochemical markers of bone metabolism. Our extensive literature search found 1 study (Schneider et al., 2007) that measured serum markers of bone formation and resorption in adolescents. As static measures require longer durations for differences to be found, measuring biochemical markers of bone turnover to assess dynamic properties of bone could be advantageous in detecting changes sooner and allow for better comparisons of results between studies.

Prepubertal interventions
Positive effects of exercise on bone indices were found in 13 of 16 studies (81%), with overall effects ranging from 0.6% to 9.5% depending on the skeletal location and the type of measure (BMC, BMD, etc) taken for studies 7-36 months in duration. The average percent improvements for BMC included 4.5%, 4%, 2%and 1.5% at the lumbar spine (LS), femoral neck (FN), femur and total body (TB) respectively. BMD gains across studies were between 0.6-3% for the LS, FN and TB. The largest gains in girls was in BMC and area of the forearm (12.5% and 13.2% respectively) using peripheral DXA after 36 months of increased physical education class time (Hasselstrom et al., 2008). The one study that used pQCT in this group (Macdonald et al., 2007) was also the study that exhibited the largest bone gains in boys after 16 months of jump training, finding an increase of approximately 25% in BSI (an index of bone structural strength) of the distal tibia. MacKelvie et al. (2004) also presented large gains using HSA, with boys seeing a 12% increase in FN cross-sectional moment of inertia. Despite the bone gains being similar between boys and girls, the number of studies that reported significant findings differed (4 vs. 7 out of 8 for girls vs. boys respectively). These discrepancies can largely be explained by the differences in the length and type PA intervention employed. MacKelvie et al. (2001) and (2002) were studies that utilized 7 months of school based physical education classes to employ a jump circuit intervention eliciting ground reaction forces 3-5 times one's body weight and demonstrated favourable gains in bone in boys but not girls. Fuchs et al. (2001) also found 7 months of jump training to be favourable to improvements in LS and FN BMC and BMD in prepubertal boys and girls. In fact the gains demonstrated in Fuchs et al. (2001) were greater than those in the MacKelvie et al. (2001MacKelvie et al. ( , 2001 studies, most likely due to the larger ground reaction forces generated (8.8 vs. 3.5-5 x body weight). Studies at 12 months (Alwis et al., 2008b;Linden et al., 2007) utilizing a weight bearing physical education intervention follow a similar trend with improvements being seen in boys but not girls. The extra intervention time has not helped to elicit a significant positive bone response in the young girls. It is not till 24 months of the same type of weight bearing PA intervention that positive gains are found in girls . It would therefore appear that improvements in bone as a result of a PA intervention would more likely occur in prepubertal boys than girls. This is particularly true after 7 months of jumping training (MacKelvie et al., 2001 and 12 months of weight bearing PA (Alwis et al., 2008b;Linden et al., 2007). Improvements in prepubertal girls were seen in studies lasting 24 months in duration ) and any studies demonstrating bone gains in a mixed gendered population (Fuchs et al., 2001;McKay et al., 2000) could be due to greater changes in the boys than the girls.

Early pubertal interventions
Eighty-three percent of the PA interventions were capable of creating a positive effect on bone strength parameters in pubertal boys and girls. Study durations ranged from 3 months to 4 years, with both the average and median duration being 12 months. The percent gains in bone ranged anywhere between 1.3-15%; again depending on the measurement location and the technique employed. Of the 16 studies conducted in this group 12 utilized DXA, 3 pQCT, 2 QUS, and 1 SXA. Three of the DXA studies also conducted HSA with 2 of the overall studies employing more than one technique to assess experimental effects on bone. The largest improvements in bone for girls was a 10.3% change in aBMD at the FN following 10 months of a mixed program using jumping, weight bearing exercise and weight training (Morris et al, 1997). This large improvement, however, could be the result of a potential selection bias. In boys, the greatest improvements were in the double digits at 10%, 11%, 14% and 15% for LS aBMD, calcaneal SOS, FN aBMD and vBMD, respectively (Sundberg et al., 2001). These finding in boys were demonstrated after 4 years of increased physical education classes that involved a mixed program of weight bearing and jumping activities.
In addition to a PA intervention, 2 of the studies also employed a calcium intervention (Courteix et al., 2005;Iuliano-Burns et al., 2003). These studies (Courteix et al., 2005;Iuliano-Burns et al., 2003) demonstrated calcium supplementation in addition to PA can elicit greater responses in bone than with exercise alone, highlighting the importance of monitoring calcium intake during intervention studies particularly during puberty. The number of interventions conducted in boys and girls was not equal as it was in the prepubertal group making the discussion on gender differences and effects of PA on bone in this group problematic. Three studies in early pubertal children by the same author (Macdonald et al., 2007(Macdonald et al., , 2008(Macdonald et al., , 2009) incorporated 16 months of 60 minute weekly classroom PA including a bone building program of 5-36 jumps per day 4 times a week. Using pQCT, DXA and HSA these studies demonstrated no significant changes in bone strength in the tibia, but improvements in tibial geometry and bending resistance in boys (Macdonald et al., 2007(Macdonald et al., , 2009. Boys also experienced improvements in lumbar spine BMC and whole body BMC, with girls seeing increases in section modulus (a measure of bending resistance) of the femoral neck (Macdonald et al., 2008). These results imply there may be gender differences in the properties of bone that improve following an exercise intervention. There are 3 reasons why the trends shown by Macdonald et al. (2007Macdonald et al. ( , 2009 failed to reach significance. Firstly, there was an uneven distribution of sample size, maturity status and gender between groups making some of the groups underpowered. Secondly, as ground reaction forces were not reported it is possible that external loads applied during the intervention was not high enough to instigate a loading response in bone. Third and most likely, the benefits of the jumping intervention could have been attenuated due to the low compliance to the program. In fact, Macdonald et al. (2008) reported significant findings for individuals with 80% compliance. This notion is supported by 3 studies that (MacKelvie et al., 2001Petit et al., 2002) demonstrated improvements in BMC, aBMD and vBMD in girls following a shorter jumping program (7 months) eliciting larger ground reaction forces (3.5-5 x body weight) and for whom study compliance was 80% (MacKelvie et al., 2001). Not only does it appear that larger loading responses are needed to elicit positive changes in bone, but also the way in which that load is applied to bone matters. A large number of studies (69%) employed specific jumping exercises as part of their intervention demonstrating that short, irregular, diverse large loads at varying times of the day are www.intechopen.com Physical Activity Interactions with Bone Accrual in Children and Adolescents 395 sufficient to instigate bone responses MacKelvie et al., 2001;McKay et al. 2005, Meyer et al., 2011Petit et al., 2002). Unlike the studies conducted in prepubertal youth, interventions prescribing weight bearing activities do not need to be conducted over long periods of time to see similar responses in bone. Barbeau et al. (2007), Courteix et al. (2005), and Morris et al. (1997) demonstrated such improvements in 7-12 months time.
Interventions in which there were no improvements in bone parameters attributed this to higher levels of leisure PA in the non-experimental groups, increased bone mass at baseline, and earlier menarcheal status Sundberg et al., 2001). All of these factors would contribute to bone indices being elevated prior to the intervention allowing for only small changes to occur and in turn masking any effects of the intervention program.

Pubertal interventions
The fewest PA interventions were conducted in pubertal youth, with all 7 involving girls and 1 including boys. The types of interventions included resistance training (Blimkie et al., 1996;Nichols et al., 2001;Witzke & Snow, 2002), jumping trials (Weeks et al., 2008), and those with a variety of different weight-bearing activities , Schneider et al., 2007, Stear et al., 2003. DXA was the predominant method used to asses bone in this population, with one study using DPA (Blimkie et al., 1996). Three of the studies that used DXA also used an alternate method such as pQCT , QUS (Weeks et al., 2008) and serum biochemical markers of bone turnover (Schneider et al., 2007). Half of the trials demonstrated significant changes (0.7-4.9%) in bone following their interventions, with 3 of the studies demonstrating non-significant trends (Schneider et al., 2007;Weeks et al., 2008, Witzke & Snow, 2002. Of those studies that reported significant trends, one included both an exercise and calcium intervention and observed bone mineral advantages at the femoral neck, lumbar spine and total body in adolescent girls receiving both interventions (Stear et al., 2003). Albeit the combination of calcium and exercise generated greater improvements, those girls receiving just the exercise also demonstrated significant changes at the hip. Schneider et al. (2007) provided all pubertal girls with 500mg of calcium per day and unlike Stear et al. (2003) only observed significant changes in thoracic BMC despite improved trends in BMD and markers of bone turnover. It is possible that these results failed to reach significance as the intervention by Schneider et al. (2007) was shorter in duration than Stear et al. (2003), 10 vs. 15.5 months respectively. Moreover, as everyone in Schneider et al.'s (2007) study was taking calcium the room for improvements may have been smaller than Stear et al.'s (2003) who observed the greatest differences between exercising calcium takers and non-exercising non-calcium consuming controls. Regardless of these discrepancies, the one thing that is clear from these two studies and those described in the early pubertal section (Courteix et al., 2005;Iuliano-Burns et al., 2003), is that calcium is important to bone health and its use during PA interventions will greatly affect results. Three investigations of the effects of resistance training on bone mineral accrual in pubertal girls were completed, with only 1 reporting significant changes in bone indices (Nichols et al., 2001). A major difference between the studies that did not find significant changes (Blimkie et al., 1996;Witzke & Snow, 2002) and the one that did (Nichols et al., 2001) was the duration of the intervention trial. It appears that with resistance training a longer trial of approximately 15 months is necessary to demonstrate significant improvements in bone, similarly to the 15.5 months of WBPA in Stear et al. (2003). In addition to resistance training Witzke & Snow (2002) used plyometric training and the utilization of this may have resulted in the strong non-significant trends, demonstrating that perhaps shorter trials that include ground reaction forces can be efficacious at improving bone. Results from studies examining jumping trials Weeks et al., 2008) 8-9 months in duration have been ambiguous. Heinonen et al. (2000) failed to measure significant changes in bone; however, Weeks et al. (2008) did observe improved total body BMC in pubertal boys but not girls. Interestingly, Weeks et al. (2008) did measure large percent changes, albeit non-significant trends, in many different parameters of bone strength in both boys and girls. These trends could be the result of the greater ground reaction forces used in this study compared to that of Heinonen et al. (2000) and could possibly have reached significant if the length of the trial were longer. A common theme in all of these studies not having significant findings or 'almost' measuring differences is poor compliance. If it were not for the issues with compliance, there is a large probability these studies would have found significant results. Another important factor as to why very few studies reported changes in pubertal youth is due to how bone is accrued in this maturity group. According to Bailey et al. (1996Bailey et al. ( , 1999 peak velocity of BMC accrual for the whole body occurs approximately 0.7-1 year after peak linear growth around the time of menarche, which corresponds to approximately 12-13 years of age in girls. The pubertal girls in the 7 studies reviewed were between the ages of 13 and 18, putting them after the point of peak BMC velocity accrual where the velocity at which they are accruing bone is actually decreasing. The schematic representation of PBM and the rate at which bone mass is accrued over time resembles a dose response curve. It would appear that the pubertal girls in these studies are nearing their PBM, putting them near the plateau of the accrual process, and therefore both the rate and amount of BMC that can be accrued during this time is less. As a result, detecting significant changes will be difficult. Just because these percent gains are small and non-significant statistically does not mean that they are not meaningful. Turner and Robling (2003) demonstrated that a 5.4% and 6.9% gain in aBMD and BMC respectively, translated into a 64% and 94% increase in the amount of force and energy a bone could absorb before failure. This suggests that even small changes in bone mass, which are marginally detectable by DXA can significantly improve bone strength. Therefore a little bone goes a long way.

The window of opportunity for bone adaptations
The early pubertal period may be the best time to generate skeletal adaptations to PA. Studies conducted in more than one maturity group demonstrated positive bone gains in early pubertal girls with no significant increases in prepubertal (MacKelvie et al., 2001;Petite et al., 2002) or pubertal  girls. When reviewing all of the intervention studies the greatest gains in bone on average, regardless of sex, skeletal location and type of activity used, was during the early pubertal years. These results are more definitive in girls as a larger proportion of intervention studies have been conducted on females across puberty, with the sample of boys decreasing with maturity. Despite this trend, longer duration intervention studies where boys most likely transitioned from pre-to early puberty also demonstrate larger gains in bone than in just prepubertal boys (MacKelvie et al., 2004). Larger skeletal gains were also observed in interventions trials that supplemented with calcium during early puberty (Courteix et al., 2005;Iuliano-Burns et al., 2003) compared to those supplementing in prepubertal (Bass et al., 2007) and pubertal www.intechopen.com Physical Activity Interactions with Bone Accrual in Children and Adolescents 397 (Schneider et al., 2007;Stear et al., 2003) stages. Moreover, the velocity for BMC accrual is highest in early puberty prior to menarche (in girls) (Bailey et al., 1996(Bailey et al., , 1997Cadogan et al., 1998, after which accrual rates decrease with age plateauing in late adolescence upon achieving PBM (Davies et al., 2005). Therefore, the 'window of opportunity' to impart the largest influences on bone development may be during early puberty.

Optimal physical activity interventions for bone adaptations
Based on our systematic review of the literature we can deduce that regular exercise can be an effective way to improve bone density, size, and shape; in turn improving the mechanical strength of bone. With the variability in the types of interventions used and how they were employed there is no clear consensus on exactly how we should prescribe exercise in order to see the greatest returns in terms of bone health. However, in reviewing the literature, regardless of pubertal stage, the duration of the trial and the intensity in which it was employed appeared to matter. If interventions were short in duration (8-10 months) those that utilized jumping activities with high ground reaction forces received the most positive results (Bass et al. 2007;Fuchs et al., 2001;MacKelvie et al., 2001McKay et al., 2005;Petit et al., 2002;Weeks et al., 2008). If weight bearing PA or resistance training was utilized the length of the intervention needed to be longer (10-24 months depending on maturity), in order to see significant gains in bone (Alwis et al., 2008a;Courteix et al., 2005;Linden et al., , 2007Morris et al., 1997;Nichols et al., 2001;Stear et al., 2003;Valdimarsson et al., 2006). In terms of frequency of exercise, Turner & Robling (2003) suggest it is better to shorten each individual exercise session than to reduce the number of sessions, as jump training has been shown to improve BMC when performed at least 3 time per week but not when reduced to 2 time per week, with gains increasing up to 5 days a week with 2 shorter session in one day. This is reflected in the interventions reviewed with significant gains in bone indices being observed in trials occurring 3-5 times per week. The most recent intervention study reviewed (Meyer et al., 2011) is a good example of these last two concepts by demonstrating that a variety of different activities in one intervention at random times of the day can be effective in eliciting bone gains. Therefore, PA is beneficial for bone health and irregular activities utilizing jump and resisting training to weight bearing activities are some of the best ways to elicit an adaptive response in bone. Not only is the variety beneficial for bone but it can also help to alleviate the boredom that accompanies exercise regimens. Remember that in terms of bone change really is good!

Methodological issues
DXA was the technique most often used in the PA intervention trials reviewed, and was used to measure BMC and BMD in various skeletal regions of the body. However, BMD assessed using DXA is an estimation of 'true' bone density and the areal density that is expressed is affected by bone size making it difficult to interpret, evaluate and compare BMD in the growing years when there are considerable changes to the size and shape of bone in children (Bailey et al., 1996;Fulkerson et al., 2004;Gordon, 2003;Schoenau et al., 2004). Moreover aBMD is a surrogate measure for bone strength and even though BMC and BMD are related to bone strength inferring information regarding strength from studies using these measures can be misleading. This fact is represented in the many studies citing increases in BMD and BMC that were not always significant. It is possible that DXA may not be sensitive enough to detect small changes in bone particularly at a time in development when small changes are difficult to come by, like later in puberty when the rate of BMC accrual is decreasing. However, even these small detectable changes in bone mass using DXA can signify improvements in bone strength most likely by favourably altering bone geometry (Turner & Robling, 2003). Therefore the best parameter for assessing the effectiveness of PA interventions on bone would be to use a technique that includes measures of bone strength but also bone shape and size. pQCT is a method that can be used to detect true vBMD, bone strength, shape and size. Unfortunately, only 5 of the studies that we reviewed utilized this method. An advantage of using pQCT to compare bone structural differences is that it has the capability to demonstrate bone strength adaptations in bone size via changes in cortical thickness or area through investigation of periosteal or endocortical expansion (Haapasalo et al., 2000;Kontulainen et al., 2002;Nikander et al., 2009). Moreover, these measurements indirectly provide an idea of the dynamic course of bone and how bone is metabolized to infer strength. However, to date only 1 study has directly measured biochemical markers of bone turnover in response to a PA intervention (Schneider et al., 2007). Measuring bone turnover would allow for detection of potential exercise effects sooner, as gains in bone markers have been demonstrated after 8 weeks of resistance training in women 20 years of age (Lester et al., 2009). Moreover, reference values for many of the markers have been set within the literature allowing for comparison across studies; something that is difficult to do for static measures of bone as the standards and definitions defining low bone mass are available only for postmenopausal women and not youth. One way of avoiding this issue is to cease relating bone mass and strength to age, and relate it instead to muscle function (Schoenau & Fricke, 2008). This new methodological concept is based on the thought that the critical property of bone is strength rather than weight and that what influences bone strength are the mechanical loads it must endure either through PA or muscle contraction. Regardless of the mode of mechanical load the stability of the bone must be adapted to muscle strength, in a sense creating a functional muscle-bone unit (Schoenau & Fricke, 2008). Such an analysis removes the concept of a 'peak bone mass', which in fact is something we are not capable of measuring for an individual. Instead this approach allows for determination and comparison of bone deficits irrespective of age as bone strength is related to the strength and function of muscle (Schoenau & Fricke, 2008). Moreover, this approach moves away from looking at bone as a separate entity but as functionally linked system.

Psycho-social factors
It is also important to consider the psycho-social factors that are believed to affect bone health; these include osteoporosis beliefs, knowledge and practises. Women's willingness to adopt healthy behaviors depends on their level of knowledge of osteoporosis (Cook et al., 1991;Jamal et al., 1999). Majority of research examining calcium intake and PA with respect to osteoporosis knowledge and beliefs, and as preventative behaviours have been investigated in post menopausal women (Tudor-Locke & McColl, 2000). A few researchers have examined these criteria in younger women (Kasper et al., 1994(Kasper et al., , 2001Wallace, 2002), let alone in adolescents (Anderson et al., 2005;Schrader et al., 2005). A lack of knowledge about osteoporosis risk factors (insufficient calcium intake and daily PA), as well as perceptions of low risk for developing osteoporosis, has been reported among college women (Kasper et al., 2001) and adolescent females (Anderson et al., 2005). Moreover, studies have suggested exercise self-efficacy and barriers to exercise are the best predictors of weight bearing exercise and dietary intake (Wallace, 2002), with educational interventions targeting youth demonstrating improvements in bone health knowledge, increases in intake of calcium rich foods and calcium self-efficacy (Schrader et al., 2005;Sharma et al., 2010). Therefore, knowing which factors will help children and adolescents adopt healthy 'bone' behaviors is important to making the exercise interventions we reviewed a reality. Based on our literature review we know that structured and controlled PA interventions are effective in eliciting bone gains in youth. In order for youth to get involved in osteoporosis preventative behaviors such as PA they need to be able intervene in their daily lives on their own. McWannell et al. (2008) conducted a study to determine whether a structured high impact exercise program would be more effective in improving BMC and BMD than a lifestyle intervention program promoting PA in middle school children 10-11 years of age. This study demonstrated that the structured high impact PA program significantly improved total body BMC and BMD compared to controls after 9 weeks, with the lifestyle intervention seeing insignificant trends for bone gains. Moreover, a health plan-based lifestyle intervention designed at improving both diet and PA in adolescent girls outside of school demonstrated significant improvements in BMD and bone metabolism due to greater consumption of calcium and vitamin D (DeBar et al., 2006). However, when a larger focus was placed on PA and the adolescent girls taught how to properly conduct exercises a selfled PA program proved to be just as significant in improving bone strength parameters as a structured teacher-led PA program (Murphy et al., 2006). More importantly, those girls involved in the self-led PA program continued to exercise after the intervention had ceased, whereas the teach-led group did not. Therefore, it is not only important to get youth physically active in order to improve bone health, it is just as important to develop the personal skills necessary to direct their own activity.

Conclusions
With the current growing inactivity and unhealthy dietary habits, the body composition of youth is changing making this systemic review regarding the different types of exercise interventions, those utilizing resistance training vs. ground reaction forces, relevant. For long-term gains, it appears that short-term high-impact exercises undertaken early in childhood (pre and early puberty) if sustained into adulthood has a persistent effect over and beyond that of normal growth and development. Benefits in total body, lumbar spine, thoracic and femoral neck BMC (2.3-4.4%) as well as BMC at the hip (1.4%) have respectively been observed 3 (Gunter et al., 2008b) and 5 years  following the jumping intervention by Fuchs et al. (2001). It is therefore redundant in some respect to conduct more PA interventions, unless more advanced techniques of measuring bone are used, as it is apparent from this review that PA in a structured controlled environment is effective in creating positive gains in bone. The next step is to influence change by schools either adopting these activities into their physical education curriculums or providing youth with the tools to administer this change on their own. Therefore, the examination of behavioral, social-psychological variables in addition to physical determinants of skeletal development provides a holistic multi-faceted conceptual framework of bone health that will provide the tools to better disseminate knowledge on positive bone building activities in hopes of creating life-long PA practices. Osteoporosis is a public health issue worldwide. During the last few years, progress has been made concerning the knowledge of the pathophysiological mechanism of the disease. Sophisticated technologies have added important information in bone mineral density measurements and, additionally, geometrical and mechanical properties of bone. New bone indices have been developed from biochemical and hormonal measurements in order to investigate bone metabolism. Although it is clear that drugs are an essential element of the therapy, beyond medication there are other interventions in the management of the disease. Prevention of osteoporosis starts in young ages and continues during aging in order to prevent fractures associated with impaired quality of life, physical decline, mortality, and high cost for the health system. A number of different specialties are holding the scientific knowledge in osteoporosis. For this reason, we have collected papers from scientific departments all over the world for this book. The book includes up-to-date information about basics of bones, epidemiological data, diagnosis and assessment of osteoporosis, secondary osteoporosis, pediatric issues, prevention and treatment strategies, and research papers from osteoporotic fields.