Implanted metal devices can often experience loosening within the host bone (proximal loosening) as a result of stress shielding. Stress shielding can be described as localized degradation in bone strength due to a decrease in physiological loading of certain areas because of the presence of the stiffer metal implant. As this can occur following even normal activity,Fatigue testingof hip implants is required to understand how abnormal loading profiles can arise and to evaluate endurance properties by simulating the dynamic loading of the implant during gait.