Plyometrics

With the increasing popularity of Crossfit gyms, boot camps, and home training DVD's, ever more people are beginning to discover Plyometrics. This is great news since plyometric training is a terrific way to train explosive power in a functional manner. However, the demands placed on the body by plyometric training means it is not for everyone, and inadequate pre-conditioning or inexperience can result in serious injury.

Mechanics

During a plyometric movement the muscle is rapidly stretched and then contracted. This is referred to as the Stretch-Shortening Cycle (SSC). The rapid and deep stretch that precedes the muscle contraction produces three reactions within the muscle that increases the muscles ability to produce greater power.

1. The cells in the agonist muscles and tendons are rapidly stretched, and this means that potential elastic energy is created.
2. A quick stretch in a muscle reflexively activates the muscle spindles. These intramuscular organs increase the activity of the muscle cells, generating a greater potential for contractile force.
3. The stretching phase also generates neurological stimuli that begin communicating with the central nervous system. This opens the neurological path way, priming it for the impending contractile stimulus.

Rapidly transitioning from the stretch phase to the contraction phase takes advantage of these reactions during contraction, and great power can be produced. Pausing at the end of the stretch will result in the loss of the plyometric qualities of the movement. The quick squat that precedes a vertical jump produces plyometric reactions.

Safety

The rapidity of the SSC means that the Golgi tendon organs (Organs in the tendons that communicate to the brain when the load on the muscle is excessively dangerous.) will not have the time to send warnings to the central nervous system. This in conjunction with the incredible loads that can be generated by certain plyometric movements, can result in debilitating injuries. While extensive research has not been done to determine exactly what the extent of risk is, the NSCA has put together base safety guidelines.

NSCA Guidelines

• Trainee must be at least 13 years old
• Trainee must have at least three months of resistance training experience
• Trainee must not be recovering from an injury to involved body part
• Trainee must have sufficient strength, speed, and balance capacity

Strength

For lower body plyometrics: Trainees should have a 1RM squat of at least 1.5 times body weight.

For upper body plyometrics: Trainees weighing 220 lbs or more should have a 1RM bench press equaling 1.0 times bodyweight. Trainees weighing less than 220 pounds should have a 1RM bench press equaling 1.5 times body weight.

Speed

For lower body plyometrics: Trainees should be able to squat 60% of their body weight five times in five seconds or less.

For upper body plyometrics: Trainees should be able to bench press 60% of their body weight five times in five seconds or less.

Conclusion

Form is as always crucial, and every trainee who is performing plyometrics should at least learn proper form and appropriate programming under the supervision of an experienced trainer. With proper incorporation into a training program, and careful execution plyometrics can be incredibly beneficial on many fronts.

Good introductory plyometric exercises include jumping rope, skipping, and hopping in place. Not hardcore enough? Think of who you always see doing these. Heavyweight boxers and little girls - proving that they are serious work as well as safe and natural!

References

Earle, Roger W. and Bechle, Thomas R. 2003. NSCA's Essentials of Personal Training.
Champaign, IL : Human Kinetics. (pg. 426 - 439).
Earle, Roger W. and Bechle, Thomas R. 2003. Essentials of Strength Training and Conditioning.
Champaign, IL : Human Kinetics. (pg. 414 - 426).