Reactivity

Reactiveness is the amount of displacement an athlete can achieve utilizing momentum with a ground contact of less than 250ms or with a reactive ground contact. Reactive ground contacts and plyometrics are synonymous terms, that describe the ground contact interaction. Reactiveness is a special type of impulse. The load type of reactiveness is level 5 - high force and high velocity. 

Things you'll Learn with this Sport Science Lesson: 

• Comparing reactive ground contacts versus a typical impulse
• The High Impulse Position Vs. Low Impulse Position and the Importance of Pre-Activation (experiment)
• Elasticity and The Stretch Shortening Cycle – The Law of Tension
• The Cues of Reactiveness
• 7 Common problems of an athlete learning reactiveness
• The Determinants of Reactiveness

Comparing Reactive Ground Contacts Vs. A Typical Impulse

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Remember a reactive ground contact is a special impulse with a ground contact of less than 250ms. The time constraint creates the following differences between a reactive ground contact and a typical impulse:

• Total ground contract time
• The amount of momentum that must be controlled we call this the Reactiveness Level
• The position of the leg – depth 2 or higher. Bending deeper is not possible with this time constraint.
• Fast Speed of Landing.
• Pre-activation.

After comparing reactive ground contacts with a typical impulse, why would an athlete choose one strategy over another?

Time is one correct response. Reactive ground contact are faster than a typical impulse.

But there is another reason to choose the reactive ground contact.A factor that we didn’t discuss is the amount of displacement? Or how far the athlete will travel after a ground contact. Interestingly, better athletes have a greater displacement in a reactive ground contact versus a typical impulse.

WOW! How is this possible?It’s because better athletes utilize momentum by storing and release this energy (elasticity) to create a better impulse despite the shorter ground contact time.

Interestingly, utilizing momentum is much more energy efficient than producing an impulse with equivalent force. 

“How you utilize force may be more important than how you produce force”

Elasticity and The Stretch Shortening Cycle - The Law of Tension 

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Elasticity is a tissues ability to store and release energy. We learned about this quality in the Law of Tension.  Elasticity is based on the elastic properties of a tissue. In Figure 6, we explore the elastic properties of various tissues – bone, muscle, and tendon.

As you can see both muscle and tendon are able to store and release energy. But the tendon does this much better than muscle.

The Stretch Shortening Cycle The overall period from the storage of momentum which begins with initial ground contact (primarily in the tendon) to the completion of the impulse is called the stretch-shortening cycle. The key phases of a stretch-shortening cycle or reactive ground contact are: Activate muscles so the tendon is Maintain muscular activity while the energy is stored in elastic tissues (such as the tendon, and ….) Release and Impulse. The energy that is store is released while the athlete creates an impulse. The coordination of energy release plus muscular contraction is extremely powerful. Complete Impulse. The athlete finishes their impulse. To understand this concept further, let’s look at what is happening when we jump on a trampoline and compare this to what’s going on in the muscle and tendon during a reactive ground contact during – see Below

With respect to the stretch-shortening cycle appreciate the following points.

How high the athlete jumps is a function of:

• Motor Coordination. The coordination and timing of the trampoline spring back and the impulse
• Impulse. The magnitude of the impulse
• Elasticity. The properties of the spring.
• Momentum. How high is the athlete – the higher the more momentum
  
  

During the impulse phase the athlete can’t bend their knees much – beyond depth 2 because there isn’t enough time. Developing force and power in this Power Position is very important

• Stacked with Cocked Ankle
• Ball of the foot contact in the Power Position
• Pre-activation.
• Crisp! Reactive Ground Contact. We only have 250ms. Think fast, like lightening.
• Explosive Law of Flow. Make sure you complete the impulse. Full hip extension.
• Direction of Force Application.
• No Deformation. (Law of Lines)

The Cues of Reactivity

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• Too Slow- takes longer than 250ms.
• Too Much Joint Range- during force absorption
• Poor Impulse. Insufficient impulse
• Line Violation. Especially dynamic valgus, thoracic collapse and foot collapse
• No pre-activation.
• Thud Noise vs crisp reactive ground contact
• Poor Foot Contact- not cocked, landing on toes and too much ankle range of motion
  

Common Problems 

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Develop Pre-Activation

Develop Motor Control of Pre-activation. Appropriately challenge the system.

Long Term Athletic Development. 

Learn the reactive stacked position (Plyometrics, Olympic Lifting)

Reactive Impulse Development [HIP]

via Full Spectrum Training. How quickly can you apply force and the direction of force application in the stacked reactive position.

Develop Elasticity 

Develop the Foot Power Position

Injury Management and Load Management

Determinants of Reactiviteness

Force Absorption

- eccentric phase

Force Utilization

-amortization phase

Force Production

-concentric phase

Stretch Shortening Cycle

Spring like mechanism

1. Force Absorption -think compressing a spring

2. Force Utilization  - as short ground contact as possible

3. Force Production- think releasing the spring