What is the blue boxed area of the propulsive phase of the countermovement (CMJ) signal called? It’s called the weighing phase and as the name suggests, it is a phase of quiet standing on a force plate where the force plate/s quantify your body weight.
Does this phase serve any purpose? For sure!
First, you can use it as a method of calibration checking, in that if you have a known mass, you can check if the force plates are measuring it correctly.
Second, it is important in that a lot of force plate software uses your weight to calculate relative measures which you can compare across different populations e.g. N/kg – Newtons per kg of body mass. That is, heavier individuals have greater forces (standing still force, weight) as a function of their additional mass. If you wanted to compare individual’s force capability on a level playing field, then you would normalise to body mass.
A third consideration is that your clients force outputs could simply change as a function of putting on weight (hydration status, fat mass, etc.) and not due to better activation or muscle mass, so the relative measures might provide a more granular understanding of the changes taking place. If you are comparing changes overtime with the same individual, the use of relative measures is a little less important and you can use absolute measures (e.g., 1600 N), however, always keep an eye on the change in your clients standing still force/weight and subsequent effects on other measures.
Another function of the quiet standing or the weighing phase when using a dual force plate system, is that you get insights into standing imbalances and how your clients are unweighting and potentially protecting injured limbs. An example, of this type of asymmetry is shown on the diagram.
The signals on the diagram are from dual force plates. The blue signal is from both legs combined, the purple and orange signals are from the individual force plates for the left and right leg respectively.
Dual force plates have many advantages, and you can see one right from the outset of the signal of an athlete cleared to return to play after ACL reconstruction. If the person on the force plate was evenly balanced across both legs, the signals would be overlapped. However, for this athlete you can see there is a weight shift, and the athlete is unloading his left leg i.e., there is less force (~20%) coming through this leg whilst quiet standing.
So, what does this all mean? Here are some thoughts for you…
Given that you are receiving quantitative information from the force plate, you will have a more data driven/evidence-based approach to understanding the accuracy of your diagnosis and efficacy of programming.
Take home, even in quiet standing in the weighing phase, the force plate can provide you with a stack of valuable information.
As an adjunct, how would you begin addressing this weight shift in the rehab phase? I included a picture of single limb loading with wearable resistance to get you thinking about this. I have seen wearable resistance reduce asymmetry instantaneously, which makes sense. However, what is perplexing is when the wearable resistance is unloaded the reduced asymmetry is retained or even reduced further. Any thoughts on this?
If you want to find out a little more about force plates and the CMJ, then head to this free e-book. If you want to take a look at the CMJ signal in a lot more depth and its diagnostic and programming implications click here. Finally, if you want to upskill yourself on assessing and addressing asymmetries which includes the above then click here. Stay curious.