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 X2 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.

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A race should be run at a pace that is sustainable for a long duration. But pacing can be tricky. On a cool, dry day after a good night’s sleep, you could go much faster than your usual, but if you’re recovering from an illness or the day is unusually hot, for example, this will feel much harder.

This is because your brain perceives the effort your body is exerting and not your actual pace. Numerous studies (1, 2) have shown that breathing rate (BR) is linearly related to the rate of perceived effort (RPE) and is a great way to objectively and accurately measure internal effort. This is because both BR and RPE are determined by the same central command signals sent out by the brain to the muscles when you engage in hard exercise, unlike heart rate, which is influenced by various external factors.

By looking at the average breathing rate during your workout session, you can tell how intense the session was. You can even look at what parts of the session you spent in different breathing zones; this is particularly useful if your session had multiple pace changes, especially in interval training.

The Frontier X2 can be configured to alert you when you cross certain breathing rate thresholds, so you can be alerted with a buzz when you are crossing into a higher effort zone than what you had planned for the workout. This can be used to optimize your training in different effort zones.

The amount of exercise your body can take in a particular week without overtraining or undertraining depends on your fitness level. This level can be measured by looking at your average activity for the past four weeks.

Research, spanning decades (1,2) has shown that the sweet spot for the load you take in a week is between 0.8 to 1.3 times the average load your body experienced in the last four weeks. A load of more than 1.3 increases your risk of injury and a load of under 0.8 increases your propensity to decondition.

Most wearable devices, and even fitness coaches prescribe training based on pace or heart rate or some other measure of external load. For example, the external load in resistance training is the load lifted, while in running or swimming, it is the distance covered or speed/time profile. However, the measures of external load are specific to each exercise, and they do not allow you to compute the total load on your body across a week packed with different exercises.

Frontier X2 accurately measures internal load using breathing rate as an indicator of effort. This is an objective internal measure that is independent of exercise type, allowing the computation of weekly loads spanning different activities from running to cycling, swimming, and cross-fitness.