In the last post we looked at assessment of muscle strength. RFD, or rate of force development is another important metric describing muscle performance. It represents the rapidity of force production, and if you want to get accurate values, then your assessment requires even more attention to detail! So, let’s look at assessing rate of force development with hand-held dynamometry: avoiding common mistakes, and choosing the right device!
What Is Rate Of Force Development (RFD)?
I’ve covered this before (here and on a pod here), however, let’s have a quick recap. RFD is typically assessed during isometric contractions and represents the slope of the force-time (F-T) curve.
RFD can be measured in several different ways, assessing different parts of the force-time curve, however no matter which index you chose, the requirement is the same – an explosive muscle contraction from the participant.
The Force-Time Curve and Indices of RFD
Common Mistake #1: How To Instruct Participants
RFD is an inherently variable index, especially the early phase. Why? The early phase of an MVC and the slope of the F-T curve is determined principally by neural characteristics. This means, optimising neural drive (input into the muscular), recruitment of fast twitch motor units, synchrony of firing of motor units etc.
To achieve this and to assess an individuals true maximal performance, the contraction needs to be performed explosively. The focus is on the SPEED of contraction, often irrespective of the peak. And this can require a good few practice attempts by the participants – it’s not something we routinely do.
A sub-maximal effort or graded contraction, can often arise from commands that lack urgency and volume and as such the result will not provide you with an accurate estimate of RFD. Seriously. People need to be primed and ready to explode into the load cell, with maximal intent. Your verbal instruction here plays a vital role.
YOU NEED TO PROVIDE
- Clear instructions prior to assessment (what they need to do)
- Practice attempts prior to assessment (practice the test)
THEN
- Clear concise command (‘”Ready. GO!”) with
- High volume
Check out my assessing the plantar flexors as an example in the previous post. It’s loud!
If you’re interested in reading a bit more on this topic, here’s a paper that tests the influence of verbal instruction on the measurement of explosive neuromuscular performance.
Common Mistake #2: Selecting The Wrong Device
As I’ve mentioned several times previously (e.g. see here) and extensively in my hand-held dynamometer (HHD) Comparison Tool (accessible here), not all HHDs and devices are the same. And for the busy clinician, it can be a minefield to decide which HHD to choose.
Here’s what you need to know, if you’re really looking to obtain quality and usable RFD data to help inform your practice, treatment, rehab, RTP criteria. The device needs to have a FAST SAMPLING FREQUENCY. What does that mean? For a detailed explanation of this and why it matters, here’s a post I wrote a while ago. Briefly, sampling frequency refers to the number of samples of a signal that is obtained per second.
For events that happen very quickly (like RFD), fast sampling frequencies matter, else we risk losing information and obtaining inaccurate assessments of performance. So how fast is fast enough?
Firstly, sampling frequency is measured in Hz, 1Hz is 1 data point per second, 100Hz is 100 data points per second and so on. To obtain sufficient data points to make an accurate estimation of RFD performance, a minimum sampling frequency of 500Hz is required. Ideally I’d say 1000Hz. This isn’t just my personal opinion, albeit formed over >2 decades of experience and testing, it’s to do with the rapidity of rise of force over small time intervals.
It’s widely regarded that high sampling frequencies are required to accurately measure RFD, especially in the early phase of the force-time curve and from explosive contractions. Several data-driven studies suggest a minimum sampling frequency of 500Hz for peak force (PF) and indices of RFD [e.g. Thompson 2019] and faster (1000Hz/>) where for high fidelity data is required. Sampling frequencies below 1000Hz have been shown to underestimate peak and early-phase RFD due to insufficient temporal (time-based) resolution to capture the steep initial rise in force [e.g. Maffiuletti 2016].
We’re actually undertaking the first empirical research to investigate the sensitivity and accuracy of a multitude of hand-held dynamometers that feeds into this very topic. Here’s a sneak peek behind the scenes.
Different devices have different sampling frequencies, and this will (we hypothesise) influence how accurately they can measure peak and RFD characteristics.
Which Device To Choose?
If you haven’t already, you need to download your copy of the HHD Comparison Tool, this will guide you into making the right decision.
In here there’s a side-by-side comparison of most of the commercially available HHDs, details sampling frequency, plus a heap of other important data – see below for a snapshot. You will notice, that there are only one or two HHDs that have a sampling frequency of 500Hz or more.
Summary
So, what’s the take home from this?
There are many things that we need to attend to in order to obtain an accurate assessment of RFD, however, the first two critical factors are:
- Give a loud, clear concise command
- Use a device with an adequate sampling frequency
Once we’ve achieved these, then the other critical methodological factors should be attended to, but save that for another time.
As ever, thanks for reading. Feel free to share with friends and colleagues
ENROLLING 28th JULY
References
Jaafar H, Lajili H. The Influence of Verbal Instruction on Measurement Reliability and Explosive Neuromuscular Performance of the Knee Extensors. J Hum Kinet. 2018 Dec 31;65:21-34. LINK
Thompson BJ. Influence of signal filtering and sample rate on isometric torque‑time parameters using a traditional isokinetic dynamometer. J Biomech. 2019;83:235–242. Abst LINK
Maffiuletti NA, Aagaard P, Blazevich AJ, Folland J, Tillin N, Duchateau J. Rate of force development: physiological and methodological considerations. Eur J Appl Physiol. 2016;116(6):1091–1116 LINK
