Do You Target Unloading Strategies in Jump Training? You Should...
If you're a recurrent reader of this blog, you probably know that my team and I have a love affair with jumping. This is because jumping is awesome. Duh. But on a serious note, jumping performance, when tested appropriately, can reveal a great deal of information regarding athletes' physical abilities, adaptations to training, and readiness to train or participate at maximum capacity. This is why so many of us use jumping performance tests when working with athletes.
Because so many smart folks have applied their skills to sports science applications, the research and practitioner communities have a wealth of information to consider when choosing a jump testing platform or creating their own (shameless plug). One of the vogue methodologies during jump assessments is to focus on phase-specific strategies that contribute to gross performance. There are numerous phase-deconstruction methods, but there is one particular method that is quite obviously the superior method and should become universally adopted (shameless plug #2). All joking aside, there does appear to one jump strategy for which we are starting to recognize as important to jumping ability: unloading.
Importantly. it does not matter if you apply the phase deconstruction method I developed, the one John McMahon (aka the Force Plate Coach) and colleagues described, the one the Gathercole et al article described, or any of the other methods circulating the literature, as unloading can be isolated as long as you can trudge through the similar-yet-not-so-similar nomenclature. What's important is to know 1) what unloading is and 2) why you should care about it.
What is "Unloading"? Well, the unloading phase is quite simple. It is the time period that starts when the jump begins and ends when the local minimum of force occurs (See Figure 1). It represents how an athlete stops applying the equal & opposite amount of force to counter their force of gravity and stand still to begin the countermovement. Typically, this takes only 0.15 s to 0.3 seconds, depending on the athlete and their strategy to start a jump. But, more importantly, it represents the athlete's strategy related to the conversion of gravitational potential energy to kinetic energy. If you use any of the phase deconstruction methods above, that's okay. You don't have to change your preferred phase deconstruction method to obtain information about athletes' unloading strategy.
Figure 1. Visual Definition of the Unloading Phase of a Jump.
Why is the Unloading Action Important? When considering the value of the unloading action, it's important to to remember that when starting a jump by converting potential to kinetic energy, athletes do not simply "release" their resistance to gravity to "fall" downward. It is absolute fact that Earth's gravity pulls the athlete downward once they stop resisting, but jumpers actively pull their body downward to supplement the pull of gravity. We know this from a growing body of research alluding to the fact that the involved musculature performs net concentric actions during unloading. Importantly, unloading time is a significant temporal predictor of jump explosiveness in athletes. In addition, more explosive jumpers complete the unloading phase faster (p = 0.04; Cohen's d = 1.08) than less explosive jumpers. Finally, unloading time and coinciding force manipulation can be modified using simple verbal cues. Collectively, this body of work indicates both that unloading is an important strategy when seeking to alter jump performance and unloading strategies can be manipulated using non-invasive cues.
What metrics should we extract from the unloading phase? Well, this one is rather simple actually. Based on current evidence mentioned previously, the duration of the unloading phase should be the first (and only?) metric we concern ourselves with during vertical jumping. This might change as the literature starts to include more studies on this. For those of you wild and crazy outlaws testing horizontal jump performance in your athletes, we have learned that the rate of change of force application (i.e., yank or the rate of force development) along the anterior-posterior axis during unloading (see Figure 2 for a visual description) is a significant predictor of horizontal jump explosiveness. So, the unloading metric selected should be determined according to jump type.
Figure 2. Visual Definition of Anterior-Posterior Unloading Yank during Horizontal Jumping.
To close this post, the main thing we need to remember when assessing jump performance is the manner in which the jump is executed so that we can isolate metrics that tell a clear story explaining a performance or strategy change. That's why force platform companies like ForceDecks and Hawkin Dynamics are so useful to practitioners (FYI: I have no affiliation with either company and use separate force platforms and my own analysis programs), regardless of how they define the phases of a jump. These companies provide so many metrics from which the practitioner can choose based on their own curiosity. Even if those companies do not directly provide unloading metrics as defined herein (which I don't think they do yet - it takes time for the literature to reach the community, y'all), they tend to listen to their clients and provide means (access to raw data) for practitioners to get those metrics should they choose. It's up to the practitioner to take ownership of their assessment and make their own decisions about what's important for their athletes and objectives. Hopefully this post helps convince you that unloading strategies might be one of those important strategies.
See you next time, folks.