Muscles are made of many individual muscle fibers. Muscle fibers are long, cylindrical cells giving skeletal muscles their striped or striated appearance. Muscle fibers are the primary component responsible for muscle contraction.
Masochism and sadism are both about the enjoyment of pain. Masochism refers to the enjoyment of experiencing pain while sadism refers to the enjoyment of inflicting pain on someone else. Interestingly, both masochism and sadism are eponymous words. English has thousands of such words, taken from the names of people both real and fictional.
This glossary of BDSM (bondage, discipline, dominance, submission, sadism, masochism) terms defines terms commonly used in the BDSM community.