Resistance training in a previously immobilized knee extensor increases strength primarily through what mechanism?

• A. Increase in active muscle fibers
• B. Hypertrophy
• C. Splitting of current muscle fibers
• D. Increase in bone density

Answer: (B)

When a previously immobilized knee extensor is retrained, strength increases mainly because the muscle fibers grow in size. The resistance stimulus promotes protein synthesis and addition of contractile elements within existing fibers, increasing the cross-sectional area (hypertrophy) and allowing the muscle to generate more force. Neural improvements in recruitment help as well, but the dominant long-term change driving greater strength is hypertrophy. Other options aren’t the primary drivers: simply increasing the number of active fibers or splitting fibers aren’t typical primary mechanisms in this rehab context, and bone density increases reflect skeletal, not direct muscle strength, adaptations.