In-Season Monitoring Using Drop Jumps
At the higher levels of sport, “monitoring” in some form or another is commonplace. Heart rate monitors, external workload devices, Moxy, sRPE, jump testing, bike testing, isometric assessments – most of us do something to regularly assess something about our athletes.
Here with the New Jersey Devils, we are no different. We use external workload monitoring, WattBikes, isometric testing, and force plates for regular monitoring.
For the last few years, like most people using jump testing to assess fatigue/readiness, we have been using a standard, hands-on-hips, countermovement jump (CMJ).
This summer, as a performance team, we undertook a detailed review of our testing and monitoring protocols and decided to move to a hands-on-hips drop jump (DJ) from a 30cm/12in box. When I tell this to my counterparts around the league, I usually get raised eyebrows and a look of surprise. The rest of this article will explain why we made the decision to switch, and what we've learned from it.
Why We Changed to Drop Jumps
Over the last few years, it has become common knowledge that using CMJ jump height alone is a problematic metric. We know that fatigued athletes will change their jump strategy (increased dip, longer eccentric phase) to maintain jump height. This has led to performance teams monitoring a plethora of metrics from the CMJ, including peak power, eccentric braking force, RFD, and center of mass displacement, among others. Part of our performance team review was to discuss which CMJ metrics should be investigated, as in previous seasons we had only used jump height. We looked outside of our organization for expert knowledge.
Fortunately, when I was studying for my Masters' degree in Strength and Conditioning at Middlesex University in the UK, one of my fellow students was Chris Bishop, who later went on to become the chairman of the UK Strength and Conditioning Association, and earned his Ph.D. in…jump testing.
I had several calls with Dr. Bishop, hoping for him to give me a magic answer to my question. Instead, he surprised me with a comment early on, which went something like this: “If you have to dig so deep into specific metrics to find your answer, maybe think about using a different test.”
We went on to discuss the merits of the drop jump, which was once popular but had fallen out of favor over the last decade or so. While you can dig deeper into the metrics of this test (Dr. Bishop recommends Reactive Strength Index (RSI), Jump Height, Ground Contact Time, and Leg Stiffness to truly understand what is going on), using RSI alone will give you a good starting point to understand fatigue, as this test is less susceptible to conscious or subconscious changes in jump strategy.
Since the world of pro hockey is hectic, to say the least, we have to consider the integration of sports science with the minimum of disruption, and timeliness of feedback. Athletes quickly lose interest in the assessment if, when asking about their result, you reply with “well, your jump height was X, but give us 30 minutes to check exactly what that means.” As we felt more confident in a singular metric from the DJ over the CMJ (at least for immediate feedback), we made the decision, starting from training camp, to use the DJ as one of our weekly readiness monitoring tests.
What We've Learned
There is a big learning effect with this test. We knew that would be the case, so during training camp we had all athletes do the DJ every day they were in. Once we got into the season, we tried to repeat the test twice a week – once following a day off, and once following a game, to see if it was sensitive to fatigue (it is). At the time of writing, we are deep into March and playing games every other day, so we have realistically cut this down to once a week.
Coaching cues and feedback are important, but they might not have the effect you're looking for. We keep our coaching cue simple for this test – “Jump as high as you can but be as quick off the floor as possible.” More than once, an athlete has pointed out the contradictory nature of this message, and I'm not sure I have come up with a good comeback yet.
Feedback has also been an issue. At first, we were displaying live results on a screen in the gym, but athletes quickly figured out that if they were super quick off the floor, their RSI score was higher.
As competitive athletes, they all want to be top of the leaderboard, but this led to a drastic reduction in jump height, defeating the point of the test. In the end, we took away all visual feedback and I verbally gave them their RSI score. Now we get much more stable results.
As mentioned above, we compliment the drop jump with a 6-second WattBike sprint. Our working theory is that a DJ represents CNS activity, whereas the bike sprint represents peripheral fatigue. I have run correlation analyses, and while there is no relationship when you look at absolute scores (supporting the idea that they measure different things), there is a relationship when you compare the difference from the mean, suggesting that if an athlete performs well above their average on one test, they also perform well above their average on the other test, and vice versa. This makes sense since we know CNS fatigue and peripheral fatigue are linked.
So What Does All of This Mean?
The use of drop jumps appears to be a valid and reliable method of assessing fatigue/readiness to perform with elite hockey players and may be superior to countermovement jumps in this population in the applied setting. We can give immediate feedback to athletes and coaches and be confident that jump strategy is not masking underlying fatigue.
However, it is also prudent for the performance team to look past one metric when detecting fatigue to determine possible causes. To this end, I recommend everyone to read the article A framework to guide practitioners for selecting metrics during the countermovement and drop jump tests by Dr. Chris Bishop.
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