I think it’s time to revisit this topic. Prompted by a recent publication, in this post we’re going to cover RFD vs strength, outline how they are different and then the specificity needed to rehabilitate and train these separate qualities of muscle performance.
The Problem
This paper (link) came up in a search I performed the other day, using search terms “power” “RFD” and “falls”. What do you notice?
Strength is in the title, yet the first sentence of the abstract describes RTD and RFD (I’ll explain what these are below). Delve further and you clearly see that they’re not focussed on strength at all, but RTD and RFD.
Why am I annoyed? Well, strength and RFD are NOT the same thing and inappropriate interchangeable use of terminology like this causes confusion for non-neuromuscular physiologists and can really impact treatments and rehabilitation. In peer-reviewed publications, we really should do better,
RFD Is Not Strength
I spend a lot of time teaching rehab professionals how and why RFD (rate of force development) and strength are not the same thing, to empower them to develop specific and effective rehab programmes. I get why people are confused. It’s because this kind of thing isn’t typically taught in professional qualifications, like in physiotherapy etc. … And it’s also because in the media there’s this interchangeable use of a multitude of terminologies to refer to muscle performance.
The lay person can be forgiven for using the terms strong and powerful interchangeably to refer to muscle performance. How would they know the difference? But there is a difference and to become strong and to become powerful require different approaches to training and rehabilitation. As such, the editorial process needs to make sure trusted sources of information, like peer-reviewed publications are clear and correct. Sadly, as we can see from above it doesn’t always happen.
For me, saying RFD and strength are the same is like saying the shoulder and elbow are the same. They’re related, but they’re definitely not the same!
What is RFD?
RFD is an acronym for rate of force development. RFD is typically measured during isometric contractions and represents the slope of the force-time curve. (For a more detailed discussion on RFD see here). You may also see the acronym RTD, this stands for rate of torque development, and is typically obtained from isokinetic, dynamic contractions.. As either name suggests, this quality of muscle performance describes the rate by which force is developed – i.e. how quickly force can be produced.
There are several ways in which we can measure and report RFD. See in the figure below, we’ve partitioned the slope of the force-time curve into 50ms epochs. However, not many commercially-available dynamometers and software packages enable you to do this (for more detail on dynamometry see here and here) and they typically report over a larger portion of the curve (e.g. zero to peak, zero – 100ms etc).
No matter how you report it you get the idea of what we’re looking at in terms of performance.
What Is Strength?
Strength, as you can probably identify from the figure above, is the maximal amount of force produced, described above as Peak Force. As you can also probably see, this is most definitely not the same as RFD. They are separate indices of performance, driven by different physiological events, requiring different specificities of training to optimise performance.
RFD vs Strength Training
There are some common physiologic determinants that contribute to RFD and strength e.g. neural drive, fast twitch recruitment, muscle cross sectional area, but different relative proportions. And even within the RFD index itself, for example, the early part of the force-time curve (early phase RFD) is driven principally by neural factors, and the latter part (late phase RFD) is driven more by morphologic factors.
Simply put, to optimise abilities for maximal force output i.e. to get stronger, we need heavy loads, few repetitions per set and time to recover between sets (~2 mins or longer depending on the application).
To optimise abilities for fast force production i.e. fast rate of force development, we need to focus on the speed of contraction. There are many ways in which we can train for RFD and power, and it’ll depend on the application, but the key difference here is to be EXPLOSIVE in intent during each rep.
If you want more guidance on this check out the series below 🙂
More Guidance For Your Practice
To find out more about power and RFD sign up to the this free mini webinar series. Recordings available for those out of the time zone.
- 3rd March Clarifying the difference between muscle power and RFD
- 4th March Muscle power, an important mediator of morbidity & mortality?
- 5th March Reversibility of power deficits in the young & old
- 6th March Training power for sports performance
As always, thanks for reading 🙂
