1 3 Eur J Appl Physiol DOI 10.1007/s00421-013-2742-4 ORIGINAL ARTICLE Cardiac electrical conduction, autonomic activity and biomarker release during recovery from prolonged strenuous exercise in trained male cyclists Glenn M. Stewart · Justin J. Kavanagh · Gus Koerbin · Michael J. Simmonds · Surendran Sabapathy Received: 5 June 2013 / Accepted: 27 September 2013 © Springer-Verlag Berlin Heidelberg 2013 troponin T (hs-cTnT) concentrations were measured before exercise, and after 60 min and 24 h of recovery. Results The root mean square of the successive differ- ences of RR, PR and QT intervals was significantly reduced during recovery (p < 0.05). Normalised low- and high-fre- quency power of RR intervals significantly increased and decreased, respectively, during recovery. Approximate entropy of PR and QT intervals, and the QT-variability index significantly increased during recovery. All meas- ures except mean QT interval (pre 422 ± 10 ms vs 24 h post 442 ± 11 ms, p = 0.013) returned to pre-exercise val- ues after 24 h. Serum hs-cTnT was significantly elevated 60 min after exercise (pre 5.2 ± 0.7 ng L -1 vs 60 min post 27.4 ± 6.2 ng L -1 , p = 0.01) and correlated with exercis- ing heart rate (R 2 = 0.89, p < 0.001). Serum electrolyte concentrations were unchanged (p > 0.05). Conclusion The results suggest suppressed parasympa- thetic and/or sustained sympathetic modulation of heart rate during recovery, concomitant with perturbations in atrial and ventricular conduction dynamics. Exercise- induced hs-cTnT release was heart rate dependent. Keywords Heart rate variability · QT-interval variability · Autonomic modulation · Endurance exercise · Cardiac- specific biomarkers · High-sensitivity cardiac troponin · Electrolytes Introduction Non-invasive measurements of cardiovascular dynam- ics are increasingly popular for interpreting physiological manifestations of cardiac health and disease. Measure- ments of heart rate variability (HRV) provide non-inva- sive insights into the fluctuations of sympathetic and Abstract Purpose Although markers of myocyte injury, electro- lyte disturbances and an autonomic imbalance have been reported following exercise, the effect of prolonged strenu- ous activity on cardiac electrical conduction is not well understood. This study examined atrial and ventricular con- duction dynamics during recovery from exercise. Methods Electrocardiographic intervals were obtained from eight highly-trained males before, during recov- ery (15, 30, 45 and 60 min post-exercise) and 24 h after a prolonged bout of strenuous exercise. Time-domain, frequency-domain and non-linear analyses of the RR, PR and QT intervals were analysed to investigate the effect of exercise on autonomic modulation and cardiac electrical conduction. Serum electrolyte and high-sensitivity cardiac Communicated by Keith Phillip George. G. M. Stewart (*) · J. J. Kavanagh · M. J. Simmonds · S. Sabapathy School of Rehabilitation Sciences, Griffith University, Gold Coast, QLD, Australia e-mail: g.stewart@griffith.edu.au G. M. Stewart · M. J. Simmonds · S. Sabapathy Heart Foundation Research Centre, Griffith Health Institute, Griffith University, Gold Coast, QLD, Australia J. J. Kavanagh Centre for Musculoskeletal Research, Griffith Health Institute, Griffith University, Gold Coast, QLD, Australia G. Koerbin ACT Pathology, The Canberra Hospital, Canberra, ACT, Australia G. Koerbin Faculty of Education, Science, Technology and Maths, University of Canberra, Canberra, ACT, Australia