When muscles are used for and extended period of time what builds up in the muscle tissue?

When muscles are used for an extended period of time, lactic acid builds up in the muscle tissue. This is because when muscles contract, they use energy in the form of adenosine triphosphate (ATP). When ATP is broken down, it produces lactic acid as a byproduct. If muscles are used continuously, the body cannot remove the lactic acid quickly enough, and it builds up in the tissue. This can cause the muscles to feel sore and fatigued.