Heart rhythm abnormalities (beating too fast or too slow) affect more than 2 million people per year in the UK, and the consequences are a significant clinical burden. At the most malignant end of the spectrum this can result in people dying suddenly but other patterns of heart rhythm abnormality are also a leading and underestimated cause of stroke. There are data indicating genes are important, but our knowledge is limited. Heart rhythm abnormalities can be detected as changes in electrical signals, and these are identified using an electrocardiogram (ECG), a test that shows the hearts electrical activity as line tracings on a piece of paper. Prior work has indicated that measures taken from the ECG, for example heart rate, during and after exercise are strongly associated with the risk of sudden death. We propose to study the effects of exercise on heart rhythm, and will determine the genes influencing this response, as we believe exercise will amplify inherited differences. UK Biobank is a national resource for scientific research, and it has ~ 95,000 individuals who have had an exercise ECG. In a pilot study we have derived a series of measures from the ECG taken at rest, during exercise and after exercise using custom-made methods from 85 individuals. The results indicate that we can successfully extract the exercise ECG measures we are interested in. We now wish to expand the study to everyone who has had an exercise ECG, and perform tests to assess if exercise ECG measures are inherited, and further genetic studies to discover new genes. The results from this study will provide important information on genes influencing our response to exercise and recovery that will provide new insights into mechanisms, and the project also has the potential to find new genes that we can target to develop new therapies.