Fast acting, lactic acid, white muscles, myoglobin.

Answer

### Step-by-Step Solution:

1. **Understanding Muscle Types**:
- There are two main types of muscle fibers: **white muscle fibers** and **red muscle fibers**.

2. **Characteristics of White Muscle Fibers**:
- White muscle fibers contain **less myoglobin**. Myoglobin is a protein that binds oxygen, and its lower presence in white fibers means they are less efficient at using oxygen.
- They have **more sarcoplasmic reticulum** and **less mitochondria** compared to red muscle fibers. This structure allows them to contract quickly but limits their endurance.

3. **Characteristics of Red Muscle Fibers**:
- Red muscle fibers contain **more myoglobin**, which allows them to store more oxygen and perform aerobic respiration.
- They have **more mitochondria** and **more blood capillaries**, which support sustained, long-term activities.

4. **Energy Production**:
- White muscle fibers primarily rely on **anaerobic respiration**, which does not require oxygen and leads to the production of **lactic acid**. This is why they are referred to as "fast-acting" muscles; they can generate quick bursts of energy but fatigue quickly.
- In contrast, red muscle fibers utilize **aerobic respiration**, which is more efficient for prolonged activities.

5. **Conclusion**:
- In summary, white muscle fibers are characterized by being fast-acting, producing lactic acid due to anaerobic respiration, and having less myoglobin compared to red muscle fibers.