What chemical would build up in the muscles to alert them to a switch from aerobic to anaerobic functioning?

The chemical that would build up in the muscles to alert them to a switch from aerobic to anaerobic functioning is : :Lactic acid
Lactic acid is produced after we worked our heart strenuously, which alert our body to tense up the muscle end slow down any activity that we currently doing to prevent potential organ failure

The first means of speeding up reactions mentioned by the speaker is raising the enzymes