Background: Current guidelines for cardiac rehabilitation advocate moderate-intensity continuous exercise training (MICT) at ≈60% peak oxygen uptake (VO2peak). High-intensity interval training (HIIT), with repeated bouts of exercise at >80% VO2peak appears to be an effective alternative, resulting in greater improvements in VO2peak (1), but concern remains about its link with thromboembolic events (2). Exercise activates coagulation in an intensity-dependent way, potentially leading to thrombophilia (3). We therefore conducted the first study to examine in-vivo coagulation in maximal exercise, HIIT and MICT using Fourier transformation rheology. Methods: Sixteen active men (age: 21±3 years, stature: 1.80m ± 11.6cm and mass: 85.8kg ± 3.9kg) were recruited and after random assignment into either a HIIT or MICT group, undertook a cycling challenge to volitional exhaustion on a mechanically braked ergometer (Monark, Varberg, Sweden). VO2 was sampled using an online metabolic cart (Medgraphics UK, Gloucester, UK) and power output corresponding to 40% and 80% VO2peak (HIIT) or 60% (MICT) was noted. Participants returned 3 days later and performed an acute, isovolumic bout of HIIT or MICT. Pre/post exercise, blood was sampled from an intravenous cannula. 10 mL of blood was drawn and discarded, then exactly 7mL of fresh whole blood was injected into a double walled concentric rheometer (Discovery Hybrid 2, TA Instruments, DE, USA), for analysis of blood fractal dimension (Df) using previously described methods (4). Data were analysed using a 2-way repeated measures ANOVA and Bonferonni corrected paired sample t-tests and independent sample t-tests. Significance was established at P<0.05 and data were expressed as mean ± SD Results: There were no differences in Df prior to exercise between groups (1.81±0.01, HIIT vs. 1.74±0.11, MICT). Df increased in both groups after maximal exercise (1.95±0.08, HIIT vs. 1.87±0.10, MICT, P<0.05). Acute HIIT did not alter Df (1.82±0.07) but acute MICT increased Df vs. pre-exercise (1.83±0.16, P<0.05). Conclusion: An acute bout of MICT at the current recommended training intensity for cardiac rehabilitation increased Df (figure 1, B). The reference range for Df is 1.74±0.07 (5), which suggests MICT activated coagulation and increased the density and cross-linkage of the insipient clot outside the reference range. Conversely, an acute bout of HIIT did not increase Df. Our data conflict with the present literature that used less sensitive in-vitro markers of coagulation (6,7) and suggest that HIIT is a safe, viable exercise regimen and deserving of further examination in patients.