A 35-year-old male bodybuilder presents with marked skeletal muscle hypertrophy after 5 years of intense resistance training. Which of the following statements about skeletal muscle hypertrophy is NOT correct?

Explanation

## Skeletal Muscle Hypertrophy: Mechanism and Adaptation ### Definition and Primary Mechanism **Key Point:** Skeletal muscle hypertrophy is an increase in muscle fiber **diameter**, not an increase in fiber number. This distinguishes hypertrophy from hyperplasia. ### Mechanisms of Muscle Hypertrophy | Mechanism | Role in Hypertrophy | |-----------|--------------------| | Increased protein synthesis | Primary driver; mTOR and IGF-1 signaling upregulated | | Decreased protein degradation | Reduced ubiquitin-proteasome pathway activity | | Satellite cell activation | Myogenic precursor cells fuse with myofibers, donate nuclei | | Myonuclei increase | Allows greater capacity for protein synthesis per fiber | | Fiber diameter expansion | Direct result of increased contractile protein accumulation | | Fiber number | **Remains unchanged** in typical resistance training | ### Role of Satellite Cells **High-Yield:** Satellite cells are quiescent myogenic stem cells located between the sarcolemma and basal lamina. Upon mechanical stimulus (resistance training): 1. Satellite cells activate and proliferate 2. Differentiate into myoblasts 3. Fuse with existing myofibers 4. Donate nuclei to increase the myonuclei pool 5. Enable sustained protein synthesis in enlarged fibers **Clinical Pearl:** The myonuclei number increases proportionally with fiber size. This is why chronic resistance training leads to a persistent increase in muscle mass — the enlarged fibers have more nuclei to sustain protein synthesis. ### What Does NOT Happen in Typical Hypertrophy **Warning:** Muscle fiber hyperplasia (increase in fiber number) is NOT the primary mechanism of hypertrophy in humans. Hyperplasia may occur in some animal models or extreme conditions, but in typical resistance training, fiber number remains essentially constant. The increase in muscle mass comes entirely from increased fiber diameter. ### Mnemonic: HyperTROPHY vs HyperPLASIA - **Hypertrophy (TROPHY):** Size of each fiber ↑ | Number of fibers → unchanged - **Hyperplasia (PLASIA):** Number of fibers ↑ | Size of each fiber → unchanged