To Force Plate or Not To Force Plate

To Force Plate
or
Not To Force Plate

4th January 2020

Seaton Humphreys

If you’re time sensitive, or even one of my slack friends, scroll to the bottom for a summary

I constantly say that my girlfriend and I make for the perfect storm of a retailer’s dream; Jazz loves to buy things, and if I buy something, I automatically want the best. Not the most expensive, but the best. The joy of researching to buy something new is half the fun of the making the ultimate purchase. When deciding on what equipment to buy for the Athlete Origin facility, I set upon my usual ways; having been at Athletes Authority and being able to make significant contributions to the equipment has obviously given me a wide scope of exposure to training tools. However, when starting a junior dedicated facility, I had to reconsider what I needed at my disposal. Some were easy, some were plain to see why, some were definite, and some played with my emotional need to buy the best….The force plates are my pride and joy (ask anyone at Athletes Authority), they’re my baby, and a way to delve into force profiles unlike any other device, and with the Force Decks by Vald, the ease of use and speed at which the variables are calculated make for a undeniable useful tool; though after gathering data for over a year on them, I started to get a hunch about youth athletes testing strength measures on them.

When I was at Uni, we performed a maximum voluntary muscular contraction test using the Interpolated Twitch technique (2) on a very expensive testing machine called a Kin Com. This provided a very neat way of determining if you had full control over your muscular contractions. Maximally contract, zap you, and see the difference. That’s not what they did all the time, after years of research they were able to safely say that 1.3 x BW Squat and 1 x BW bench + 2 years of consistent gym time was a fairly good indicator of whether you had full control or not. The issue with this is the invasive nature of the test, I can’t exactly start sticking electrodes in kids nerves and give the thumbs up to the parents in the background. Although I never intended to perform this test, it is a pretty definitive way of determining someone’s training level.

Although this has never been directly correlated to someone’s classification of trained or un-trained, when we first acquired the force plates at Athletes Authority, I started using another test called the Dynamic Strength Index (DSI (3)). Now the dynamic strength index is a comparison of the force capabilities from a truly dynamic movement; a counter movement jump, and a truly strength movement; an Isometric mid-thigh pull (IMTP (1)), which is where you pull against an immovable bar as hard as you can. Both tests are supposed to yield similar mechanics to achieve the same outcome, except one you fly and the other you are stuck pulling a bar. Generally what this index is supposed to divulge is how much strength someone has in reserve if they can maximally flex, and how much someone can actually access when moving like they would on a field; Karl my old boss used to say it’s like a bank balance. From this information we essentially used it help direct training for your needs; can’t jump that high, but can pull a bar hard = more power & speed training; can jump really high, but don’t have much left in the tank = lets deposit some more strength in your bank account.

For adult athletes the IMTP was great, for youth athletes there were already a couple of red flags;

It’s time consuming to teach
If not taught correctly it could be dangerous (the last thing you want is a young athlete going home from testing and saying to their parents, the next day that their back is sore)
Generally it’s not performed well anyway, people cheat and leverage alternate methods of pulling on the bar to get higher scores.

After seeing this continuously repeat, I followed my hunch, and although the numbers aren’t conclusive, I’d rest my hat on it for now until I or someone else proves me wrong.

Pretty easy and obvious conclusion, more adults can pull a bar harder than youth. But then it got me thinking about all the interpolated twitch work we did whilst at Uni; how did the trained vs. un-trained adults compare?

Again, un-trained adults were not capable of pulling a bar harder than their counterparts. Pretty obvious again, lastly, I wanted to look at the magnitude of difference between the IMTP and CMJ capabilities to answer, if the youth athletes IMTP is higher than their CMJ, how much larger? 200 Newtons? 400 Newtons? More than 400 Newtons? Once I delved into this, it showed the different characteristics of youth vs trained adults & un-trained adults.

Youth sat at 50% of all of them being able to produce larger than 400N differences between the 2 tests, and Trained adults 85%. Within the groups data though, the majority of the youth’s 50% were those still classified as youth, but had been training with us for the better part of the year, and the adults who weren’t able to produce more than 400N differences, were those coming into Track peaking seasons where we wouldn’t want them to be carrying extra strength for the sake of having more strength without being able to use it. However, what really got me was the percentage of youth who were within 200N or not able to produce more from the IMTP, 39%! Compared to 14% of un-trained adults & 6% of trained adults. Having the CMJ and IMTP that close together would yield about a 0.9 on the DSI, which is uncommon, and therefore render the magnitude essentially as another indicator of not being able to have any significant differences.

This is a bittersweet moment for me. I’ve confirmed my hunch (every researcher is probably saying “yeah but with confirmation bias”), but that means I need to leave my force plates out of Athlete Origin. The validity of using them with youth athletes just isn’t there for me, nor with that limitation can I justify the price for assessing jump profiles of youth who’s mechanics are constantly changing. For now, I’ll be using the relative metrics of 1.3x BW Squat, and 1 x BW Bench to assess my youth athletes muscular contraction capabilities.

In the coming blogs I’ll divulge a little further as to what and why of my though process of including certain equipment into the Athlete Origin lean start up.

Summary

Over the year of collecting strength data on Force Plates I started to see a trend in junior athletes not being able to produce an adequate amount of strength to justify the use of the testing techniques such as Dynamic strength index’s and Isometric mid-thigh pulls (IMTP) commonly used on adults. When I assessed the data, it showed that indeed Youth athletes (for the most part) are incapable of producing large enough forces to warrant using a Dynamic Strength Index marker. Furthermore, there was a difference between youth, untrained adults, and trained adults, which helped with the decision that the observations aren’t isolated to untrained (both youth and adult) and trained, but clearly between all 3 groups; Youth, Un-trained adults & trained adults. Consequently, I decided not to purchase force plates for the Athlete Origin facility.

References

De Witt, J. K., English, K. L., Crowell, J. B., Kalogera, K. L., Guilliams, M. E., Nieschwitz, B. E., … & Ploutz-Snyder, L. L. (2018). Isometric midthigh pull reliability and relationship to deadlift one repetition maximum. The Journal of Strength & Conditioning Research, 32(2), 528-533.

Marshall, P. W., Finn, H. T., & Siegler, J. C. (2015). The magnitude of peripheral muscle fatigue induced by high and low intensity single-joint exercise does not lead to central motor output reductions in resistance trained men. PloS one, 10(10), e0140108.

Thomas, C., Jones, P. A., & Comfort, P. (2015). Reliability of the dynamic strength index in college athletes. International journal of sports physiology and performance, 10(5), 542-545.