Hi there and welcome to the third in this series about assessing performance and dynamometry. To obtain objective measures of muscle performance is important to help judge improvements and in support of treatment decision-making, but there are several things that influence the quality of data. In this post we’ll be specifically asking why does sampling frequency matter in dynamometry?
What is Sampling Frequency?
Sampling frequency simply refers to the number of data points sampled by a device per second and it’s measured in Hz.
- 1 Hz = 1 sample per second
- 10Hz = 10 samples per second
- 1000Hz = 1000 samples per second
You get the gist!
How Fast Does Sampling Frequency Need To Be?
This relates to the frequency of signal that you’re sampling. In most applications where we’re assessing performance using dynamometry in rehabilitation and performance-based situations we’ll be assessing muscle strength (see here on how to best measure), perhaps rate of force development, and less so electromyographic activity (see here for an article on that).
Each of these difference elements of performance require different sampling frequencies to accurately capture true performance. Let me explain.
Consider an ECG; we’re all. pretty familiar with this:
This is a fairly rudimentary example but it illustrates the point. Let’s say the person being assessed has a heart rate of 60bpm and the sampling frequency of your device is 40 Hz, how many data points would you expect to obtain per second, and how do you think that this would influence the accuracy of the visualisation of the PQRST trace?
On the left is the ECG data recorded at a sampling frequency of 500Hz and on the right is what the data might look like with a sampling frequency of 40Hz. Looks very different, right? Just 4 samples per second leads to a very different outcome.
So, how does this relate to assessing muscle performance?
Sampling Frequency and Muscle Performance
To understand this we need to look into the force-time curve and understand the rapidity of muscle force production. In well-conditioned people and indeed in non-athletes who are habituated and familiar to isometric testing, peak force (i.e. muscle strength) can occur within 300ms of force onset. That’s 1/3rd of a second!
Rate of force development (RFD) is a measure of the gradient of the force-time curve i.e. how quickly muscle force is being produced (more about RFD on a podcast I recorded here). As you can imagine and indeed see from the figure below, this is an index of performance that would require fast sampling frequencies.
Why Does Sampling Frequency Matter In Dynamometry?
The implications of obtaining a sub-optimal number of data points per second would mean that our interpretation of the slope of the line (the RFD) is in accurate, most likely underestimated. Or we could miss the peak of the force-time curve and underestimate muscle strength performance.
There are several ways in which we can measure RFD, but that’s a detailed post for another time, however, what we can say is that testing muscular performance accurately requires maximal effort from the individual, and If we’re aiming to assess RFD muscular efforts need to be explosive.
This is a detailed topic, but let me tell you that assessing RFD by zero-peak (that’s measuring the slope of the curve from force onset to maximal force), which is an index many hand-held dynamometers (HHDs) report, is influenced to a lesser degree by sub-optimal sampling frequencies than assessments of early phase RFD. Fine, so why bother with faster sampling frequencies? Well look at the figure below. Even with adequate speed of data acquisition zero-peak may grossly underestimate RFD performance. So, just be aware that the information you’re procuring.
How Fast Does Sampling Frequency Need to Be?
High sampling frequencies are required to accurately measure RFD, especially in the early phase of the force-time curve and from explosive contractions. So if you’re really wanting to use RFD data to inform important treatment decisions, especially with individuals capable of fast contractions you need to select a tool that has minimum 500Hz sampling frequency. Anything slower and you risk losing information and compromising your decision-making.
I’ve put together a tool to help you navigate the HHD market. Grab your free copy below
Summary
So, what can we take from all of this? The main aim of this article, for me, was to get you thinking about the technical specifications of the tool that you’re using – or intending to use. Do you think that it really delivers the type, quality and accuracy of data that you want to aim your treatment and training decisions?
It might not, but knowledge of this can help you avoid ill-informed decision-making and alter the metrics that you’re using to accurately judge performance. Don’t forget to download the Dynamometer Comparison Tool where we consider this topic a little further.
As ever, thanks for reading.
Claire
