Similar Changes In Vo2max Over A 14-month Period In Very Active And Inactive 12-13-year-old Girls

During puberty, upregulations of hormones increase fat-free mass (FFM), which is closely linked to adaptations in circulatory- and respiratory systems. Together, these adaptations lead to increased maximal oxygen uptake (VO2max) regardless of the children’s level of training and physical activity. If endurance training can add to the effects of growth on VO2max remains to be elucidated. PURPOSE: To assess the changes in VO2max, blood volume (BV), hemoglobin mass (Hbmass), left ventricular mass (LVM), left ventricular end-diastolic volume (LVEDV) and stroke volume at rest (SV) during a 14-month period in very active compared to inactive 12-13-year-old girls. METHODS: Seven girls participating in cross-country skiing (END, age: 12.6 ± 0.7 years) were compared to eight untrained and mostly inactive girls (UNT, 12.4 ± 0.3 years). Training volume was reported four times through a questionnaire with follow-up interviews. FFM, VO2max, BV, Hbmass and cardiac parameters were assessed at baseline and after 14 months and adjusted for FFM. Between-group differences were assessed with a Students t-test, and an ANCOVA was used to compare %-change from baseline to 14-month follow-up. RESULTS: Average weekly training hours during the 14 months were 9.1 ± 1.3 h for END vs. 0.5 ± 0.6 h for UNT. At baseline, no difference was found for FFM (END: 35.2 ± 4.9 kg, UNT: 33.6 ± 5.2 kg), but VO2max relative to FFM was higher for END (70.5 ± 6.6 ml·kgFFM-1·min-1) compared to UNT (56.3 ± 5.8 ml·kgFFM-1·min-1)(p < .001), as was SV (END: 1.5 ± 0.3, UNT: 1.2 ± 0.1 ml·kgFFM-1)(p < 0.01). No differences were seen for BV, Hbmass, LVEDV or LVM relative to FFM. The change in FFM did not significantly differ between groups, and no significant differences between groups were seen for %-change relative to FFM for VO2max (END: -1.3 ± 7.7%, UNT: -6.1 ± 7.6%), LVM (END: -1.4 ± 11.3%, UNT: 5.3 ± 11.5%), LVEDV (END: -7.1 ± 9.9%, UNT:-5.7 ± 12.1%) or SV (END: 0.0 ± 25.4%, UNT: 5.8 ± 14.4%). However, BV (ml·kgFFM-1) changed more in UNT (19 ± 15%) than END (4 ± 4%)(p < .05), which was also true for Hbmass (g·kgFFM-1)(UNT: 18.8 ± 10.5%, END: 3.3 ± 4.1%)(p < .01). CONCLUSION: A high amount of endurance training during puberty in already trained girls yield no physiological changes beyond the change in FFM and may negatively affect hematological adaptations.