Bone mass develops throughout childhood and adolescence until a peak bone mass is achieved during early adulthood. Fracture risk later in life can be predicted at a large extent by peak bone mass. Occurence of sarcopenia and osteoporosis (i.e. loss of mone mass) during late adulthood has been strongly associated with the degree of bone mineralization during early life. Nearly 50% of total bone mineral content (BMC) reached during adulthood is obtained during pre-adolescence rendering this period critical for skeletal health and is considered as an optimal period for bone/skeletal growth since during this time bones are more adaptable to osteogenic stimuli such as exercise-induced mechanical loading. Organized sport activities and/or nutrition appear to affect profoundly bone mineral density (BMD), BMC, bone geometry, and overall skeletal health during preadolescence offering an effective type of prevention of osteoporosis, a condition very difficult to treat later in life. Evidence suggest that some modes of exercise activities may be more effective (osteogenic) for bone development due to the magnitude and type of mechanical strain placed on long bones causing them to be more dense. Weight-bearing activities (e.g. running, jumping etc.) are believed to be more osteogenic than non-weight bearing activities. However, more research is required in order to determine: i) whether weight-bearing activities are more osteogenic than non weight -bearing activities during childhood and ii) the osteogenic potential of a large number of sport activities used by school-children as compared to a control treatment of no participation in organized sport activities. The present trial attempted to compare a large number of different sport activities in respect to their osteogenic potential based on training variables that are thought to affect osteogenesis while at the same time allows direct comparison of exercise modes that are entirely different. Therefore, the goal of this investigation was to determine the osteogenic potential of a large number of exercise training activities in boys and girls of 8-12 years of age during an entire primary school season.

Healthy, previously untrained, pre-pubertal boys and girls (N=335) were assigned to 16 different groups: 1) physical education, i.e. children participated only school in physical education classes (control group), 2) football (soccer) training, 3) basketball training, 4) volleyball training, 5) wrestling training, 6) martial arts training, 7) tennis training, 8) track and field training, 9) taekwondo training, 10) rhythmic gymnastics training, 11) artistic gymnastics training, 12) dance training, 13) swimming training, 14) climbing training, 15) two weight-bearing training modes, and 16) one weight-bearing and one non-weight bearing activity. Exercise training was performed three times per week for nine months and each training session had a 60-minute duration (except for the physical education classes at school in the control group). Anthropometric measurements (body height, body mass, and length and circumferences of various body segments), blood sampling, measurements of body composition (using dual X-ray energy absorptiometry or DEXA and skinfold calibers), bone measurements (bone density and bone mineral content at lumbar spine, both hips, both wrists and whole body using DEXA), and performance (cardiorespiratory fitness, muscle strength, muscle power, flexibility and motor ability) were performed at baseline and after the completion of a 9-month training intervention. Nutritional intake and habitual physical activity were measured at baseline, mid-training and post-training (using diet recalls and accelerometry, respectively). Intensity and volume of training was measured once every three months using heart rate monitoring, accelerometry, Global Positioning System (GPS) devices and jump measurement. Furthermore, two other studies were also performed as a part of this project: a) assessment of physical activity during physical education classes for primary school (using accelerometry, GPS instrumentation and jump measurement) and b) a smaller number of participants in the football, track and field, swimming and tennis training groups provided blood samples before and after a training session at baseline.

Inclusion criteria

• were 8-12 years and pre-pubertal
• were healthy and had no prior bone fractures or related surgical operation
• had not been involved in organized sport activities previously
• their body fat was <30%, e) had no history of growth irregularities
• were not receiving agents or drugs that affect bone tissue (e.g. Gonadotropin-Releasing Hormone (GnRH) agonists, antiresorptive, bisphosphonates, etc.)

Exclusion Criteria:

• had prior bone fractures or related surgical operation
• had been involved in organized sport activities previously
• their body fat was >30%
• had history of growth irregularities
• were receiving agents or drugs that affect bone tissue (e.g. GnRH agonists, antiresorptive, bisphosphonates, etc.)
• missed more than 10% of training sessions