Anaerobic work becomes painful because of the accumulation of ..........

Explanation

**Step-by-Step Solution:**

1. **Understanding Anaerobic Work**: Anaerobic work refers to physical activity that occurs in the absence of sufficient oxygen. This typically happens during intense exercise when the body requires energy quickly.

2. **Role of Muscles**: During anaerobic work, muscles need energy in the form of ATP (adenosine triphosphate). When oxygen levels are low, the body resorts to anaerobic respiration to produce ATP.

3. **Anaerobic Respiration Process**: In anaerobic respiration, glucose is broken down without oxygen, leading to the production of ATP and byproducts. One of the main byproducts of this process is lactic acid.

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