The local committee for the Protection of Human Subjects approved this study (CAAE: 61097822.0.0000.0068). We evaluated a Brazilian male marathoner (H.L.S.F.) who ran 366 consecutive marathons in 366 days (from 28^thAugust 2022 to 28^thAugust 2023). The participant provided written informed consent.

The pre-participation clinical evaluation (PPE) was performed according to the Brazilian Society of Exercise and Sports Medicine.⁵ We performed an anamnesis, clinical examination composed by 12-lead resting electrocardiogram (ECG), maximum cardiopulmonary exercise test (CPET), bioelectrical impedance analysis, and blood test. In addition, the athlete presented an echocardiogram at PPE that was realized three months before our study.

We periodically repeated all tests. In the third month of follow-up, we also included measures of cardiac structure, left ventricle (LV) and right ventricle (RV) functions by echocardiography (ECHO) and repeated until the end of this study.

In addition, a multi-professional team composed of a dermatologist, endocrinologist, orthopedist, physiotherapist, nutritionist, physical trainer, and psychologist supported the athlete over the period.

The CPET was performed using the maximum effort on automatic treadmill (Embramed – Model –Atlanta, United States). A computerized respiratory gas analyzer (Vynthus CPX – Pulmonary Function/Cardiopulmonary Exercise Testing Instrument, Hoechberg, Germany) was used to evaluate pulmonary ventilation, maximum oxygen consumption (VO₂max), and carbon dioxide production (VCO₂). The variables were measured breath-by-breath. The first anaerobic threshold (AT) was identified by the V-slope technique observed at the first point of dissociation of the VE/VO₂ curves and the lowest value for partial oxygen pressure at the end of expiration (PetO₂), before this parameter started to increase progressively. The second ventilatory threshold (respiratory compensation point) was identified by the inflection in the VE/VCO₂ curves and the maximum value of partial pressure for CO₂ at the end of expiration (PetCO₂), before a progressive decrease in this response.⁶ Heart rate (HR) was continuously recorded during the CPET using a 12-lead electrocardiogram and software CardioSoft v6. The CPET was performed at PPE, after three months, and monthly until the end of the study. The athlete ran the marathons in the morning, and CPETs were performed in the afternoon.

The images were collected by the Vivid E9 (GE Healthcare; Oslo, Norway). The athlete underwent 2-dimensional ECHO. The 4-chamber color Doppler and then pulse-wave Doppler were used to assess peak flow velocities across the mitral valve. We assessed parasternal long-axis views, septal and LV posterior wall thickness in diastole, as well as LV chamber dimensions at end-diastole and end-systole. LV volume at end-diastolic and LV volume at end-systolic were assessed by apical 2-and 4-chamber views, which then allowed the estimation of stroke volume and left ventricle ejection fraction by Simpsons biplane method. Specific speckle-tracking software was used to estimate strain in all segments. All the images were collected according to the American Society of Echocardiography and the European Association of Cardiovascular Imaging.⁷ The first echo was performed 3 months after starting the study and periodically evaluated during the follow-up. The athlete ran the marathons in the morning, and echocardiograms were performed in the afternoon, always before the CPET.

Data are periodically presented based on evaluations and reported as mean ± standard deviation (SD). The Statistical Package for Social Science (SPSS) version 23 was used to describe the variables.

At PPE, 43-year-old, height: 1.83 m, weight: 76.9 kg, VO₂max: 52 ml/kg/min, body fat: 12.6%, systolic and diastolic blood pressure: 120/80 mmHg, blood glucose: 92 mg/dL, total cholesterol: 185 mg/dL, high-sensitivity C-reactive protein (hs-CRP): 0.08 mg/dL, creatine phosphokinase (CPK): 183 U/L, and high-sensitivity troponin T (hs-TnT): 7.1 ng/L.

In the first CPET, we used a specific ramp protocol (protocol: velocity 1) composed of increments of velocity and inclination each minute until the athlete’s exhaustion (protocol details are described in supplementary material). The CPET duration was 10 minutes, the maximum velocity was 8.2 mph, 11.0% of inclination, VO₂max was 52.0 ml/kg/min, and the respiratory exchange ratio (RER) was 1.11. RER ≥ 1.10 is considered a maximum test.⁶ The first AT was around 6.0 mph, and the HR was 150 bpm. The athlete was instructed to run the marathons at moderate intensity close to the first AT obtained by maximum CPET.

Clinical measures, body composition, biomarkers, and lipids profile are described in Table 1. These variables showed minimal physiological variations with no abnormal alterations. The resting HR was measured from CPET in the supine position after 2 minutes at baseline. The slight variation of resting HR may be related to pre-test anxiety.

Table 1

Variables	August 2022, Pre-study	2022 November	2023 February	2023 May	2023 July	August 2023, End of Study	Mean - SD	Normal Value or Range
Clinical
SBP (mmHg)	120	110	110	110	120	120	115 ± 5	< 120
DBP (mmHg)	90	80	80	80	80	80	80 ± 5	< 80
Resting HR (bpm)	90	81	96	80	67	77	82 ± 10	< 100
Body composition
Weight (kg)	76.9	74.2	71.4	71.0	71.7	70.5	72.6 ± 2.5	-
Height (m)	1.83	1.83	1.83	1.83	1.83	1.83	1.83	-
BMI (kg/m2)	23.0	22.2	21.3	21.2	21.4	21.1	21.7 ± 0.7	18.9 - 24.9
Lean mass (kg)	67.2	65.5	64.2	63.9	64.7	63.6	64.9 ± 1.3	-
Fat mass (kg)	9.7	8.7	7.2	7.1	7.0	6.9	7.8 ± 1.2	-
Fat mass (%)	12.6	11.7	10.1	10.0	9.7	9.8	10.7 ± 1.2	< 20.0
Biomarkers and Lipids Profile
hs-CRP (mg/L)	0.08	0.08	0.06	0.08	0.07	0.06	0.07 ± 0.01	< 0.1
CPK (U/L)	183	184	190	190	183	104	169 ± 36	46 - 171
hs-TnT (ng/L)	7.1	6.0	8.8	10.0	8.0	9.0	8.2 ± 1.4	< 16.8
Creatinine (mg/dL)	1.14	1.01	1.15	1.02	0.99	1.06	1.06 ± 0.07	0.7 - 1.30
Potassium (mmol/L)	4.8	4.3	4.5	4.3	4.7	4.5	4.5 ± 0.2	3.5 - 5.1
Sodium (mmol/L)	137	136	141	138	139	140	139 ± 2	136 - 145
NT-proBNP (pg/mL)	-	-	22.1	24.4	10.8	14.4	17.9 ± 6.4	< 125
TC (mg/dL)	185	180	-	199	191	204	192 ± 10	< 190
LDL (mg/dL)	111	111	-	130	126	132	122 ± 10	< 130
HDL (mg/dL)	53	48	-	52	49	56	52 ± 3	> 40
TG (mg/dL)	99	81	-	75	64	78	79 ± 13	< 150

HR: heart rate; SBP: systolic blood pressure; DBP: diastolic blood pressure; BMI: body mass index; hs-CRP: high-sensitivity C-reactive protein; CPK: creatine phosphokinase; hs-TnT: high-sensitivity troponin T; NT-ProBNP: N-terminal proBNP; LDL: low density lipoprotein; HDL: high density lipoprotein; TG: triglycerides; TC: total cholesterol.

The mean HR during the marathons was 140 ± 10 bpm, VO₂ max remained at 49 ± 1 ml/kg/min, and pulmonary ventilation at 113±4 L/min (Figure 1 A-B). From the CPET performed in January, we changed the protocol velocity 1 to protocol D to avoid the risk of muscle injury during CPET (supplementary material). This alteration could explain a slight drop in VO₂max and ventilation. However, the athlete reached the maximum HR (HR > 95% predicted by age), and the RER was 1.08 ± 0.04. Moreover, no signs of malignant arrhythmia and cardiovascular disorders were found during CPET, which assured us of the safety of continuing with the marathons. Furthermore, no significant changes between 12-lead ECG in the pre-study and 12-lead ECG at the end of the study were found (Figure 1B-C).

Figure 1

Cardiac structural variables, LV systolic function, RV systolic function, and myocardial work are described in Table 2. These variables showed minimal physiological variations within normal values.

Table 2

Variables	October After 3 months	2022 November	2023 March	2023 June	2023 July	August,2023 End of Study	Mean - SD	Normal Value or Range
Cardiac structures
Aortic root (cm)	3.4	3.3	3.4	3.3	3.3	3.4	3.4 ± 0.1	3.0 - 3.7
RA diameter (cm)	3.8	3.5	3.6	3.3	3.4	3.3	3.5 ± 0.1	3.0 - 4.0
RV basal (cm)	3.8	3.8	3.4	4.0	4.0	3.3	3.7 ± 0.3	2.5 - 4.1
RV mid (cm)	3.1	3.1	2.8	3.1	3.3	2.7	3.0 ± 0.2	1.9 - 3.5
Interventricular septum (cm)	0.9	0.8	0.9	0.8	0.9	0.8	0.9 ± 0.1	0.6 - 1.0
LV posterior wall thickness (cm)	0.9	0.8	0.8	0.8	0.8	0.8	0.8 ± 0.1	0.6 - 1.0
LV end-diastolic diameter (cm)	5.4	5.3	5.4	5.3	5.5	5.2	5.4 ± 0.1	4.2 - 5.8
LV end-systolic diameter (cm)	3.5	3.7	3.6	3.4	3.4	3.3	3.5 ± 0.1	2.5 - 4.0
LV diastolic volume (ml)	157	141	153	149	152	149	150 ± 5	80 - 150
LV systolic volume (ml)	62	63	58	59	60	62	61 ± 2	70
LV mass index (g/m2)	92	77	87	78	90	76	83 ± 7	49 - 115
LA volume index (ml/m2)	18	20	21	16	25	22	20 ± 3	< 34
Left ventricular systolic function
LV ejection fraction (%)	60	62	62	61	60	59	61± 1	> 52
LV global longitudinal strain (%)	16	18	19	19	20	19	19 ± 2	≥ 18
Right ventricular systolic function
TAPSE (mm)	21	24	24	22	24	25	23 ± 2	> 17
S’ (cm/s)	14	13	14	14	15	19	15 ± 2	> 9.5
FAC (%)	49	46	53	51	46	50	49 ± 3	> 35
RV strain (%)	21	24	23	25	23	22	23 ± 1	> 20
Myocardial work
GWE (%)	95	-	92	92	97	94	94 ± 2	> 96
GWW (mmHg)	84	-	182	154	49	106	115 ± 53	73 - 87
GWI (mmHg%)	1576	-	1953	1688	2148	2025	1878 ± 238	1907 - 2113
GCW (mmHg%)	1680	-	2194	1987	2224	2209	2059 ± 233	2186 - 2369

RA: right ventricle; RV: right ventricle; LV: left ventricle; LA: left atrium; TAPSE: tricuspid annular plane systolic excursion; S’: tricuspid lateral annular systolic velocity wave; FAC: fractional area change; GWE: global work efficiency; GWW: global work wasted; GWI: global work index; GCW: global constructive work.

In addition, the athlete had been involved in a muscle strengthening program based on weight training, mobility, flexibility, and stretching, twice a week, 2 to 3 sets to each muscle group with 15 repetitions at 70% of your maximum capacity during the marathons. The average daily calorie consumption was 4.716 ± 806 kilocalories (composed by carbohydrate 68 ± 6 %, lipids 17 ± 5%, and protein 15 ± 4%). Psychological work was also performed by the athlete, composed of 102 therapy sessions that assessed personal, motivational, resilience, ability to deal with frustration, family involvement, routine division, and adaptation over the period. In January 2023, the athlete was diagnosed with pubalgia and needed physical therapy treatment that remained until August 2023. The athlete documented all the marathons and distances using two smart watches simultaneously and used 27 pairs of running shoes. The average marathon time was 5 hours, 8 minutes, and 28 seconds; the fastest was 3 hours, 54 minutes, and 48 seconds; and the slowest was 10 hours, 21 minutes, and 6 seconds.

These results highlight the importance of a careful cardiovascular assessment in athletes undergoing endurance exercise and provide unique insight into the relationship between high-volume, intensity marathons and cardiovascular adaptations.

We recognize some limitations in our study. From the CPET performed in January until August, we adjusted the protocol velocity to reduce the risk of muscle injury during CPET. This alteration may explain a slight drop in VO₂max and ventilation, which could limit the scientific utility of VO₂max as a follow-up variable. However, VO₂max remained around 49 ml/kg/min, suggesting a sustained functional capacity. The Echo was not performed in the pre-evaluation study because the athlete presented an echocardiogram with normal values at PPE that was realized three months before our study. However, during the follow-up, the parameters were within the normal limits. This study relates to one male marathoner, and these results should be interpreted with caution when extrapolating to a broader population.