text: In the human ventricles, repolarization can be seen on an ECG (electrocardiogram) via the J-wave (Osborn), ST segment, T wave and U wave. Due to the complexity of the heart, specifically how it contains three layers of cells (endocardium, myocardium and epicardium), there are many physiological changes effecting repolarization that will also affect these waves. Apart from changes in the structure of the heart that effect repolarization, there are many pharmaceuticals that have the same effect.
  On top of that, repolarization is also altered based on the location and duration of the initial action potential. In action potentials stimulated on the epicardium, it was found that the duration of the action potential needed to be 40–60 msec to give a normal, upright T-wave, whereas a duration of 20–40 msec would give an isoelectric wave and anything under 20 msec would result in a negative T-wave.
  Early repolarization is a phenomenon that can be seen in ECG recordings of ventricular cells where there is an elevated ST segment, also known as a J wave. The J wave is prominent when there is a larger outward current in the epicardium compared to the endocardium. It has been historically considered to be a normal variant in cardiac rhythm but recent studies show that it is related to an increased risk of cardiac arrest. Early repolarization occurs mainly in males and is associated with a larger potassium current caused by the hormone testosterone. Additionally, although the risk is unknown, African American individuals seem more likely to have the early repolarization more often.