In the endurance training world there are currently many different methods of prescribing exercise intensity; all of which have rigorous scientific research backing their use. So we are left wondering which method is best to use. To be honest, there is probably no one best method, but there are some basic principles that need to be consistent in your training prescription.
The most commonly used method of intensity prescription is the five zone intensity distribution model, which has its roots in the work of Finish scientist Martii Karvonen from the 1950s. This method uses a percentage of maximum steady state HR to determine running and cycling speed or power. In summary, there is a certain HR that you can maintain for an extended period of time before fatigue slows you down i.e. steady state. And there is a certain HR that you can maintain for very short periods of time i.e. threshold and VO2max. Fatigue will occur for numerous reasons, but essentially it comes down to the failure of energy delivery to working muscles.
When we exercise, the body produces lactic acid as a normal by-product. Think of lactic acid as the fumes coming out of your car from petrol being burned to run your engine. The body re-uses this lactic acid to make more energy (yes the body is cool like that!). Generally the word on the street is that lactic acid development is a bad thing during occurrence; but it is in fact normal and it is reusable. The human body is eco-friendly in its recycling of by-products for energy production. It simple means that you are exercising. Referring to the graph below, in low intensity exercise, lactic acid accumulates at a slower rate than that which the body can reuse as energy; essentially lactic acid does not accumulate in the blood. At a certain intensity of exercise, there is a slow increase in blood lactate accumulation, which the body can sustain for a limited time. This is termed lactate turn point one (LT1). In high intensity exercise, there is a rapid accumulation of lactic acid which exceeds the body’s ability to reuse. An accumulation of lactic acid occurs, and the body slows down rapidly due to a lack of energy available. This is termed lactate turn point two (LT2). The five zone intensity distribution model is essentially based on these two physiological occurrences during exercise. Zone 1 and zone 2 are below LT1, zone 3 is somewhere between LT1 and LT2, zone 4 is just below and beyond LT2, and zone 5 is well beyond LT2.
Confused yet? That’s why it is important to note that the five training zones are not strict points on this graph. The only way to measure an exact turn point is with lab tests measuring blood lactate at certain exercise intensities (HR, speed, power, etc.). Each zone is a continuum of increasing exercise intensity and changing blood lactate. To simplify training zone distribution, we like to use the three zone training model. In this model, training intensity distribution is based on these two changes in blood lactate. Obviously this needs to be measured in a lab, but research has translated these blood changes into field based formulas that ‘guestimate’ three training zones. And so when prescribing exercise dosage, we simply say remain below LT1 for long aerobic efforts, and go into zone 3 (above LT2) for anaerobic efforts. Zone 2 is a grey area and hotly debated topic in the sports physiology community. Some evidence suggest that training in this zone 2 is not at all useful when training is focused on zone 1 and zone 3 (polarised training). Other evidence indicates that a mixture of using all three zones can lead to fitness adaptions and performance improvements (threshold training).
Getting exercise intensity distribution correct is an art. Even the most experienced coaches may not get it 100% correct, but as long there is a measurable target aimed for, the deviation from perfect becomes smaller. And to land this discussion, it is so important to learn how to feel where your body is. To guide this, we offer the following advice. If you can hold a conversation whilst exercising you are likely in zone 1. If you can kind of talk, but feel short of breath, you are likely in zone 2. If you feel like you need to stop, your legs and lungs are on fire, and talking is not an option, you are definitely in zone 3.
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