The heart is a muscle. Like all muscles, it needs a supply of oxygenated blood to meet its metabolic demand. During intense exercise, this demand may not be met, causing damage to your heart cells (cardiomyocytes). In the long term, this can lead to conditions such as fibrosis or arrhythmias.
Oxygen demand-supply balance is typically measured in cardiology labs as part of the standard treadmill test. The ECG signal is very sensitive to changes in the oxygen supply to your heart muscles and shows detectable distortions in specific segments when the supply is low.
'Cardiac Strain' is a measurement based on the ST-segment deviation recorded by the Frontier X from the CC5 Lead position. This deviation could be due to a variety of reasons, including oxygen deprivation to a part of your heart, electrolyte imbalance and conduction defects. Please note that Cardiac Strain does not capture all aspects of cardiac function. Furthermore, Cardiac Strain is not sufficient to make any clinical assessment, and is not intended to be used for diagnostic or treatment purposes.
During exercise, a deviation of less than 0.1 millivolts is considered insignificant, and above 0.2 millivolts is considered significant, and may suggest that your heart is being over-strained. However, it is also possible to have a high value of Cardiac Strain due to an unusual ECG morphology. This can be determined by visual analysis of the ECG by a Cardiologist or trained expert.
Various studies (1,2) have been conducted on a number of apparently healthy marathoners who, unknown to themselves, over-strain their hearts. Older athletes are especially susceptible as their coronary vessels tend to be less open than those in younger hearts.