Search databases and terms

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 S1 File. 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 [15,16]. The search covered all records up to 25 November 2025, without language restrictions. Search terms were developed using free-text principles [17] 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.

Inclusion and exclusion criteria

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.

Study selection process

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.

Data extraction and categorization

The dataset is provided in S2 File. 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.

Risk of bias assessment

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.

Methodological limitations and future research priorities

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.

Physiological adaptations

Twenty-two studies examined physiological responses to IRONMAN® 70.3, predominantly in small cohorts of trained male triathletes aged 25–39 years [18–36] (Tables 1 and 2). Nineteen studies investigated acute post-race changes, while three captured both acute and chronic adaptations [31,33,35]. Most measurements were conducted immediately before and after the race, with delayed follow-up assessments performed under laboratory conditions.

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Table 1. Physiological adaptation.

https://doi.org/10.1371/journal.pone.0352506.t001

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Table 2. Injury prevention.

https://doi.org/10.1371/journal.pone.0352506.t002

Across eight studies, body composition, hydration and electrolyte responses were measured (male-only cohort, n = 6–42) [5,6,13–15,48–50]. 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) [33]. Professional triathletes exhibited higher total body water (63.7% vs. 61%) and intracellular water, compared with recreational athletes [31]. 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 [31]. Amateur triathletes showed higher fat mass (23.9%) and lower muscle (48%) and lower bone mass (11.3%) compared to professionals [5,6]. 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) [22]. Calcium increased by 0.8 mmol/L [34], whereas potassium and chloride showed no significant alterations [20–22,34]. 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 [25,32]. Elevation in hepatic enzymes AST and ALT were reported but remained below harmful thresholds and normalized within 48 h [30].

Four studies documented respiratory conditions in triathletes, with documented cases of asthma, airway hyperreactivity and swimming-induced pulmonary edema (SIPE) [37,41,42,51]. 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 [42]. A case study of a female athlete underlined the cold-water exposure [51] and use of 50 mg Sildenafil was in one cohort mentioned (sex not reported, n = 71) [41]. Airway hyperresponsiveness and asthma were more common in endurance triathletes (sex not reported, n = 30) [37].

Four studies responses cardiac responses and reported transient reductions in systolic performance following IRONMAN® 70.3 were demonstrated [43–45,52]. 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 [52]. Three additional studies (male-only cohort, n = 9–14) reported no late potentials, and elevated troponin concentrations returned to baseline within 48 h [43–45].

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 [23–26,28–30,32,34,35]. Vitamin C and E supplementation showed no reduction in oxidative stress responses [24]. Experienced athletes demonstrated stronger oxidative and inflammatory responses compared to less trained individuals [26].

Race related complications and medical strategies

Twenty studies reported race-related complications in IRONMAN® 70.3 (Tables 2 and 3). Overall injury incidence was approximately 10% [39,53], with most events occurring after 6–7 hours of racing [39,53]. The most common reported issues included hyperthermia and dehydration in one race [39], while another study identified gastrointestinal complaints (27.6%), musculoskeletal injury, and myalgia (25.4%) as the predominant issues [54].

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Table 3. Medical support.

https://doi.org/10.1371/journal.pone.0352506.t003

Complications varied by race segment. Injuries occurred most frequently during the running segment, followed by cycling, and least often during swimming [55]. Swimming-related risks included drowning and swimming-induced pulmonary edema [39,51], while cycling incidents often involved trauma requiring medical intervention [54,55]. Severe outcomes were rare, with a fatality rate <0.01%, primarily due to cycling accidents, drowning, or cardiovascular events [39], with one report estimating a cardiovascular death rate of approximately 1 per 50,000 athletes [39].

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 [54,56], but only 2 of 17 athletes sought follow-up care [56]. Deep vein thrombosis was documented in a case report, potentially linked to bradycardia or contraceptive use [57]. Monitoring of hematocrit (n = 70 male, 17 female) proved unreliable, as values fluctuated without exceeding the clinical threshold of >55% [58]. 21 of 29 (sex not reported) triathletes demonstrated successful completion of IRONMAN® 70.3 with prosthetic joints [63], and rehabilitation after a fracture appeared to benefit from extracorporeal shock-wave therapy in a case report [60].

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 [64]. Compression stockings did not reduce medical complications (male-only cohort, n = 36) [46], while ulnar neuropathy appeared frequently but without long-term sequelae (sex not reported, n = 777) [47]. Structured on-course medical systems reduced treatment times and improved outcomes in IRONMAN® 70.3 [39,53]. 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 [61].

Performance determinants and race characteristics

Twenty-five studies investigated predictors of IRONMAN® 70.3 performance, including environmental and race-related factors (Tables 4 and 5) [8–10,13,14,66–74,81]. 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 [10,68,69,73]. Across IRONMAN® 70.3, cycling consistently emerged as the strongest predictor of overall performance [10,13,14,68,69,73], accounting for 50–60% of race duration [1,82], 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 [73]. V̇O₂max explained 67% of the variance in overall race time [74].

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Table 4. Performance analysis.

https://doi.org/10.1371/journal.pone.0352506.t004

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Table 5. Environmental conditions and performanceperformance.

https://doi.org/10.1371/journal.pone.0352506.t005

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 [66]. A conservative pacing strategy with a glycogen preservation were detected as particularly important for maintaining running performance in the final race segment [68].

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 [67]. Strength training has been shown to produce better results (male-only cohort, n = 16) [65,75]. Transition efficiency, which improved among top-10 finishers over time, although sex differences in transition times persist across age groups [71,63]. Swim pacing remained relatively stable across race distances and was associated with overall performance in amateur but not professional triathletes [81,70].

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 [69,77], while the fastest individual split occurred at Zell am See [77]. Among male amateurs, peak results were recorded in St. Pölten [72], and triathletes from Belgium, Denmark and Switzerland achieved the best outcomes across events [7]. 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 [78,48]. Optimal running temperatures were estimated at 5–7 °C (both sex, n = 146,924) [78], consistent with fast performances reported in Zell am See (both sex, n = 852,721), where race-day temperatures ranged from 11–20 °C [77]. Heat-acclimatized triathletes (male-only cohort, n = 19) showed attenuated performance impairments [79], while faster triathletes (sex not reported, n = 34) demonstrated greater reductions in plasma osmolality under heat stress [48]. Travel from cold to warm climates impaired sleep but normalized within 48 hours without inducing immune suppression (sex not reported, n = 12) [49].

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) [83], furthermore, it was shown that promotional race material often differed from actual environmental conditions [80]. Course topography contributed to split-time variance in cycling and running [67,68,84], and higher competitive density corresponded with faster aggregated field results in professional triathletes [69,72].

Nutrition strategies

Ten studies examined race-day and preparatory nutrition for IRONMAN® 70.3 triathletes (Table 6). Higher carbohydrate intake was associated with improved performance across studies [85–89], and IRONMAN® 70.3 triathletes consumed more carbohydrates and water than full-distance IRONMAN® triathletes without reporting increased gastrointestinal symptoms (both sex, n = 221) [89]. Sodium supplementation was beneficial for triathletes with high sweat loss [22,90]. 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 [23]. 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 [91].

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Table 6. Nutritional strategies.

https://doi.org/10.1371/journal.pone.0352506.t006

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) [92]. One study (male-only cohort, n = 11) found reduced post-race immunosuppression after creatine supplementation for 20 g/day for 5 days [93]. Finally, supplement use was associated with a risk of inadvertent doping [94].

Peak performance and participation trends

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 (Table 7). Peak overall performance occurred in men aged 18–39 years and women aged 25–39 years [3–5,75,95]. Stones and Hartin reported peak performance at 34 years in men and 35 years in women [3,4,75], while amateur triathletes performed best between 25–29 years [72]. 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 [3,5,75]. Performance decline began earlier in women (from 30–34 years) than in men (from 35–39 years) for both amateur and professional triathletes [3–5].

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Table 7. Age-related difference in performance.

https://doi.org/10.1371/journal.pone.0352506.t007

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) [3,4]. 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 [4]. Running showed the steepest age-related drop, while cycling exhibited the smallest decline [4,5].

Eight studies analyzed sex differences in IRONMAN® 70.3 (Table 8). 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 [63]. Women performed lower across all age groups, with narrowing sex gaps at older ages [3–5,72,95–97]. Nikolaidis et al. reported that the performance gap decreased from age 50 onward [97], while the largest sex difference among top 10 triathletes occurred in the 60–64 age group [63]. 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 [63,96]. In cycling, the smallest sex gap occurred between 70–74 years (~17:05 minutes) [96], and 11.2% in top triathletes [5,63].

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Table 8. Sex-specific differences.

https://doi.org/10.1371/journal.pone.0352506.t008

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 [78]. Participation also increased in triathletes >50 years, though more slowly [78]. However, male triathletes were more likely to complete the race than women [96]. Between 2004–2020, the 35–39 age group was the most represented, with 186 triathletes aged ≥70 years included [96].

Performance predictors and training characteristics

Five studies examined training strategies and characteristics for IRONMAN® 70.3 (Table 9). Recreational triathletes complete 20–24 weeks of preparation with 6.5–11.5 h·wk ⁻ ¹ training [84,98]. 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) [98], with greater improvements in triathletes with higher fat mass and more endomorphic somatotype (male-only cohort, n = 14 male) [84]. Cycling training showed the strongest association with overall performance and explained >50% of variance [84,98,99], whereas swimming contributed the smallest training portion and showed no transfer effects to cycling or running [84,98]. Zone-2 training improved running performance (male-only cohort, n = 18), whereas medium-intensity intervals did not affect total race time [81]. Standardized training load metrics (e.g., TSS, power-based data) were largely absent, limiting comparability between studies [81,84].

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Table 9. Training program.

https://doi.org/10.1371/journal.pone.0352506.t009

In addition to training characteristics, several predictors were associated with training outcomes and race performance. Anthropometric factors, especially lower body fat percentage in men, training experience, total body volume, and balanced rest days predicted faster race times (n = 155 male, 54 female) [99]. Personal goals, previous race times, and goal commitment were additional predictors [99]. Endurance-related traits such as ethos and identity improved adherence to training (n = 200 male, 168 female) [100].

Synthesis of results

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https://doi.org/10.1371/journal.pone.0352506.t010

Physiological responses

This summary shows that the IRONMAN® 70.3 induces marked but short-lived alterations in hydration status, electrolyte concentrations, oxidative and inflammatory activity, muscle-damage biomarkers and renal and hepatic indices [20–35]. Despite the magnitude of these disturbances, most parameters normalize within hours until 48 hours and no values exceeding the pathological threshold [25,27,30,34,35], underscoring robust homeostatic recovery mechanisms in trained triathletes. This rapid resolution mirrors patterns observed in other endurance modalities but appears less pronounced than in full-distance IRONMAN® events, consistent with a lower cumulative load and a shorter duration of exertion [101,102].

A reduction in fat mass combined with a significant increase in circulating free fatty acids suggests a shift toward a lipid-dominated metabolism, which is characteristic of prolonged moderate-intensity exercise [33]. In contrast to full-distance IRONMAN®, with a reported skeletal-muscle loss [33], muscle mass is preserved during the IRONMAN®70.3 [33,103], likely due to the shorter Olympic-distance events [103,104].

Preservation of muscle tissue aligns with sustained reliance on lipid oxidation at moderate exercise intensities and with the shorter total race duration, which reduces cumulative catabolic strain [103].

Elevated intracellular and total body water before race have been associated with superior performance [31,105], although interpretation should consider the susceptibility of bioimpedance techniques to acute fluid shifts [48]. Age-related differences in anthropometry, ranging from ectomorphic profiles in younger triathletes to increased endomorphy in triathletes above 35 years, reflect long-term morphological adaptations [106]. Amateurs consistently display higher fat mass and lower body water percentage, than professionals [5,6,48]. This patterns are consistent with training induced plasma volume expansion and metabolic adaptations [5,6,48].

Electrolyte changes were modest and remained within physiological rages. The decline in sodium without clinical hyponatremia support individualized sodium and fluid strategies during exercise [22,107]. Evidence for supplementation other electrolytes remains inconclusive, due to variability of sweat rate, environmental conditions and personal intake [20–22,34,108].

A sharp increase in creatine kinase, myoglobin, and lactate dehydrogenase following competition indicates muscle stress caused by physical exertion; however, these levels return to normal within 48 hours, suggesting a reversible process [25,27,34,35,109]. Biomechanically, temporary disturbances in Ca² ⁺ homeostasis, the formation of reactive oxygen species (ROS), and increased free fatty acids contribute to membrane instability without causing structural damage [101,102]. The lower magnitude compared to endurance competitions supports a classification as moderate systemic stress within the context of adaptability [33]. An increase in biomarkers of muscle damage is therefore best interpreted as temporary permeability of the sarcolemma membrane rather than structural myofibrillar damage [25,27,34,35].

Temporary proteinuria, creatinuria, and hematuria caused by physical exertion reflect glomerular permeability and renal hemodynamics [25,32,110,111]. Elevated AST and ALT levels, which normalize within 48 hours, are more indicative of functional enzyme release from muscle and liver than of hepatocellular damage [30,112,113]. These patterns are consistent with those seen in other endurance sports and suggest transient organ stress with rapid recovery [112].

The increase in markers of oxidative stress and cytokines (IL-6, IL-10, TNF-α) reflects the formation of reactive oxygen species (ROS) and immune activation during prolonged exercise [23–26,28–30,32,34,35]. These reactions are not exclusively harmful but serve as signals for repair mechanisms [26,29,30,32,34,35]. Stronger reactions in experienced athletes likely reflect a higher absolute training load, not maladaptation [26]. Vitamin C and E supplementation showed no reduction in these markers [24]. This is consistent with the finding that exogenous antioxidants do not attenuate the oxidative signaling triggered by physical activity [24].

The higher prevalence of airway hyperresponsiveness, asthma, and SIPE suggests a susceptibility of the airways in endurance triathletes [37,41,42,51,114–118]. Respiratory conditions, including asthma and airway hyperreactivity, remain relevant due to exposure to environmental irritants, airway drying and high ventilatory flow [114–116]. SIPE is associated with various pathophysiological factors, such as cold-water exposure, hypertension, and female gender. This underscores the involvement of hemodynamic and environmental factors [42,51]. In long-term swimming events, especially in cold water, are similar patterns described [42,51]. Elevated proportions of Th17 cells suggest a possible immunological contribution to airway inflammation, although the causal relationship has not been conclusively established [37].

Cardiac responses showed a transient decrease in systolic performance and β-adrenergic responsiveness, consistent with exercise-induced cardiac fatigue [43–45,52,119]. A preserved cardiac output and normalization of troponin levels within 48 hours suggest functional, reversible changes rather than damage [44,52]. Possible mechanisms include downregulation of β-adrenoceptors, altered Ca² ⁺ homeostasis, and metabolic stress [44,45]. Compared to full-distance competitions, the extent of cardiac dysfunction appears to be lower, which is consistent with the reduced overall workload [44,45,52]. The incidence of cardiovascular death is low, estimated at ~1 per 50,000 triathletes [39,120]. Although experienced endurance triathletes often present with benign ECG variants, the long-term prognostic significance of acute biomarker elevations remains uncertain, reinforcing the value of individualized risk stratification [120–122].

Implications across different distances

Across all areas, the IRONMAN® 70.3 shows consistently weaker responses compared to the full-distance triathlon, which supports the dose-response model of physiology:

• Metabolism: Fat utilization present in both races, but lower cumulative deficit. preserved muscle mass
• Muscle damage: Elevated markers in both races, but lower amplitude
• Cardiac stress: Transient in both races, but less pronounced dysfunction
• Recovery: Generally faster normalization (≤ 48 h)
• Lower dehydration, electrolyte imbalance, and cardiac stress

Interpretation

• The IRONMAN® 70.3 appears to be physiologically tolerable for trained individuals
• Individual sodium and hydration strategies are advisable
• Targeted monitoring of compromised physiological systems is recommended for risk individuals
• Inflammatory responses should be viewed as adaptive signals, and a rest period should be planned following the race.

Medical considerations and injury risk

Medical issues in IRONMAN® 70.3 arise from the combined effects of environmental load, prolonged exertion and segment-specific mechanical demands [39,53]. Overall risk remains low, but clinically relevant events do occur. Rare but severe events, like cardiovascular incidents, drowning, thrombotic events, and SIPE highlight the importance of individual risk assessment and environmental conditions [39,51,123].

Most medical encounters cluster in the final third of the race, informing strategic deployment of medical personnel and resources [39,53]. This pattern reflects cumulative fatigue, dehydration, and thermal strain. Musculoskeletal complaints are more common during running, suggesting the importance of eccentric loading [54]. Gastrointestinal symptoms are common among endurance athletes; this is attributed to reduced blood flow to the internal organs during prolonged physical exertion [54,124].

Organizational factors also play a crucial role in the safety of triathletes. The targeted deployment of medical resources is particularly beneficial during the running segment [39,53]. Evidence from structured race-medical systems demonstrates that proactive, segment-specific planning reduces complication severity and improves triathlete outcomes [39,53,60]. Environmental factors such as heat and UV radiation also act as additional risk factors for accidents [64]. Ventilatory efficiency and body mass acting as key determinants of recovery [62].

Compared with full-distance IRONMAN®, IRONMAN® 70.3 showed fewer injuries, shorter treatment times, and lower complication severity [39,53,60]. Whereas, in Olympic-distance competitions, a lower incidence and milder disease progression have been reported, which is consistent with the shorter duration of exposure [53,60].

Implications across different distances

• Full-distance IRONMAN® shows higher incidence and severity of complications, and the Olympic-distance shows lower overall complication rates.
• Longer recovery and increased cumulative load the longer the race duration.

Interpretation

• Complications are primarily due to cumulative fatigue and dehydration, not to individual events
• Most medical issues are temporary and reversible, which demonstrates the overall safety of the sport for trained athletes
• Serious incidents are rare and are associated with individual risk factors and environmental influences
• There is a correlation between the duration of the competition and the severity of complications
• Organizational strategies and medical planning reduce medical risk.
• Behavioral factors (e.g., high training volume) can indirectly increase risk

Performance determinants and environmental modifiers

Performance in IRONMAN® 70.3 reflects the interplay among physiological capacity, tactical pacing, environmental conditions and technical execution. Cycling is consistently the strongest predictor of overall race time due to its mechanical demands, aerodynamics and the downstream consequences for running performance [24,27,32,69,125–127]. This aligns with its contribution of 50–60% to total race duration [1,82]. Pacing analyses show mid-race deceleration across performance levels, suggesting that conservative intensity regulation may mitigate premature fatigue and preserve neuromuscular output for the run segment [7,27,69]. Swimming showed weaker associations with overall performance and its predictive value diminished at higher competitive levels, indicating a ceiling effect among professionals [70,73,81]. These findings are also supported by the general literature on endurance sports and are further corroborated by the observation that accumulated competitive experience, as reflected in personal best times, is a strong predictor of performance in shorter triathlon formats [8]. However, the applicability of this relationship to middle-distance races likely depends on additional factors such as environmental stressors and technical execution.

Technical and biomechanical factors are modifiable contributors [65,67,75,128]. Distinct neuromuscular activation patterns between road and time-trial bicycle configurations influence both cycling efficiency and subsequent running performance [67,128]. Strength training has also been associated with improved outcomes [65,75,129]. Transition efficiency, which differs by sex and competitive level, further contributes to outcome variance, particularly among high-level triathletes [71,63].

Environmental load shapes race dynamics profoundly. Heat, humidity, and altitude consistently impair performance, while cooler temperatures and prior heat acclimation attenuate physiological strain [19,22]. Acclimatization to the heat mitigated this disadvantage somewhat [79], underscoring the importance of pre-competition preparation. Other factors, including sleep disturbances following travel and fluid-electrolyte changes under thermal stress, further reduce performance [48,49]. These findings underscore the need to incorporate environmental factors into strategies for pacing, preparation, and career planning.

Race characteristics and organizational context also contribute meaningfully. Course topography, elevation gain, wind exposure and competitive density influence mechanical load, pacing strategy and group dynamics even in non-drafting events [67,68,84,130]. Social-organizational factors, including event structure, crowd density and urban versus rural settings, also modulate engagement and pacing behavior rather than direct performance [50,80]. Together, these findings support a performance model in which success depends on aligning physiological capacity and technical execution with environmental and contextual demands.

Comparisons with other triathlon formats reveal both common and distance-specific influencing factors. In a review study, swimming pace remained largely stable across all race distances and predicted overall performance in amateur triathletes, but not in professional triathletes, suggesting that its predictive value decreases at higher performance levels [70,81]. In the full-distance IRONMAN®, as in the IRONMAN® 70.3, cycling also dominates, with the relative importance of pace management and energy management increasing as the duration of the race increases [70,81]. At the Olympic distance, personal best times were identified as the best performance predictor [8], underscoring the importance of accumulated competition experience. This is also evident in the fact that the fastest performance among women was achieved at the 2024 Olympic Games [9], whereas no single fastest race could be identified among men [8,9]. In the full-distance IRONMAN®, the best performances were recorded in Copenhagen, Hawaii, and Barcelona [7]. These differences highlight the need to interpret performance in the IRONMAN® 70.3 within its specific physiological and tactical context.

Implications across different distances

• Cycling remains a key factor in both IRONMAN® 70.3 and full-distance races, but has a relatively greater influence in middle-distance races due to its higher percentage of the total race distance
• Fatigue is more pronounced during the run at longer distances, and pacing strategy becomes increasingly critical
• The predictability of overall performance based on swim time decreases as performance level and distance increase. For elite triathletes, it is a minimal distinguishing factor.
• Environmental influences are comparable across all formats, but their cumulative effect is greater in longer races.
• In Olympic-distance races, speed, tactics, and experience take precedence over energy conservation.

Interpretation

• Performance is best explained by a trimodal model that integrates physiology, technique, and environment.
• Cycling sets the performance limit, while running under fatigue conditions determines the final result.
• Pace strategy is a primary, modifiable factor, particularly for non-elite triathletes.
• Environmental stressors act as performance inhibitors and require individual adaptation strategies.
• Technical marginal gains, such as transition times, become decisive at higher competitive levels.
• The current evidence base is robust in terms of large-scale observational data but limited in its mechanistic depth.

Nutritional strategies

The studies found suggest nutritional strategies in IRONMAN® 70.3 are primarly determined by carbohydrate availability, hydration, and electrolyte balance are central to sustaining race intensity and supporting stable pacing across segments [85–89]. Higher carbohydrate intake consistently enhances performance and is well tolerated at intakes exceeding those commonly reported in full-distance events [85–89,131]. The absence of gastrointestinal symptoms suggests that nutritional strategies can be implemented more variably in IRONMAN® 70.3, probably due to the shorter duration and lower cumulative physiological stress [89,132].

Individual hydration and sodium replacement remains critical for maintaining hydration and plasma osmolality, particularly under heat stress [64,115]. The marked decrease in blood amino acid concentration during competition underscores the importance of adequate daily protein intake and timely recovery after training [23]. The decrease in blood amino acid concentration during competition shows no correlation with fatigue during the race. This suggests that amino acid availability is not a primary limiting factor for acute performance [23,91]. Instead, the focus should be on adequate daily protein intake and recovery after training [90,107]. This results are consistent with more general endurance recommendations highlighted targeted intake of sodium, carbohydrates, water, fat, nitrates, caffeine and protein, including 1.2–2.0 g/kg/day protein, although these guidelines have not been separately validated in the context of IRONMAN® 70.3 [90,107].

Nutritional patterns may also influence RED-S risk in female triathletes, underscoring sex-specific considerations in fueling strategy [133,134]. The association between higher fat intake and a lower incidence of a relative energy deficit during training (RED-S) in female triathletes demonstrates that total energy availability, and not just macronutrient composition, is crucial for a good outcome [92,133]. This underscores the need for individualized and gender-specific nutritional planning. Creatine supplementation shows mixed evidence, with isolated studies reporting reduced immune perturbation but no consistent performance benefit, as supported by both primary studies and reviews [93,107,135,136] The risk of supplement addiction emphasizes the need for informed triathlete education and quality-assured products [94].

Implications across different distances

• Higher reliance on energy intake during the race, which increases with race duration
• Hydration and sodium importance increases with duration and environmental stress
• Low-carbohydrate strategies appear to be particularly disadvantageous in middle- and long-distance formats
• Supplement evidence (e.g., creatine) remains weak across endurance disciplines

Interpretation

• Carbohydrate availability is the most important nutritional determinant of performance
• Fluid and sodium balance are critical under heat and high sweat loss
• Protein is mainly relevant for muscle recovery, not acute performance
• Amino acid depletion reflects metabolic demand rather than actual performance limitation
• Supplementation provides limited benefit and carries potential risks
• Individualized adjustments are essential due to variability in physiology and race conditions

Age, sex and discipline-specific factors

The studies demonstrate age, sex, and discipline-specific demands interact with performance trajectories in IRONMAN® 70.3 align with established age- and sex-specific endurance patterns. Peak performance occurs in IRONMAN® 70.3 typically between 18–39 years in men and 25–39 years in women, and exhibiting narrower sex differences with advancing age [3–5,75,95]. Across triathlon formats, peak age increases with race duration from ~26 years in Olympic-distance triathlon to ~34 years in full IRONMAN® and ≥35–40 years in ultra-endurance events, reflecting the rising contribution of experience, pacing expertise and long-term aerobic development [3–5,75,95,137–139,140].

Discipline-specific age reveals further differences. For example, in the full-distance IRONMAN, the best performance in swimming is around 29 years, followed by cycling at around 31 years and running at around 35 years [139]. These results suggest an increasing importance of metabolic efficiency and fatigue resistance in disciplines with later peak performance [139]. In the IRONMAN® 70.3, this is reflected in the dominant role of cycling and the accumulated fatigue in the running segment, thus reinforcing the interaction between the order of the disciplines and the performance result. Across all formats, running shows the steepest age-related decline in performance, while cycling performance remains relatively constant [4,5].

Age-related performance decline is driven largely by reductions in V̇O₂max around 1% per year after age 30 and progressive neuromuscular remodeling, with V̇O₂max decreasing progressively after early adulthood [50,141–146]. A cause of age-related performance decline shows progressive muscle loss from the age of 50–60, which can be partially mitigated by hypertrophy-oriented training [95,147]. Women, however, show earlier muscle loss in perimenopause, which can be reduced by hormone replacement therapy [147]. The observed delayed decline in performance with age between amateur and elite athletes suggests that the losses can probably be mitigated by differences in training history and physiological reserve [3,95]. The decline in running compared to cycling supports the hypothesis that weight-bearing and neuromuscular demands accelerate performance deterioration [4,5] although in general findings high-intensity training remains effective into older adulthood [84,98,148].

Sex-specific differences in performance persist across all age groups but narrowing with advancing age, suggesting differential age trajectories between woman and men [3–5,96,97]. Across triathlon races, sex gaps range from 12–18%, largest in running (17.1%) and smallest in cycling (13.4%), while swimming typically shows the smallest absolute differences [72,149]. These differences are generally explained by physiological factors, including different muscle phenotype, higher fat mass and, lower V̇O₂max, alongside sex-specific endocrine influences [6,150–152]. However, increased fat burning and delayed glycogen depletion can mitigate these differences during longer competitions and have also shown positive effects in the swimming segment [6,11,12,153]. In opposite men show greater testosterone driven erythropoiesis and muscle cross-sectional area [6,11,12]. Discipline-specific characteristics can modulate gender differences, with mechanical and metabolic demands playing a role [63,96,149].

Training characteristics show both shared and IRONMAN® 70.3 specific features. High-volume, low-intensity training, supplemented by high-intensity sessions (polarized/pyramidal models), is a general endurance training principle that has been observed across all triathlon distances [154–157]. Structured preparation for IRONMAN® 70.3 typically spans 20–24 weeks with moderate weekly training volumes (6.5–11.5 h wk^-1) for recreational athletes, in contrast from higher training loads typically required for full-distance events [84,98,154,158]. Increasing weekly training volume beyond 14 hours/week did not necessarily improve performance in a full IRONMAN®, suggesting a diminishing benefit at higher training loads [154,158]. Regardless of the format, training volume increases with race distance, but the optimal intensity distribution remains unclear [81,155–157]. An important difference is that cycling training proves to be the most important performance factor in IRONMAN® 70.3, both in training and in competition, and explains a large part of the performance variance, whereas the relative importance of the disciplines varies depending on the distance (e.g., swimming in the Olympic distance, running in the full IRONMAN®) [10,84,98,99].

In addition to physiological and training-related variables, multifactorial influencing factors such as experience, recovery, and psychological characteristics affect all endurance sports, regardless of the race distance [81,99,100,159–161]. Early sport participation and pain-resilience conditioning are additionally associated with improved long-term outcomes, indicating the importance of developmental factors [81,159,160].

Olympic-distance triathlon emphasizes speed and tactical execution, with an earlier peak performance (26 years) and a greater influence of swimming performance [10,95]. IRONMAN® 70.3 represents a transitional format where cycling is the dominant performance factor and pacing is important, whereas full-distance IRONMAN® places a greater emphasis on running endurance and energy management [10,139]. Ultra-endurance formats demonstrate that performance is increasingly determined by experience and fatigue resistance rather than maximum physiological capacity [95,137,138].

Additional contextual factors, such as regional differences in the athlete population, can also influence performance characteristics. For example, the age distribution among top runners varies by origin, with Asian and African athletes achieving peak performance at a younger age compared to European athletes [162]. Although these results were not specifically investigated for triathlon, they suggest that demographic and training culture factors may contribute to the observed performance patterns.

However, in IRONMAN® 70.3, the interaction of these factors with race tactics is particularly important, as the mean duration and intensity require both metabolic efficiency and tactical control. From a practical perspective, these results show that endurance principles such as aerobic development, structured training, and recovery must be adapted to the specific demands of the IRONMAN® 70.3. The focus should be on cycling efficiency, pacing strategy, and the integration of discipline-specific fatigue management.

Implications across different distances

• The age of top triathletes increases with race duration, from a peak of around 26 years (Olympic distance) to approximately 34 years (IRONMAN®) to 35–40 years (ultra-endurance).
• Experience and race pace become increasingly important with longer race durations.
• The importance of the disciplines for overall performance shifts: swimming (Olympic distance), cycling (IRONMAN® 70.3), and running (IRONMAN®).
• The gender differences are greatest in running and smallest in swimming.
• Training volume increases with race distance, while the optimal intensity distribution remains unclear.

Interpretation

• Cycling is considered the most stable performance factor across all age groups.
• Running contributes most to the general age-related decline in performance.
• Gender differences are primarily physiological, but are partially mitigated in longer competitions.
• Besides physiology, holistic factors such as experience, psychology, and recovery are crucial.

Key message

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https://doi.org/10.1371/journal.pone.0352506.t011