Over past several weeks, there have been many posts about weakness and atrophy. Many have concerns and questions about weakness and strength and thus we try to draw comparisons of our own symptoms to those with ALS. In this case we want to know what real true and objective weakness in ALS compared to something we may be experiencing.
As I was reading an article on muscle physiology and the differences between athletes who require explosive strength those who require endurance, the article made a profound statement that hit home with me. The statement was something like this:
Initial gains in strength is a result of improved recruitment of the motor units….
In the physical examination, the physician grades strength on a scale of 0-5 and compares left and right sides at the same time. The scale is generally as follows:
0-2 = total immobility up to slight movement against gravity
2-3 = slight movement up to full range of movement against gravity (but not against resistance to opposing force)
3-5 = movement against gravity up to full normal strength (resistance to opposing force)
Many at this forum have shared quotes from their doctors about how obvious ALS presents itself and if this is true, abnormalities would likely show with weakness in the physical exam. Also, other neurological changes like changes in reflexes would be expected. As one member puts it…ALS will present like a skunk at a picnic! ( that’s my personal favorite)
However, many continue to site references about ALS patients who do not present with initial weakness during the exam, but in rebuttal it is generally accepted that an EMG will detect abnormalities early on in the absence of weakness and other neurological changes.
This post is not addressing that issue, however, I would like to invite anyone to site additional evidence beyond the one published correspondence between the Vancouver General Hospital ALS clinic and the Mayo Clinic (the infamous 6.7%). This is not a challenge but just a general call for current and accurate information in support with what our neurologists seem to think the way ALS will present.
The real purpose of this post is to address the basic facts about abnormal and normal findings on an EMG as it relates to what strength and weakness actually is.
Our muscles are comprised of three different type of fibers. Each plays a huge role in a muscle contraction that will define actual strength as an EMG sees it. Typically these fibers are classified as Type 1,Type 2 and Type 3. Also, it is important to note that during a muscle contraction, these fibers once they contract do not stay in a constant state of contraction.
Type 1 fibers are the first recruited fibers that start a muscle contraction. They have little force and react slow and resist fatigue therefore they will fire at a lower frequency rate.
Type 2 fibers are next to be recruited in and have more force thus they contract faster, They also resist fatigue but fire at a slighting higher frequency rate.
Type 3 fibers are recruited under full force muscle contraction. They are very fast and have a lot of force, but they fatigue very rapidly and fire a even higher frequency rate.
During a muscle contraction we may feel like the muscle is in constant state of contraction, but on the smallest level, these fibers are alternating at various frequencies otherwise the muscle would fatigue very rapidly and we would only be able to muster short bursts of strength without much result.
A motor unit is comprised of the neuron and the fibers it branches out to. The neuron is typically called the anterior horn cell. The rectus formis muscle, one of the muscles of the thigh has approximately 900 motor units with each unit branching to approximately 1000-2000 fibers. In ALS it is the neuron that degenerates, which in turn removes its muscle fiber from contributing to the “work load” of a contraction thus it is called a motor neuron disease and weakness is the result of decreased motor unit recruitment.
There are two ways strength is achieved. The first is to increase the firing frequency of the muscle fiber (increasing frequency increases the quantity of work) and the other is to recruit more motor units or more muscle fiber (more fibers to do the work). In real life, both are occurring at the same time, but an EMG can isolate the function of the various fiber types during a contraction thus give valuable information on the condition of the muscle which in turn is the condition of the nerve supply.
Normal vs. Abnormal Results
Over the course of history with EMG studies, it has become known that motor units are recruited in a muscle contraction at a rate of 10Hz. If the doctor calibrates his EMG machine to read at 100ms/cm and his screen is divided into 10cm hash marks, then he would expect to see 1 waveform potential every hash mark on the screen.
In a muscle contraction, Type-1 fibers will fire first at an expected frequency of around 5Hz. Actually the frequency is calculated by observing the time between two waveform potentials on the screen and dividing into 1000 since there are a 1000ms in 1 second. As more strength is needed, firing frequency will increase up to 10Hz where upon additional motor units will kick in and start at 5Hz and as work load is increased, firing frequency increases. As a general rule, increasing the work force of a contraction will result in an increase the firing rate and recruitment of more motor units. Each time a new motor unit potential contributes to the work, the firing frequency of the previous motor units in action will increase by 5Hz. When the patient reaches full contraction doctor will note the highest firing frequency and then divide it by the number of potentials on the screen. This establishes RECRUITMENT RATIO and is the basis for normal and abnormal findings. The ratio should always maintain at around 5 at the beginning of a contraction all the way to full contractions end.
We now know that when a motor unit (and all the fibers it serves) reaches a frequency of 10Hz, additional motor units will be called in. Therefore if the RECRUITMENT RATIO increases, mathematically it means less motor units being recruited and conversely if the ratio decreases below 5 then additional motor units are being recruited too early. Both are abnormal.
Since we know strength is directly related to the recruitment of motor units, if there are less units to be recruited for any given amount of work then it’s a good indication that something is damaged. Conversely, if motor units are recruited to early this would indicate that additional motor units are needed to maintain a minimal contraction where only a few should be enough to maintain the work load. Thus we have objective weakness verified by EMG.
Another way to look at strength in EMG is to consider that motor units are actually being recruited at different time intervals and not synchronously. Therefore during a full strength contraction, the patients should be recruiting a full compliment of motor units in the muscle- all of them firing but at different times. The waveform should look like chaos. That is any one motor unit potential can not be clearly seen. This chaos is called interference and represents a normal finding by fact that there are many motor units available to contribute to a muscle contraction. In contrast, if only a few motor units are available, the waveform will have a more orderly appearance and individual potentials can be identified on the screen when they shouldn’t.
At this point, there needs to be some indication as to whether or not abnormal findings are a result of a muscle disorder, or nerve damage and disease. It is generally understood that in ALS or acute nerve damage that the frequency (firing rate) of the early motor units (i.e type-1 fibers) needed to maintain minimal contraction, will be higher. In other words when the doctor asks you to minimally contract the muscle, those motor units that should be firing at around no more than10Hz start firing at 15Hz or higher then nerve damage is indicated. In simple terms in order for these motor units to maintain a minimal level of work, they have to fire faster and faster. There is no help on the way from additional motor units. In a muscle disorder, firing rate (frequency) stays the same but additional motor units are recruited too early.
So to conclude this rather lengthy and complicated post, motor unit recruitment is at the heart of muscle strength. If you’ve had an EMG showing normal recruitment pattern and firing frequency, you wouldn’t be a suspect for ALS or a muscle disease.
There are many more features in an EMG that are evaluated, but in terms of strength and muscle contraction, this is what separates normal from abnormal. If ALS patients present only with muscle fasciculation, then the EMG should do a good job to pick up these abnormalities in recruitment pattern-motor neuron diseases do not preserve the motor unit.
End of discussion for now. Hope to hear from many of you on this!
Adapted from Dr. Friedheim Sandbrink, MD staff Department of Neurology, Veterens Affairs Medical Center Washington DC “MOTOR UNIT RECRUITMENT IN EMG”
As I was reading an article on muscle physiology and the differences between athletes who require explosive strength those who require endurance, the article made a profound statement that hit home with me. The statement was something like this:
Initial gains in strength is a result of improved recruitment of the motor units….
In the physical examination, the physician grades strength on a scale of 0-5 and compares left and right sides at the same time. The scale is generally as follows:
0-2 = total immobility up to slight movement against gravity
2-3 = slight movement up to full range of movement against gravity (but not against resistance to opposing force)
3-5 = movement against gravity up to full normal strength (resistance to opposing force)
Many at this forum have shared quotes from their doctors about how obvious ALS presents itself and if this is true, abnormalities would likely show with weakness in the physical exam. Also, other neurological changes like changes in reflexes would be expected. As one member puts it…ALS will present like a skunk at a picnic! ( that’s my personal favorite)
However, many continue to site references about ALS patients who do not present with initial weakness during the exam, but in rebuttal it is generally accepted that an EMG will detect abnormalities early on in the absence of weakness and other neurological changes.
This post is not addressing that issue, however, I would like to invite anyone to site additional evidence beyond the one published correspondence between the Vancouver General Hospital ALS clinic and the Mayo Clinic (the infamous 6.7%). This is not a challenge but just a general call for current and accurate information in support with what our neurologists seem to think the way ALS will present.
The real purpose of this post is to address the basic facts about abnormal and normal findings on an EMG as it relates to what strength and weakness actually is.
Our muscles are comprised of three different type of fibers. Each plays a huge role in a muscle contraction that will define actual strength as an EMG sees it. Typically these fibers are classified as Type 1,Type 2 and Type 3. Also, it is important to note that during a muscle contraction, these fibers once they contract do not stay in a constant state of contraction.
Type 1 fibers are the first recruited fibers that start a muscle contraction. They have little force and react slow and resist fatigue therefore they will fire at a lower frequency rate.
Type 2 fibers are next to be recruited in and have more force thus they contract faster, They also resist fatigue but fire at a slighting higher frequency rate.
Type 3 fibers are recruited under full force muscle contraction. They are very fast and have a lot of force, but they fatigue very rapidly and fire a even higher frequency rate.
During a muscle contraction we may feel like the muscle is in constant state of contraction, but on the smallest level, these fibers are alternating at various frequencies otherwise the muscle would fatigue very rapidly and we would only be able to muster short bursts of strength without much result.
A motor unit is comprised of the neuron and the fibers it branches out to. The neuron is typically called the anterior horn cell. The rectus formis muscle, one of the muscles of the thigh has approximately 900 motor units with each unit branching to approximately 1000-2000 fibers. In ALS it is the neuron that degenerates, which in turn removes its muscle fiber from contributing to the “work load” of a contraction thus it is called a motor neuron disease and weakness is the result of decreased motor unit recruitment.
There are two ways strength is achieved. The first is to increase the firing frequency of the muscle fiber (increasing frequency increases the quantity of work) and the other is to recruit more motor units or more muscle fiber (more fibers to do the work). In real life, both are occurring at the same time, but an EMG can isolate the function of the various fiber types during a contraction thus give valuable information on the condition of the muscle which in turn is the condition of the nerve supply.
Normal vs. Abnormal Results
Over the course of history with EMG studies, it has become known that motor units are recruited in a muscle contraction at a rate of 10Hz. If the doctor calibrates his EMG machine to read at 100ms/cm and his screen is divided into 10cm hash marks, then he would expect to see 1 waveform potential every hash mark on the screen.
In a muscle contraction, Type-1 fibers will fire first at an expected frequency of around 5Hz. Actually the frequency is calculated by observing the time between two waveform potentials on the screen and dividing into 1000 since there are a 1000ms in 1 second. As more strength is needed, firing frequency will increase up to 10Hz where upon additional motor units will kick in and start at 5Hz and as work load is increased, firing frequency increases. As a general rule, increasing the work force of a contraction will result in an increase the firing rate and recruitment of more motor units. Each time a new motor unit potential contributes to the work, the firing frequency of the previous motor units in action will increase by 5Hz. When the patient reaches full contraction doctor will note the highest firing frequency and then divide it by the number of potentials on the screen. This establishes RECRUITMENT RATIO and is the basis for normal and abnormal findings. The ratio should always maintain at around 5 at the beginning of a contraction all the way to full contractions end.
We now know that when a motor unit (and all the fibers it serves) reaches a frequency of 10Hz, additional motor units will be called in. Therefore if the RECRUITMENT RATIO increases, mathematically it means less motor units being recruited and conversely if the ratio decreases below 5 then additional motor units are being recruited too early. Both are abnormal.
Since we know strength is directly related to the recruitment of motor units, if there are less units to be recruited for any given amount of work then it’s a good indication that something is damaged. Conversely, if motor units are recruited to early this would indicate that additional motor units are needed to maintain a minimal contraction where only a few should be enough to maintain the work load. Thus we have objective weakness verified by EMG.
Another way to look at strength in EMG is to consider that motor units are actually being recruited at different time intervals and not synchronously. Therefore during a full strength contraction, the patients should be recruiting a full compliment of motor units in the muscle- all of them firing but at different times. The waveform should look like chaos. That is any one motor unit potential can not be clearly seen. This chaos is called interference and represents a normal finding by fact that there are many motor units available to contribute to a muscle contraction. In contrast, if only a few motor units are available, the waveform will have a more orderly appearance and individual potentials can be identified on the screen when they shouldn’t.
At this point, there needs to be some indication as to whether or not abnormal findings are a result of a muscle disorder, or nerve damage and disease. It is generally understood that in ALS or acute nerve damage that the frequency (firing rate) of the early motor units (i.e type-1 fibers) needed to maintain minimal contraction, will be higher. In other words when the doctor asks you to minimally contract the muscle, those motor units that should be firing at around no more than10Hz start firing at 15Hz or higher then nerve damage is indicated. In simple terms in order for these motor units to maintain a minimal level of work, they have to fire faster and faster. There is no help on the way from additional motor units. In a muscle disorder, firing rate (frequency) stays the same but additional motor units are recruited too early.
So to conclude this rather lengthy and complicated post, motor unit recruitment is at the heart of muscle strength. If you’ve had an EMG showing normal recruitment pattern and firing frequency, you wouldn’t be a suspect for ALS or a muscle disease.
There are many more features in an EMG that are evaluated, but in terms of strength and muscle contraction, this is what separates normal from abnormal. If ALS patients present only with muscle fasciculation, then the EMG should do a good job to pick up these abnormalities in recruitment pattern-motor neuron diseases do not preserve the motor unit.
End of discussion for now. Hope to hear from many of you on this!
Adapted from Dr. Friedheim Sandbrink, MD staff Department of Neurology, Veterens Affairs Medical Center Washington DC “MOTOR UNIT RECRUITMENT IN EMG”