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cofactor · 4
Strength comes back fast in the first weeks after a break, and the nervous system rather than the muscle does most of that work, for the same reason a beginner's strength rises first.
The strength gained in the first weeks of lifting comes mostly from the nerves, and so does fast force: the rate of force development depends mainly on how many muscle fibers are called on in the first few dozen milliseconds of a contraction and how fast motor units fire. In one small study, 12 weeks of fast-contraction training left the muscle's own contraction time unchanged; what changed was that motor units switched on earlier and fired faster.
Part of a one-repetition maximum is a skill the nervous system practices. When light and heavy loads are both taken to failure, muscle grows about the same, but the maximum rises more with heavy loads; untrained people who did nothing but single maximal attempts for 8 weeks raised their maximum as much as a high-volume group, though their muscles grew less.
A beginner's rapid strength gains come largely not from more muscle but from the nervous system learning to use existing muscle better: in a classic study, most of the strength gained in the first weeks of training came from neural adaptation. Crediting all of it to muscle makes the inevitable slowdown look like going backward.
regulates · 2
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.
Before puberty, children do get stronger from strength training, but the progress lands mainly in the nervous system: more motor units are recruited, they fire more densely and the opposing muscle learns to let go, while muscle size barely changes. After puberty, the same training converts more readily into thicker muscle. This is a simplification that helps understanding, not a switch anyone has measured.