The Stretch-Shortening Cycle: Can We Finally Try to Properly Define the Amortization Phase?

As a (primarily-focused) jumping researcher, I often incorporate strategy metrics intended to reflect neuromuscular function during the stretch-shortening cycle, sometimes called the reversible muscle action. Recently, one of our lab members (John Krzyszkowski, PhD Candidate) brought up an interesting, often over-looked point: we don't really know how to define the reversible action/stretch-shortening cycle (SSC), which is, in general, the transition between eccentric-to-concentric actions.

Let's first familiarize ourselves with the "textbook" definitions. Zatsiorski, Kraemer, & Fry define the SSC is defined as shown in Figure 1 below, where there is eccentric action (i.e., stretch) followed by a concentric action (i.e., shortening).

Figure 1. Representation of the stretch-shortening cycle.

Haff & Tripplet go into a bit more detail, as they deconstruct the SSC into eccentric, amortization, and concentric phases, as shown in Table 1 below.

Table 1. Descriptions of the stretch-shortening cycle phases.

The eccentric and concentric phases of the SSC are fairly clear, and we've been able to show their existence and extract their locations within the countermovement jump using only force platforms (and this rationale can be translated and applied to other plyometric movements like the drop-jump [from a vertical jump of course]). The kicker is, "what in the bloody hell is amortization and how can we isolate it during a jump or related movement?"

I'm not a big fan of the definition described for the amortization in Table 1 above (i.e., in the action column) . For instance, the pause between eccentric and concentric actions is so short that it can't really be defined as a phase. This is because the pause between eccentric and concentric phases of the countermovement jump only lasts 1 millisecond), presuming a 1000 Hz sampling frequency on a force platform. The commonly studied (Barker et al 2018; Chowning et al 2020; McHugh et al 2020; Heishman et al 2019) "amortization force" or "force at zero velocity," which is extracted at the instant of the pause, has shown to be a valuable strategy or driver metric with respect to jump output. Still, I'm sure there's other unexplored force and time variables neighboring that force magnitude that can help explain performance changes or neuromuscular adaptations.

I think the reason we've yet to see a "good" definition for the amortization phase is because sports biomechanics researchers spend wayyyyy too much time exploring mechanisms of overuse running injuries (here's proof: ~12,500 studies!). and not enough time on the important stuff. I know I've just triggered some folks to the level that Gus Fring would trigger ol' Hector Salamanca. But, I don't care. It's the truth :).

On a more serious note, perhaps the most complex thing about trying to define amortization is that the involved muscles creating joint articulations act eccentrically and concentrically at different times. If we try to use the net sum of joint power (hip power + knee power + ankle power), which is how the eccentric and concentric phases are currently defined, we would run into the same problem described above. We could employ published method using a 1-centimeter depth threshold, where the amortisation (I'll kindly leave that typo in here for all my compadres "across the pond") phase lasts the whole duration of time during which the depth resides within +/- 1 cm of the its maximum. The problem with this method is that it does not account for the actual depth of the countermovement. What I mean is the 1 cm threshold does not represent the same thing for a 20 cm depth versus a 35 cm depth. So, a percent threshold of the depth might be a more precise way to define the amortization phase. Still, I don't think we should be defining the amortization phase, which if you recall is the transition between eccentric and concentric actions, by using a kinematic criterion. Call me a stickler, but that's probably a hill I'll die on (thanks, Ross Miller, for indirectly teaching me this phrase - I think I use it way too much now).

So, the big question, which I do not have an answer for yet (we are working on it), is how do we define this phase? It's clearly a valuable and much needed definition, otherwise we wouldn't say "amortization" when discussing training or plyometric testing as frequently as a "valley girl" says "like" in conversation.

If you haven't figured it out by now, this post is really a call to arms for all the smart folks I know to bang heads and think about this, so we can really define this critical time period within sporting movements. My initial thought is that we can use time between the hip and knee joint transitions from eccentric to concentric dominance, which would occur roughly between 50% and 65% of the countermovement jump. This would ensure the phase has a clear period of time, but is that time "too long"? Another possible issue is that it ignores the ankle's transition from eccentric to concentric dominance. This might not be as critical because the ankle really doesn't decide to do anything (eccentric or concentric) during a countermovement jump until the final ~20% of the movement. The ankle is kind of like me in the kitchen. For instance, my wife and will be cookin' up dinner, and she will throw marinade on some meat we're about to put on the grill (I was scolded by my Southern friends for calling the "grill" a "barbecue" - but I blame my west-coast California roots for that). I'll show up right towards the end and add a little more of that same marinade and then take all the credit for how delicious it turns out and my wide just sits there like, "we for real, y'all?!?" Hopefully my little tangent explains my thoughts on why excluding the ankle from an amortization definition might not be a bad thing. If not, I tried.

My second thought is that we can use a +/- threshold related to the net sum of joint power (to include all the joints). The issue with this method is that there's no way (in my mind) how we can identify a meaningful threshold value. So, the threshold would likely be a wild, albeit slightly-educated guess. Maybe not the best approach?

So, what do you fun folks think? How should we start to explore more accurately defining the amortization phase? I welcome all comments and feedback here (I included that option on these posts so y'all can have a constructive discourse, because my thoughts aren't the Gospel and y'all probably have some kick-ass ideas of your own).

Okay, that's all for this week, party people. L8R (you know what "L8R" means, right?. I mean, we all had AOL Instant Messenger at one point didn't we? If you didn't, you sure did miss out amirite?!?).