Patients post stroke showed substantial changes in biomechanical properties of the ankle and knee joints. However, it is not clear what the underlying mechanisms are in the hypertonic calf muscles in stroke survivors. Biomechanical properties of the medial gastrocnemius muscle in both stroke survivors and healthy subjects were investigated in vivo and noninvasively. A programmable electrical stimulator was used to activate the medial gastrocnemius selectively and a GE LOGIQ-9 ultrasound machine was used to register the muscle images across the fiber lengths to examine its biomechanical properties in vivo. It was found that 1) stroke survivors showed shorter muscle fiber length; 2) stroke survivors' muscles had smaller pennation angles both anteriorly and posteriorly; 3) stroke survivors demonstrated an active torque-angle curve shifted towards dorsi-flexion, compared with healthy control subjects.