The organic substance produced when a muscle generates energy in the absence of oxygen is lactic acid.

Here's why:

* Anaerobic Respiration: When oxygen is limited, muscles switch to anaerobic respiration. This process breaks down glucose to generate energy (ATP) without using oxygen.

* Lactic Acid Formation: Anaerobic respiration produces a byproduct called pyruvate, which is then converted to lactic acid.

* Muscle Fatigue: Lactic acid buildup in the muscles contributes to muscle fatigue and soreness.

Let me know if you have any more questions!