The eccentric portion of a lift is what you call the lowering of a weight under control. Straight from Wikipedia here “the force generated is insufficient to overcome the external load on the muscle and the muscle fibres lengthen as they contract. An eccentric contraction is used as a means of decelerating a body part or object, or lowering a load gently rather than letting it drop.”
However what happens when you accelerate an eccentric and then quickly reverse it is, at the reversal point, your muscle fibres create an isometric contraction by locking up against that negative force and resistance. They lock up to resist mechanical deformation. If they didn’t then you would be unable to absorb the force and would either collapse or get injured. This “locking up” is largely involuntary because of the forced stretching. In this video Tim has to lock up after sticking the jump lest he crumple into a heap.
Now, because of the velocity component, the level of maximal tension during that isometric, although extremely brief, will also be extremely high.
It’s that extremely high yet brief level of tension in the muscles and tendons at the isometric “stop” that mainly does the positive things with accelerated eccentrics, drops and explosive reps, depth jumps, kneeling jumps etc. The intramuscular force will be extremely high. Therefore, the body adapts to develop extremely high levels of tension very quickly.
These changes are mediated through
The result is a fairly quick increase in strength, power, and reactive strength. I have used these methods to increase my athletes stand broad jump and peak power puts significantly.
Suggested by Siff and Verkhoshansky called Actively Accelerated Eccentrics. The athlete should perform this movement by deliberately pulling the bar downwards on the eccentric phase of the movement which will help to increase stretch reflex contribution and more fully prepare the body for sport movement. The athlete can pull down the weight but this is tricky to moderate, having a band or a partner doing the pulling seems to a lot safer and practical. Exercises that lend them selves well to this method are band resisted squats (sink fast and explode from the bottom), kettle-bell swings, partner assisted kettle-bell swings,.
A common question is “If eccentrics or plyometrics create greater tension levels then why not just do fast eccentrics all the time?”
Be smart with your employment of these methods or any explosive method. Because it is not a matter of saying which is better. It’s a matter of employing a method that is more appropriate at a given time.
A typical prescription for these types of exercises might be 50-60% of the duration and volume that you would use other traditional methods. So use them when you need a quick increase in explosive ability but not all the time. I usually use 4-6 sets of anywhere from 3-10 reps depending on the movement.
Using these moves in brief cycles we can get real improvements in strength, power and explosivity.
Nice collection of video’s here with more accelerated eccentric methods, featuring partner, passive and accelerated eccentrics ,http://www.strengthperformance.com/video/video/listForContributor?screenName=1o95dvqsotlkf