If you want muscle that is quick as well as strong, what should you train? A small laboratory study offers one clue. Eight young men did eight weeks of jump training. Afterwards, researchers tested single fibers from their thigh muscles and found the fast-twitch fibers were up to 30% bigger and contracted up to 29% faster.1 It was a small study with no comparison group, so it is a clue, not a rule. Here is what the researchers did, what they found, and what the study cannot tell us.
What the Study Did
Eight healthy men took part. Their average age was 23, they were recreationally active, and none had done jump training before. For eight weeks they trained three times a week, 24 sessions in all, adding up to 5,228 jumps. No added weight, just jumping.
The jumps got harder as the weeks went on. They began with standing jumps and countermovement jumps, moved to drop jumps from a 40 cm box, and then to triple jumps and hurdle jumps, including on one leg. Every jump was at maximal effort, with a deeper knee bend on landing. The number of jumps rose gradually over the first four weeks to keep the risk of injury low, and every session was supervised. Sessions lasted about 20 minutes at the start and about 45 minutes at the end.
Before and after, the men did leg strength and jump tests, and the researchers took a small sample from the vastus lateralis, a large muscle at the front and side of the thigh.
What Changed in the Muscle Fibers
Muscle fibers come in slow-twitch and fast-twitch types. Fast-twitch fibers contract quickly and produce power, the kind of effort a jump or a sprint asks for. To show what “fast” means here, before training the fast-twitch fibers called type IIa contracted about three times faster than the slow-twitch fibers.
After eight weeks, testing fibers one at a time in the lab, the researchers found:
- Size. Fast-twitch fibers grew 22% (type IIa) and 30% (the mixed type IIa/IIx). Slow-twitch fibers grew too, by 23%.
- Speed. Top contraction speed rose 29% in type IIa fibers, 22% in the mixed IIa/IIx fibers and 18% in slow-twitch fibers.
- Power. Peak power per fiber rose 34% in type IIa and 49% in IIa/IIx fibers, and 25% in slow-twitch fibers.
- The whole body. The countermovement jump rose 13%, leg press strength rose 12%, and a short shuttle run got 4% faster.
What the Study Did Not Show
This is a small study, and it is worth being plain about its limits.
- It had eight young, already-active men and one group measured before and after. With no comparison group, it cannot separate the effect of jumping from everything else.
- The mix of fiber types did not change significantly. The share of type IIa fibers rose from 33% to 41%, but that gap was not statistically significant. The fibers the men already had got bigger and faster. The study did not show that jumping turns slow fibers into fast ones.
- The fibers were tested one at a time in a lab dish at 15 degrees Celsius, so a living, working muscle may behave differently.
- The authors say they have no evidence for why contraction speed went up.
- The results apply to these men. They tell us nothing about women, older adults or people with injuries.
How Jumping Differs From Lifting
The authors compared their results with earlier studies of heavy lifting. In those studies, lifting mainly increased fiber size and force, and contraction speed did not change. Here, speed rose in every fiber type. They suggest jump training may be an interesting addition or alternative to existing programs.
That is a comparison across different studies, so treat it as a hint. It does not mean jumping replaces lifting. It suggests the two may train different things.
What This Means If You Want to Try It
This part is our view, not the study's. The most useful lesson in the study is how it was done. Jumps were built up slowly over several weeks, simple jumps came before hard ones, sessions were supervised, and there were three a week. We would treat that as a ceiling on ambition, not a target. Start with small jumps, land softly, and add more over weeks, not days.
Jumping is high-impact. If you have an old injury, or you are returning to exercise after pregnancy or surgery, talk to your doctor or physio before you add jumps.
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- Malisoux L, Francaux M, Nielens H, Theisen D. Stretch-shortening cycle exercises: an effective training paradigm to enhance power output of human single muscle fibers. J Appl Physiol. 2006;100(3):771-779. doi:10.1152/japplphysiol.01027.2005. The training program, participant details and all figures in this post are from the full text, which is free to read. Eight men, one group, no comparison group. ↩
