Most runners determine cardiac safety limits during exercise using age-based maximum Heart Rate. But heart rate only tells you how many times your heart is beating, not how well it’s doing.
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 of this distortion, recorded by Frontier X, for every heart-beat of the user.
A higher value of Cardiac Strain indicates a higher level of Oxygen deprivation in your heart muscles, and a lower value of Cardiac Strain indicates a higher availability of oxygen to your heart muscles.
A value of <0.1 millivolts is considered mild, 0.1 to 0.2 is considered moderate, and above 0.2 millivolts is considered severe Cardiac Strain, and is not recommended for long periods of time.
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. A notable example is Micah True, the central character in ‘Born to Run’, who passed away at the age of 58 due to a cardiac abnormality.
*The ECG and Cardiac Strain displayed in our app is not meant for clinical use.