When there is a rapid stretch followed by a rapid contraction, muscles are able to exert more force than if this were not the case. This is thought to be due to the stretch reflex. Put a little too simplistically, muscles don’t “want” to be stretched out, they “want” to remain short. So when they are stretched out past a certain point, there is a protective mechanism that begins to shorten them. Athletic movements that seek to harness the stretch reflex seek to take advantage of this; a fast stretch, the protective mechanism kicking in, combined with a very fast contraction can equal a really powerful movement.
You can see this on a vertical jump: Move into a quarter squat. Hold the bottom position for ten slow seconds. Without a counter-movement, try to jump as high as possible. Now perform the entire movement as quickly as possible without the pause. Chances are excellent that you jumped higher in the second condition.
This is one of those phenomenon that is observable in sports, but is difficult to pin down what is going on scientifically. Over the decades there have been theories and counter-theories. One of my questions has always been: how can muscles produce more force than they are normally capable of without being injured?
The latest issue of Exercise and Sport Science Reviews has a fascinating little paper on an aspect of this. In their paper, Roberts and Konow discuss the role of tendons in rapid tasks like sprinting, agility, and jumping. Their view is that tendons have a protective role during these tasks in the sense that the tendon acts as a mechanical buffer during rapid deceleration. The authors essentially argue that tendons may temporarily absorb energy from the muscles and lengthen, which may ultimately protect muscles during rapid eccentric movements.
It needs to be pointed out up front that this is a limited review article. The authors review their research and discuss the research of other individuals, but this is neither an experimental paper (we did x and found y and it means z) nor is it a paper that reviews several hundred studies. The authors discuss research they have done with turkeys as well as observational studies on humans. This kind of topic is one of my few exceptions about disregarding animal research. The fact is that very few people will allow for a study to be done and their tendons and muscles to be cut out to be studied, so one needs animal subjects for something like this.
Basically what the authors are finding is that during these fast eccentric movements (for example, landing after stepping off the box during a depth jump), there are several phases. In the first phase the tendon is lengthening while the muscle fibers are shortening. This allows for the tendon to absorb the force that is being developed in this phase. The lengthening of the tendon is such that, even though the muscle fibers are shortening, there is a net lengthening of the muscle tendon unit. The tendon stretches faster than it recoils. So while the muscles are lengthening (second phase), the tendon is shortening at a rate slower than it lengthened. This is allowing the force to be transferred to the muscles at a slower rate, protecting the muscle fibers. In the figures that are drawn from their research, the authors are reporting that the first phase (tendon lengthening & muscle fiber shortening) is occurring in around .05 seconds, with the second phase occurring over the following .05 seconds (i.e. the whole thing takes about .1 seconds).
This information is interesting because it suggests that there may be a protective mechanism that limits force output during these fast stretch movements. The authors suggest that this may be fine-tuned as a result of training, i.e. like other protective mechanisms it may be set very conservatively. To support this they cite the rapid adaptations that are seen as a result of eccentric training.
References:
Roberts, T.J. and Konow, N. (2013). How tendons buffer energy dissipation by muscle. Exercise and Sport Sciences Reviews, 41(4): 186-193.