Glucose and fatty acids are the main source of fuel for muscular activity in our organism. 
Both of them require the presence of oxygen to be “burned” completely, to form carbon dioxide (CO2) and water and release the energy needed for locomotion. 

As the intensity of the effort increases, so does the demand of energy and oxygen, together with a rise in heart rate and a reduction in peripheral vascular resistances (dilatation of blood vessels). 

When the capacity of transfer and use of oxygen gets close to its maximum (between 75 and 90%), an increasing part of the energy required by the effort is obtained through the transformation of glucose into lactic acid. 
This process does not require oxygen and it is therefore called anaerobic glycolysis. 
Fatty acids are not involved in this metabolism, as they can be burned only aerobically (with the presence of oxygen). 

Indeed, even in rest conditions our body produces a minimal amount of lactic acid, easily getting rid of it. 
When the cyclist speeds up, the amount of produced lactic acid increases: up to a certain intensity, the organism is capable of getting rid of as much lactate as it is produced, with no significant increase of its concentration in muscles and blood. 

Liver, heart, kidneys and other muscles (not “busy” in the effort) are all involved in the process of “eliminating” the lactic acid. 

Above a certain critical level of effort, which varies from athlete to athlete, the produced lactic acid is more than the body can get rid of, therefore accumulating in the muscles and blood: the anaerobic threshold (AT) is the maximum intensity at which there is still balance between lactate production and elimination. 

 

On average the AT corresponds to a concentration of 4 mM/L of lactic acid, with an individual variability between 3 and 6 mM/L. 
Intensities corresponding to the AT can be maintained for a maximum of 40-60 minutes of continuous effort, as they quickly exhaust muscular glycogen. 

Intensities that are higher than the AT can be maintained only for a few minutes, as the accumulated lactate determines an increase in hydrogen ions (H+), with consequent acidosis within the muscular fibers, inhibiting their efficiency.