Prophylactic anticoagulation is a standard practice in patients with sudden lower limbs paralysis. Thromboprophylaxis is usually continued until the patient regains independent mobility. The duration of anticoagulation in long-term immobile patients is unknown. Spinal cord injury patients are the only population that was comprehensively studied and prophylactic anticoagulation is discontinued after 4 months as the risk of venous thromboembolism drops dramatically after 3-4 months. Development of muscle spasticity has been traditionally considered to be the reason for this low risk as lower limbs spasticity/spasms might be able to improve the calf muscle pump action. We are presenting the evidence from physiological studies of the lower limbs vascular system that cast doubt over this explanation and present an alternative hypothesis backed by several clinical circumstantial evidence suggesting that the vascular changes following long term lower limbs inactivity which are universal to all immobile patients is probably the main protecting factor. We suggest that prophylactic anticoagulation is necessary only on the first 4 months following the acute onset of immobility in all neurologically impaired immobile patients regardless of their muscle tone state.