Introduction. The incidence of deep venous thrombosis (DVT) in patients with lower limb cast immobilization occurs in up to 20% of patients. This may result from altered calf pump function causing venous stasis. Our aim was to determine the effects of below knee cast on calf pump function. Method. Nine healthy participants were enrolled in this research and ethics approved prospective study. Four foot and ankle movements (toe dorsiflexion, toe plantar flexion, ankle dorsiflexion, ankle plantar flexion) and weight bearing were performed pre and post application of a below knee cast. Baseline and peak systolic velocity within the popliteal vein was measured during each movement. Participants with peripheral vascular disease, varicose veins, deep venous thrombosis or previous foot and ankle surgery were excluded. Results. The mean patient age was 34 years (range 28–58), the majority were female (n = 6). With cast in situ all movements resulted in a statistically significant increase in peak systolic velocity (p = <0.05). There was no significant difference in peak systolic velocity at the popliteal vein pre or post cast application. Discussion. This is the first study to examine the effect of a lower limb cast on calf muscle pump function. Despite cast immobilisation, toe and ankle flexion and extension movements significantly increase peak systolic velocity measured at the popliteal vein. Conclusion. Toe and ankle flexion and extension exercises significantly increase calf pump function in patients with below knee cast immobilization. We recommend that all patients treated in below knee cast immobilisation are advised to perform regular toe and ankle exercises to reduce venous stasis and risk of deep venous thromboembolism