The external knee adduction moment (EKAM) is associated with knee osteoarthritis (OA) in many aspects including its presence, progression, and severity. Despite of its importance, there is a lack of EKAM estimation methods that can provide patients with knee OA a real-time EKAM biofeedback during training and be used for routine clinical evaluations outside motion analysis laboratories. Thus, a practical real-time EKAM estimation method, which utilizes kinematic variables from a simple 6-DOF goniometer, was developed to provide patients with knee OA a real-time feedback of their EKAM during stepping on elliptical trainers (ETs) to reduce the damaging EKAM. Feasibility of the proposed method was verified on seven healthy subjects. Combined with advantages of ETs (e.g., functional weight-bearing stepping, mitigation of delivery of impulsive forces), the real-time EKAM estimation method is expected to benefit patients with knee OA.