No matter what physical activity you are partaking in, your level of exertion will fall somewhere on what we call the 'Intensity Spectrum'. From only minimal exertion, to absolute maximum effort - at any given moment, your activity will fall somewhere within these two extremes. The lower the intensity, the longer that effort can be maintained. The higher the intensity, the sooner fatigue will force you to slow down or stop completely. The primary adaptations you are seeking from a particular workout, will determine the specific location(s) on this spectrum that you will be targeting.
However, the precise intensity you are deemed to be operating at, can vary greatly depending on which particular metric you are using. There are numerous ways to gauge the current demand of an activity. Power, Pace, Heart Rate, RPE, Blood Lactate, VO2 and Gas Exchange, are all valid means of determining intensity. All of them have their place, none of them are perfect, some are more useful than others in certain situations, and some are more practical for most situations. There is certainly not a perfect correlation between any of these means for gauging intensity. There is however, a general relationship that allows us to make our best educated guess on any given occasion - depending upon what we have at our disposal.
Below, you will find a broad overview of the intensity spectrum and the primary physiological adaptations you can expect to receive at each level.
BELOW AEROBIC THRESHOLD (AeT)
THP LEVELS 1 & 2
When operating below your Aerobic Threshold, you are working almost entirely via aerobic pathways. This is to say, fatty acids and lactate are the primary fuel sources you are combusting to produce ATP (the energy currency of human cells). Below AeT, as lactate is being used to fuel aerobic metabolism, it will not be accumulating in the blood, and will remain at a baseline level (typically below 2.0 mmol/l). Although pace, power and heart rate ranges are provided to prescribe workouts below AeT, a simple talk test can accurately determine an appropriate intensity without the reliance on any other metrics. When operating below your Aerobic Threshold, your breathing rate will be comfortable and talking in full sentences will be possible. If talking becomes more challenging, then know that you have likely reached (or exceeded) AeT.
As an endurance athlete, the majority of your training will be performed below your Aerobic Threshold. When you sufficiently increase the volume of your training at this low intensity, you can expect positive adaptations to your physiology. The most significant of these adaptations, is an increase in your Aerobic Capacity (VO2max). Your VO2max can be thought of as 'the size of your engine', and no matter what event you are targeting, a higher VO2max is always beneficial to endurance performance. It is a measure of how much oxygen your body is able to use in order to produce energy during exercise. The more oxygen you can use, the more energy you can produce aerobically. The more energy you can produce aerobically, the less your body has to rely on anaerobic pathways - resulting in a higher Anaerobic Threshold (AnT).
Another significant positive adaptation you can expect to receive from extensive training below your Aerobic Threshold, is an increase in the number of mitochondria you have in your muscles. We refer to this as mitochondrial content or mitochondrial volume density. Your mitochondria are often referred to as 'the powerhouses of your cells', as it is here that nutrients are broken down and turned into ATP. The more powerhouses you have available, the greater your overall capacity to produce power through aerobic metabolism. There is in fact a well established linear relationship between a high number of mitochondria and a high VO2max. Mitochondria only have a very short life (between 18 and 26 days), so this can explain why fairly short periods of inactivity can significantly reduce an athlete's VO2max.
@ AEROBIC THRESHOLD (AeT ... aka LT1, VT1)
THP LEVEL 2/3
As exercise intensity increases, you will reach what we call the Aerobic Threshold (AeT). This is the approximate location on your intensity spectrum, where the reliance on carbohydrates to fuel energy production, exceeds the number of ATP produced from fat. It is therefore sometimes referred to as 'the crossover point' in substrate utilisation. The increasing reliance on carbohydrates, results in a greater production of lactate, leading to a rise in blood lactate concentration above baseline levels (typically around 2.0 mmol/l). Although pace, power and heart rate ranges will be provided in training to estimate the location of your AeT, individual differences will always occur. Therefore, a more consistently accurate way of determining this intensity, is to perform a simple talk test (as demonstrated by Carl Foster et al., 2008). As exercise intensity increases below AeT, you will reach a point where you notice a distinct increase in your breathing rate. Whilst you will still be able to string a few words together, talking in complete sentences will no longer be comfortable. This increase in breathing rate, is your body's response to the increasing reliance on carbohydrates to produce energy. As blood lactate accumulates faster than it can be cleared, you are forced to breathe faster in an effort to remove the extra carbon dioxide being produced.