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IRONMAN® 70.3, commonly known as the Half IRONMAN®, consists of a 1.9 km swim, 90 km cycling and 21.1 km run, placing it between Olympic-distance and full-distance IRONMAN® triathlon [,]. Participation has increased markedly across age groups and both sexes in recent years, reflecting broader growth in endurance sports and event accessibility [–]. Triathlon performance depends on the integrated demands of swimming, cycling and running, influenced by discipline-specific physiological, biomechanical, environmental, and metabolic factors [,]. Previous research has examined these determinants across different triathlon distances [,–]. However, findings are often based on small sample sizes, heterogeneous study designs and mixed race formats, limiting their generalizability and direct applicability to IRONMAN 70.3®.

Although the increasing number of participants suggests a greater need for specific information on IRONMAN® 70.3, research on this topic remains fragmented, with small, predominantly male samples, heterogeneous methodologies and inconsistently reported environmental and physiological metrics. No prior synthesis has systematically integrated evidence across performance determinants, physiological adaptations, environmental modifiers, nutritional strategies and medical aspects for this distance. The present review therefore consolidates multidisciplinary findings to support triathletes, coaches, clinicians and race organizers, and identifies key gaps, including sex-specific responses, age-related differences and mechanistic pathways that require targeted investigation.

This systematic review was conducted in accordance with PRISMA 2020 recommendations, with all methodological steps documented prospectively following an initial scoping phase. The PRISMA 2020 checklist is provided in . Registration in PROSPERO was not possible because retrospective submission is not permitted. A comprehensive search was performed in PubMed, Scopus, SciELO, EBSCO and Google Scholar, selected for their relevance to sports and health sciences research [,]. The search covered all records up to 25 November 2025, without language restrictions. Search terms were developed using free-text principles [] and included variations of: ((Ironman AND Half Triathlon) OR (Ironman AND 70.3) OR (Ironman AND Half AND Triathlon) OR (middle-distance Triathlon)). Google Scholar results were screened through the first seven pages, with entries on pages 6 and 7 excluded due to irrelevance. Three additional studies were identified via reference and citation tracking.

Studies were eligible if they investigated the IRONMAN® 70.3 distance, involved human participants, and reported physiological, anthropometric, nutritional, environmental, medical or performance-related outcomes. Observational (cross-sectional, longitudinal, retrospective), experimental and interventional designs were included. Studies analysing other triathlon distances were eligible only when IRONMAN® 70.3-specific outcomes were clearly distinguishable. Exclusion criteria encompassed animal or in vitro studies, multi-stage or ultra-endurance events, ultra-swimming, ultra-cycling, mountain biking, cross-country skiing and studies lacking extractable 70.3-specific data.

After removal of duplicates, 86 studies met the inclusion criteria. Title and abstract screening were performed independently by three reviewers IvK, BK and KW, followed by full-text assessment when eligibility remained unclear. Any discrepancies were resolved through discussion until consensus was reached.

The dataset is provided in . For each included study, data were extracted on participant characteristics, sample size, race conditions, environmental context, performance metrics, training variables, anthropometric data, nutritional strategies, physiological and biochemical markers, immune and inflammatory responses, and medical outcomes. Extracted variables were categorized into thematic domains to support structured synthesis and cross-study comparison.

Given the heterogeneity of study designs, risk of bias was assessed using tools appropriate to each methodology: the Newcastle–Ottawa Scale for observational studies, the Cochrane Risk of Bias Tool for interventional trials, and the NIH Quality Assessment Tool for cross-sectional and physiological research. Major methodological challenges included small samples, male-dominated cohorts, inconsistent measurement timing, variable laboratory versus field conditions and incomplete reporting of environmental characteristics.

Comparative analyzes with other races such as the full distance IRONMAN® and the Olympic distance triathlon were used to determine IRONMAN® 70.3 specificities and general findings. Data heterogeneity, stemming from non-standardized performance metrics, scarce interventional designs, and variable environmental conditions, impeded comparability. Small sample sizes in studies on physiological adaptation, training programs, and nutritional strategies further constrained interpretability. Generalizability is additionally limited by a pronounced male bias due to participation patterns and the rarity of female-specific cohorts. Additionally, the prevalence of cross-sectional designs and inadequate control of environmental factors limited causal inferences.

Following the screening process, a total of 86 studies were included in this review (). The included studies comprised case studies, observational studies, and experimental studies and primarily focused on trained, male, and young triathletes, although the study populations varied in terms of sample size and gender distribution across the different outcome categories. The results are organized in descending order by number of studies found into physiological adaptations, competition-related complications and medical strategies, performance determinants and competition characteristics, nutritional strategies, age and gender in peak performance, as well as training characteristics and performance predictors.

Twenty-two studies examined physiological responses to IRONMAN® 70.3, predominantly in small cohorts of trained male triathletes aged 25–39 years [–] ( and ). Nineteen studies investigated acute post-race changes, while three captured both acute and chronic adaptations [,,]. Most measurements were conducted immediately before and after the race, with delayed follow-up assessments performed under laboratory conditions.

Across eight studies, body composition, hydration and electrolyte responses were measured (male-only cohort, n = 6–42) [,,–,–]. Bioimpedance analyses demonstrated reductions in fat mass (e.g., from 13.91% to 9.45%) and increases in circulating free fatty acids (0.15 to 1.69 mEq/L) []. Professional triathletes exhibited higher total body water (63.7% vs. 61%) and intracellular water, compared with recreational athletes []. The fluid losses during the race were comparable, 0.98l for professional athletes and 0.75l for recreational athletes, without association with overall race time []. Amateur triathletes showed higher fat mass (23.9%) and lower muscle (48%) and lower bone mass (11.3%) compared to professionals [,]. Electrolyte responses were found under ad libitum intake conditions. It reported modest declines in key electrolytes including sodium declined by 3.38 mmol/L (141.72 to 138.37 mmol/L) []. Calcium increased by 0.8 mmol/L [], whereas potassium and chloride showed no significant alterations [–,]. No values fell below clinical thresholds, and none were associated with performance outcomes.

During IRONMAN® 70.3, measurable alterations were found across multiple physiological systems, including skeletal muscle, renal and hepatic markers, pulmonary function, and cardiac responses. In four studies (n = 12–34 male, 4 female) markers of muscle damage documented significant increases, in creatine kinase (3- to 10-fold), myoglobin (32.8 to 654.8 mg/L) and lactate dehydrogenase (318.4 to 479 U/L; 41.71 to 191 U/L), due to the absence of pathological threshold values. Three studies (male-only cohort, n = 6–12) documented reversible post-race proteinuria, creatinuria and hematuria [,]. Elevation in hepatic enzymes AST and ALT were reported but remained below harmful thresholds and normalized within 48 h [].

Four studies documented respiratory conditions in triathletes, with documented cases of asthma, airway hyperreactivity and swimming-induced pulmonary edema (SIPE) [,,,]. SIPE occurred in approximately 1.4% across different race distance (n = 749 male, 669 female) and was associated with hypertension, cold-water exposure, fish-oil intake and female sex. Symptoms included dyspnea, reduced oxygen saturation and auscultatory crackles []. A case study of a female athlete underlined the cold-water exposure [] and use of 50 mg Sildenafil was in one cohort mentioned (sex not reported, n = 71) []. Airway hyperresponsiveness and asthma were more common in endurance triathletes (sex not reported, n = 30) []. Four studies responses cardiac responses and reported transient reductions in systolic performance following IRONMAN® 70.3 were demonstrated [–,]. In one cohort (n = 9 male, 8 female), only male triathletes required higher dobutamine doses post-race to achieve an increase of 25 beats/min in heart rate or 10 mmHg/cm² in contractility, indicating reduced β-adrenergic responsiveness []. Three additional studies (male-only cohort, n = 9–14) reported no late potentials, and elevated troponin concentrations returned to baseline within 48 h [–].

Oxidative stress and inflammation responses increased after competition. Across ten studies (n = 6–26 male, 3 female and 13–57 sex not reported), oxidative stress markers (oxygen radicals, DNA breaks) and inflammatory cytokines (IL-6, IL-10, TNF-α) increased post-race and returned to baseline within 48 h [–,–,,,]. Vitamin C and E supplementation showed no reduction in oxidative stress responses []. Experienced athletes demonstrated stronger oxidative and inflammatory responses compared to less trained individuals [].

Twenty studies reported race-related complications in IRONMAN® 70.3 ( and ). Overall injury incidence was approximately 10% [,], with most events occurring after 6–7 hours of racing [,]. The most common reported issues included hyperthermia and dehydration in one race [], while another study identified gastrointestinal complaints (27.6%), musculoskeletal injury, and myalgia (25.4%) as the predominant issues []. Complications varied by race segment. Injuries occurred most frequently during the running segment, followed by cycling, and least often during swimming []. Swimming-related risks included drowning and swimming-induced pulmonary edema [,], while cycling incidents often involved trauma requiring medical intervention [,]. Severe outcomes were rare, with a fatality rate <0.01%, primarily due to cycling accidents, drowning, or cardiovascular events [], with one report estimating a cardiovascular death rate of approximately 1 per 50,000 athletes [].

Medical support and clinical outcomes were investigated in several studies. Hospitalization was required in 7.5–12% of injuries, most commonly for syncope, wound care, or hyponatremia [,], but only 2 of 17 athletes sought follow-up care []. Deep vein thrombosis was documented in a case report, potentially linked to bradycardia or contraceptive use []. Monitoring of hematocrit (n = 70 male, 17 female) proved unreliable, as values fluctuated without exceeding the clinical threshold of >55% []. 21 of 29 (sex not reported) triathletes demonstrated successful completion of IRONMAN® 70.3 with prosthetic joints [], and rehabilitation after a fracture appeared to benefit from extracorporeal shock-wave therapy in a case report [].

Preventive and organizational factors were also reported. Sun exposure reached 1.2 SED/h during cycling and 3.0 SED/h during running, exceeding the minimum erythema dose of approximately 1.0 SED for fair-skinned individuals up to 5.0 SED for dark-skinned individuals []. Compression stockings did not reduce medical complications (male-only cohort, n = 36) [], while ulnar neuropathy appeared frequently but without long-term sequelae (sex not reported, n = 777) []. Structured on-course medical systems reduced treatment times and improved outcomes in IRONMAN® 70.3 [,]. Most studies focused on physiological aspects, however one psychological study reported increased maladaptive or addictive training behavior in individuals training more than 10 h·week ⁻ ¹ and in triathletes competing in longer events [].

Twenty-five studies investigated predictors of IRONMAN® 70.3 performance, including environmental and race-related factors ( and ) [–,,,–,]. In large overview studies (both sex, n = 8,340–16,611), the primary determinants included driving dynamics, environmental conditions, competitor density, course topography, age and sex [,,,]. Across IRONMAN® 70.3, cycling consistently emerged as the strongest predictor of overall performance [,,,,,], accounting for 50–60% of race duration [,], correlations with finishing time reached 0.85. Running showed correlations of 0.75 in women and 0.82 in men, while swimming correlations were 0.46 and 0.63, respectively []. V̇O₂max explained 67% of the variance in overall race time []. Performance strategies were examined in relation to race execution, pacing, and biomechanics. Analyses (n = 858 male, 380 female) revealed speed declines of 1.14 km/h in the second half of cycling and 0.37 km/h (professionals) to 0.6 km/h (amateurs) in running, with larger decrements observed in men, middle-aged, and lower-performing triathletes []. A conservative pacing strategy with a glycogen preservation were detected as particularly important for maintaining running performance in the final race segment [].

Biomechanical and technical factors further modulate performance outcomes. In a small cohort (n = 3 male, 2 female), the TRI position was associated with optimized force application during cycling []. Strength training has been shown to produce better results (male-only cohort, n = 16) [,]. Transition efficiency, which improved among top-10 finishers over time, although sex differences in transition times persist across age groups [,]. Swim pacing remained relatively stable across race distances and was associated with overall performance in amateur but not professional triathletes [,].

The race venue and environmental conditions were associated with performance results. The fastest top-100 IRONMAN® 70.3 performances were observed at the World Championship and the European Championship in Elsinore [,], while the fastest individual split occurred at Zell am See []. Among male amateurs, peak results were recorded in St. Pölten [], and triathletes from Belgium, Denmark and Switzerland achieved the best outcomes across events []. Environmental load exerted a measurable influence on performance in IRONMAN® 70.3, with speed declined at temperatures exceeding 25 °C, humidity above 70%, and at altitude [,]. Optimal running temperatures were estimated at 5–7 °C (both sex, n = 146,924) [], consistent with fast performances reported in Zell am See (both sex, n = 852,721), where race-day temperatures ranged from 11–20 °C []. Heat-acclimatized triathletes (male-only cohort, n = 19) showed attenuated performance impairments [], while faster triathletes (sex not reported, n = 34) demonstrated greater reductions in plasma osmolality under heat stress []. Travel from cold to warm climates impaired sleep but normalized within 48 hours without inducing immune suppression (sex not reported, n = 12) [].

Environmental and organizational factors contributed to the race outcomes. Social-environmental factors, including media exposure, urban race settings and structured event organization, were associated with increased novice participation in South Korea (sex not reported, n = 200) [], furthermore, it was shown that promotional race material often differed from actual environmental conditions []. Course topography contributed to split-time variance in cycling and running [,,], and higher competitive density corresponded with faster aggregated field results in professional triathletes [,].

Ten studies examined race-day and preparatory nutrition for IRONMAN® 70.3 triathletes (). Higher carbohydrate intake was associated with improved performance across studies [–], and IRONMAN® 70.3 triathletes consumed more carbohydrates and water than full-distance IRONMAN® triathletes without reporting increased gastrointestinal symptoms (both sex, n = 221) []. Sodium supplementation was beneficial for triathletes with high sweat loss [,]. During an IRONMAN® 70.3 race, essential amino acids decreased by 27.1%, branched-chain amino acids (valine, leucine, isoleucine) by 32.8%, and non-essential amino acids by −24.4%, with no correlation to fatigue []. Dietary interventions were limited. However, a case report describing a 32-week low-carbohydrate, high-fat diet led to the worst recorded performance including depression and irritability []. Among twenty elite premenopausal female triathletes, higher intake of animal and saturated fats was associated with a lower incidence of relative energy deficiency in sport (RED-S) []. One study (male-only cohort, n = 11) found reduced post-race immunosuppression after creatine supplementation for 20 g/day for 5 days []. Finally, supplement use was associated with a risk of inadvertent doping [].

Ten studies investigated age-related performance patterns in IRONMAN® 70.3 across large datasets ranging from approximately 5.000 to over 800.000 male and female triathletes (). Peak overall performance occurred in men aged 18–39 years and women aged 25–39 years [–,,]. Stones and Hartin reported peak performance at 34 years in men and 35 years in women [,,], while amateur triathletes performed best between 25–29 years []. Longitudinal data indicated a U-shaped performance curve in men but not in women, with the largest discrepancy between 18–24 and 45–54 years [,,]. Performance decline began earlier in women (from 30–34 years) than in men (from 35–39 years) for both amateur and professional triathletes [–]. Discipline-specific analyses showed peak performance at different ages: in swimming (28 years women, 26 years men), cycling (35 years women, 34 years men) and running (31 years women, 32 years men) [,]. The age-related performance decline also differed by discipline, with swimming predicted performance reduction between 35–49 years in women and 40–54 years in men []. Running showed the steepest age-related drop, while cycling exhibited the smallest decline [,].

Eight studies analyzed sex differences in IRONMAN® 70.3 (). In the eight large overviews, women were underrepresented, reflecting lower female participation rates in IRONMAN 70.3 ® events. However, the top-performing studies showed a 1:1 ratio []. Women performed lower across all age groups, with narrowing sex gaps at older ages [–,,–]. Nikolaidis et al. reported that the performance gap decreased from age 50 onward [], while the largest sex difference among top 10 triathletes occurred in the 60–64 age group []. Swimming showed the smallest sex difference at ~2:13 minutes in the 18–24 age group, although a top-10 cohort showed a 37.5% difference in swimming [,]. In cycling, the smallest sex gap occurred between 70–74 years (~17:05 minutes) [], and 11.2% in top triathletes [,]. Participation trends were reported in large datasets (both sex, n = 146,924–840,100). From 1997–2017, participation increased in both sexes, particularly among triathletes aged 30–49 years, with fastest improvements observed in runners over 40 years and more triathletes with paces of 5:00–6:59 min/km []. Participation also increased in triathletes >50 years, though more slowly []. However, male triathletes were more likely to complete the race than women []. Between 2004–2020, the 35–39 age group was the most represented, with 186 triathletes aged ≥70 years included [].

Five studies examined training strategies and characteristics for IRONMAN® 70.3 (). Recreational triathletes complete 20–24 weeks of preparation with 6.5–11.5 h·wk ⁻ ¹ training [,]. Over this period, V̇O₂max increased by 2.7 ml/kg/min and maximal power by 14.6 W in triathletes aged 39 ± 9.9 years (n = 19 male, 13 female) [], with greater improvements in triathletes with higher fat mass and more endomorphic somatotype (male-only cohort, n = 14 male) []. Cycling training showed the strongest association with overall performance and explained >50% of variance [,,], whereas swimming contributed the smallest training portion and showed no transfer effects to cycling or running [,]. Zone-2 training improved running performance (male-only cohort, n = 18), whereas medium-intensity intervals did not affect total race time []. Standardized training load metrics (e.g., TSS, power-based data) were largely absent, limiting comparability between studies [,].