Muscle Phenotypes

• Power output determines a muscle’s contractile performance, and changes in response to use and disuse
• Endurance training elicits changes in fiber type, increased capillary density, and increased mitochondrial density
• BOX 21.1 In Endurance Training, VEGF Production Precedes Angiogenesis, and Angiogenesis Precedes Changes of Fiber Type
• Resistance training causes hypertrophy and changes in fiber type
• Both resistance-trained and endurance-trained muscles continue to remodel during taper
• Combined resistance and endurance training can improve performance
• Hypertrophy also occurs in cardiac muscles

Atrophy

• Humans experience atrophy in microgravity
• Disuse influences the fiber-type composition of muscles
• Muscles atrophy with age
• Some animals experience little or no disuse atrophy
• BOX 21.2 No Time to Lose

Regulating Muscle Mass

• Myostatin
• The PI3K–Akt1 pathway

Summary