Plyometrics - An Explanation

Plyometric exercise refers to activities that enable a muscle to achieve maximal force in the shortest possible time. The goal of plyometric exercises is to develop and/or increase power. To understand plyometrics better, one must grasp the three different phases.

Eccentric Phase:
In this phase, the muscle is being stretched and elastic energy is stored.

In the picture above, the calf muscle (Gastrocnemius) is being stretched as the ankle moves into a position of dorsiflexion. (Ankle dorsiflexion is an anatomical movement term reflecting the decrease in the angle between the foot and the shin bone.)

Amortization Phase:
This phase refers to the time between the eccentric phase and the subsequent concentric ('shortening') phase. During this very brief instant, nerves from the brain are sending signals to nerves that stimulate the muscles.

Concentric Phase: In this phase, the muscle is now being shortened. In addition to the muscle force being produced from the muscle itself and its contraction properties, stored energy from the eccentric phase is also released and contributes to the total force production.

In the picture above, the contraction of the calf muscle during the push-off phase (ankle plantarflexion) would reflect the concentric phase.

So, basically, the muscle functions much like that of a rubber band. If it is stretched, it can produce additional force because it possesses elastic properties.

How do coaches often mess up the application?

The key component to plyometric exercise is ground contact time. In other words, how fast can the gymnast get off the ground when they are performing these exercises?

What I see far too often in most gyms is that coaches prescribe too many of a plyometric exercise within a set. As you see in the amortization phase, much of what we're really training is the neuromuscular system. By doing too many repetitions in a set, fatigue begins to accumulate. However, depending on the conditioning level of the athlete, that fatigue may not be felt in the muscles. But, the nervous system itself gets very fatigued. As a result, the athletes slow down, ground contact times increase, and you've turned a power exercise into a conditioning circuit.

In typical strength training, power is developed by utilizing 1-5 repetitions per set. I would apply this same concept to plyometrics since they are power exercises. Total volume is based on the number of foot contacts. If you desire 20 foot contacts of a specific exercise, then I would perform 4 sets of 5 and allow sufficient rest time between sets for neural system recovery.

While not an official term that I have ever seen in textbooks, if your goal is to develop power endurance (the ability to maintain one's power over a particular duration), then I think that you have three options. First of all, you can increase the rep range, but I would not exceed 10 repetitions per set. Or secondly, you could perform your plyometric exercises in a more fatigued state. (I.e. end of practice, following a hard strength training/conditioning session, etc.) Third, you could decrease the rest periods between sets.

Developing power endurance may be useful for the athlete so that they have enough reserve power at the end of a floor routine to make the final tumbling pass.

Hopefully, this gives all of you a bit more understanding of my opinion of how to best incorporate plyometric training into your strength and conditioning programs.