How does strength training primarily affect muscle fibers?

Strength training primarily affects muscle fibers by increasing their diameter, a process known as hypertrophy. When subjected to resistance, muscle fibers undergo micro-tears, which then heal and adapt by growing larger and stronger. This increase in muscle fiber diameter is a key physiological response to strength training, as it enhances the muscle's ability to produce force.

The other options do not directly capture the primary adaptation of muscle fibers in response to strength training. While strength training can lead to improved blood flow due to increased vascularization over time, enhanced blood flow is more of a secondary effect rather than a primary alteration of muscle fibers. Additionally, while strength training can improve endurance and reduce perceived fatigue levels, the direct impact on muscle fibers that is most prominent is the increase in size. Lastly, decreasing muscle size contradicts the basic principles of how strength training works; it is designed to promote growth and strength, not reduction.