The study aimed to determine the effect of acute, one-time physical effort performed under different environmental temperature conditions on erythrocyte deformability in healthy young men. This exploratory randomized parallel-group study involved 30 men randomly assigned to an experimental group exercising at −10 °C in a climatic chamber and a control group exercising under thermoneutral outdoor conditions. Erythrocyte deformability was assessed using the elongation index (EI), reflecting erythrocyte elasticity and the ability to pass through microcirculation vessels. Participants performed an incremental 20 m shuttle run test. Venous blood samples were collected before and immediately after exercise, and erythrocyte deformability was analyzed using a Lorrca analyzer across a shear stress range of 0.30–60.00 Pa. A two-factor repeated-measures analysis of variance was applied. An increase in EI after exercise was observed in both groups, predominantly at higher shear stress values, indicating enhanced erythrocyte deformability under conditions of increased shear forces. However, the magnitude of post-exertion changes differed between groups. At lower shear stress levels (0.30 Pa and 0.58 Pa), EI tended to decrease after exercise. These findings indicate that a single bout of physical effort influences erythrocyte deformability, while the potential effects of cold exposure on this response remain uncertain and warrant further investigation.

Purpose: Physical activity induces numerous modifications in the morphological, rheological, and biochemical properties of blood. The purpose of this study was to evaluate changes in blood rheological and biochemical indicators among runners. Also, we assessed how the rheological and biochemical properties of blood in people who practised running characterised the range and direction of exercise modifications and allowed for the diagnosis of transient adaptive effects. Methods: This study included 12 athletes who regularly trained in middle- and long-distance running (6–8 times a week) and presented a high sports level (national and international class). The athletes performed a 30 min warm-up consisting of 15 min of jogging and exercises. After a 10 min rest, they completed a 3 km run with submaximal effort. Blood samples were collected at baseline and after the effort. Results: No statistically significant changes were revealed in erythrocyte, leukocyte, platelet, iron, ferritin, transferrin, erythropoietin, or C-reactive protein concentrations in the examined runners. The same applied to the elongation index at a shear stress within the range of 0.30–60.00 Pa, amplitude and total extent of aggregation, aggregation half-life, and aggregation index. A significant increase (within standard limits) was only observed in fibrinogen concentration after running. Conclusions: The lack of post-exercise changes in blood rheological and biochemical indicators in the investigated runners points at an efficient haemorheological system. This, in turn, reflects well-executed training and remarkably well-trained adaptive systems responsible for regeneration.