When tendons elongate to a greater extent during a jumping movement that is preceded by a countermovement, the muscle lengthens less. This produces two effects. Firstly, the greater elongation of the tendon means that more elastic energy is stored during the countermovement, which is then released in the subsequent jumping phase. Secondly, the smaller elongation of the muscle means that countermovement depth can be greater for the same shortening velocity in the subsequent jumping phase, because the muscle never lengthened that much to begin with. Since shortening velocity determines force, this allows the same muscle force to be produced, despite the larger joint range of motion.
Starting out, athletes should always err on the conservative side and only perform 10-20 maximal effort jumps in a training session. Because of the explosive nature of a vertical jump, the body can only perform a handful before performance starts to drop. Training beyond this point will not improve jumping height and will only lead to injury. At the completion of a training session, it is generally recommended to rest 48 hours before completing another intense training session.
Three weeks after I received that counsel, on a rare afternoon when I felt fully rested, I dunked a volleyball on a 9' 11" rim. Again, I knew I could never swing my arms while palming a basketball the way I’d swung them while palming that volleyball, but I’d be lying if I said it didn’t feel badass. Thirteen failed attempts later, I did it again. Then two more times, each one an unexpected thunderclap. All of the explosive Olympic lifting I’d been doing was paying off, but my problem wasn’t going anywhere: How could I get my hand and a basketball over the cylinder? A lob to myself off the backboard? A big bounce off the blacktop?

Vertical jump measurements are used primarily in athletic circles to measure performance. The most common sports in which one's vertical jump is measured are track and field, netball, basketball, football, and volleyball, but many sports measure their players' vertical jumping ability during physical examinations. In addition, single and multiple vertical jumps are occasionally used to assess muscular strength and anaerobic power in athletes.[3]
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