Plyometric training also works by decreasing your ground contact times when you run or jump. The ground contact time is also known as the amortization phase. The amortization phase refers to the time between your foot contacting the ground and being able to leave again (i.e. the time in between the muscle stretching and being able to contract again). The amortization phase is in essence the time spent paying off the time spent on the ground. Analysis of great jumpers and sprinters shows that they spend very little time on the ground during their activities. Thus, by decreasing this time by specific training you should be able to improve jumping and sprinting skills.
Plyometrics can be broken down into several categories. All the various categories are important and have their place in a program. These categories include:
1. Light reactive exercises- such as jump rope, toe and ankle bounces, side- to-side line jumps, and low stair jumps. These exercises are characterized by relaxation and lack of “strain”.
2. Moderate standing “power” or “up” variety exercises- these exercises are characterized by less reactivity or stretch and rather a focus on applying max
force from more of a standstill either vertically or horizontally. These, along with lighter load/high speed weight training are great for improving the rate of force development. They include for example: on-box jumps (jumping on a box from the ground), standing broad jumps, 1-leg step-up jumps and un-weighted squat jumps. They develop the ability to apply max force instantaneously without a large involvement of the reflexive stretch shortening cycle.
3. Moderate reactive exercises- these are true plyometric exercises, but of the less intense variety. They include low-box depth jumps (jumping off a box impacting the ground, absorbing the energy, and immediately transferring that energy into
an explosive jump, lateral cone jumps, jumps over hurdles and a variety of other exercises.
4. High intensity reactive exercises- generally these are the original plyometric exercises depth jumps and drop jumps. They can also include other intense methods such as high barrier jumps and other various hybrids.
5. Short response reactive exercises- these exercises are characterized by a very short amortization (ground contact) phase. Most of them tend to be 1-legged variety jumps. Examples include: power skipping, one-leg bounding, single leg speed hops, 1-legged speed box jump, and 1-legged jumps for height or distance.
Furthermore, plyometric exercises can be broken down into 3 simple methods of performance. These include:
1. Jumps- land with both feet
2. Hops- done with single leg
3. Bounds- take off on one foot land on the other
The good thing about plyometric training is that all the individual components of the reactive stretch-shortening cycle are trainable and respond to training.
Over time, plyometric training results in the following:
1. Recruitment of most motor units and their corresponding muscle fibers.
2. Development of explosive power.
3. Development of the nervous system so it will react with maximum speed to the stretching of the muscle developing the ability to contract rapidly with maximum force, thus leading to better plyometric strength.
4. An increase in rate of force development
5. A shift in muscular fiber type from type I (slow contracting), into type IIA (fast contracting), and type IIA into type IIB (fastest contracting).(Paddon-Jones 2001)
6. Increase in plyometric or reactive strength.
7. An increase in the proficiency of the central nervous system (CNS)
8. An increase in the ability to transform eccentric force absorption into concentric force output.
It is important to note that plyometrics, especially more intense methods such as depth jumps and shock jumps, can fatigue the nervous system to a large degree and are best used within an intelligent plan. The nervous system actually takes about 5 times longer to recover then the muscular system. Plyometrics increase muscle recruitment but the nervous system still has to fire to recruit those muscles and it is like a battery that needs recharging.
The fatigue of the nervous system can be illustrated like this. Imagine going for 3 whole days without any sleep. Even if you were completely inactive during those 3 days and hadn’t used your muscles at all, you’d still likely be very weak and tired. Remember that your CNS is like your computer or central controller and needs recharging once it’s worn down. Plyometrics may not create much muscle burn or soreness, but because of the way they involve the nervous system, intense plyometric drills can actually take as long or longer to recover from then any other training method.
The problem with many athletes and coaches is that they don’t feel that intense plyometric drills are all that difficult. After all, getting up on a box and jumping down 20 times may not be considered as difficult to some people as going out and running a mile! Because of this, many people prescribe and/or do way too much volume in depth and/or shock jumps and young athletes without much of a training base to begin with do way too much volume of them and wonder why they don’t see much improvement. So make sure you pay attention to the total amount of intense plyometric training you do.