Final answer:
The number of skeletal muscle fibers is set at birth and remains constant throughout life; however, the size of these fibers can change due to factors like exercise and hormones, leading to either hypertrophy or atrophy.
Step-by-step explanation:
The number of muscle fibers at birth compared to the number of muscle fibers in an adult remains relatively consistent, as it is genetically determined and does not change.
What changes is the size of the muscle fibers, which can grow larger, a process known as hypertrophy, or become smaller, known as atrophy.
Muscle hypertrophy leads to increased mass and bulk due to the increased production of myofibrils and sarcomeres within the muscle fibers.
Conversely, muscle atrophy results from decreased use of a muscle, leading to a reduction in the number of myofibrils and sarcomeres, which makes the muscle fibers become narrower.
Skeletal muscle fibers are formed during early development when embryonic myoblasts fuse to become multinucleated fibers, capable of producing large amounts of proteins and enzymes necessary for muscle function.
This number of fibers and their potential for contraction contribute to muscle strength.
Factors such as exercise, hormones, and stress can influence whether a muscle undergoes hypertrophy or atrophy, impacting the muscle's strength and bulk.