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cofactor · 2
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.
In a jump, the calf's muscle fibers first shorten and stretch the tendon, storing elastic energy; in the last 100 milliseconds or so before take-off the fibers hardly shorten any more, and the tendon recoils quickly to release that energy, with a higher peak power in that phase than the fibers produce themselves. Tendons get stiffer and stronger under load, but more slowly than muscle, which is why jump volume should rise slowly.
regulates · 2
Pain between the kneecap and the shin is often linked to repeated running and jumping, and is commonly called jumper's knee. In a cohort of young volleyball players, the more hours they trained, the higher their risk of jumper's knee, an observed association; stiff landings raise the impact, and teaching people to bend the hips and knees together and land on the front of the foot clearly lowers it.
Children can do jump training too. A systematic review of 7 studies found that children aged 5 to 14 clearly improved their running and jumping after jump training, and that it was safe when parents consented, the children agreed to take part and safety rules were built in; the studies, though, were all of low quality.
contrast · 2
Power and strength are often treated as the same thing, but they are not: power is force times speed, and maximal strength is only its ceiling. In a randomized trial, relatively weak men who did 10 weeks of heavy squats raised their jump power about as much as men who did loaded jumps, and gained much more maximal strength; a review likewise says that without a fair amount of strength it is hard to have a lot of power.
Strength training for older people is often taken to mean lifting slowly, yet power declines earlier and faster with age than strength. In a meta-analysis of 20 randomized trials, healthy adults over 60 who lifted the weight as fast as possible and lowered it slowly improved their everyday physical function slightly more than with ordinary strength training; the evidence is low-certainty; neither group reported serious adverse events, but 14 of the 20 trials did not report adverse events completely, so that carries limited weight.