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The normal development of bone is critical for prevention of bone medical conditions that could lead to a lower quality of life. Physical activity has always been seen to provide multiple health benefits when undergone regularly, including reducing osteoporosis; a medical condition in which the bones become brittle.
Bone mineralisation is an important development from birth and throughout life. With ‘normal’ bone development, it decreases the chance of bone medical conditions occurring in individuals, and what happens during childhood can affect the bone mineral density (BMD) in later life. BMD is largely defined as the amount of mineral in the bone per a unit area. The World Health Organisation (WHO) defines ‘normal’ bone density in the terms of a T-score, this scale is also used to identify individuals that may be a risk or may already have osteoporosis. On this scale, a T-score of above -1 can be considered normal, and these values are created for individuals by undertaking a dual energy X-ray absorptiometry (DXA) scan .
Bone formation (osteogensis) is the process of osteoblasts synthesising extracellular organic matrix, with this matrix mineralising to create the formation of bone . Support cells; osteoblasts (bone formation), and remodelling cells; osteoclasts (bone absorption), are present upon the bone surface. Physical activity (PA) has shown positive associations with BMD ; this shows an interest into promoting physical activity to benefit children and increase their BMD as much as possible before adulthood, in hope that this increase will be advantageous in later life.
Physical activity is defined by WHO as additional energy expenditure created through the skeletal muscles producing bodily movements. PA has seen to have a variety of health benefits, including increased prevention of diabetes, cardiovascular disease, obesity, depression and osteoporosis . The current recommendations within children stands at a minimum a 1 hour/day of moderate to vigorous physical activity .
This article looks at the influences of PA upon bone mineralisation development within children, both positive and negative associations. A hypothesis states that PA has increased benefits over risks on the impact of bone mineralisation development. Conclusions have assessed the positive and negative impacts of PA, and consideration of the recommended intensity of PA for peak bone development will be examined. Studies reviewed will be longitudinal unless otherwise stated to increase the robustness of the conclusions.
As previously stated, the most effective way to measure BMD would be through a DXA scan compared to the use of X-rays. Development of BMD shows that before puberty, sex differences make no impact; but after puberty, males BMD continues to rise to late adolescence, with females BMD scarcely rising . To increase BMD further, weight-bearing physical activities has been recommended.
A cross-sectional study looked at pre-pubertal girls who were serious competitors within swimming and gymnastics, compared to an age and gender matched control group, . Swimmers had been training intensively for 8-12 hours/week, gymnasts 10-15 hours/week and control group would only complete 2 hours/week; all training was considered high intensity and had been consistent for 3 years prior to testing. Gymnasts were shown to see a significantly greater BMD than swimmers and control groups in the mid-radius (+16.3%), distal radius (+30.5%), L2-L4 (+9%), and the femoral neck (+13.7%); with no significant differences between the swimmers and the control. This study would suggest that intensity of PA needs to be also matched with weight-bearing loads to provide a large osteogenic stimulus, as swimming proved to not apply great impact loadings (mechanical strains) upon bone and therefore had little influence over BMD. A study  found that over a 6 year period physically active boys and girls showed a 9% and 17% increase, respectively, in bone mineral content compared to the less active children.
PA is also important when considering lowering the risks of bone fractures and disease. When looking at infants, preterm infants have a higher risk of developing osteopenia of prematurity . This is because the after 24 weeks of pregnancy, large processes of bone mineralisation from calcium and phosphate accretion occurs. Therefore; if the infant is premature, they would be missing part of this vitally important process. Preterm infants also require supplementation of calcium and vitamin D as well as human milk to prevent poor BMD. There is some evidence that passive daily exercise can be beneficial for preterm infants as it potentially could improve strength of the bone, and the bone mineral content.
In 1995 a study  investigated the effect of PA on the bone mineralisation of preterm infants. Risks highlighted with the effect of having a preterm infant; such as if the birth weight is under 1kg, showed a 50% estimation for developing osteopenia of prematurity and a 70% estimation of consequent fractures. This shows how it could affect the infant from childhood through to adulthood if ‘normal’ ranges of bone mineralisation are not met. Mean calcium, phosphorus and vitamin D intake did not differ for the infants throughout the study, this reduced the cause of any significant results to be dietary.
PA has also shown to sometimes have a negative effect upon bone development through the increased risk of fractures. A study in 2009  investigated 2692 healthy children, from birth to 10-11 years of age. This study assessed whether the amount of vigorous physical activity (VPA) and time spent outdoors in summer had an impact on the fracture risk of children. Results showed that children who participated in daily episodes of VPA doubled the risk of receiving a fractures compared to those who did less than 4 episodes. Also children who spent more than 28 hours/week outside during the summer also doubled their risk of getting a fracture.
Most studies seem to be in agreement that with increased weight-bearing physical activity, bone development within children has a positive association. It has shown to not only be a preventative measure from medical bone conditions, but similarly with increased variables such as BMD, the susceptibility to fractures is reduced when performing daily activities, . Contradicting evidence suggests that intensity of a training programme may want to be further researched, with a vigorous physical activity showing a negative effect upon bone development through the forms of fractures. This is proposed to be due to the increased exposure to fracture injury. Studies also show that there is a gender imbalance, with boys being more at risk to girls. More research for this negative impact of physical activity needs to be conducted to support this argument to identify an ‘optimal’ intensity for maximal bone development without the significant fracture risk. Therefore the hypothesis would be rejected until further research has been produced assessing the risk of fractures through PA.