Many students find the concept of lactate threshold or OBLA difficult(remember that they are different!). I think it’s the ‘threshold’ bit. A threshold is a level above which things can rise. Think of a pain threshold or a wages threshold. Lactate threshold is similar. Another area that appears to confuse is the relationship between lactate threshold and VO2 max. So let’s try to clarify things.
The breakdown of the energy substrate glycogen is called glycolysis. During glycolysis, glycogen is first converted to glucose and then broken down to pyruvate. This process takes place in the sarcoplasm of muscle fibres and does not require the presence of oxygen; it is an anaerobic process. Hence the process can be called anaerobic glycolysis. This process is the primary source of energy and ATP when exercise continues at a high intensity for a period of 10 or more seconds. In this energy system, the partial breakdown of glucose to pyruvate provides the energy for some ATP resynthesis.
In the presence of oxygen, the pyruvate formed during glycolysis is further broken down into carbon dioxide and water in the mitochondria through Kreb’s cycle and the Electron Transfer Chain. But when the demand for energy to resynthesise ATP exceeds the amount that can be supplied by this oxidative means, the pyruvate is temporarily converted to lactate (lactic acid).
In other words, if too much pyruvate is being produced, not all of it can be broken down aerobically, and some is converted into lactate. The energy system involved is sometimes called the lactic acid or lactate anaerobic system. The lactate being produced first builds up in the muscles, but then diffuses into the blood. Lactate sampling of blood is one way that levels of exercise intensity and correspondingly, fatigue, can be measured. Because the harder you exercise, the more you use the lactate anaerobic pathway to resynthesise ATP, and hence the more lactate that is produced in your muscles and diffuses into your blood.
High levels of lactate in the muscles causes fatigue. Probably this is because of the high levels of acidity due to the presence of lactate, which in turn inhibits the actions of enzymes. When this happens, exercise intensity must be reduced or stopped to allow the body to remove the excess lactate.
Another limitation of this system is that only a relatively small amount of ATP can be resynthesised from anaerobic glycolysis. The lactate anaerobic system is however, important to the athlete, because it provides a rapid supply of resynthesised ATP for energy.
For example, exercises performed at near-to-maximum rates and lasting up to 2 minutes depend heavily upon the lactate anaerobic system to provide ATP energy. Also, in many events, the ‘sprint finish’ relies on the lactate anaerobic system for its energy supply.
The following diagram summarises the lactate anaerobic system:
During heavy exercise both carbon dioxide and lactate are produced as waste products, which increase levels of acidity in the blood causing an increase in ventilation rate (remember the sequence from AS – ↑acidity – chemoreceptors – medulla – sympathetic nerves – breathing muscles). But the relationship between workload and ventilation rate is not linear, as might be expected.
As can be seen, the relationship between ventilation rate and workload changes at about 150 Watts. This upward breaking point equates to the anaerobic threshold, which used to be taken as the point where exercise was becoming largely anaerobic. But measuring changes in breathing rates is not an accurate method of measuring lactate production. A more accurate and easily available measure is OBLA (onset of blood lactate accumulation), which is said to have been reached when blood lactate levels reach 4 millimols/L. But this measure fails to take into account performer’s resting lactate levels (there is always lactate in the blood, even at rest). The lactate threshold is considered the more accurate representation of increased lactate production as it takes into account variations in resting lactate levels, and is that point where levels of lactate increase by 2 millimols/L above resting levels.
Lactate is not a waste product or, as I have seen in some books, a poison! It is a valuable substrate for aerobic energy. Once produced, some of the lactate is converted into glucose by the liver, but the majority is converted back into pyruvate in skeletal muscle when sufficient oxygen is present and is used as an energy source. In practice, lactate is not only always present in blood (ask your teacher where it comes from!) and additional amounts are produced during the mildest of exercise, because anaerobic energy production invariably occurs at the start of any exercise, giving us our oxygen deficit. But this lactate is generally removed as quickly as it is produced, and any remaining is removed during the slow component of EPOC.
When lactate production exceeds the rate at which it can be removed, then it begins to accumulate, firstly in the muscles where it is being produced, and then in the blood as it diffuses out from the muscle. In practice, lactate threshold or OBLA occurs when levels of exercise reach an intensity where much of the energy needed for that level of exercise is being produced anaerobically rather than aerobically. Because we use oxygen for our aerobic energy production, and VO2 always increases as exercise levels increase, lactate threshold occurs at a certain percentage of the VO2 max, depending on the level of fitness of the subject.
The following graphs show this. The first shows the essentially linear relationship between a performer’s oxygen uptake and exercise intensity (different levels of activity):
The upper limit for oxygen uptake is the VO2 max which may be reached during maximum sprinting. If we then add their blood lactate levels the graph looks like this:
Lactate threshold (OBLA) corresponds to the sudden increase in lactate levels and occurs somewhere around 50% of the maximum oxygen uptake.
If we look at a similar graph for an elite performer we can notice several differences :
Firstly, the oxygen uptake line goes higher; the elite performer has a higher VO2 max. Secondly, the lactate line goes higher; the elite performer produces/can tolerate more lactate. Thirdly, the elite performer doesn’t produce hardly any lactate, even when running. Fourthly, the lactate threshold occurs at a higher percentage of the oxygen consumption – more like 75%.
I get my students to draw these results onto pre-prepared graph templates, adding in the workload axis and oxygen uptake/lactate levels following general group discussion as to what levels of firstly oxygen uptake and then lactate, might be expected.
As fitness increases the lactate threshold becomes delayed. Non-elite performers may have a lactate threshold that is 50-60% of their VO2 max, whereas elite performers may have a lactate threshold that is 80-90% of their VO2 max.
Somewhat surprisingly, training has a limited effect on VO2 max, because VO2 max is largely genetically determined. The big difference in performance comes from the improving lactate threshold. When we exercise we tend to work at or just below our lactate threshold. In other words, at a level where fatigue (caused by lactate) is not going to cause our performance to deteriorate. The fitter we are the higher our lactate threshold and hence the harder we can work.