Cardiorespiratory, hematological and body composition changes in maturing girls and boys with different training backgrounds

Abstract

This thesis is based on one longitudinal study, monitoring a group of children from age 12 to age 15. The main objective was to establish the association between growth, maximal oxygen uptake (VO2max), hematological variables and cardiac dimensions in girls and boys during puberty. We also aimed at investigating the effects of inclusion of high volumes of endurance training in active children. Further, in a cross-sectional study, bioelectrical impedance analysis (BIA; InBody 720) was validated against dual-energy X-ray absorptiometry (DXA) for assessing body composition.

Seventy-eight children volunteered to participate in the study, and came for repeated measurements at age 12, 13 and 15. They were assigned to an endurance training group (End group; 40 girls and boys) and a non-endurance training group (non-End group; 38 girls and boys) based on the types of sport and volume of endurance training they were undertaking. Total weekly volume of training at age 14-15 was 10.7 (2.8) hours and 8.3 (3.7) hours in the End and non-End group, respectively, while weekly volume of endurance specific training was 7.3 (1.8) hours and 1.5 (1.1) hours, respectively.

VO2max was determined by an incremental running test to exhaustion on a treadmill and haemoglobin mass (Hbmass) and blood volume (BV) were assessed using the optimized carbon monoxide rebreathing method. Cardiac dimension measurements were performed using two- and three-dimensional echocardiography. Anthropometric measurements included body mass (BM), height and sitting height, and body composition which was assessed using InBody 720.

There were significant sex differences in the rate of development of VO2max and intravascular volumes in absolute values and relative to BM from age 12 to 15, with on average a larger increase in boys. The timing of the appearance of pronounced sex differences was aligned with significant differences in the development of body-composition, and when related to fat free mass (FFM), the age effect of sex from age 12 to 15 disappeared.

There was on average no interaction between age and training group in the development of all measures of VO2max, and intravascular volumes in absolute values from age 12-15. For Hbmass and red cell volume (RCV) relative to FFM, there was no difference between boys, however for girls, there was a difference in the age effect between the training groups, with no increase in the End girls over the three years.

There were no differences between the training groups in the alteration of cardiac dimensions relative to FFM; however, the End group experienced a smaller increase in left ventricular posterior wall thickness in diastole (LV PWd) than the non-End group. This led to a decrease in relative wall thickness (RWT) in the End group indicating eccentric remodeling, while RWT stayed unchanged in the non-End group compared to what was seen in both groups at baseline.

There were very strong to almost perfect correlations between absolute values for VO2maks, intravascular volumes, and cardiac dimensions and FFM at both 12 and 15 years of age. Also, no change in FFM-relative values of VO2max and no or minor changes in vascular volumes relative to FFM during the study period, indicate that these variables develop in parallel with FFM.

In boys, time to exhaustion (TTE) increased in both groups but increased significantly more in the End boys than in the non-End boys, while the development of VO2max did not differ between the training groups. This suggests that endurance training may have had a significant effect on determinant factors other than VO2max. This pronounced difference between the training groups was not seen in girls.

Validating InBody 720 against DXA, showed that InBody 720 underestimated FM and overestimated FFM. However, the correlation between the devices was almost perfect for both FM and FFM. Taken together, this indicate that InBody 720 is a valid method in estimating changes in FFM during growth.

In conclusion, the present longitudinal study suggests that the development of VO2max and intravascular volumes was dependent on sex but not on volume of endurance training. Using multiple linear regression analysis for developing a model for predicting changes in VO2max from measurements of changes in anthropometrics, heart dimensions and intravascular volumes, indicated that a model which included changes in FFM and Hbmass was best suited. This model was able to account for 74% of the variance in the change in VO2max.