Muscle atrophy is the decrease in muscle strength due to a decrease in muscle mass, or the amount of muscle fibers. Atrophy can be partial or complete, varying in the extent of muscle weakness. Muscle atrophy is often a result of disease such as cancer, AIDS, congestive heart failure, chronic obstructive pulmonary disease, renal failure, and burns. Starvation can also result in muscle atrophy. Simple disuse of muscle, either due to a sedentary lifestyle, or because of bed rest, can also cause muscle atrophy.

Muscle atrophy is typical to some extent during aging. Atrophy over time due to aging is known as sarcopenia. Though not completely clear, it is suspected that the cause of sarcopenia is a combination of the decline of satellite cells to regenerate cells of skeletal muscle fibers, as well as a decreased sensitivity or availability of hormonecues, including growth factors, that stimulate maintenance muscles through regeneration of muscle fiber cells from satellite cells.

Loss of muscle not due to atrophy or sarcopenia is indicative of diseases that result in structural defects of muscles (muscular dystrophy) or autoimmune responses that degrade muscle structure (myopathies).

Muscle hypertrophy is an increase in the size of a muscle through an increase in the size of its component cells. It differs from muscle hyperplasia, which is the formation of new muscle cells. Depending on the type of training, the hypertrophy can occur through increased sarcoplasmic volume or increased contractile proteins.

A range of stimuli can increase the volume of muscle cells, including strength training or anaerobic training. These changes occur as an adaptive response that serves to increase the ability to generate force or resist fatigue in anaerobic conditions.

Several biological factors such as age and nutrition can affect muscle hypertrophy. During puberty in males, hypertrophy occurs at an increased rate. Natural hypertrophy normally stops at full growth in the late teens. An adequate supply ofamino acids is essential to produce muscle hypertrophy. 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, as in anabolic steroids, will increase results. It is also considered a performance-enhancing drug, the use of which can cause competitors to be suspended or banned from competitions. In addition, testosterone is also a medically regulated substance in most countries, making it illegal to possess it without a medical prescription.

Normal versus atrophied sarcomeres: Schematic of filament arrangement in normal, functional sarcomeres, versus atrophied sarcomeres following 17-day space flight