While biologists have uncovered the cellular mechanisms that lead to muscle growth, the practical applications have traditionally remained the expertise of bodybuilders. Resistance exercise such as weightlifting is the most common way to stimulate muscle protein synthesis, which leads to muscle growth or hypertrophy when supported by adequate nutrition.
While there is no one-sized-fits-all formula for muscle growth, there are tried and true techniques to progressively overload muscles. An appropriately proscribed exercise program will adjust training variables such as exercise selection, load (weight), volume (number of reps and sets), frequency (training sessions per week), and intensity to the individual. Classic bodybuilding practices such as increasing the time-under tension, adjusting the lifting tempo, overloading the muscle through drop-sets, and completing forced reps and slow eccentrics, all create mechanical stress and metabolic damage to stimulate new growth. See this article to read about the effects of training parameters on hypertrophy and this infographic for keys to maintain long-term muscle growth.

New muscle growth is initiated by the activity of satellite cells, stem cells within the muscle that proliferate and fuse to existing cells to create new muscle fibers. Local and systemic signaling mechanisms help modulate the activation, proliferation, and differentiation of satellite cells. For instance cell-signaling cytokines are part of the immune response that activate satellite cells, while hormones such as insulin-like growth factor (IGF-1), testosterone, and growth hormone (GH) direct protein metabolism and subsequent muscle growth.
Plateaus in muscle growth are common problems for bodybuilders. After performing the same exercises for several weeks in a row, the muscle do not experience as much damage and do not grow to the same extent, a phenomena know as the repeated bouts effect.
While there is meaningful overlap between muscle size and strength, it is possible to gain strength without increasing muscle size. Strength gains can be achieved by upgrading your central nervous system to recruit and coordinate more muscle fibers. Changes in the fascia, tendons, and bones also affect the contracting muscles ability to transmit force. In contrast bodybuilding techniques are primarily concerned with building new muscle fibers, regardless of absolute strength. If you're looking for ways to focus your training on size opposed to strength, read "Size Vs. Strength: Are You Lifting Too Heavy?"
Skeletal muscle (opposed to cardiac or smooth muscle) doesn't exist in isolation. Connective tissue (facia), nerves, and vascular tissue are crucial for muscles to function properly. Without facia, bones and joints, muscles would have no way of translating tension into movement. The importance of facia has often been overlooked, but this matrix of tissue not only helps transmit force through your body, it is crucial for the health of your muscle tissue. Facia can become dehydrated and brittle, limiting motion and preventing muscles from working properly. A type of massage known as myofacial release can help stretch and loosen the fascia to restore blood flow and motion.
Skeletal muscles vary considerably in size, shape, and arrangement of fibers. In fact, muscles can be classified into a number of different fiber types depending on how strong and long they can contract. Slow twitch (Type I) muscle fibers are specialized for endurance but generate less force per contraction than fast twitch (Type II) fibers, which contract forcefully but only for short periods of time.
Muscles often work in pairs, contracting and relaxing to coordinate movements. Feedback from sensory nerves allows the nervous system to constantly adjust the force and type of contraction to produce smooth and efficient movement. Concentric muscle activation is the most familiar type of movement. It occurs when a muscle tenses as it shortens, like when your bicep shortens as your lift your utensil towards your face. The opposite are eccentric contractions that create tension while the muscle is lengthening (e.g. lowering a weight to the ground with control). Lastly, isometric or static muscle contractions create tension in both the agonist and antagonist muscles so forces are balanced and there is there is little movement.
On Earth, we must constantly use certain muscles to support ourselves against the force of gravity. In space, however, very little muscle contraction is needed to support our bodies or move around. As a result, astronauts must constantly workout their muscles or else they would weaken and deteriorate, process called atrophy.
Losing muscle is a natural part of aging, a process known as sarcopenia. To learn more about the repercussions of muscle loss and how slows this inevitable decline check out: What is Sarcopenia?

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