Prolonged sitting for three hours increases the low-frequency/high-frequency heart rate variability ratio by approximately 20–30% in healthy young men, which is associated with elevated diastolic blood pressure and leg fluid retention.

When a person sits still for hours, fluid collects in the legs due to gravity and lack of muscle movement. This reduces the amount of blood returning to the heart, which lowers the pressure sensed by blood vessels in the neck. In response, the brain sends stronger signals to narrow blood vessels throughout the body, raising blood pressure in the lower arteries and increasing heart rate. The same fluid buildup in the legs continues to worsen because the narrowed vessels make it harder for blood to flow back up.