How does resistance training primarily affect muscle fiber types?

Resistance training primarily affects muscle fiber types by increasing the quantity and efficiency of fast-twitch fibers. Fast-twitch fibers, also known as type II fibers, are designed for high-intensity, short-duration activities like sprinting or lifting heavy weights. When subjected to resistance training, these fibers adapt to become larger and stronger, allowing for improved performance in strength and power activities.

The process involves a phenomenon known as hypertrophy, where the muscle fibers undergo growth due to mechanical overload, leading to an increase in protein synthesis within these fibers. Additionally, resistance training can enhance the metabolic capabilities of fast-twitch fibers, allowing them to perform better under conditions of stress and fatigue.

While some resistance training programs may lead to a slight transformation of fiber type — for example, a potential shift from type IIx to IIa, which provides a balance of endurance and power — the primary and most significant change involves the enhancement of the fast-twitch fibers themselves. This adaptation is crucial for athletes and individuals focusing on improving their strength and explosive power.

In conclusion, the increase in fast-twitch fibers due to resistance training is a key reason for enhanced muscle performance, making this the correct choice regarding how resistance training affects muscle fiber types.