Updated: Jun 6, 2020
"Every dynamic movement beings with an eccentric muscle action." - Cal Dietz.
Every dynamic movement consists of three parts as it relates to the muscles involved in the movement. We have eccentric(lengthening), isometric(static), and concentric(shortening). Training these three parts individually can be incredibly beneficial for sports performance and is often referred to as Triphasic training. Today I'll discuss the first part, eccentric training.
An eccentric muscle action can be referred to as the lengthening phase as the muscle is stretched due to tension produced. In other words, the attachment(distal and proximal) of the muscles moves in the opposite direction of each other. An example would be the lowering phase of a squat.
When training the eccentric phase of a movement, we're looking at two physiological components in the body that contributes to force production. The strongest reflex in the body - the stretch reflex and in a close second, we have the stretch-shortening cycle.
The entire force production of the stretch reflex is composed of the sum of two proprioceptive nerve signals :
Muscle spindles, which serve as a neuromuscular stimulator
Golgi Tendon Organs(GTO), which serve as a neuromuscular inhibitors
Muscle spindles are intramuscular sensory receptor which runs parallel to the muscle fibers. The job of the muscle spindles is to send information to the brain into the central nervous system(CNS), regarding how much force is operating on the muscle. The spindles will relay information to the brain(afferent neural pathway) on how hard to contract. The greater the signal of the spindles, the greater the resulting signal of contraction from the brain(efferent neural pathway). The muscle itself does not know if you're trying to lift a weight or cut on the field or turn in the water. Its only interest is to obstruct the eccentric lengthening of the muscle. With that said, the fundamental purpose of the muscle spindle is to tell the brain how hard it must contract to overcome a load.
Oppose from the muscle spindles that tell the brain how hard to contract, the Golgi tendon organ(GTO) tells the brain when to relax. GTO are found within the tendons. Dissimilar to the muscle spindles, which act on changes in length, the GTO measures changes in the force being placed on the muscle. Inhibitory mechanisms are necessary to prevent the muscle tissue from exerting more force than the connective tissue(tendons) can tolerate. So if to much force is put on the muscle during a eccentric movement, the GTO will signal an inhibitory muscle reflex to prevent damage to the muscle structure.
An example would be you dropping the weight during a lift because the force is simply too great for your body to handle. However, we know that his mechanism can be overridden and allow a person or athlete to produce levels of force that are much higher than their "normal" max. Superhuman strength has been seen more than once when, for instance, a mother lifted a car because their kid was trapped under it. However, significant damage was seen on the muscle structure when the autogenic inhibitory mechanism(GTO) was deactivated. The GTO is perceived as something good; it's looking out for you. But it's more like an overprotective mother. Most GTO acts as a kill switch and is set 40 percent below that the muscle structures before severe damage occur. Ultimately the GTO can be trained to produce more force, and an untrained GTO is destructive for explosive power because it limits the athlete's potential to absorb high levels of force.
When training the eccentric phase of dynamic movement, you are training these two proprioceptive structures of the muscles. To maximize an athlete's force production, an athlete must use methods in training that will decrease the inhibitory effect of the GTO and, at the same maximizing the nerve signal response of the muscle spindle. If an athlete isn't used to handling high levels of eccentric force, the resulting concentric muscle action will be weakened(i.e., force production).
The athlete who can eccentrically absorb more energy(kinetic energy) will able to produce force concentrically and therefore take advantage of the second physiological muscle action - the stretch-shortening cycle(SSC).
Think about stretching a rubber band. "Energy" is created when the band is stretch so the band can return to its normal length. The SSC within the muscle and tendon as it relates to the eccentric phase works in the same way. Kinetic energy is created and stored when a muscle and its attaching tendon are stretched within these two components. This energy can later be used during the concentric phase. This effect is also known as elasticity, which means that a structure is able to return to its normal length after being stretch(lengthen).
It is essential to understand that the energy that can be absorbed by the SSC is dependant on the combined effect of the stretch reflex and the GTO. You can think of it this way: Stretch reflex - GTO reflex = Force production.
The sum difference between the two reflexes is the remaining energy that is absorbed by the SSC and is applied during the concentric phase. In the end, it comes down to physics and the law of conservation of energy. The amount of force an athlete can load eccentrically into a muscle/tendon is directly proportional to the amount of force he can apply.
How to apply it
Eccentric specific work is extremely taxing on the nervous system not only for the physiological components just discussed but also the stress that it puts on each motor unit. Therefore, the eccentric work should be programmed at the beginning of the workout when the athlete's nervous system is fresh and at the point in the athletes season where the focus is on "base" work and physiological adaptions. For athletes with less experience with strength, training eccentrically should be limited to large compound exercises, such as a back squat. In contrast, more experienced athletes can add more eccentric training throughout the workout. Here are some general parameters for eccentric loading:
More reps would indicate a shorter time during the eccentric phase and vice versa.
If an athlete can produce more force, he can apply more force, simple as that. However, it is vital to notice, train if you train slowly you'll become slow. Therefore, always maximize the speed during the concentric phase and eccentric training has to be later combined with more isometric and concentric focused training as well as high-velocity training, since sport is about producing more force in less time. Strength is not what sport is about. Strength has a role in sport but id ineffective if it can't be applied at high velocities.
Example video of a 3 seconds eccentric back squat.