This is due to a stagnation of blood in the veins caused by prolonged immobility, which can promote blood clot formation in veins.

其原因是由于长时间不动，造成静脉血液停滞，促使静脉中血凝块形成。

youdao

She would want to avoid prolonged periods of immobility (long car, plane or train rides) but some of it is just bad luck of the draw.

因此应该避免长时间的不动(长途汽车，飞机或者火车旅行)，但其中也有一些是运气不好。

youdao

A. representative explained that there was a statement in their literature to the effect that circulatory problems could arise from prolonged immobility.

BBC: Your comments on Economy Class Syndrome

However, the prolonged immobility of joints associated with wheelchair use can worsen contractures, in which the limbs draw inward and become fixed in that position.

CNN: Muscular dystrophy