**Question:** Which is true about Type-2 muscle fibres?

A. Type-2 muscle fibres are exclusively involved in fast twitch contractions.
B. Type-2 muscle fibres are only present in the skeletal muscles of infants and young children.
C. Type-2 muscle fibres are primarily responsible for endurance activities.
D. Type-2 muscle fibres are characterized by a higher number of mitochondria compared to Type-1 muscle fibres.

**Correct Answer:**

**Core Concept:** Type-2 muscle fibres are a subtype of skeletal muscle fibres that exhibit different physiological properties compared to Type-1 muscle fibres. They are characterized by their faster-twitching speed and different physiological properties.

**Why the Correct Answer is Right:** Type-2 muscle fibres are indeed involved in fast twitch contractions, which are essential for power and speed-related activities. This is due to their higher number of actin and myosin filaments, which enable them to generate force more rapidly but at the expense of endurance.

**Why Each Wrong Option is Incorrect:**

**A.** Type-2 muscle fibres are not exclusively involved in fast twitch contractions. They also contribute to slow twitch contractions, although to a lesser extent than Type-1 muscle fibres.

**B.** Type-2 muscle fibres are present in the skeletal muscles of adults, not just infants and young children. They are a prominent feature of the adult human skeletal muscle.

**C.** Type-2 muscle fibres are not primarily responsible for endurance activities. Type-1 muscle fibres are the primary determinant of endurance capacity due to their oxidative properties and slower contraction speed.

**D.** While Type-2 muscle fibres do have a higher number of mitochondria compared to Type-1 muscle fibres, this is not their defining characteristic. Type-2 muscle fibres are characterized by their fast twitch contractions and their resistance to fatigue during high-intensity, short-duration activities.

**Clinical Pearl:** A proper understanding of muscle fibre types is crucial for understanding muscle function, muscle fatigue, and exercise performance. Muscle fibre type distribution can also be influenced by factors such as genetics, training, and disease states.