Understanding the Plyometric Continuum with ForceDecks

Sasha Birge – Physical Therapist, ATC, CSCS at VALD

The Plyometric Continuum (PC) refers to a progressive training approach to improve the body’s ability to store and release elastic energy in the musculotendinous unit for sport-specific activities. The exercises in each phase focus on enhancing utilization of the stretch-shortening cycle (SSC).

This article discusses the application of ForceDecks in each stage of the plyometric continuum including:

• Key tests to use,
• Key metrics to examine,
• How to apply the findings.

Stretch Shortening Cycle

The SSC refers to a 3-stage event whereby a musculotendinous unit is quickly stretched (eccentric phase) until it reaches a certain length (amortization), and then rapidly recoils and shortens (concentric phase) to produce an action of the body.

For this process to be most effective, and maximal elastic energy to be stored, it must be done quickly. Because only the first and third phases actually involve movement, the focus of training will be on improving the concentric and eccentric components. Amortization represents a transition between concentric and eccentric movements, so won’t be given much attention as a training consideration on its own.

Plyometric Continuum

The term “plyometrics” is used to describe the application of the SSC. Simply, this is the rapid lengthening and shortening of the musculotendinous unit to produce quick movement of the body.

Like many other training adaptations, improving plyometric ability involves a progression from simple to more challenging, by gradually introducing greater demands. This periodization model is commonly known as the plyometric continuum and is broken down into 5 stages.

The stages of the plyometric continuum involve isolating the movement components of the SSC and then integrating them all together; these steps correspond to the 5 stages listed in the table.

1. Eccentric Absorption

The initial stage of the PC focuses on enhancing the eccentric component of movements. This is often referred to as the “force absorption” stage. This muscle lengthening effect can be achieved by rapidly moving the joints in the lower body from a position of extension to flexion. The “tall-to-short” or “drop squat” is often used as an assessment (and training exercise) of eccentric absorption. 

Although it is not a natively auto-analyzed test in ForceDecks, this movement can be detected and analyzed (and trained) using the Squat Assessment test type (see video below). 

When analyzing the results, we look at metrics related to eccentric movements. This pattern can be considered the precursor to landing – just like learning to fly, one must learn to land before taking off; that is what this test helps us understand. Here are some common metrics to consider: 

• Peak Force: Maximal force produced throughout the entire movement; as the athlete becomes more proficient with this action, the peak force should increase. 
• Maximum Negative Displacement: How far down the athlete moves during the action; this may increase as the athlete becomes more comfortable moving quickly into the flexion pattern. 
• Eccentric Peak Force: Because the emphasis of this action is on the eccentric phase, this value should be equal to peak force. It is helpful to capture the eccentric peak force because it can then be compared to the same metric during phase 3, or countermovement jump.
• Eccentric Peak Velocity: The Speed at which the athlete moves; this is represented as a negative number due to the direction of the movement. 

For an athlete returning from an injury, it is helpful to use a limb symmetry index to determine when they can progress to the next phase of the continuum. By assigning a target value, clear guidelines can be used as a criterion for progression.

2. Concentric Development

Concentric development is geared toward improving the muscle shortening component of the SSC and is also referred to as “force expression”. Directly after the eccentric movement, and the brief moment representing amortization, the muscles rapidly contract, and the joints return from a position of flexion to one of extension. This muscle shortening action can be assessed (and trained) using a Squat Jump on ForceDecks.

This assessment focuses on the “take-off” or concentric-only movement, hence we focus on concentric metrics. This extension pattern of the Squat Jump also happens to begin exactly where the drop squat assessment left off: at the “bottom” or end range position of the squat

Concentric development examines how efficiently an athlete leaves the ground. An understanding of asymmetry can also be applied to these metrics to determine progression to the next stage of the PC. Here are a few suggested metrics:

• Jump Height: peak vertical displacement after take-off. Look for jump height to increase as an athlete’s concentric power improves.
• RSI-modified: jump height divided by the time required to achieve take-off (contraction time). A higher RSI-modified result is better and can be achieved by either increasing jump height or decreasing contraction time. Either way, both would indicate an improvement in performance.
• Concentric Peak Velocity: maximal upward speed achieved before take-off. The athlete should be able to increase their speed as they become more powerful.
• Concentric RFD: how quickly force is developed during the take-off. This demonstrates an athlete’s ability to accelerate, and should improve with training targeted towards enhancing power production.

3. Jump Integration

The Jump Integration stage is a combination of the first two stages of the PC. This entails performing the eccentric component followed directly by the concentric component. This is the first stage in which the stretch-shortening cycle is introduced. It occurs during the transition from the lengthening to shortening of the muscles, as the joints change from a flexion to extension moment. Jump integration can be assessed by performing a Countermovement Jump (CMJ).

The CMJ puts the first 2 pieces of the puzzle together, combining eccentric absorption and concentric development:

• The lengthening of the muscles and pre-stretch of the tendons,
• The subsequent recoil of the elastic structures generating the take-off,
• The flight phase of the jump, and the landing.

The metrics relevant to the CMJ help us understand the big picture of how an athlete navigates a jumping action. In addition to being part of the continuum, the CMJ is a great tool for monitoring neuromuscular fatigue when performing this assessment on a regular basis. Here are a few suggested metrics:

• Jump Height: peak vertical displacement after take-off. Look for jump height to increase as an athlete’s concentric power improves. CMJ jump height will improve as an athlete becomes more “elastic” by enhancing their ability to utilize the SSC.
• RSI-modified: the height of the jump relative to the time from start of movement to take-off. The RSI-modified ratio can increase as the athlete improves their jumping ability, but also as they become more efficient at utilizing the SSC as this will decrease the contraction time.
• Eccentric Peak Velocity: the maximal downward speed achieved before take-off. This will improve as the athlete becomes more proficient at introducing higher speeds to the eccentric phases of lower extremity exercises.
• Concentric Peak Velocity: the maximal upward speed achieved before take-off. Modifying the tempo of lower extremity exercises to shorten the duration of the concentric effort helps improve velocity.

4. Continuous Jumps

Continuous jumps are the penultimate stage of the PC. It progresses Jump Integration by adding frequency, which is achieved by exhibiting bouts of the SSC in rapid succession. Continuous jumps can be performed and assessed by using the Hop Test.

By assessing continuous jumps in rapid succession, it is possible to observe the change in performance over time; this can help to understand the athlete’s endurance.

The metrics for the hop test can demonstrate behavior throughout the test, illustrating how the athlete’s ability changes in response to progressive volume. “Mean” (average) and “fatigue” (implied by decreases in values) are unique to the Hop Test as they convey information about all the repetitions; these values can be helpful in determining progression to the final component of the continuum – Shock Method. Here are metrics to be considered for analysis:

• Mean RSI: the average relationship between time spent on the ground versus time in the air. This will improve as an athlete focuses on endurance and repeated efforts of lower extremity ballistic training.
• Contact Time: time spent on the ground between jumps. Contract time will decrease as the athlete becomes more “elastic” or “springy”, but it is also important to provide appropriate cueing for this exercise to ensure emphasis on shorter ground contact time.
• Flight Time: time spent in the air. This will improve as the athlete becomes more capable of producing force in their take-off.
• Mean Active Stiffness: The average amount of force produced during contact periods in ratio to how far an athlete needs to drop down in preparation for the next take-off. Stiffness is an adaptation that occurs over time in the musculotendinous unit as a response to resistance training. The increase in stiffness will contribute to greater reactive strength.

5. Shock Method

Shock Method is the final stage of the PC and involves increasing the intensity of the SSC stimulus by increasing the eccentric load. This is achieved by initiating the movement with a landing from altitude. 

Landing from greater heights will increase the force production demand required to transition from the lengthening to shortening phase, making the plyometric action more challenging. Shock Method is commonly assessed using a Depth Jump, however on ForceDecks one can use the Drop Jump (DJ) for analysis. The “Shock” itself refers specifically to the overload during the eccentric phase only. For the purposes of the continuum, the Drop Jump is used because it exhibits an elastic collision, which is considered more “sport specific”, and incorporates all 3 phases of the SSC. Another test which could be used in this stage, which isolates the landing phase, is the single leg Land and Hold Test.

The DJ is a progression from the CMJ in the Jump Integration phase due to the increased intensity. This is achieved by enhancing the magnitude of the stretch shortening cycle via initiation of the movement with a drop landing. 

The demands of the DJ require greater eccentric force production to decelerate the body as it lands from a greater height, and higher stress on the SSC when quickly applying the brakes and transitioning into a take-off. 

The metrics for the DJ reflect the athlete’s ability to utilize their reactive strength by measuring the way the athlete quickly transitions from a land directly into a jump. Just like the CMJ, the DJ is a slightly more advanced way of monitoring athlete neuromuscular fatigue by assessing the athlete’s efficiency over time. When reviewing the DJ results, consider these metrics:

• RSI: the relationship between time spent on the ground versus time in the air. RSI in the DJ will improve along with factors mentioned above, such as: utilization of the SSC, eccentric and concentric force production, and stiffness.
• Contact Time: time spent on the ground between jumps. As with the Hop Test, this is largely predicated on cueing, and will improve with all the same training components listed above.
• Flight Time: time spent in the air. Flight time in the drop jump is largely predicated on stiffness and power production, as the athlete aims to minimize deformation in order to store elastic energy, and in the case of the DJ, release that energy as quickly as possible in order to jump higher.
• Active Stiffness: the amount of force produced during the contact period in ratio to how far an athlete needs to drop down in preparation for take-off. This improves with exposure to resistance training and ballistic plyometric exercises.

It is important to note that there may be subphases or progressions of the Plyometric Continuum. For example, the assessments can be performed unilaterally or an external load can be applied. One may choose to assess these variants before moving on to the subsequent phase.

The results of the PC should help to inform the development of targeted training programs. In order to improve athletic performance in force and power generation, high performance managers (or health professionals) should focus on the specific deficits that the data reveals.

Exercises should be chosen that reflect the movement component of the PC you are focusing on. The tests themselves (“tall to short” squat, Squat Jump, Countermovement Jump, Hop Test and Drop Jump) can also be used as training exercises, and can be progressed in the same way as the subphases listed above.

When it comes to progressing from one stage the next, a practitioner may use limb symmetry index as their criteria as supported by research. This usually requires designating a level of symmetry (often below 15% indicating comparable values to the non-injured or otherwise preferred side). Use of limb symmetry index may differ depending on factors such as demands of the sport and limb dominance.

For questions on how to get the most out of ForceDecks assessments with athletes in both a rehab and/or performance setting, please reach out here.

Sasha is a licensed Physical Therapist and Athletic Trainer, with a background in Strength and Conditioning; he currently works at Alta Allied Health in Brisbane. Sasha uses the VALD technology regularly with patients and athletes he sees in order to help assessment physical impairments, monitor progress through rehab, and inform return to play decision-making. The ForceDecks are a mainstay in Sasha’s clinical practice.