A small study involving 11 non-obese, healthy men aged 20-35, has found that blood flow in leg arteries is significantly impaired after three hours of sitting, but not if they walk for five minutes every hour.
When people sit, slack muscles do not contract to effectively pump blood to the heart. Blood can pool in the legs and affect the endothelial function of arteries, or the ability of blood vessels to expand from increased blood flow. Impaired endothelial function is an early marker of cardiovascular disease.
In the study, participants sat for three hours without moving their legs, while the functionality of the femoral artery was taken at baseline and every hour. This showed that the expansion of the femoral artery as a result of increased blood flow was impaired by as much as 50% after just one hour.
In another session, they again sat for three hours, but also walked on a treadmill for 5 minutes at a speed of 2 mph at the 30-minute mark, 1.5-hour mark and 2.5-hour mark. In this case, arterial function stayed the same throughout the session.
Thosar, S. S., Bielko, S. L., Mather, K. J., Johnston, J. D., & Wallace, J. P. (2015). Effect of Prolonged Sitting and Breaks in Sitting Time on Endothelial Function: Medicine & Science in Sports & Exercise, 47(4), 843–849. http://doi.org/10.1249/MSS.0000000000000479