Early strength gains come mostly from the brain, not bigger muscles
When someone starts lifting weights, they often get stronger quickly before their muscles visibly grow. Most of that early progress is the nervous system learning to recruit and coordinate muscle fibres more efficiently. Actual growth, called hypertrophy, comes later as muscle cells add contractile units and fluid.
Hypertrophy means skeletal muscle getting larger because its cells grow. Scientists describe two flavours: sarcoplasmic growth, tied to storing more glycogen, and myofibrillar growth, where the contractile fibres themselves thicken. The exact trigger is still debated, but the leading theory is mechanical tension, which switches on signalling pathways such as mTOR that ramp up the building of new muscle protein.
Not all swelling lasts. The pump felt during a workout is transient hypertrophy, caused by extra blood rushing to working muscles, and one explanation is that small molecules such as lactate draw water into muscle cells. Lasting growth needs structural change. Long, low-intensity aerobic exercise generally does little for size; endurance athletes instead store more fuel in their muscles and grow new blood vessels.
Biology sets much of the stage. A twin study estimated that about 53% of the variation in lean body mass is inherited, along with about 45% of the variation in muscle fibre make-up. On average men carry about 60% more muscle than women, partly because testosterone is a major growth hormone, and natural growth usually levels off at the end of the teenage years. Researchers have also found that partially restricting blood flow with cuffs during light exercise can produce growth comparable to heavy lifting, which interests people recovering from injury and older adults.
The finer points remain contested. Slowing a repetition to keep muscles under tension longer boosted protein building in one 2012 study, yet another found normal-speed training grew fibres more, and reviews suggest total workload matters more than tempo. Working through a full range of motion, especially with the muscle stretched, appears to help, and there is growing interest in eccentric contractions, where a muscle lengthens under load, as when lowering into a squat.
Source: Muscle hypertrophy
