AAMC FL Practice Exam

Which of the following is a characteristic of slow twitch muscle fibers?

1. Low capillary density

2. Less mitochondria

3. Higher levels of oxygen-binding proteins

4. Fast contraction speed

The correct answer is: Higher levels of oxygen-binding proteins

Slow twitch muscle fibers, also known as type I fibers, are specialized for endurance and prolonged activity, which is why they have several distinct characteristics. One key feature is their higher levels of oxygen-binding proteins, specifically myoglobin. Myoglobin is similar to hemoglobin but is found in muscle tissue, where it binds and stores oxygen, facilitating a steady supply of oxygen during aerobic respiration. This characteristic allows slow twitch fibers to sustain activity over a longer period without fatigue, making them more efficient for endurance activities like long-distance running or cycling. In contrast, slow twitch fibers have a higher capillary density compared to fast-twitch fibers to enhance oxygen delivery. They also contain more mitochondria, which are the powerhouses of the cell, providing energy through aerobic metabolism. Additionally, slow twitch fibers have a slower contraction speed, enabling them to contract more steadily over time rather than generating powerful but quick bursts of force. Understanding these distinctions emphasizes the functional adaptations of muscle fibers to various types of physical demands.