Running Performance Decline in IRONMAN-Distance Triathlon: A Scoping Review of Existing Evidence and Research Gaps

In an IRONMAN-distance triathlon, run performance is a particularly strong correlate of overall race outcome, alongside cycling. Although individual research areas such as pacing, performance prediction, and laboratory physiology have been reviewed separately, they have not been integrated to determine factors contributing to running performance decline or to identify key knowledge gaps. This scoping review aims to map the current evidence on running performance decline in IRONMAN-distance triathlon and identify key gaps, with a particular focus on age-group athletes. Following the Arksey and O'Malley framework and PRISMA-ScR guidelines, we searched PubMed and SPORTDiscus for studies reporting quantitative running performance outcomes in IRONMAN races or bike-to-run transition protocols. Of 896 deduplicated records, 70 studies were included. Findings were organised into five sub-questions addressing the extent of evidence, quantification of decline, physiological and biomechanical changes, underlying mechanisms, and inter-individual variability. Pacing patterns and performance predictors are well described, whereas mechanistic research relies almost entirely on short, controlled protocols. Critically, no study identified by this review quantified the "between-context decline," defined as the difference between an athlete's standalone marathon performance and their actual IRONMAN running performance. Addressing these gaps requires observational studies using real IRONMAN race data and individual-level analyses, particularly for age-group athletes.