Controlling Stimulation Intensity 1) Hi welcome. In this lesson we are looking at how we control the overall stimulation intensity. We have seen that since the 1800’s science has known that electricity can generate muscle contractions, but to create a useful and safe tool for exercise or rehabilitation we need to have fine control over how the electricity is delivered. 2) We have learned so far that the overall stimulation intensity is controlled by three things - the Frequency, Current and Pulse Width. Whilst these control the overall intensity, the relative level of each will produce different training possibilities. 3.1) Imagine that we are introducing a small amount of electrical energy into the electrodes on a muscle. This might be represented by either of the two patterns on the right. The term Frequency relates to how often the pulses occur. We might say, for example, that there are 50 pulses per second - this would be described as a frequency of 50 Hz. The term pulse width describes how long each pulse lasts. In RehaMove FES applications this is measured in microseconds and may be anywhere from 10 to 500 microseconds in duration. The third term, current, represents the height of each pulse and with the RehaMove application the value is likely to be up to 130 milliamps. Notice how small these pulses are. 3.2) One pattern consists of a train of positive pulses that repeat over time. 3.3) The second pattern consists of a train of positive and negative pulses that again repeat over time. 4) Remember Galvani’s experiments with frog’s legs. He noticed that the frogs leg muscle could be made to twitch when stimulated by electricity delivered to the sciatic nerve. It wasn’t too long after these first experiments that science started to get a better
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Derek Jones
understanding of the relationship between the stimulus being delivered and the response of the muscle. In this image we can imagine a single pulse of energy being delivered and the muscle responding by contracting and then relaxing over a few milliseconds. 5) Suppose now a second pulse were to be applied before the muscle had a chance to totally relax. The result is a more sustained muscle twitch. 6) Now we imagine a third pulse comes along before the muscle has completely relaxed and the result is an even more sustained contraction. 7) There is an important point here which is highlighted in any and every text book on electrical stimulation. At low frequencies, below about 20 Hz, a muscle will respond to stimulus by twitching as long as the total energy is sufficient to generate an action potential. As the frequency increases above 20 Hz the tension generated in the muscle will both increase and become sustained. We could refer to the generation of this smooth strong contraction as a tetanic contraction. 8) This diagram illustrates the relationship between frequency of stimulation and force generated over time. Low frequency stimulation generates a small amount of force and force increases as the frequency increases; plateauing as we reach 50 Hz and beyond. This assumes that the other two parameters, current and pulse width are constant but sufficient for the total stimulation to generate action potentials. 9) This is useful information. When we use low frequencies we tend to activate the “slow twitch”, fatigue resistant, muscle fibres. This is a good tactic to reduce the effects of muscle atrophy which is a factor in many clinical situations. When we use higher frequencies of 30 Hz and beyond we are going to activate fast twitch, fast fatigue fibres. This will tend to generate muscle strength and size. 10) The RehaMove FES unit generates what are called bipolar rectangular pulses. The fact that this has both positive and negative pulses helps to prevent skin irritation and increases effectiveness. The pulse shapes are high fidelity - in other words close to pure Page 2 of 4
Derek Jones
rectangular form. This helps efficiency as the area of each pulse is related to the intensity of stimulation. It helps us to use less energy overall in producing a particular level of response. Many inexpensive FES units will produce waveforms that have poorly controlled wave shapes. This means that much higher levels of stimulation current and longer pulse widths would be needed to produce the same effect as with a high quality system. 11) The RehaMove system generates current levels up to 130 milliamps, Frequencies up to 50 Hz and pulse widths up to 500 microseconds. 12) We have looked at how changing stimulation frequency affects the generation of a response in muscle. But what about the effect of stimulus amplitude (the current) and pulse duration on the strength of contraction? In general, the relationship between current and pulse duration necessary to generate a muscle contraction is as shown here. Longer pulse durations require less current to produce a muscle contraction and vice versa. 13) Combining this with what we have learned of the effect of frequency, we can imagine something like this chart. Combinations of current and pulse width will produce the same level of muscle contraction. We can imagine a current of 40 milliamps combined with a pulse width of 120 microseconds producing the same effect as a current of 80 milliamps and a pulse width of 100 microseconds. 14) There are two terms used to describe this characteristic - the Rheobase and the Chronaxie - and these terms are defined here. However, we won’t need to be concerned with these for our purposes. 15) As we will learn, the RehaMove system allows us various ways of setting and adjusting frequency, current and pulse width. Most often we will use either an automatic or manual adjustment of pulse width to increase or decrease the strength of muscle contractions. We will typically set current values and frequency for each muscle group but may adjust them during a session. 16) This chart and the next one summarises what we would expect with different stimulation frequencies
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Derek Jones
17) By choosing our stimulation frequency with care we can influence the muscle fibre types we are targeting, whether we train for muscle strength or endurance, relax muscles or even accelerate wound healing. That’s all for this lesson.
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Derek Jones