Recently I had a couple of clients ask - "Why does the RehaMove system only deliver FES frequencies up to 50Hz - your competitor systems go to 100Hz?" When you really look closely, the truth is not actually what it seems.
There is a bit of explanation needed here because the true technical performance of a system can then be revealed.
Back in 2012 I wrote about how pulse shape makes a difference in FES applications. At that time I was responding to this practical observation:-
Why is that clients can apparently use less stimulation current intensity to generate muscle power with our RehaMove FES bike when compared with some of the competitor systems?
I wrote at the time that the answer might lie in the quality of the pulse shape our system uses to trigger muscle contraction.
There are three parameters that describe the character of the electrical signals delivered to the muscles via the electode pairs. These parameters are signal frequency, current and pulsewidth.
The electical waveforms delivered by effective FES systems are typically, so called, bipolar in form. In other words a positive-going part followed by a negative-going part. The height of the pulse (related to current), the width of the pulse and the frequency with which these pulses are delivered are also controlled by the FES system.
I was originally a control engineer by training and so you and I both know that frequency relates to waveforms or patterns of change that repeat over a period of time. Every time the pattern repeats in shape we have completed a cycle. Frequency relates to the number of complete cycles per unit time.
If we are measuring the pattern of change in Hz - we are effectively describing the number of complete cycles per second of the repeating pattern.
If we were able to examine the output from a RehaMove FES system we would see the following pattern of change generated by the stimulator.
One cycle starts, let's say, at the leading edge of the positive going pulse and ends at the leading edge of the next positive going pulse.
We can see there are +ve pulses immediately followed by -ve ones; hence the terms bipolar to describe the pattern of change.
Therefore 50 Hz means that, with the RehaMove generated FES waveform, we will have 50 positive going pulses and 50 negative going pulses in a second.
Now if we look at the pattern below, on the same time scale as the previous diagram we can see just a single positive going and then a single negative going pulse.
This pattern, in terms of stimulation energy, is actually delivering half of the energy available with the RehaMove unit.
I am led to believe by our technical advisors that when some competitor systems say they generate 100 Hz they are actually generating the equivalent of 50 Hz generated by the RehaStim 2. They are using the approach in the second diagram.
You might be thinking - OK you are splitting hairs here - does it really make any difference?
I guess one issue is perception. When clients are trying to compare systems and understand differences it matters because we want to compare "like with like".
It also matters in a practical way. When we use a term like "Hz" it has a precise meaning in engineering. Research shows that slow-twitch muscle fibres are stimulated with a frequency of about 30 Hz and fast-twitch fibres with about 35 Hz. If there is confusion about when a "Hz is really a Hz" it may lead to unexpected results and confusion.
The fact is that with the RehaMove system - setting 30Hz or 35Hz gives you exactly that. With some rival systems you may need to use 60 or 70Hz to give the same result in terms of muscle response.
When you combine this frequency issue with the pulse shape issues discussed last year it explains some of the differences we see in practice.