Heart Rate Zones

Heart Rate (HR) zones are designed to measure physical exertion levels. The maximum HR is typically calculated as 220 - age, meaning a 35-year-old's max HR would be 185. In theory, during maximum exertion, an individual's HR should reach 100% of their max HR. Resting HR is often used as an indicator of physical and cardiovascular health, with a lower rate indicating better health due to a more efficient circulatory system. Regular cardiovascular exercise can lead to a lower resting HR and greater cardiovascular efficiency. HR monitoring during cardio exercise is also useful, particularly when running a marathon, as it can be challenging to count pulse rates. It is recommended to maintain a sustainable HR range between zones 2 and 3, though there are many variations of HR zone names.

Simply speaking, metabolism is how the body converts glucose into energy. To be able to carry out metabolic processes in the presence of oxygen means that the body is still able to supply oxygen effectively at an adequate rate. This is how aerobic metabolism can continue.

However, once the rate that oxygen can be delivered is decreased (because it's being used up by skeletal muscles elsewhere in the body), the body needs to switch over to anaerobic metabolism, and start to produce energy in the absence of oxygen. It also works out that the anaerobic pathway is faster, in terms of milliseconds, as compared to the aerobic pathway, which takes place over a few seconds or minutes.

This means that the aerobic pathway is energy efficient but time inefficient. Conversely, the anaerobic pathway is time efficient but energy inefficient.

There comes a tipping point where the body favours the less energy-efficient way to produce energy, when it transitions to anaerobic metabolism. This is called the anaerobic threshold (AT). This varies from person to person depending on age and fitness levels, but generally should fall between 80% - 90% for the average non-athlete.