<P> Humans are genetically predisposed with a larger percentage of one type of muscle group over another . An individual born with a greater percentage of Type I muscle fibers would theoretically be more suited to endurance events, such as triathlons, distance running, and long cycling events, whereas a human born with a greater percentage of Type II muscle fibers would be more likely to excel at sprinting events such as 100 meter dash . </P> <P> Independent of strength and performance measures, muscles can be induced to grow larger by a number of factors, including hormone signaling, developmental factors, strength training, and disease . Contrary to popular belief, the number of muscle fibres cannot be increased through exercise . Instead, muscles grow larger through a combination of muscle cell growth as new protein filaments are added along with additional mass provided by undifferentiated satellite cells alongside the existing muscle cells . </P> <P> Biological factors such as age and hormone levels can affect muscle hypertrophy . During puberty in males, hypertrophy occurs at an accelerated rate as the levels of growth - stimulating hormones produced by the body increase . Natural hypertrophy normally stops at full growth in the late teens . As testosterone is one of the body's major growth hormones, on average, men find hypertrophy much easier to achieve than women . Taking additional testosterone or other anabolic steroids will increase muscular hypertrophy . </P> <P> Muscular, spinal and neural factors all affect muscle building . Sometimes a person may notice an increase in strength in a given muscle even though only its opposite has been subject to exercise, such as when a bodybuilder finds her left biceps stronger after completing a regimen focusing only on the right biceps . This phenomenon is called cross education . </P>

Muscles are made of bundles of muscle fibers