## **Core Concept**
Slow-twitching muscles, also known as Type I muscles, are specialized for endurance and are rich in myoglobin, which stores oxygen for prolonged activity. These muscles are primarily used for aerobic exercises and are characterized by their red color due to the high myoglobin content.

## **Why the Correct Answer is Right**
The correct answer includes options that accurately describe features of slow-twitching (Type I) muscles. These muscles are indeed **rich in myoglobin (a)**, which allows them to store oxygen and facilitate prolonged, low-intensity contractions. They are **composed of smaller fibers (b)**, which enables a greater density of mitochondria and myoglobin, supporting their endurance capabilities. Slow-twitch muscles are **red in color** due to their high myoglobin content, not white, and they have a **low glycolytic enzyme activity** and **low ATPase activity**, which contrasts with fast-twitch muscles.

## **Why Each Wrong Option is Incorrect**
- **Option C:** Contains larger amounts of glycolytic enzymes - This is incorrect because slow-twitch muscles rely more on oxidative phosphorylation than glycolysis for energy production. They have lower glycolytic enzyme activity compared to fast-twitch muscles.
- **Option D:** White in color - This is incorrect because slow-twitch muscles are red due to their high content of myoglobin, not white. White muscles are characteristic of fast-twitch muscles.
- **Option E:** It has high ATPase activity - This is incorrect because slow-twitch muscles have low ATPase activity. High ATPase activity is a feature of fast-twitch muscles, which allows for rapid contraction but leads to quicker fatigue.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **slow-twitch muscles are highly resistant to fatigue** and are crucial for endurance activities such as distance running and cycling. These muscles are **rich in mitochondria and myoglobin**, which supports their aerobic metabolism.

## **Correct Answer:** B. a, b