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Muscle hypertrophy refers to the increase in the size of muscle fibers, which occurs as a physiological response to resistance training or strength training. When muscles are subjected to stress, such as lifting weights, micro-tears occur in the muscle fibers. In the recovery process, the body repairs these fibers, leading to an increase in their size and strength, a process facilitated by adequate nutrition and rest. This is a central goal for individuals engaged in strength training, as larger muscle fibers can contribute to greater overall strength and improved physical performance.

The other options do not accurately describe muscle hypertrophy. A decrease in muscle fiber size directly contradicts the definition of hypertrophy and refers more to muscle atrophy, which typically occurs due to lack of use or certain medical conditions. The ability to maintain a static position relates to muscular endurance rather than hypertrophy, while improvement in flexibility pertains to the elasticity of muscles and does not directly involve changes in muscle fiber size. Thus, recognizing muscle hypertrophy as an increase in muscle fiber size is critical for understanding strength training outcomes.