L | R Impulse Index: Observations from the Private Sector
I make mistakes frequently. It’s part of the learning process. Perhaps, one of the biggest mistakes I’ve made in my 20-year coaching career is not keeping meticulous records. It’s interesting to hear debates on social media consistently backed by published research. I think that’s a great start (better to be backed by solid research than not), but the biggest form of “research” that coaches should take to heart in the applied setting is longitudinal data from their OWN unique population. This takes time, repetition, diligence and organization. What’s good? What’s reliable? Those questions aren’t that easy to answer. In the world of metrics and data, longitudinal information wins the gold medal, published research, second.
I recently saw a Twitter post from a fellow colleague regarding her use of force plates in the return to play setting.
This got me thinking and led me to my own investigation (i.e., longitudinal data stream). We have used force plates for over two years in the private setting and have had the opportunity to test 100’s of hockey players. We look at a few metrics and break them down into buckets: health, strategy, and performance (thanks to my friend Eric Renaghan). Our health metric is L|R Braking Impulse index and L|R propulsive Impulse Index. We would use these numbers as a baseline should an athlete get injured and use them as objective markers in the RTP process. According to Hawkin Dynamics the definitions of these metrics are as follows:
L|R braking asymmetry index: “The asymmetry between the left and right vertical impulses applied to the system center of mass during the braking phase.”
L|R propulsive impulse index: “The asymmetry between the left and right vertical impulses applied to the system center of mass during the propulsion phase.”
Injury and asymmetry are complex issues in the world of performance. Injury is multi-factorial with 100’s of confounders at work and asymmetry, many would argue, is a natural human quality, not just from the repetition of playing sport. In fact, therapists such as PRI (Postural Restoration Institute) practitioners would state that the body is designed asymmetricaly based on viscera. Research is also variable with cutoffs and thresholds. This can get confusing. Side to side differences within 15% of the contralateral limb have been recommended for functional tests involving jumping (2) for RTP, while lower asymmetry levels may affect performance in healthy athletes (1).
So, I decided to put my longitudinal data to the test to answer the following questions:
What does asymmetry look like in a asymptomatic population of athletes re: jump testing? How big is this asymmetry? (I used >15% threshold)
Which metric is more reliable? In essence, which metric has less variability?
Here is the investigation, by the numbers:
N = 160 asymptomatic athletes
Ages 14-18 years old
380 jumps were analyzed (note: athletes in this investigation jumped multiple times and an average score was used)
I calculated the mean and standard deviation for each metric while also calculating the normal distribution. I then graphed this distribution via bell curve and calculated the 1st, 2nd and 3rd standard deviation from the mean.
L | R Braking Impulse Index
68% of the jumps tested had an asymmetry of between -11 – 10.8
27.2% of the jumps tested had an asymmetry of between -11- -21.8, and 10.8- 21.7 (68 tests were >15%)
5% of the jumps tested had an asymmetry between -21.8 - -32.7, and 21.7 – 32.5.
L | R Propulsive Impulse Index
68% of the jumps tested had an asymmetry of between -6.26 and 7.15
27.2% of the jumps tested had an asymmetry of between -6.26 - –12.97, and 7.15 – 13.86 (13 tests were >15%)
5% of the population of healthy athletes had an asymmetry of between 21.7 – 32.5, and -21.8 - -32.7
Questions Answered (Maybe?):
What does asymmetry look like in a healthy population of athletes re: jumps? How big is this asymmetry?
18% of all jumps tested (68 0f 380) had L | R Braking Impulse Index asymmetries >15%. This may pose problems when looking at this metric solely for RTP purposes. Is this normal variability? An underlying issue? An accurate measure still may not be a good measure when close to 20% of the tests showed >15% asymmetry in asymptomatic athletes.
3.4% of all jumps tested (13 0f 380) had L | R Propulsive Impulse Index asymmetries >15%.
Which metric is more reliable? In essence, which metric has less variability?
Although the contraction profiles are different it appears that L | R Propulsive Impulse Index is less variable, thus a more reliable measure.
Limitations:
Relatively small sample size
Randomization, learning effect, and testing time were not controlled. I would argue that in most applied cases, this happens more often than not.
Closing Remarks:
One metric alone should never rule the RTP space. It’s important for coaches in the applied setting to understand the metrics within their unique populations. This takes time. The other major concern I have is the over reliance on numbers and metrics at the expense of first principal knowledge (physics, programming, phycology, physiology, and biomechanics). These, I would argue, enable the practitioner context and narrative so the numbers can “speak.”
Looking at the longitudinal data, teasing out metrics that may be less reliable, having a strong coaches eye, and sound understanding of first principles knowledge are critical factors in epistemic growth. Common practice doesn’t always equate to best practice.
References
1. Bishop C, Read P, McCubbine J, and Turner A. Vertical and horizontal asymmetries are related to slower sprinting and jump performance in elite youth female soccer players. J Strength Cond Res 35: 56-63, 2021.
2. Myer GD, Paterno MV, Ford KR, Quatman CE, and Hewett TE. Rehabilitation after anterior cruciate ligament reconstruction: criteria-based progression through the return-to-sport phase. J Orthop Sports Phys Ther 36: 385-402, 2006.
