Getting what you ask for
The importance of cueing for the capacity you're trying to investigate with isometric tests on force plates.
My name is Ryan Mc Laughlin. I hold a Bachelors Degree in Sport Coaching & Performance, a Masters Degree in Strength & Conditioning and am currently studying towards my PhD in applied sport science. This and the upcoming pieces are based on anecdotal experiences and aiming to answer common questions I get asked on a regular basis from other practitioners.
Conducting a force plate test with recent advancements in modern day technology is relatively straight forward. Gone are the days of intimidating, expensive testing technology with the barrier to entry often being a PhD in the topic of force plate analysis or sometimes even more.
With accessibility being at an all time high, the key focus now must be on the human element. How well the athlete executes the test is extremely important for collecting high quality data. In my opinion however, how the practitioner cues the test is even more important.
For this topic I’ll use an isometric test as an example. Isometric tests are typically performed with maximal effort and are seen as a safer method of assessing strength due to the static nature of the test when compared to a heavy deadlift or squat close to an individuals one rep max.
I’ve often been told and have told others “You can’t go wrong getting strong”, and still completely agree with this statement. In the context of force plate testing however, we must always consider the type of strength we are trying to investigate. How much force they can produce, or how quickly they can produce force. Producing high levels of force in small amounts of time is obviously the overall performance goal and with most athletes, the performance requirement. However, by using force plates, we are able to zone in on either of these qualities in isolation.
For my first example of “Peak Force”, otherwise described as the most amount of force the athlete can exert into the force plate, time is typically not a constraint. Coaching cues like “push as hard as you can” or “bend the bar”, will allow us to see high forces relative to the athletes capacity. The force trace typically shows a more gradual increase until the peak becomes apparent. Peak Force is typically measured in Newtons (N) and can be quickly converted to it’s Kilogram (KG) value if you divide the N value by 9.806. For example, a peak force of 4000N, divided by 9.806 gives us a force value of 407.9kg. I find athlete’s can understand this and buy into the testing process a bit more when I present it as a KG value.
The above force trace shows what I’d expect from a typical peak force assessment. While the nice thing about dual force plates is the insight to asymmetries as seen with the blue (left) and orange (right) traces, the bilateral force shows a gradual build towards achieving the peak force at the end of the movement.
Cues I’ve found to be successful when examining maximum strength in the IMTP (isometric mid thigh pull) or ISO Squat tests;
“Pull / Push as hard as you possibly can.”
“Bend the bar”
Both of these cues are paired with constant and loud verbal encouragement from the practitioner.
Secondly as we move to investigating how quickly an athlete can produce force, RFD (rate of force development) comes into the spotlight. RFD is measured in Newtons per second (N/s) and gives us an insight into how quickly force is produce. Using force plate technology like ForceDecks allows you to quickly get a RFD number but also identify how RFD occurs in the early stages of movement initiation such as 100ms and 200ms. As with the peak force assessment and even more crucial with RFD, ensuring the athlete starts from a stable amount of force is key to ensure high quality data. What this looks like in practice is to have the athlete get into position, get them to apply a small amount of pretension (no more than a 10% increase on baseline force) and hold that steady until instructed to execute the effort. Pretension is also more commonly understood as ‘taking the slack out of the bar’.
With coaching cues emphasising speed of movement such as ‘Push as fast as you can’ or ‘Explode’ we can typically expect a close to vertical increase in the individuals force trace. RFD in a visual form is essentially measuring how steep the force trace is on that initial execution of the movement.
Cues I’ve found to be successful when examining RFD in the IMTP (isometric mid thigh pull) or ISO Squat tests;
“Push as fast as you can”
“Explode”
“3-2-1 Push”
As usual, these cues are paired with constant and loud verbal encouragement from the practitioner.
RFD is a crucial quality of strength in both performance and rehabilitation. One key insight I’ve learned over the last few years is as an athlete begins to heal from an injury and regain strength, they will be able to perform well in a peak force assessment alot earlier than in a RFD assessment. Caution on their part will usually be the cause of the delay but it’s increasingly important to understand that returning an athlete to being ‘strong’ is only a part of the puzzle.