In some examples, a system may be used for delivering cardiac therapy or cardiac sensing. The system may include an in implantable medical device including a housing configured to be implanted on or within a heart of a patient, a fixation element configured to attach the housing to the heart; and a sensor configured to produce a signal that indicates motion of the implantable medical device. Processing circuitry may be configured to identify one or more impingements between the housing and another structure, such as a tissue of the heart, based on the signal from the sensor and provide an indication of the one or more impingements to a user.