Bone mineralization increases with age, height and body mass throughout childhood (Jurimae, 2010), and maximal bone mineral density (BMD) accrual occurs in years surrounding puberty (Maimoun et al., 2013). The transition from prepuberty to early puberty is a period of increased bone adaptation to mechanical loading due to the velocity of bone growth and endocrine changes at this time period (Hind and Burrows, 2007). Mechanical loading factors such as weight bearing and muscle forces are important in BMD accrual (Ho and Kung, 2005). Pubertal maturation also influences BMD accrual together with body composition factors (Cobayashi et al., 2005). Therefore, body fat mass (FM) and fat free mass (FFM) contribute to bone development by increasing compressive forces during skeletal loading (Ho and Kung, 2005). Accordingly, high impact mechanical loading is one of the most important factors determining BMD accrual during growth and maturation (Hind and Burrows, 2007), and young athletes whose skeletons are subjected to forces of high intensity, such as rhythmic gymnasts, present significantly higher BMD values at the load-bearing sites of the skeleton in comparison with athletes of low-impact activities (e.g., swimming) and healthy untrained controls (Gruodyte et al., 2009).
There are substantial data to support the view that body FM and FFM are both positively related to BMD during pubertal maturation in girls with different physical activity patterns (Gruodyte et al., 2010). It also appears that jumping ability seems to correlate well with different BMD values in growing and maturing female athletes (Gruodyte et al., 2009). Various types of vertical jumps have been used to evaluate jumping ability in young athletes (Gruodyte et al., 2009; Kellis et al., 1999). It has been suggested that maximum vertical jump performed from the standing position with countermovement (CMJ) and also rebound jumps for 15 (RJ15s) and 30 (RJ30s) seconds represent well jumping abilities in young athletes (Kellis et al., 1999). However, to the best of our knowledge, no studies have been conducted to examine longitudinally the influence of body composition and jumping performance on BMD accrual in prepubertal rhythmic gymnasts entering puberty. Accordingly, the aim of the present 3-year longitudinal investigation was to study the increases in BMD values during pubertal maturation in rhythmic gymnasts and their age- and height-matched untrained control girls, and to evaluate the relationships between body composition and jumping performance parameters with increases in BMD values when entering from prepuberty to puberty. Therefore, in addition to whole body (WB) BMD, which is the most recommended measurement site for bone health in children and adolescents (Crabtree et al., 2014), BMD measurement was also done at the femoral neck (FN), since this skeletal site is associated with muscle power (Baptista et al., 2016) and used in rhythmic gymnastics trainings (Vosoberg et al., 2016).
A total of 50 Estonian prepubertal girls participated in this study, who were divided into rhythmic gymnasts (n = 25) and untrained control (n = 25) groups. They were followed for the next 3-year period, when they all had reached puberty. Rhythmic gymnasts were recruited from local training groups and had usually trained 6-14 h/week (4-7 training sessions per week) for the past 2 years before starting the study. All RG had very similar training lessons, including rhythmic gymnastics, ballet and acrobatics. All RG were competing at the national level (Vosoberg et al., 2016). Controls had compulsory physical education classes twice a week at school. Participation only in school physical education classes was inclusion criteria for control subjects (Vosoberg et al., 2016). The groups were matched for age ([+ or -] 0.5 year) and height ([+ or -] 2 cm) at the beginning of the study (Jurimae et al., 2016). All procedures were approved by the Medical Ethics Committee of the University of Tartu, Estonia and were explained to the children and their parents, who signed an informed consent form.
Height, body mass, body mass index (BMI), pubertal stage, bone age, body composition, BMD and jumping performance were obtained at baseline (T0) and after 3-years (T3). Therefore, the subjects were evaluated as close as possible to the baseline date at T3 measurement and within the same month of both measurements sessions. Both absolute values as well as changes between T3 and T0 ([DELTA] scores) were used in the analyses (Vosoberg et al., 2016).
Materials and procedure
Biological maturation: Pubertal development of the participants was assessed by self-report using an illustrated questionnaire according to the Tanner classification method (1962), which has been used previously in our laboratory (Jurimae et al., 2016; Parm et al., 2011; Vosoberg et al., 2016). Bone age was assessed with an X-ray of the left hand and wrist (Greulich and Pyle, 1959).
Bone mineral density and body composition: Bone mineral density (BMD; g x [cm.sup.-2]) of whole body (WB) and femoral neck (FN) were measured using dual-energy X-ray absorptiometry (DXA; DPX-IQ, Lunar Corporation, Madison, WI, USA) equipped with proprietary software, version 3.6. In addition, WB fat percentage (body fat%), fat mass (FM, kg) and fat free mass (FFM, kg) were also determined. Participants were scanned in light clothing while lying flat on their backs with arms at their sides. The fast scan mode and standard subject positioning were used for WB measurements and results were evaluated by the same examiner. To reduce the impact of the operator variability factor, one qualified observer analyzed all scans over the 3-year period. The precision of measurement expressed as coefficient of variation was less than 2% for all bone mineral and body composition measurements (Ivuskans et al., 2013; Parm et al., 2011).
Jumping performance: The maximal vertical height (in cm) of two-footed hands-on-the-hips vertical jumps were measured using a contact mat (Newtest OY, Finland) (Jurimae et al., 2008). The participants performed two tests: 1) a countermovement...