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CSCS Stretch Shortening Cycle Explained

Jun 30, 2022

Edited by: Danielle Abel

The stretch shortening cycle occurs in fast movements such as jumping and sprinting. It involves 2 concepts - a pre-stretch of a muscle and then a counter-movement where the muscle is actually contracting. 

Phases of Muscle Contraction

What we know about the phases of muscle contraction: Example: Vertical Jump

  1. Eccentric: As we're going down, we are dipping down and the quads and glutes are lengthening...we're going through an eccentric muscle contraction. We're controlling the motion of lengthening.
  2. Concentric: Then on the way up, we're going through a concentric muscle action or shortening of the muscle as we jump up.

Amortization:

Between the eccentric and concentric phases, there is another phase: the amortization phase. This phase is really important for strength, power, and speed and is where the stretch shortening cycle starts to give us recoil. During the amortization phase, there is stored energy from the tendon lengthening and stretching. That stored energy is dependent on the length of the amortization phase and will recoil back during the concentric phase. 

 

During the amortization phase, the stretch shortening cycle works in 2 ways:

  1.  Neurophysiological Component:
    1. As the muscle lengthens, it's going to stretch the muscle spindle fibers (You can check out my video on muscle spindles if you want to learn more about this), but they're intrafusal muscle fibers that sense muscle stretch. 
    2. Those fibers send a signal back to the agonist muscle to cause a contraction.
    3. In the case of a vertical jump:
      1. We are stretching the muscle spindle fibers within the quadricep as we are eccentrically activating.
      2. Then, that muscle spindle response is going to go through the spinal cord and back to the quadricep muscles to cause them to contract even more rapidly in the concentric phase.
  2. Mechanical Component:
    1. During quick movements like jumping and sprinting, we are stretching our tendon and connective tissue. It will recoil and give us energy during the concentric portion of the motion.
    2. This is mechanical because it is not really going through the nervous system. It is just the tendon itself stretching and shortening

Training Application

How does this actually apply to training? When we're going through a vertical jump, we have a couple of options:

  1. Static vertical jump: Where you squat and hold it for 3 seconds and then jump straight up.
    1. If you test an athlete they might be able to jump 12 inches for a static vertical jump. That 12 inches really corresponds to just the active muscle contribution to jumping. It's not involving a stretch shortening cycle because they held that 3-second isometric at the bottom before jumping.
  2. Counter-movement vertical jump: Where you throw your arms down as you squat and then throw then up as you jump.
    1. This is a more common way to test vertical jump because this is truly how sport movements work. With the counter-movement, not only are we taking advantage of the active component of the muscle shortening, but we are also adding on both the neurophysiological component of that muscle spindle activation which is going to aid in that jump height, AND the mechanical tension aspect.

Muscle, Tension, & Spindles

All three things combined:

Active muscle, the mechanical tension from tendon, and the muscle spindle activation will allow that athlete to maybe jump 20 inches instead of 12 inches. Depending on the type of movement that you're doing, you can actually double the force from the muscle just from the stored tendon energy and the muscle spindle activation.

This is a really significant contribution to speed and power movements. How do we actually most effectively train the stretch shortening cycle?

  1. We know the faster the counter-movement, the more contribution we get from the muscle spindles and the mechanical aspects of the stretch shortening cycle.
  2. Therefore, our training for the stretch shortening cycle really needs to involve fast movements.

A common way to do this is with a depth jump (stepping off of a box, falling a certain distance, and then landing with short ground contact time and immediately jumping).

Muscle Actions

Let's think about the muscle actions involved here...

As we're stepping off and falling we're getting some momentum going down to the ground, we're going to hit the ground and then quickly have to transfer momentum from going down to going up. This can be a big change in momentum which will involve a large eccentric stretch on the muscle and then a quick rapid concentric contraction to jump after that. 

If this is programmed effectively, this can be a really effective way to train the stretch shortening cycle.

The key to programming depth jumps is that you need a short enough ground contact to time to still get a good effect from the muscle spindles and from the mechanical aspects of the tendon.

For example: An athlete jumps from a 50-inch box.

  1. The eccentric phase will be so long and so forceful that the ground contact time will be extended.
  2. If the ground contact time extends too long, the muscle spindles will not act and effectively transition from stretch to shortening.
 

Calculating Depth Jump Height

If you're trying to calculate depth jump height, for most athletes, 30-32 inches seems to be about optimal. If you go much higher than that, it will be too much force for the athlete to absorb in a short amount of time.

Much lower than that,  and there will not be enough stretch to get an effective shortening response.

Another reason that increased ground contact time is less effective is that the golgi tendon organ begins to activate. If we think back to our rate of force development curve, we're hitting max muscle activation and putting peak tension on the tendon. Activation of the golgi tendon organ is counterproductive for activating the stretch shortening cycle and would cause decreased activation of the agonist muscle.

Most of our training for the stretch shortening cycle will occur at the early portion of the rate of force development curve (.1 to 5 second range). This area is truly the peak rate of force development, not maximal force.

Let's say you apply all these principles and you effectively train your athletes to improve their stretch shortening cycle.

Tendon Strengthening

You might be thinking, what else is going to improve in their muscles? A common misconception is that the athletes will get more elasticity in their tendons or their tendons will become more bouncy. 

In reality, the response from the musculotendinous unit is actually increased stiffness. That increased stiffness makes you absorb force quicker which would allow you to express force quicker when we think about that stretch shortening cycle. This will result in improved reactive strength index (RSI).

RSI= jump height / ground contact time

If we are training our athletes with depth jumps, vertical jumps and plyometrics, they're effectively improving their muscle stiffness and their muscle spindle response. These athletes will then improve their RSI and complete higher jumps with shorter ground contact times.

Additionally, you can see benefits in the stretch tolerance profile of the athlete. They will be able to handle greater forceful eccentric motions.

If at the start of the season they were only doing depth jumps from 24 inches and they improve their RSI, they may actually improve to 30 inch depth jumps, allowing them to do greater forceful movements in their sport.

Overall Application

How can you use your new knowledge of the physiology of the stretch shortening cycle to immediately improve your athletes vertical jump heights or your own vertical jump height?

  1. Example: An athlete is doing a vertical jump by throwing their arms down and then jumping up, however, the movement of their arm swing down is actually fairly slow. They haven't been coached on how to perform the vertical jump, but if you were their strength coach, you could cue them to throw their arms down faster and then quickly transition into that upward motion.

This will make the eccentric component faster which would allow for greater activation of the stretch shortening cycle in the amortization phase...

 = jumping 1 or 2 sticks higher!

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