Which type of muscle fibers are typically increased in proportion due to resistance training?

The correct choice is fast-twitch muscle fibers. These muscle fibers are specifically designed for explosive strength and rapid contraction. Resistance training, especially when it involves heavy weights or explosive movements, places significant stress on the muscles, prompting adaptations that typically lead to an increase in the size and number of fast-twitch fibers. These fibers have a greater capacity for hypertrophy, meaning they can grow larger and stronger in response to the mechanical overload associated with resistance training.

Fast-twitch fibers are characterized by their ability to generate quick bursts of power and force, making them critical for activities like sprinting, weightlifting, and any high-intensity training. The adaptation to resistance training not only enhances their size but also improves their efficiency and power output, which is essential for maximizing performance in strength-based activities.

In contrast, smooth muscle fibers are primarily found in the walls of hollow organs and blood vessels and are not involved in voluntary movement or resistance training. Slow-twitch muscle fibers, which are more resistant to fatigue, are typically engaged during endurance activities and are not increased significantly in response to resistance training focused on strength. Cardiac muscle fibers, meanwhile, are specialized for pumping blood and do not adapt or increase in proportion due to voluntary resistance training exercises.