Skeletal muscleStriated muscle
The voluntary muscle attached to bone that moves the body: the tissue training targets.
Skeletal muscle is the voluntary muscle attached to your skeleton: the tissue that moves you, holds you upright, and makes up somewhere around 40% of body mass in a lean adult. It is what training is aimed at, and what dieting badly costs you.
Structurally it is a nested set of bundles. Fibres are made of myofibrils, myofibrils of sarcomeres, and sarcomeres of the interleaved actin and myosin filaments that slide past each other to shorten the muscle. Force is produced by that sliding, and the arrangement is why a muscle is strongest through the middle of its range.
It is a metabolic organ as well as a mechanical one. Muscle is the largest sink for glucose in the body, so how much you have and how often you use it is a substantial part of insulin sensitivity, and it holds the amino acid reserve the body draws on during illness and injury.
That is why muscle mass tracks so strongly with outcomes in later life. Low muscle strength is now the primary criterion for diagnosing sarcopenia, ahead of muscle size, and it predicts falls, fractures, hospital stays and loss of independence.
It adapts to what you ask of it, in fairly specific ways. Heavy loads build force production, high volumes build size, endurance work builds the mitochondrial and capillary machinery, and stopping altogether reverses whichever of those you had. The tissue is unusually responsive at every age tested, including the ninth decade.
It is also expensive to maintain, which is the honest reason it disappears when it is not used. The body is not sentimental about tissue it is not being asked for.
In practice, the case for resistance training is not aesthetic. Our lean body mass calculator estimates how much you have; the training volume calculator tracks whether you are asking enough of it to keep it.