High repetition resistance training of 15 or more repetitions primarily affects which type of muscle fibers?

High repetition resistance training, typically consisting of 15 or more repetitions, primarily targets Type I muscle fibers, also known as slow-twitch fibers. These fibers are characterized by their endurance capabilities and are well-suited for sustained activities that require prolonged effort, such as long-distance running or cycling.

Type I fibers have a higher density of mitochondria, which enables them to utilize oxygen effectively for aerobic energy production. This allows them to resist fatigue over extended periods, making them ideal for high-repetition workouts. The training stimulates these fibers to adapt by increasing their oxidative capacity and enhancing muscular endurance.

In contrast, Type II muscle fibers (both IIa and IIb) are more suited for short bursts of high-intensity effort and generally perform better with lower repetitions, focusing on strength and power instead. The other classifications mentioned, like Type III and Type IV fibers, are not widely recognized in standard muscle fiber classifications and are generally not discussed in the context of resistance training and muscle endurance. Hence, the focus on Type I fibers in this scenario provides a clear understanding of the physiological responses to high repetition training.