Methods
Full details of the protocol have been previously published8 and this was a pre-planned ancillary analysis. Healthy participants were recruited in 2016 prior to undertaking training their first marathon (Virgin Money London). Inclusion criteria age was less than 35 years at recruitment, no past significant medical history and no previous marathon running experience. All procedures were in accordance with the principles of the Helsinki declaration. All participants gave written informed consent and the study was approved by the London-Queen Square National Research Ethics Service Committee (15/LO/0086).
All measurements were conducted before training started over a three-week period (three consecutive weekends after ballot place announcement), six months prior to the marathon.

Data acquisition and analysis

Cardiopulmonary exercise testing (CPET) was performed according to exercise testing guidelines 9. Protocols were individually determined with work rate (15-30 W) increased every minute until voluntary exhaustion, aiming for 10 minutes of exercise. To permit concurrent transthoracic echocardiography, studies were performed using a semi-recumbent cycle ergometer (ERG 911 S/L, Schiller, Baar, Switzerland). A 1-minute rest period was included followed by a 3-minute warm up. Heart rate (HR), blood pressure and oxygen saturation were monitored throughout. VO2 was continuously measured using a calibrated breath-by-breath analyser (Cosmed Quark CPET, Rome, Italy). Participants were verbally encouraged to exercise until maximal exertion. VO2peak was expressed as the highest value from an average of 30 seconds during the final stage of the exercise test. To fully assess submaximal efforts, the oxygen uptake efficiency slopes (OUES) was calculated automatically from VO2against the logarithm of VE (logVE)10.
Echocardiography was performed using a GE Vivid E95 platform (Vingmed-General Electric, Horten, Norway) equipped with a phased-array transducer (1.4-4.6 MHz). A detailed protocol was collected at rest, at 5 minutes into exercise (excluding the 3-minute warm-up period), and when the respiratory exchange ratio (RER) was above 1.0. This included the apical 4-chamber view (with and without tissue velocity imaging (TVI), the apical two-chamber, apical long axis view, parasternal short axis view at the base and apical level, and a PW at the level of the Left ventricular outflow tract (LVOT) (one centimetre below the aortic valve). An abbreviated protocol was obtained every 1 minute and 15 seconds and included the apical 4-chamber view (with and without TVI). As part of the protocol 12-lead ECG and cardiac magnetic resonance was undertaken but not included in this analysis.