Purpose: The overall objective of this thesis isto contribute to a more detailed understanding of the relationship between endurance training organization and adaptive responses. Three independent studies, and five original papers, have been published towards this objective. Peer-reviewed studies describing training characteristics in elite endurance athletes have been published since the 1980`s. In these studies, different methods of quantifying training patterns during longer time frames have been used, with athlete self-report (SR) in training diaries being the most common. While extensively used, athlete SR diary data has not been evaluated for accuracy and validity. In addition, there are several pitfalls concerning quantification of training intensity distribution (TID). The aims of papers Iand IIwere therefore 1) to validate the accuracy of SR training duration and intensity distribution in elite endurance athletes, and 2) compare three methods of TID quantification employed byelite endurance athletes. Results from these two methodological papers secured a fundamental platform for analysis of further training-characteristicinstudies including reliable methodological interpretations, during an annual cycle in World-Class athletes. The aim of paper IIIwas to describe training characteristics across the annual cycle in Olympic and World Champion endurance athletes. Through observations of high intensity training (HIT) organization patterns in paper III, we formulated hypotheses to be tested experimentally. The aim of paper IVwas to compare the effects of different intensity zone periodization models during 12 weeks on endurance adaptions in well-trained cyclists. Finally, the aim of paper Vwas toquantify the time-course of developmentof performance, physiological and hormonal responses during a 12-week HIT period ingroups prescribed different interval training prescriptions. Methods: In papers Iand II, 29 elite cross-country (XC) skiers from the Norwegian national team (mean maximal oxygen uptake (V.O2max) 80±5 (Männer) and 70±5 mL7kg.min (Frauen)) performed, in total, 570 training sessions during a ~14 day altitude camp. Paper Icompared SR training duration with recorded training duration from heart rate (HR) monitors, and compared SR intensity distribution with the intensity distribution derived from summated expert analyses of all session data. In paper II, the proportion of training in the zonesof low intensity training (LIT), moderate intensity training (MIT) and HIT was quantified using total trainingtime or frequency of sessions, and compared through a time in zone (TIZ), session goal (SG) or a hybrid session goal/time in zone (SG/TIZ) approach. Simple conversion factors across different methods were calculated. In paper III, 11 Olympic or World Champion XC skiers and biathletes (mean V.O2max Männer 85±5 and Frauen 73±3 mL/kg.mi) SR one year of day-to-day training leading up to the most successful competition of their career. Training data were quantified and divided in phases and distributed into training forms, activity forms and intensity zones.Papers IVand Vare derived from a randomized controlled experimental trial executed as a coordinated multicenter study involving three test centers. Sixty-nine well-trained male cyclists (mean V.O2max61±6 mL/kg.min) were randomly assigned to one of three training groups, all of whom performed a 12-week interventionconsisting of 2-3 prescribed HIT sessions perweek in addition to ad libitum LIT.Groups were matchedfor total training load, butincreasing HIT (INC)group (n=23) performed interval training as 4x16min in cycle 1 (week 1-4), 4x8min in cycle 2 (week 5-8) and 4x4min in cycle 3 (week 9-12). Decreasing HIT (DEC)group (n=20) performed interval sessions in the opposite cycle order as INC, andmixed HIT (MIX)group (n=20) performed all threeinterval prescriptions in a mixed distribution during each cycle. Interval sessions were prescribed as maximal session efforts.Laboratory exercise testsand measuresof resting blood hormones were conductedpre, and at the end of weeks4, 8 and12of the intervention. Main results: In paper I, SR training was nearly perfectly correlated with recorded training duration (r = .99), but SR training was 1.7% lower than recorded training duration (P<0.001). No significant differences were observed in intensity distribution in the LIT area between SR and expert analysis comparisons, but small discrepancies were found in the MIT and HIT area (P<0.001). In paper II, comparing TIZ, SG/TIZ and SG methods, 96.1, 95.5 and 86.6% of total training time or frequency of sessions was spent in zone 1 (P<0.001), 2.9, 3.6 and 11.1% in zone 2 (P<0.001), and 1.1, 0.8 and 2.4% in zone 3 (P<0.001), respectively. Estimated conversion factor from TIZ or SG/TIZ to SG was three (x 3) in the MIT/HIT range. Paper IIIdemonstrated that gold medal winning XC skiers trained ~800 h.yr-1(of this ~500 h sport-specific), of which 94%endurance training (90% LIT and10% HIT). Total training volume progressivelyincreased during the general preparation (GP) and decreased 32% (mainly aerobic cross-training) from GPto competition period (CP). Absolute volume of HIT remained stable acrossallphases, although HITpatterns became more polarized inCP. Paper IVdemonstrated a 5-10% improvement in key components of endurance performance among already well-trained cyclists completing the training intervention. However,no significant adaptation differences were observed amongthe three training groupsdiffering in sequencing of prescribed HIT sessions(P>0.05). Anindividual response analysis indicated similar likelihood of either large, moderate or non-responses in each training group (P>0.05).Of the total change in power output corresponding to 4 mMol/L blood lactate concentration(Power4mM) andpeakoxygenuptake(V.O2peak) during 12 weeks, INC achieved 98 and 70%, and MIX 149 and 92%, respectively, whilst DEC achieved only 34 and 38%, during the first 4 weeks of intensified training (paper V). However, changes in PPO during cycle 1 accounted for 77, 64 and 89% in INC, MIX and DEC groups, respectively, of total change. INC (4x16 min) revealed a moderate effect size (ES) compared to DEC (4x4 min) when comparing delta changes in Power4mM(ES: 0.7) and V.O2peak(ES: 0.7)during cycle 1. Pooling the three training groups, total-(TT), free-testosterone(FT)and free testosterone-cortisol ratio(FTCR)decreased significantly by 22, 13 and 14% (all P<0.05), respectively by the end of the first 4-week training cycle.Insulin-like growth factor-1 (IGF-1) increased significantly by 10% (P<0.05). Conclusions: The present thesis demonstrates that daily SR training is a valid method of quantifying training duration and intensity distribution in elite endurance athletes, although additional common reporting guidelines would further enhance accuracy. Our evaluation of three common HR based TID approaches provide practical and useful tools to compare and convert different methods used by athletes. A one year, day-to-day description of the training to Olympic-andWorld champion XC skiers shows annual training patterns that can provide a valuable reference for upcoming athletes. However, we questioned if training patterns where TIDbecame more polarized in CP wereanappropriatetradition based on best practice.Our experimental approach suggests that different HIT organization patterns have little or no effect on training adaptation when the overall training load is the same. However, we found that most of the progressionin specific performance outcomes wasachieved already during the initial 4 weeks of training, though dependent on interval training prescription. Hence, a 4x16 min interval prescription 2-3 times per week appears to induce greater adaptions in Power4mMand V.O2peakcompared to a 4x4 min interval prescription. Resting levels of anabolic hormones were found to first decline and then rebound over 12 weeks, with the period of decline associated with greater adaption
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