Knowing your thresholds is a useful way to quantify effort and distribute your training load, but understanding what they mean and how to use them is vital to realizing performance improvement. In a way, the term “threshold” can be a bit confusing, as it is commonly used to represent different things. So, we’re going to break down what thresholds really mean and how that information impacts your training.
Arguably, the most useful way to use thresholds is to identify the body’s need for different fuel sources. The human body generally uses a mixture of fatty acids and carbohydrates to generate fuel for the working muscles (ATP**). However, this mixture is not always the same. In general, more fat is used for fuel at lower efforts, while high intensity workouts require more carbohydrates (1*). The reason for this is that the breakdown of fatty acids is a slower and more efficient process resulting in less waste, i.e. exhaled CO2 (4*). When it comes to understanding your thresholds, in most cases, a person’s breathing pattern during exercise closely matches their body’s use of fuel (and subsequent CO2 production). Practically speaking, measuring the first and second ventilatory thresholds gives us a glimpse into how the body is utilizing the fuels available.
When starting out at a low effort, whether running or cycling, one can easily breathe and keep up a conversation with other people. Talking at this effort level should be as easy as when you are walking. However, if we were to increase our effort, we would eventually hit a point where we’d become uncomfortable keeping up a fluid conversation. At this point, faster and deeper breathing is necessary to sustain the effort, and so sentences would need to be broken up into smaller pieces. At this point, it’s safe to assume that we have passed our first ventilatory threshold, VT1.
This effort level varies by person. Some can easily maintain a jog, or even a run, while others need only walk briskly to reach this point. At this intensity, your body’s fatty acid utilization is at its highest. When training close to your VT1, the body is receiving a very high stimulus for fat metabolism. Training at this effort level will help you become more efficient in using oxygen and burning fat. Formation of slow twitch (type I) muscle fibers is increased, and mitochondrial density and function is positively affected. One can see the evidence of this fat metabolism stimulus when looking at elite endurance athletes, as they have a significantly higher number of mitochondria and fatty acid oxidation compared to the normal population (5*).
The key to building a strong aerobic foundation is allowing your mitochondria to rely on fat as a fuel source. When your effort increases beyond your First Threshold (VT1), carbohydrates become an increasingly prominent source of fuel. This is due to an increase in Type II muscle fiber recruitment, also known as fast twitch fibers. This results in your muscles down-regulating the use of fat as a fuel source. This happens because the mitochondria within the muscle cells need a more accessible and more easily digested fuel source. A byproduct of increased carbohydrate consumption is increased CO2 production, which triggers faster and deeper breathing in order.
This point of the body’s intensity and effort is also associated with increased blood lactate content. Working muscles start shuttling lactate to other tissues when they have reached their capacity to metabolise their own lactate production (6*). If the effort is increased beyond this point, there will come a time when the athlete can no longer sustain any fatty acid oxidation and the ATP needed for muscle contraction is only fueled by the breakdown of carbohydrates. This effort level is closely correlated with what has been termed the “anaerobic threshold”, or second ventilatory threshold, VT2 (7*).
At the second ventilatory threshold, the breathing rate starts to increase significantly, and the athlete has difficulty speaking more than a few words at a time. It is important to note that this point does not represent what has become known as the “maximum lactate steady state” (MLSS), but rather is an indication of the complete switch to carbohydrates as a fuel source (8* 9*). In a study published in Oxidative Medicine and Cellular Longevity in 2019, it was indicated that at efforts close to VT2, no specific benefit for either mitochondrial respiration or their volume in the muscle was obtained (11*). Therefore, training at or close to VT2 can provide temporary benefits for lactate tolerance in an athlete when a race is approaching, but might otherwise only contribute to extra fatigue. The “temporary” part of the previous sentence is the key here, as the lactate tolerance changes will eventually be overshadowed by the decrease in mitochondrial density and respiration due to not enough training at low or high intensities.
When continuing to increase the effort level above VT2, you will soon reach your VO2max. This point, as indicated by its name, is the absolute maximal oxygen uptake the body is capable of as measured in milliliters per minute per kilogram (ml/min/kg). The most efficient way to increase VO2max for the vast majority of people is to lose weight, although increases in the uptake of oxygen itself (ml/min) can also be seen when exercising regularly (10*). At your body’s VO2max, your breathing frequency can often go up to 50-60 times per minute and lead you to start to hyperventilate, which causes increased fatigue.
Training in short efforts at or above VO2max can provide improvements in mitochondrial respiration, improving their ability to consume oxygen and use fuel at high loads (11*). Typical training efforts might last from half a minute up to 2 minutes in length. At this intensity, the body is only burning carbohydrates for fuel and there is no fatty acid oxidation. Due to this, lactate build-up is high, and the body can only sustain the effort for a short period of time. Since there is such a high effort load on the system, there’s usually a longer recovery time in both the short term (within a session) and longer term (within week structure). Therefore, these sessions should only be used 1-2 times per week, balanced by longer and easier sessions at lower intensities closer to VT1.
**ATP (Adenosine triphospate) is a molecule produced by our cells when metabolising either carbohydrates, fatty acids or, to a lesser extent, proteins. ATP is then used for muscle contraction and is therefore essential for all life.
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