What substance forms in the muscles when glucose is broken down at a high rate and cannot be metabolized into adenosine triphosphate (ATP) as a result of insufficient oxygen?

Lactate.

Explanation:

Adenosine triphosphate (ATP) is known as the energy currency of the cell. Our body uses ATP for doing any work and for the transport of molecules and other life process.

The glucose is broken down for the ATP formation. The anaerobic glycolysis occurs in case of muscles in the limited supply of the oxygen. Here, the pyruvate is broken down into the lactate that cannot be metabolize in the body. This lactate results in the muscle cramp.

Thus, the answer is lactate.

Lactic acid

Explanation:

Muscle contraction needs energy in the form of ATP which allows the formation of cross-bridge.

The ATP is formed by the process of cellular respiration which takes place in the presence of oxygen but when the amount of oxygen decreases in the muscle, to produce energy the cell perform anaerobic respiration.

The pyruvate formed as a result of the glycolysis and decarboxylation gets accumulated in the muscles as it does not enter the Krebs cycle. The accumulated pyruvate gets converted into the Lactic acid.

Thus, Lactic acid is the correct answer.