Plyometric exercises are a specialized form of training that takes advantage of the stretch reflex and neuromuscular system to produce powerful muscle contractions. The stretch-shortening cycle is a natural response to rapid muscle lengthening prior to a muscle contraction that results in a more powerful and faster response by the nervous system. Stretch-shortening cycle training, commonly called plyometric training, is associated with jumping exercises, but includes other types of activities, such as sprinting, throwing and resistance training routines, that take advantage of the stretch reflex mechanism.
Plyometrics is one of the most important training elements for all types of power development that will improve the reactivity of the neuromuscular system. In fact, plyometric training is considered a type of reactive strength, also known as elastic strength. The characteristics of plyometric exercises are “reflex-induced” and take advantage of the stored energy in the muscles and tendons (Ritzdorf, 2003).
Knudson (2012) describes why additional force from the stretch-shortening cycle is beneficial in the contraction phase: “Movements that require a muscle group to shorten considerably will not be able to create maximal forces” (p. 85). This is because the stretch-shortening cycle generates additional force during the concentric phase of the muscle contraction.
According to Bompa (2000), “Plyometrics develop the nervous system so that it will react with maximal speed to the lengthening of muscle; in turn, it will develop the ability to shorten (contract) rapidly with maximal force” (p. 26).
Plyometric Training Considerations
Plyometric training progressions must be well planned with limitations for specific athletes. Ground contact time is the key variable: the quicker the reaction, the less ground contact time, the more effective the training exercise.
Body weight and strength to body weight ratio are two factors that need to be considered when designing plyometric training. Athletes with body weight over 100 kilos should not depth jump from heights over 60 centimeters and only perform low-intensity single-leg movements.
Athletes who cannot back squat over 1.5 times their body weight should not perform high-intensity plyometrics or single-leg exercises at medium intensities. All athletes need to improve jumping abilities at lower intensities before progressing to more difficult exercises.
Plyometric Training Design
Plyometric exercises are periodized similarly to other types of strength movements with variations in intensity, volume, and frequency. Training will progress from low intensity and medium volume to high intensity and low volume over several weeks. Plyometric training can begin with simple sprint drills and build up into more dynamic sprint drills, followed by jumping movements.
Plyometrics Training Concepts
- Training can develop reflexive activities to create more force
- Objective is to produce high forces in a short time
- Muscles must be preactivated for reflex stretch
- Minimize landing (eccentric) deceleration
- Maximize takeoff (concentric) acceleration
- Maximize force output and minimize time to create large force output
- The velocity of the stretch is more important than the amount of stretch.
- Low-impact sprint-related exercises to explosive sprint-related exercises
- In place jumps to moving jumps
- Double-leg jumps to single-leg jumps
- Jumps with a pause to continuous jumps
- No obstacles to obstacles (low to higher)
- Low amplitude jumps to high amplitude jumps
- Jumps without external resistance to jumps with external resistance.
Intensity Factors for Plyometrics
- Stretch-shortening cycle speed (initial rate of force production)
- Time-dependent (how fast)
- Performance measurement (how far)
- Eccentric load (drop height/obstacle height)
- Complexity of movement (one movement or multiple movements)
- External resistance.
Plyometric Training Tips
- Use minimal contact times between jumps
- Use obstacles that are safe to land on or jump over
- Use proper rest periods between sets
- Use lower intensity exercises if ground contact time is diminished.