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synergy · 3
↔Bone
When muscles contract they pull on bone, and this loading is one of the main mechanical signals for rebuilding bone. With age, sarcopenia and osteoporosis often appear together (a combination called osteosarcopenia). Strength training loads muscle and bone at the same time, which makes it one way to look after both.
Multi-joint lifts such as the squat, deadlift, bench press, row and overhead press work several muscles at once and also train the nervous system's ability to drive them, so coaches generally treat them as the backbone of strength training. The key to building muscle is taking each set close to failure: in the Schoenfeld 2017 meta-analysis, lighter weights for more reps built about as much muscle as heavy weights, as long as sets were taken close to failure.
During exercise, muscle releases signaling molecules, and brain-derived neurotrophic factor (BDNF) in the brain rises briefly; in animals, these changes promote neural plasticity. They are candidate explanations for how exercise improves mood and thinking, but whether they are the main reason in people has not been shown.
cofactor · 3
Muscle cells sense leucine, which flips mTORC1, the switch for making protein. On how much one meal needs, Morton 2018 cites a plateau in the synthesis response at about 0.24 g per kilogram of body weight in younger adults and about 0.40 g in older adults; larger amounts still add something and last longer, just with diminishing returns (Trommelen 2023). What decides how much muscle you build is the daily total: for people doing strength training, the benefit keeps rising up to about 1.6 g per kilogram a day (Morton 2018).
The moment a nerve signal arrives, the sarcoplasmic reticulum releases its stored calcium into the cell; calcium binds to troponin, and only then is myosin's binding site exposed. Without calcium, no cross-bridges form between the filaments and the muscle cannot contract.
The pump that returns calcium to the sarcoplasmic reticulum uses up one magnesium-bound ATP on every trip; without magnesium, ATP cannot be used and calcium cannot get back to storage. But a sound mechanism does not mean magnesium treats cramps: in the trials pooled by Cochrane, magnesium did essentially nothing for the most common cramps, those with no clear cause.
regulates · 7
Motor neurons send their commands across the neuromuscular junction, and only then does the muscle contract. A muscle that loses its nerve supply wastes visibly within weeks. Magnesium affects how excitable nerves and muscles are, and very low blood magnesium can cause muscle twitching and spasms.
Sarcopenia means muscle mass, strength and physical performance declining together. Part of it can be trained back: in Fiatarone 1994, very old, frail nursing-home residents who did progressive strength training gained a great deal of strength while those who did not train barely changed, and their walking speed and stair climbing improved; the difference in thigh-muscle cross-sectional area was not statistically significant, and a nutritional supplement without training did not improve the main outcomes. Frailty is not destiny.
Muscle also releases hormone-like signaling molecules (myokines); IL-6 released during exercise, for example, prompts the body to make anti-inflammatory molecules. This is thought to be one reason exercise benefits the whole body, but the effects of most myokines come from animal and mechanism studies, and the role of one of them, irisin, in people is still unsettled.
In the first weeks of strength training, most of the rise in strength comes from the nervous system learning to use the muscle you already have more fully (recruiting more motor units, firing them faster and coordinating them better), not from the muscle getting bigger. In the small Moritani 1979 study, muscle growth only gradually took over after several weeks. That is why strength and muscle size can move out of step.
The soreness that arrives a day or two after training (delayed-onset muscle soreness) starts with small injuries to muscle fibers from eccentric contractions, and the inflammation that follows makes pain nerves more sensitive; it usually peaks at 24–72 hours. It is not lactic acid building up: lactate is cleared within an hour of training. Repeating the same movement makes the soreness milder each time (the repeated bout effect), and being very sore does not mean the session was a good one.
Glycogen supercompensation happens in the muscle that just did the work, not through a whole-body hormone filling every muscle at once. When one leg is exercised until its glycogen runs out and carbohydrate is eaten afterward, only the exercised leg overshoots its old glycogen level; the rested leg stays where it was (Bergström 1966).
Training volume is the main driver of muscle growth: the Schoenfeld 2017 dose-response meta-analysis found that, across the studies it pooled, the more weekly sets, the more muscle grew, and it did not locate a plateau. Krieger 2010 likewise found that multiple sets build more muscle than single sets. How many days a week you train mostly decides how those sets are spread out.