INTRODUCTION: The current review examined the literature about retrospective analyses of training intensity distribution (TID) in elite to world class endurance athletes by clustering the different sport disciplines, quantification methods (QM) and time of season. METHODS: TID data was obtained from PubMed, using varying combinations of the search terms "training intensity distribution", "TID", "training intensity", "endurance training", "training characteristics, "endurance," "training" and "athletes". Inclusion criteria: peer-reviewed English articles of endurance athletes categorized as Elite Level (Tier 4) or World Class (Tier 5) with internal (e.g., heart rate [HR]), external (e.g., velocity, power) and/or subjective (e.g., rating of perceived exertion [RPE]) QMs. All different QMs were categorized. The TID data expressed in percentages or in absolute numbers was included. RESULTS: 33 reports quantified the TID of 425 athletes (67 females) from different endurance sports. 162 different TIDs could be extracted from all studies and clustered according to the sport, different seasonal phases, and different QMs. The TIDs derived from cycling (9 studies), rowing (n=7), running (n=6) skiing (n=6), swimming (n=2), hand cycling, triathlon and ice speed skating (each n=1). 69 (42%) TIDs were based on single-case reports of which 44 TIDs stemmed from cross-country skiing. 45 TIDs reported an entire season, n=7 mean values for multiple seasons, n=65 preparatory periods and n=54 competition periods. TIDs were categorized in 9 different QMs: HR time-in-zone (HR-TiZ), HR session goal - number of sessions (SGSession), HR session goal - total time/session (SGTime), HR session goal/time-in-zone (HR-TiZ/SG), session RPE (sRPE), RPE time-in-zone (RPE-TiZ), velocity time-in-zone (V-TiZ), power time-in-zone (PO-TiZ), race pace time-in-zone (RP-TiZ). 7 studies directly compared different QMs in their reports. 66 TIDs (41%) showed a polarized TID with 34 (52%) of these reported by single case analysis in cross-country skiing. 72 TIDs (44%) showed a pyramidal TID. Analysis of the most comparable TID QMs (HR-TiZ,V-TiZ, PO-TiZ) show proportion ranges for the moderate (Z1), high (Z2) and severe (Z3) intensity domain of 20-96%, 0-70% and 0-19%, respectively. Independent of the QM and time of season 91% (n = 147) of all analysis report Z1 proportions *65%, whereas 70% of all analyses report Z2 and Z3 proportions < 10%. CONCLUSION: Current evidence on TID in Tier 4 and 5 athletes is equivocal and influenced from a high number of single case reports. The QM influences the TID pattern which must be recognized in the TID debate in endurance sports. The number of reports considering pyramidal and polarized TIDs are approximately equal. The most comparable QMs using the TiZ-approach highlight high variation in proportion ranges in the different intensity zones. Most noticeable, high volume of low intensity training zones (* 65%) is applied across all sports, seasonal phases, and quantification methods.
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