Categorizing Plyometric and Stretch Shortening Cycle Exercises

I haven’t posted in a couple of months, and I wanted to write up an article on something a little different than normal. A while back (maybe two years ago) I was in a conversation about including different types of plyometric exercises in training programs for my athletes. I can’t remember exactly what I said, but I mentioned categorizing them something like this:

  1. Lower Leg Plyometrics
  2. Biarticular Muscle Plyometrics
  3. Upper Body & Trunk Plyometrics

As I was digging through some old notes, and putting together a presentation on tendon function this week, I found a better explanation. Depending on who you ask, the definition of plyometrics may only include exercises with what strength and conditioning coaches call a “shock phase”. That is, there needs to be some overcoming of an eccentric load or external force by the concentric phase of the movement. While I am in agreement with this, I might cross a couple of lines in this blog post so from now on, I’ll use the term elasticity training.

As strength coaches, we all intuitively understand the value of elasticity in athletic performance. It makes athletes more reactive and helps them run faster, jump higher, and can optimize the economy of movement. So for now, let’s hit a little bit of background on elasticity.

A spring element, by definition follows a very simple behavior. Springs deform when a force is applied and recoil to their resting shape when force is released. Materials can act like springs when loaded in tension, like a rubber band, or in compression, like a rubber ball. Both kinds of loading are important in nature. When springy materials deform, they store energy in the form of elastic strain energy, and when they recoil this energy is released [Roberts, Azizi 2011].

In the context of exercise physiology, we are fortunate to have two spring elements involved in enhancing movement speeds – the series and parallel elastic components. In the musculotendinous unit (MTU), the parallel elastic component is comprised of a number of structures within the sarcomere including the major ones: titin and interstitial connective tissue. The mode of contribution for these structures is to increase passive stiffness or tension of the contractile component. That is to say, they help resist fascicle lengthening when stress (force) is applied to the tissue. The series elastic component on the other hand, is primarily concerned with the tendon’s role in the MTU. It still lengthens and shortens, but as we will see later, it does so against a(n) (ideally) relatively isometric contractile component. Below are two proposed models of two Hill-type skeletal muscle models.

 

F1.large

[Hoffman et al. 2012]

My preferred explanatory model is the right side image (B) based on two premises:

  1. Parallel elastic component structures operate within the contractile component’s sarcomere. Therefore, they must be able to operate independently of the series elastic component.
  2. Model A does not appear to allow independent length change in the series elastic component. Although the endo- and perimysium are continuous with the tendon, I don’t think it is a fair representation to include the SEC the way it is included in Model A.

With that said, let’s look a little more closely at the tendon now. Specifically, the below image of a stress-strain curve.

stress-strain

[Latash, Zatsiorsky 2015]

This image illustrates the tendency of tendon tissue to change length (strain) in response to applied force (stress). Basically, from 0-4 on the x-axis, the tendon has capacity to change length up to a certain point within the physiological range in response to applied external forces. These external forces are applied in the form of ground contacts in running or elasticity training activities like hopping, bounding, or landing from height. Obviously, at some point there is the potential for microscopic failure or rupture, occurring when the external force (stress) exceeds the physiological loading capacity of that tendon. This physiological window is predicated on the training status of the athlete as well as physical maturity.

From a performance perspective, consider the combination of both models we’ve examined up to this point.

comboimage

We have the muscle on the far right represented by the CE (contractile element), and the PEC (parallel elastic component). On the left we have the SEC (series elastic component) – tendon, and its associated stress-strain characteristics. At this point it is appropriate to recognize the viscoelastic property of tendon as that which allows it to resist strain (viscocity) as well return energy (elastic) once stress is applied. At a glance, most high force, low ground contact time stretch-shortening (SSC) activities involve an isometric contractile component which defers lengthening (stretch) and shortening (shorten) cyclic (cycle) activity to the tendon. This is what gives these activities the appearance of bouncing or athletic reactivity.

So how does this work or what does it look like in application? The next image is the best visual representation I have ever seen of three tendon functions in stretch-shortening cycle activities.

TendonActions

[Roberts, Azizi 2011]

I have also made an effort to plot these functions on a force-velocity curve:

Edited F-V

Force-Velocity curve with SSC (stretch-shortening cycle) functions approximately plotted. Right side F-V characterized by concentric force production (increased velocity = lower force due to shortened time frames for cross-bridge cycling). Left side F-V characterized by ballistic force activities (increased velocity = increased force) facilitated by elastic properties of SEC and PEC.

Load Attenuation – Internally Mitigating High External Forces

Starting with the highest force, highest velocity action on the far left, let’s talk about how that aligns with tendon function C (attenuation). Roberts & Azizi 2011 proposed an energy flow in their schematic of “Body>Tendon>Muscle”. This means the energy  produced is a product of body weight and gravity, and is first dissipated by tendon, and then muscle and tendon cooperatively as the athlete initiates contact with the ground. Consider the following Video:

This guy performs a quick and reactive (energy conservation) depth jump in between the short and large boxes. At about (0:03) and again shown at slower speed at (0:11) you can see him launch off of the larger box. When he hits the ground following this higher jump you notice that the amortization phase is much longer. This is a prime example of force attenuation, and the flow of energy can be stated as follows:

  1. Massive ground reaction force upon the athlete initiating the landing (foot contact)
  2. Tendon temporarily stores the resultant force during the shock phase
  3. Once total force encountered by the body is reduced, remaining force is absorbed by the contractile component

**The proposed mechanism for force attenuation by the tendon serves to protect the muscle fascicles which could potentially be damaged if they were to attenuate the entirety of the resultant force themselves. In this case it has been proposed that the muscle remains relatively isometric until the force is reduced to a level that may be safely attenuated/absorbed via fascicle lengthening [Roberts, Azizi 2011., Roberts, Konow 2013]. At this point the energy dissipation responsibility will be shared by the tendon and contractile component.

I placed this action furthest left on the F-V curve because the initial contact is represented by a massive peak force at initial contact. The time characteristics of the amortization phase are longer than that of energy conservation activities, but the force is much, much higher in this case.

Where does this go in training?

First of all, landings are a great evaluative tool or exercise to help you identify an athlete as at-risk or not with respect to how well they manage external forces. With respect to performance, a box that high may not fit anywhere in your training plan, but load attenuation should. It can be used as a great preparatory phase progression toward true plyometric activities as a means of building loading capacity of the tendon tissue. This relates back to the physiological window on the stress-strain curve earlier. Progressing to higher box heights and shallower knee/hip flexion angles will progressively build greater loading capacity and Eccentric Rate of Force Development (stopping power), a quality of great importance when it comes to generating internal forces to tune up “Power Amplification” which we will discuss later.

Energy Conservation – Sustained Force Output

The next point on our force-velocity curve has to do with what we as coaches most readily recognize as plyometric or shock phase tendon function. In the Roberts, Azizi 2011 schematic, this is represented by an energy flow of “Body>Tendon>Body”. In this case, the contractile component absorbs none of the force, it merely remains in isometry while the tendon stores and releases energy. Consider the following videos:

In these cases we see the very familiar short ground contact time interactions of the athlete with the surface. In this case, the muscle groups (Bicep Fem., Quadriceps, And Triceps Surae) are acting isometrically during the interaction with the ground. The ankle maintains a relatively fixed position via these isometric muscle actions and lengthening/shortening is done by the tendon exclusively. If you doubt this, pause the video during one foot contact and ask yourself if the knee or hip joint flexion angles would be capable of producing sufficient muscular force for the resultant action. This energy flow can be described as follows:

  1. Foot initiates contact with the ground “shock phase”
  2. Gastroc remains isometric while tendon lengthens slightly to store energy
  3. As body weight begins to unload from the stance phase with help of the arms, tendon recoils and returns elastic energy to produce ballistic plantarflexion

In these cases, we see what we most commonly refer to as stretch-shortening cycle activities. The contractile component is fixed in isometric contraction which means the lengthening of the MTU occurs via the series elastic component exclusively. At these relatively greater muscle lengths however, it is possible that the parallel elastic components contribute to some degree as well based on the fact that the muscle must resist passive lengthening and the cross-bridges may need assistance in resisting that change in length.

I placed this lower on the F-V curve simply because of the fact there is an energy return phase if the exercise is appropriately scaled. Amortization phases longer than about 0.2 seconds in activities like this may be suggestive of energy dissipation into the contractile component. Greater ground contact times (>0.2s) may also be indicative of an athlete who does not have the appropriate strength or loading capacity to perform the activity. For example, a ground contact time longer than this in a depth jump suggests that either the box being dropped from is too high, or the box being jumped TO is too high. One or both will need to be adjusted, or the exercise dropped entirely (if boxes are of reasonable height) until the loading capacity is sufficient. In addition, externally encountered forces will almost always be higher than internally generated forces in training. Internally generated forces will be next.

Where does this go in training?

I would use these exercises following a preparatory or strength phase to condition tendons to store and return energy. Bear in mind that exercise progressions can sometimes be a bit overcautious in the sense that athletes are typically already encountering these high forces in practice or recreational endeavors. Where the strength coach fits in is in making sure that correct technique is used so that these movements are less likely to result in pathological patterns or overuse injury. With eccentric RFD in mind, these exercises fit nicely in a strength or power phase, and can be used additionally as a primer for the CNS or as the second part of a post-activation potentiation superset.

Power Amplification – Augmented Force Output

The lowest point on the eccentric half of the F-V curve is power amplification. These are some of the “non-plyometric” exercises strength coaches may have a hard time agreeing with because there is no “shock” phase. The eccentric phase is still there though, and has an energy flow described by Roberts, Azizi 2011 as “Muscle>Tendon>Body”. In these movements, the contractile component generates the force to be used in stretching the tendon. Consider the following videos:

In these movements, the contractile component shortens, which applies a stretch to the tendon. Visualize the calf muscles shortening as the athlete forcefully plantarflexes to accelerate off the ground. The shortening of the contractile component stretches the tendon for as long as the heel stays flat and the resultant recoil or return of energy produces transient power outputs which exceed the capacity of the contractile component alone. This is why these activities are lowest on the eccentric half of the force velocity curve. Internally generated forces will be lower than those encountered externally because there is less downward acceleration. This is also part of the reason that eccentric RFD is important, to apply higher force and more abrupt stretch to the tendon.

It is important to consider in this case, that the tendons to not actually ADD force to the muscle or the movement, they simply increase impulse due to their ability to release stored energy faster than the contractile component. Impulse is a force-time function as well, so decreased time of application of the same force equates to greater values.

Where does this fit in training?

These exercises can go nicely anywhere in the training cycle due to their relatively low risk. Just like the other two classes of SSC exercises, we can use these as evaluative means for looking at the movement strategies of athletes and identify them as potentially problematic. You can learn a lot about an athlete just by watching them jump, and it is not limited exclusively to the lower limbs. You can evaluate:

  1. Stopping power (ERFD) How quickly do they eccentrically load, and how sharply do they initiate upward movement?
  2. Concentric force production (Power Output), How high can they jump relative to their body mass?
  3. Core strength and bracing (Intra-abdominal pressure), Are there force leaks coming from the core during the eccentric-concentric coupling?
  4. Landing strategies (Single – knee – vs. Multiple – hip & knee – joint), How do they negotiate external loads when they return to the ground? Are there force leaks in the lower extremities (valgus)?
  5. Movement economy (Mobility), Do they retain anterior pelvic tilt and tension the hamstrings when they load and land? Or are they extensor (quad, low back) dominant in their upward acceleration?

I hope this hasn’t been a waste of time!

 

References

Hoffman, B. W., Lichtwark, G. A., Carroll, T. J., & Cresswell, A. G. (2012). A comparison of two Hill-type skeletal muscle models on the construction of medial gastrocnemius length-tension curves in humans in vivo. Journal of Applied Physiology113(1), 90-96.

Latash, M. L., & Zatsiorsky, V. (2015). Biomechanics and motor control: defining central concepts. Academic Press.

Roberts, T. J., & Konow, N. (2013). How tendons buffer energy dissipation by muscle. Exercise and Sport Sciences Reviews, 41(4), 186-193.

Roberts, Thomas J., Azizi, Emanuel. Flexible mechanisms: the diverse roles of biological springs in vertebrate movement.  Journal of Experimental Biology, 2011.

 

 

 

 

 

 

 

 

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