# The impact of physical activity and intensity on clot mechanical microstructure and contraction in middle-aged/older habitual runners

## TL;DR

This study examines how different exercise intensities affect blood clot structure and contraction in older runners, revealing a temporary hypercoagulable state.

## Contribution

The study provides new insights into intensity-dependent changes in clot mechanics and fibrinolysis in middle-aged/older habitual runners.

## Key findings

- Both moderate and high-intensity exercise increased coagulant activity and D-dimer levels, indicating a hypercoagulable state.
- High-intensity exercise led to greater clot density (df) compared to moderate-intensity exercise.
- Clot contractile force (CFmax) decreased over time, suggesting ongoing fibrinolytic activity post-exercise.

## Abstract

Exercise in healthy individuals is associated with a hypercoagulable phase, leading to a temporary increase in clot mass and strength, which are controlled by an effective fibrinolytic system. Conversely, people with cardiovascular diseases often have a reduced fibrinolytic pathway, increased clot mass and abnormal clot contraction, resulting in poorer outcomes. We assessed clot microstructure, particularly the contractile forces of clot formation, in response to two exercise intensities in middle-aged/older runners.

Twenty-eight habitual male and female runners aged over 40 years completed a 10 km moderate-intensity run; 14 of them performed a 3 km high-intensity run. Blood samples were collected at baseline, immediately postexercise and after 1 h of rest. Clot structural biomarkers df, gel time, and measurements of mature clot mechanical properties (gel time, G’Max and CFmax) were analysed alongside conventional plasma markers.

Both exercise intensities altered markers of coagulant activity (PT, APTT and FVIII) and fibrinolysis (D-dimer), indicating hypercoagulability. Compared with longer-duration lower-intensity exercise, df was greater after short-duration intensified exercise bouts. Following an hour of rest, df dropped to baseline levels. Additionally, CFmax decreased across timepoints at both exercise intensities. This effect was noted after one hour of rest compared with baseline, suggesting continuous fibrinolytic activity postexercise.

Exercise transiently induces an intensity-dependent hypercoagulable state, resulting in denser clot formation and a reduced clot contractile force due to fibrinolysis. These findings can help guide the safe commencement of rehabilitation exercise programs for cerebrovascular patients.

The online version contains supplementary material available at 10.1186/s12883-025-04074-y.