This study is enrolling participants who are already implanted with a S-ICD device (Subcutaneous Implantable Cardioverter Defibrillator, which is a device designed to monitor heart rhythms and deliver a shock to the heart to stop life threatening heart rhythms), and are undergoing a routine outpatient clinic follow up appointment to have their S-ICD checked. The aim of the study is to see if a feature in the device can recognize muscle movement and differentiate movement from irregular heart signals. The study involves doing some simple arm exercises while collecting device data, both with this feature turn on and then with it turned off. Participants will be randomized to either have the feature turned on or off initially then it will be programmed the alternative way and the same exercise testing will be repeated. The study will involve just one visit.