Final answer:
Large muscles contain many skeletal muscle fibers, the number of which is genetically fixed. Muscle size can increase through hypertrophy, leading to more myofibrils and sarcomeres, or decrease through atrophy without a loss in fiber number.
Step-by-step explanation:
Large muscles contain many fibers; the number of skeletal muscle fibers is genetically determined and remains constant throughout life.
However, muscle strength and size can be influenced by the amount of myofibrils and sarcomeres within each fiber. Factors like hormones, stress, and anabolic steroids can induce hypertrophy, leading to increased mass and bulk of skeletal muscles.
Conversely, lack of use can result in atrophy, where muscles decrease in size due to the loss of sarcomeres and myofibrils, even though the number of fibers remains unchanged.
Skeletal muscle fibers, which can be quite large, are divided into two types: slow-twitch (or type I) muscle fibers, which are better for endurance, and fast-twitch (or type II) muscle fibers, which are better for short bursts of power.
Bodybuilders, for example, typically have a higher number of fast-twitch fibers. Different muscles in the body have different numbers of fibers depending on their function.
For example, the quadriceps have many fibers for powerful contractions, whereas the extracellular muscles that move the eyes have fewer fibers, as precision is more important than force in their function.