Introduction: Timesaving specific endurance training is currently necessary in soccer training settings. A possible physiologic mechanism underlying adaptations to high-intensity aerobic interval training (HIT) is the increase of the total haemoglobin mass (tHbm). This study analyses the adaptations in maximal oxygen consumption (VO2max) and tHbm resulting from a HIT block. Improvements in VO2max were expected as a result of an increase in tHbm. Methods: Fourteen young soccer players were randomized allocated into an intervention (HIT, n=8) and a control (KON, n=6) group. The target intensity was 90-95% of the maximal heart rate (HRmax) and high blood lactate levels. HIT underwent 13 HIT sessions within ten days. Sessions consisted of 4 sets of 8x15/15 sec intermittent running and 4x4 min playing in a small field. KON underwent a conventional training program. Jump height (H) and power (Pmax), VO2max, running economy (RE) and blood parameters (tHbm and blood volume (BV)) were measured pre- and 2 weeks post-intervention. Results: The HIT group reached lactate levels of 6.6 ± 1.6 mmol/L and 93.4 ± 2.2% of HRmax during intermittent running and lactate levels of 6.1 ± 2.0 mmol/L and 90.9 ± 2.1% of HRmax during playing in a small field. There were no significant differences in VO2max, RE, BV and tHbm between pre- and post-intervention. Maximal blood lactate, HRmax and Pmax decreased post-intervention (-6.4%, -3.0% and -2.4%, respectively; p < 0.05). Discussion: The lack of differences between pre- and post-intervention prevents the establishment of a relationship between tHbm and VO2max improvements. We suggest that the selected training mode did not provide stimuli high enough to improve VO2max, partly due to the increased recovery during intermittent exercise. Furthermore, the results could also have been influenced by the recovery period after the HIT block.
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