Several factors determine your performances in endurance sports, such as muscle glycogen and capillaries, BMI, mental strength, motivation, support... The three most important physiological factors however are VO2 max, lactate threshold and exercise economy.
Your VO2 max and lactate threshold combine to determine for how long you can produce a given amount of energy, and exercise economy will determine how fast you can go with that energy.
VO2 max
VO2 max is the maximal amount of oxygen your muscles can use to produce aerobic energy.
It depends on the maximal volume of blood your heart can pump around, and on how good your muscles are at taking up oxygen.
Male elite endurance athletes typically have VO2max values of 70 to 85 ml/ kg/ min, while young untrained men have up to 40 to 50 ml/kg/min. The values in women are always about 10% less.
Training will increase the amount of blood your heart can pump around at each beat. This amount is called the stroke volume, and it is the dominant factor of your VO2max. With training you will also make more capillaries in your muscles and more mitochondria (energy plants) in your cells, which will increase the amount of oxygen your muscles cells can take up and use.
Your heart’s stroke volume depends on your heart’s capacity to fill up with a large volume of blood during the diastole, which can then be pumped into your arteries during the systole. To an extent, it is genetically determined but it also depends on the kind of training you do and for how long you have been doing it.
As you get older, your maximal heart rate decreases by about one beat a year. Inevitably, your VO2max will decline as well.
Why is VO2 max the maximum?
Earlier studies concluded that your heart’s abilities limit VO2 max.
However, according to a new model, called the central government theory, your brain limits your performances to make sure that vital organs such as your heart continue to receive enough oxygenated blood. If not, you would collapse. In other words: the brain anticipates a collapse, and restrains you to avoid it.
This theory is still controversial, but it explains for example why at altitude (where the oxygen pressure is lower), we slow down very early during our workout. It also explains why in some studies athletes can go beyond their VO2max. VO2 max is therefore not always the maximum.
Lactate threshold
The lactate threshold is defined as the point at which blood lactate starts to accumulate substantially when you exercise harder and harder. As you produce lactate continuously, it corresponds to the moment that the production rate exceeds the clearance capacities.
Energy pathways during endurance exercise
Our cells use ATP (a high energy molecule) for energy. During easy exercise, you use mainly fatty acids to make ATP, but when you increase the intensity, you start using glycogen.
In a first step, you break down glycogen into pyruvate without the help of oxygen (anaerobic glycolysis). You will also make hydrogen ions during this process. It is a very quick way to obtain energy, but you will only obtain 2 to 3 molecules of ATP per molecule glucose.
Using oxygen however, you can further metabolise pyruvate and hydrogen ions and obtain 37 to 39 molecules ATP. This is the oxidative system, and is obviously what you need for endurance exercise, but it is a slow mechanism.
If you are exercising hard, your cells need more energy. The anaerobic system can act swiftly and deliver more ATP, but the oxidative system gets overwhelmed, and hydrogen ions start to accumulate in your cells and create an acidosis (excessive amount of acid). This is responsible for a burning sensation in your muscles, interferes with glycolysis and muscle contractions, and leads to fatigue.
A marker of stress in muscle cells
As pyruvate molecules start to accumulate as well, they combine with hydrogen ions and form lactate.
Therefore lactate helps you to delay the acidosis. Furthermore, it can be transferred to the liver, brain or other muscles, and used for energy production. This means that lactate is not the culprit of fatigue, but an indirect marker of stress in the muscle cells.
It is usually expressed as a percentage of VO2max.
The efficiency of your muscle cells’ metabolism determines your lactate threshold. You can increase it by more than twofold, and if you are lucky enough to have a high VO2max as well, you will be able to sustain a high oxygen consumption -and therefore a high power output- for a long time.
Exercise-specific economy
In practice, the crucial question is: how fast will I go with the energy I can produce? That will depend on how much energy you need to exercise at a given speed. If your body uses less energy than your opponents, you will be able to go faster than them before getting tired.
The cost of exercising is very different from one person to another, and depends on biomechanics, anatomical factors and the efficiency of your muscle cells. Scientists believe that you will be a more efficient endurance athlete if you have more Type 1 muscle fibres.
Studies on elite athletes have shown that you can improve your economy considerably by training.
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