Again notice how the oxygen consumption increases in a linear fashion. Oxygen consumption increases from about 53% of VO2 max to about 80% of VO2 max and the lactate levels are only a little above the resting lactate levels. Then lactate climbs rapidly. So heart rate and oxygen consumption are linear and lactate is not. The lactate curve changes quickly as intensity gets higher and oxygen consumption becomes much higher.
Diagram of basic aerobic and anaerobic energy producing processes
Notice two things.
In a later question we will discuss where the lactate comes from that is in a resting subject.
- (1) it is not lactate but pyruvate that is the output of the anaerobic system. But pyruvate is an intermediate step, which turns almost immediately into lactate.
- (2) lactate can enter the bloodstream or turn back into pyruvate. If lactate turns back into pyruvate, it will enter the aerobic system and be used as a fuel. There will be no change in the lactate levels in the bloodstream. So lactate is produced continually, but at low effort levels, most is immediately consumed by the muscles. The output of the aerobic system is energy, water and carbon dioxide; no change in blood lactate levels are noticed. Thus, at low levels a lot of the lactate produced never enters the blood stream. At higher exercise levels not all the lactate produced by the anaerobic system can be consumed by the originating muscles. Some will spill out into the bloodstream and the lactate levels in the bloodstream will rise.
If you are familiar with lactate curves, this curve has a familiar shape. However, it differs from typical lactate curves in that it plots the rate of lactate production against VO2. It also shows the rate of lactate production in the muscles rather than in the bloodstream.
It fluctuates a little, but at rest the production of lactate Lp is much less than the processes that eliminate lactate Le. The average for this particular day is 1.44 mmol/l. During normal daily activity there is constant production and constant elimination. The difference between one time of the day and another is not totally understood, but some variance is due to food intake, which seems to affect lactate production. (The red blood cells are the only cells in the body that do not have mitochondria and thus all their energy is generated by anaerobic processes which produce lactate as an end result).
For the chart above Lp is increasing as the intensity of the exercise gets harder. Each line represents a slightly harder effort (from blue to red to green to purple to teal to brown). But for the first five effort levels, the elimination capability Le is equal to or higher than the production rate Lp. That is why each line represents a steady state. For the sixth level (brown line) Lp is greater than Le and the lactate levels continue to rise – there is no steady state. Eventually athletes have to stop at this effort level because of the increase of various metabolites which causes the muscles to get very acidic.
From 180-240 watts there is little or no change in blood lactate levels which are similar to resting levels for this particular athlete. At these effort levels Le is still much greater than Lp and the body gets rid of all the new lactate being produced.
The next chart shows what would happen to lactate in the blood if the athlete changed the effort level every 6 minutes. The athlete would change from one steady state to a higher one every 6 minutes but eventually reach a point above any steady state.
We have simplified the process by pointing out that a lactate value is just the result of two natural processes, lactate production Lp and lactate elimination Le. Now let's look at two lactate tests taken with the same athlete at two different time periods.
The early and late season tests are very similar at the first five low effort levels. But the next three readings show substantial divergence. We know from above that the lactate value is the net result of the elimination Le and production Lp processes. So which is it? Or could both have changed? We know that at least one of the two processes has changed. If this is an endurance athlete, it is a change that will let the athlete compete at a faster pace most of the time. But why do the muscles produce less lactate or eliminate more, later in the season? The answer to this is the secret of training and the main reason an athlete spends so much time working out.
This illustrates that lactate production and lactate elimination are essential parts of energy metabolism and shows why the measurement of lactate is so important. Lactate values demonstrate how well the two energy systems are developed. This will be the greatest determinant of race performance. So lactate is the key to understanding how well the athlete is conditioned for a particular event.
***Steady state exercise are those intensities that are below the maximal lactate steady state. Lactate levels, heart rates, VO2 consumption and perceived effort change only small amounts over a time period, usually at least 50 minutes and sometimes much longer.