Eccentric Yielding: Where Did It Come From & What The Hell Is It?
Updated: Apr 10, 2021
Before we get into what eccentric yielding is, we need to dig into why it came about in the first place. Back in 2019, Dr. Leland Barker and I were hell-bent on improving the way sports science researchers deconstruct the countermovement jump (CMJ) into phases. Not that there was anything "wrong" with the phases that were (and still are) being used, but we knew that they were limited in terms of what they "meant" in relation to how the neuro-muscular system functions (i.e., cause and effect). In short, the most widely used CMJ deconstruction method in contemporary research and practitioner-focused force platform systems includes the following phases: unweighting, braking, propulsion (Figure 1).
Figure 1. Widely used CMJ phase deconstruction method (see McMahon et al., 2018).
Now, I must reiterate, that there's really nothing mechanically wrong with these phases when we consider only what is happening at the body center of mass. The only bone I have to pick with this method centers on the unweighting phases. This is because it should be called "unweighted" as it describes the entire period where force production is below body weight, thereby including times where force decreases (unweighting) and subsequently increases (re-weighting). Some might say this is semantics, but I think it's a fair point. The braking phase is exactly as it sounds, as it is the time period when the body's downward velocity gets closer to zero, indicating a braking effect. Propulsion is also exactly as it sounds, as it is the time when the athlete propels the body upward to leave the ground.
Where things start to get murky with this phase deconstruction method is when we try to relate them to how we train athletes. For example, it's quite common for coaches to prescribe "eccentric" or "concentric" focused exercises. If our goal was to relate that to how it changes neuro-muscular function during the CMJ, we'd have to know when the CMJ is predominantly eccentric and concentric. There's literally tons of papers out there using the eccentric and concentric nomenclature for CMJ phases (summarized in this review), even dating as far back as the late 1970s when the terms were used instead of the aforementioned braking and propulsion phases. The problem Lee and I noticed was that there was no evidence supporting whether the terms eccentric and concentric actually associated with the time periods they were assigned to in those various CMJ deconstruction methods. Ultimately, we felt it would be extremely useful to know this information so that coaches and practitioners could have a specific period of time within the CMJ to explore to see whether their "eccentric" focused interventions, for example, actually stimulated eccentric adaptations.
From a biomechanics perspective, joint angular power (i.e., the product of the net joint torque and the joint angular velocity) provides us with a measure of muscular involvement to create (or terminate) a joint's rotation. It must be said that during a dynamic movement, the fascicles within the involved muscles are not all acting a certain way, meaning some could be acting isometrically at a given time while others are acting concentrically or eccentrically. However, the joint angular power provides the type of action dominating at a given time points, with positive joint angular power representing concentric dominance and negative joint power representing eccentric dominance. So, we can easily know when the joints act in specific ways. Because the joints don't all act eccentrically or concentrically at the same time (see Figure 2), we felt it would be best to focus on the muscular action dominating the movement by obtaining the net sum sum of the joint angular powers (see Figure 3).
Figure 2. Joint power curves for the ankle (A), knee (B), and hip (C) joints (see Smith et al., 2020).
Figure 3. The net sum of joint power plotted with ground reaction force and velocity (see Harry, Barker, Paquette 2020).
What stood out to us most when we tried to connect key events in the curves for the net sum of joint power, ground reaction force, and center of mass velocity, was that the "eccentric" phase actually begins within the unweighting phase outlined in Figure 1. We know this because the athlete is applying force by way of a negative sum of joint power (notice the negative power that occurs alongside a positive rate of force development in Figure 3?). This means you sure as hell need to revise the way you study the CMJ if you a) only refer to the time period when the total body's downward velocity decreases (i.e., braking) as the "eccentric" phase, or b) refer to the entire downward period (unweighting or unloading + braking) as the "eccentric" phase (Only if you want to maintain mechanical and functional accuracy). It also means that there's something very interesting going on during the time when the force output is below body weight. Lee, Dr. Max Paquette, and I banged heads for quite awhile trying to come up with a name that best describe both what was happening during this time period to go alongside the dominant muscle action (eccentric). We tossed around terms like "absorption" and "uptake" but those didn't do the trick. I think it was Lee who suggested "yielding", which seemed like a good term for two reasons. First, it has been used in resistance training texts to describe specific exercises where an athlete must resist a load. Second, we can see that the athlete is acting predominantly eccentric in an attempt to slow and ultimately stop their downward velocity, but in reality gravity is actually winning (Charlie Sheen voice, of course) because their downward velocity is still increasing. So, despite all the athlete's effort during this time period, they are yielding to the force of gravity. How cool, right?!
I like to use the following situation to explain what happens during the eccentric phase, particular as it relates to why we should isolate yielding and braking as eccentric sub-phases. Consider a car that is coasting down a hill, so it's going faster and faster because of gravity. The eccentric process reflects the time when the driver presses the brake pedal and the car slows down and ultimately stops. Braking reflects the time when the car is actually slowing down and eventually stops. Yielding is the time when the driver presses the brakes and the braking components (brake lines, calipers, etc.) are invited to the party. This is because there is a brief period of time where the car's increase of velocity continues even though the brake pedal is being pressed. Obviously, the velocity doesn't start slowing down until all the other working parts subsequently contribute. This means eccentric yielding is the start of the deceleration process. In my view, the stuff happening while yielding represents an athlete's efficiency when starting the stretch-shortening cycle.
So, now that we've covered how yielding came about and what it means, how does it relate to performance? Well, we've only just begun to explore it, and I hope others "see the light" and begin to help do this (yes, I am extremely biased here). Recently, I got all giddy when I saw that Dr. Daniel Cohen and Cory Kennedy included yielding in the CMJ phases described in the Kinetics and Force Platforms chapter they authored in the NSCA's Essentials of Sport Science textbook edited by Drs. Duncan French and Lorena Torres Ronda (see Figure 4).
Figure 4. Use of eccentric yielding phase in a text I did not not author (FINALLY).
From what we've seen in the data, variables obtained from the eccentric yielding phase, specifically yielding duration and yielding yank (sometimes called rate of force development) do not predict CMJ performance (Krzyszkowski, et al 2020), as defined by both jump height and RSImod (jump height divided by time to takeoff). In addition, greater force and yank asymmetry during the yielding phase does not appear correlated with jump performance, as defined by jump height, RSImod, power, or takeoff momentum in females (Harry, et al 2021). We have noted, however, that force asymmetry is quite evident during the eccentric yielding phase in collegiate men's basketball athletes, but we're still not seeing it predict CMJ performance using current analytical approaches (we are trying to get this into a manuscript right now). Finally, and perhaps most importantly, we noticed that yank during the eccentric yielding phase decreased in professional footballers (i.e., soccer) as a result of isolated/remote training during the COVID-19 quarantine even though jump performance was maintained (Cohen et al, 2020). What that means is the ability to initiate the eccentric phase and begin the process of slowing the body's downward velocity was compromised, revealing a lack of preparedness for the rapid reversible actions that are required in elite football.
From the available data I've briefly summarized here, yielding is a key part of the CMJ and represents an important aspect of neuromuscular function during stretch-shortening cycle exercises. While we have not yet isolated yielding qualities linked to better or worse CMJ performance, we have noticed that yielding qualities can indicate neuro-muscular preparedness for the demands of certain sports, especially football.
To wrap things up, eccentric yielding (IMO) should become a mainstay in CMJ analyses so we can really understand how an athlete's function during this time period influences performance. Given the current evidence highlighting it's importance for returning to competitive sport participation, eccentric yielding qualities should definitely be focused on when athletes are nearing return to competition, whether it be following an off-season, abrupt shutdowns (i.e., like during COVID-19), or return to play protocols.
Okay, party people. That's all the party I have in me today. It's time to pick up the guitar and annoy my wife with my exceptionally mediocre skills. Maybe I'll start things off with her "favorite" song, Seek and Destroy, what do you think?
Happy weekend, y'all. I'm out!