- John R. Harry, PhD, CSCS
Athlete Performance vs Strategy: Do You What You're Testing?
Sometimes I feel like a stickler for accuracy and transparency. Perhaps my most obvious sticklery-ness comes out when I hear or read the term, "rate of force development". My rally against that term is connected to the fact that movement performance can require intentional reductions of force application, which obviously is not a "development" (Merriam-Webster defines development as the act or process of growing or causing something to grow or become larger or more advanced). Moreover, there's a real descriptor for the rate of change of force, and that term is "yank". It's an uphill battle trying to shift an ingrained way of thinking, but I'm on that hill and prepared to die there :)
Okay, enough about the rate of for- I mean yank. On to today's topic...
PERFORMANCE VERSUS STRATEGY - DO WE KNOW THE DIFFERENCE?
I can't begin to quantify how many times I've seen people, myself included in the early days, say that an athlete's performance increased because they demonstrated an increased amount of force application or a reduction of inter-limb force asymmetry (or something else along those lines). It is my opinion that the increasing accessibility of data (e.g., force platforms, motion capture, etc.) has opened so many doors to explain "change" that we can easily focus on the gazillion trees that quickly catch the eye but lose sight of the lone forest and its growth.
Let's take a simple task, like landing from a jump (I know what you're thinking: "this blog post isn't about the countermovement jump?!?!?") in one athlete. Given my current interests, let's consider a collegiate basketball player. When we try to define landing performance in basketball, we have to remember that when a player lands from a jump, a number of subsequent tasks can be required, such as a second jump-landing or an immediate transition to sprinting among other tasks. Obviously, it would benefit the athlete if they were able to stop their downward motion quickly so that any secondary task can begin as soon as possible. Given this, the the most important metric of landing performance can be considered the time it takes to complete the landing process. I define the landing process as the time required to stop downward motion and not the time to required to return to stabilization or standing upright. The latter would be a landing followed by a standing up, right? How sticklery of me, I know.
Due to the aforementioned number of trees (possible metrics) in our way, we can start to lose sight of what defines performance because of those metrics we've (yes, me, too) long feared during landing, such as the peak impact force and the loading rate. Because of this, we could think of a reduction of impact force or loading rate (Figure 1) as an increase in landing performance. However, all those changes really tell us that there "could" be a decrease of overuse musculoskeletal injury risk associated with external force, such as stress fracture.
Figure 1. Changes in peak impact forces and loading rates across four test sessions in a Collegiate Men's Basketball Player. Notes - red diamonds are the percent change between tests, and circled diamonds are "true changes".
Now, if we take that same athlete's landing duration changes (Figure 2), the story is different because the changes in landing time were not "true" and the percent change was relatively small. So, did this athlete actually perform the landing better? Or have the just changed their strategy for impact force attenuation? I would argue from the data thus far that the athlete has not changed performance but has changed (improved?) their impact force attenuation strategy from these three metrics.
Figure 2. Changes in landing time, peak impact forces, and loading rates across four test sessions in a Collegiate Men's Basketball Player. Notes - red diamonds are the percent change between tests, and circled diamonds are "true changes".
FORCE METRICS =/= PERFORMANCE
While my own work has shown that other force metrics, specifically the magnitude of yank during the attenuation phase of landing (which can be thought of as the rate of force attenuation for those of us still reluctant to adapt), characterize athletes who can perform landings more quickly. BUT, similar to the loading rate data, changes in yank are really strategies that can influence performance but aren't direct explanations of performance. More importantly, something I've ignored thus far in this post is the fact that the landing data was extracted from a vertical jump test and not a drop landing test from a raised platform with a consistent height. Because of this, it's possible that the athlete's jump height changed over time. More importantly, it's possible that the athlete's landing height changed over time. This is an important specification because it's often incorrectly assumed that the time of upward flight is equal to the time of the downward return (this is what jump mats assume). In reality those times differ because the fall duration and therefore landing height is heavily influenced by the athlete's approach to landing during which preparatory muscle activation and joint flexion occurs right before impact with the ground. Remember, the athlete aims to "accept" the impact rather than go "splat" for obvious reasons. So, any change in landing performance (i.e., time) should account for the height they landed from, right? Here's my answer to that:
PERFORMANCE IS DEFINED BY THE TASK AND ITS FUNCTIONAL DEMANDS
How might we account for landing height and landing time to quantify performance? Well, look no further than the drop jump and countermovement jump literature. Time-constrained drop jump and countermovemnt jump performance is often assessed via the ratio of the height achieved and the time required to leave the ground (reactive strength index, RSI, and reactive strength index-modified, RSImod). I know, I know, there's issues with ratios when one or both of the constituent parts change or vary drastically, but let's consider the low variation in those metrics from the data I'm presenting here and agree that there's not much limiting use of a ratio. Cool. Now that we've cleared that up, let's explore possible changes in a more complete look at landing performance, using what I have coined the "landing performance index" that is the ratio of landing height and landing time. Figure 3 provides those data along with the data from the previous figures.
Figure 3. Changes in landing performance, landing height, landing time, peak impact forces, and loading rates across four test sessions in a Collegiate Men's Basketball Player. Notes - red diamonds are the percent change between tests, and circled diamonds are "true changes".
PERFORMANCE DOES NOT ALWAYS CHANGE IN PARALLEL WITH FORCE
As we can see in the complete set of results, true increases or decreases of landing performance do not always occur alongside changes in force, specifically the impact force or loading rate included here (as two examples that we monitor for this athlete - obviously there are many other force metrics available). Instead, the changes in impact force or loading rate reveal, as touched upon previously, the athlete's strategy for attenuating impact force. A change in landing performance is multi-factorial in that any of the following can explain performance change: getting better at landing (a practice thing), increased eccentric strength capacity, a shifted mindset or change of volition/interest - we see this a lot in athletes, as they often need to be reminded of why the test requires full effort and focus, and other things that would take 500 more lines of text to document. What this means is that any assessment should include both performance and strategy metrics, with clear separation between the two. This means that strategy does not explain the performance change, but it might contribute to the performance change and tell us something else about the strategy. In this case of landing, the force-related strategy change is really about providing some estimates for the athlete's change of overuse musculoskeletal injury risk.
One final note relates to the use of landing time as a performance metric. However, my opinion is that the duration of a complete movement in a time-constrained environment is a performance quality. The way in which that movement time is manipulated, as defined by the times of the movement phases, would reflect the strategy. For, example, I define a landing into two phases: loading (when the impact force increases) and attenuation (when the impact force decreases finishing at the end of downward motion) as shown in Figure 4 below. So, the duration of the landing (one portion of the landing's performance) can be maintained but strategized in different ways (e.g., increase loading time but decrease attenuation time). That exploration was surplus to requirements for this blog post, but it is something we consider in our athlete testing reports.
Figure 4. The loading and attenuation phased of landing (taken from Harry et al 2018).
To summarize this post, when athlete performance is tested and monitored, it is crucial to specify a real metric of performance to ensure that strategy is not being mistakenly monitored in place of performance. Our main motivation for this is to avoid my sticklery side from coming out, right? I hope this was useful reading for some of you. Hit me up in the comments with questions, concerns, or trolling. Until next time!