Answer:
In anaerobic respiration, each molecule of glucose yields only two ATP molecules. Compare this with the 38 molecules obtained in aerobic respiration.
Most of the energy is still locked up in the Lactic acid or ethanol molecules. In animal cells, the energy in Lactic acid can be released if oxygen is supplied to the cell. During vigorous activity, muscle cells do not get sufficient supply of oxygen to meet their requirement.
As a result, anaerobic respiration occurs to temporarily supply energy, and lactic acid is produced.
Lactic acid accumulates in the cells up to a certain level. Beyond this level, the muscle cells cannot work and this is know as muscle fatigue. The muscle cells are said to have incurred an 'oxygen debt' since they have used up more energy than was possible from the oxygen supplied to them.
During the resting stage, the muscle cells respire aerobically and 'pay back' the oxygen debt by converting the Lactic acid to pyruvic acid.
This is the connection.
