The goal of this observational study is to learn if two specific heart electrical signal patterns can help in detecting the risk of dangerous heart rhythms in athletes, and to see if exercise-based tests can be used instead of invasive hospital procedures to record this electrical signals.

The main questions it aims to answer are:

1. Can special ECG action potential duration markers (R2I2 and PERS) identify athletes who are at higher risk of sudden heart rhythm problems.
2. Can an exercise test give the same information as a non-invasive electrophysiology study.

Researchers will compare athletes who have an implanted heart device (ICD) with athletes who do not, to see if there are differences in these heart signals.

Participants will undergo:

1. ECG recordings during rest and exercise.
2. If they have an ICD or pacemaker, an ECG will be recorded during a non invasive stimulation.
3. A continuous 24 hour ECG.

Inclusion Criteria:

• Age 18 and over.
• Participates in regular physical activity for at least 3 hours or greater weekly.
• Having Implantable cardioverter defibrillator insitu (cases).
• Having a cardiac pacemaker (control).
• Having no cardiac device (control).
• Sufficient capacity and agreement to participate orally and signed written consent forms.
• Understanding of written and spoken English language.

Exclusion Criteria:

• Pregnancy, as detected by positive urine pregnancy test result.
• Inability to comply with study protocols.
• Any iatrogenic cardiac pathology (stents, CABG etc).
• Unstable ventricular tachycardia (>30 seconds).
• Any unstable malignant arrhythmia.
• Fever or presence of a clinically diagnosed febrile illness

Study population will be from a single tertiary care hospital (University Hospitals Leicester), and residents of United Kingdom

Undecided yet due to study committee and study sponsor needing to deliberate