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Measuring the Conduction Velocity of Motor Axons by Surface Electromyography

Paper Type: Free Essay Subject: Sciences
Wordcount: 4282 words Published: 8th Feb 2020

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1.1 What is electromyography?

Motor neurons carry action potentials from the central nervous system to muscle fibres they innervate, where they release acetylcholine at the neuromuscular junction, which depolarizes the cells forming an action potential and eventually causing the contraction of muscle cells. Electromyography (EMG) is a test designed to evaluate nerve and muscle health by measuring and recording induced muscle action potential waveforms caused by the release of acetylcholine (1). EMG is based around the premise that if a nerve is stimulated electrically a measurable reaction will occur somewhere along the nerve. (2) Measurements of said induced reaction can be taken intracellularly with one electrode located inside the cell and the other outside or extracellularly with both electrodes located outside. The voltage or potential between the 2 electrodes is measured after a stimulus has been supplied.

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There are various types of EMGs depending on type of electrode used. Needle EMG and fine-wire EMG both use indwelling electrodes through which they can isolate single motor unit activity and are therefore invasive and painful (3). Surface EMG is non-invasive and uses electrodes placed on the skin over a muscle to monitor global muscle motor responses, or over a nerve, to monitor sensory responses (2).

1.2 Electrode configuration in Surface EMG

To depolarize muscle cells and bring about a muscle action potential nerves are electrically stimulated by a stimulating electrode containing a cathode and an anode. To detect the electrical potential in muscle cells generated by the current supplied by the stimulating electrode we use surface recording electrodes placed on the skin above the muscle of interest (3). A reference electrode (also referred to as a “ground” or “earth” electrode through this experiment) is located on an electrically neutral tissue to use as a comparison for the signal received by the recording electrodes.

There are two possible electrode configurations in EMG, monopolar and bipolar. The less commonly used set up is the monopolar which consists of one active electrode overlying the muscle and a reference electrode located on an electrically neutral tissue (nearby bone or tendon (2)). The signal generated will be determined by the difference in potential between the two.

The bipolar configuration, as the name indicates, uses two recording electrodes. Two action potential waveforms are determined by the difference in electrical signals detected between the reference electrode and each surface electrode. Differential amplification then subtracts the signal at one electrode from that at the other to get rid of the common noise. The bipolar configuration is considered preferable due to its inferior signal-to-noise ratio. In a monopolar set up, noise will be incorporated to the signal unlike in the bipolar configuration, where it can be deducted by the differentiation of the two individual readings. (4)

Figure 1. Diagram depicting the common electrode montages A) Monopolar configuration, a single surface electrode is placed on the skin above the muscle of interest. The response obtained is compared to that of a reference electrode. B) Bipolar configuration, two electrodes are located above the muscle the individual EMG readings (trace 1 and 2) obtained are then differentiated (trace 3) to remove the common noise readings (Adapted from a figure from M.A. Cavalcanti Garcia and T.M. M. Vieira 2011 (4))

2. Technical procedure

2.1   Experimental Setup

The subject was seated in a comfortable position that allowed the examiners to easily adjust the electrodes. Firstly, the earth electrode cable was connected to the dry earth strap which was then wrapped around the lower forearm of the subject.  In this experiment surface recording disc electrons were placed on the muscle belly of the thumb to measure the action potential calculated from the difference in voltage between the positive (black) and negative (white) electrodes (Fig.2).

After wetting the stimulating bar electrode in a saline water solution, it was placed over the subject’s median nerve at the wrist with the cathode positioned towards the recording electrodes (Fig.2)



Figure2. A) Picture of the experimental set up showing the placement of the electrodes on the subject. (Provided by a fellow group) B) Diagram followed during our experiment (5)

2.2   The stimulus parameters

The stimulus used was a rectangular pulse stimulus that starts at 0mA and is raised to the set current (a value ranging between 0 and 20mA) after 0.05ms and then returns to 0mA. The duration of the isolated pulse stimulus (or pulse width) was 0.05ms. The PowerLab 26T used allows the user to vary the frequency at which the stimulus is repeated. This  experiment, however, will rely on individual stimuli. (6)

2.3 Proper signal detection and processing

EMG signals are relatively small compared to the noise, including cross talk from proximal muscles, motion artifacts, ambient noise, electrical machinery and the inherent instability of the signal. While it’s impossible to reduce all noise, certain noises such as motion artifacts, which sEMG is highly susceptible to, (3) can be reduced by following a proper set up and having an appropriate electric circuit (7,8). This means avoiding wires crossing over muscles, leaving the hand at rest not producing any voluntary movement, ensuring that the cables connected to the isolated stimulator are intertwined and testing the set up prior to performing the examination. Additionally, the manufacturer claims that the channels have individual filters to reduce internal noise. (6)

To ensure that the set up used in this experiment was correct and that the stimulating electrode was situated above the median nerve, a series of initial test were performed without taking any recordings. The first trial test aimed to ensure the readings were accurate. The stimulating bar electrode was placed on the table and readings were measured without any current being applied. The expected evoked reading should be a straight line of amplitude 0. The obtained results for the first 10 trials showed a continuous sinusoidal waveform of small wavelength. The cables connected to the earth electrode and the surface recording electrodes were then rearranged to their right connection (Fig2.B) so that two electrodes showed no difference in voltage. The reading was a flat line. Following this the subject was then instructed to push their thumb against their middle finger and apply pressure which resulted in the expected reading from voluntary muscle activation which is a series of continuous compound action potentials with varying amplitudes as a higher or lower force is applied.

The second trial is designed to locate the median nerve and to figure out the required stimulus intensity to result in contraction of the abductor pollicis brevis (APB). After placing the bar electrode in the assumed correct location, the current was increased in small intervals. The position of the electrode and the pressure with which it was held had to be modified until the subject determined that he could feel a sensation in his thumb. The following results were obtained:

Current used



Subject felt the current in his hand

10mA (trial 19)

Subject felt the current in his thumb

15mA (trial 38)

Slight twitching of the thumb

15mA are required to elicit APB muscle contraction in the subject. Once the optimal position for the stimulating electrode has been determined the placement should be marked to avoid errors in measurements.

In order to estimate nerve conduction velocity of motor neurons the median nerve of the subject should be stimulated at a minimum of two different spatial locations, i.e. a distal and a proximal stimulation (Fig.3). For the purpose of this experiment a current superficial to the median nerve at the wrist and at the elbow was supplied.

Figure 3. Diagram of the suggested distal and proximal placements of the stimulating bar electrode to trigger a depolarization of APB muscle cells A) At the wrist B) At the elbow (5)




3.Results and Data analysis

3.1 Recordings at the wrist

Using the set up illustrated before the subject’s median nerve was stimulated with a current of 15mA at the wrist. The response was recorded, Fig.5, and analysed to obtain the latency and amplitude values. Latency was measured to be 5ms and Amplitude1.13mV. Figure 4 also shows the recorded stimulus artifact which serves as evidence that the stimulus was applied even if there is no sensory or motor response and that the time recorded is correct (as it should be recorded at t=0ms).



Figure 4. Action potential recorded on muscle fibres of the abductor pollicis brevis muscle after electrical stimulation of the median nerve at the wrist with a current of 15mA.





3.2 Recording at the elbow

The location of the stimulating electrode was changed to the elbow and after several attempts to locate the median nerve correctly it was determined that the minimum current necessary for APB muscle contraction was 18mA.


Despite the current differing from the one applied at the wrist the response was recorded, Figure 6, and analysed to obtain the latency and amplitude values. Latency was 8.8ms and Amplitude 9.8mV.


Figure 5. Action potential recorded on muscle fibres of the abductor pollicis brevis muscle after electrical stimulation of the median nerve at the elbow with a current of 18mA.



3.3 Data analysis

This experiment aims to calculate the motor conduction velocity of the Median Nerve. This was achieved by stimulating the median nerve at two separate points and measuring the depolarization and repolarization of APB muscle fibres.

The amplitude measured is that of the compound action potentials. Action potentials are an all or nothing event, the stimulus either reaches the threshold and induces a muscle response or it doesn’t and there is no effect on the muscle fibres. However, compound action potentials can increase in amplitude as a stronger stimulus (increase in muscle tension or current) is applied. This is because the increased stimulus will amplify the number of nerve action potentials on the same motor unit or it will cause the depolarisation of more motor units leading, in both cases, to more action potentials (1). The compound action potential is still limited by the number of cells that can be activated. Therefore, when a certain intensity (known as maximal stimulus) is reached there will be no increase in the amplitude of the potential even if the current is increased (2). This is known as maximal response or end motor wave and varies from person to person.

The latency is the time necessary between the stimulus artefact and the beginning of fibre depolarization. The two different latency times obtained from the stimulation of the median nerve at varying distances from the APB and the distance between the two points of stimulation allow the estimation of the CV through the following formula:

Cv=Distance between points of stimulation (mm)Difference in latency times (ms)

Proximal stimulation has a larger latency as the stimuli needs to travel a larger distance along the median nerve before it can reach the muscle fibres it innervates and induce their depolarisation (Fig. 6) (9).

From the results obtained during the experiment we can estimate that the motor conduction velocity of the subject’s median nerve is equal to

Cv=280mm8.8ms5ms=73.684. m/s

This value is merely a reflection of the conduction velocity of the action potentials. If the objective is to accurately determine the conduction velocity of a nerve, more trials would be necessary. Additionally, intramuscular EMG would be preferable as it provides a more precise and reliable reading of muscle potentials by reducing the noise from surrounding muscles and allowing the examiner to measure a smaller number of motor units (4,8).  While this experiment used a bipolar montage, recent studies claim the superiority of double differential signals in CV estimation and high-spatial-resolution-EMG. Therefore, an analysis of the results obtained from monopolar, bipolar and double differential signals is recommended in order to determine the most successful procedure for neuromuscular health evaluation depending on the muscle under study (4,8,10).

Figure6. Difference in voltage readings and latency between distal and proximal stimulation. (Adapted from the instructions of the practical)




During the course of this experiment the conduction velocity of the median motor nerve was assessed through the use of electrical stimuli at distal and proximal regions. The induced muscle action potentials of the abductor pollicis brevis muscles were recorded and our results showed a conduction velocity of 73.684m/s or 1m every 13.57ms. This velocity seems to be above average as estimated by a study performed on 290 subjects that found that the mean median motor nerve velocity to be 60.45m/s for males aged 17 to 21 (11).

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However, there are many factors that could affect nerve conduction velocity. In 2006 Awang et al. (12) demonstrated the inverse relationship between age and BMI and nerve conduction velocity (Fig 7). Nevertheless, this is unlikely to be the cause for the fast conduction velocity observed in this study for while the subject is a healthy 19 years old male and would be expected to have faster nerve conduction, it is still above that measured by Singh et al. in subjects between 17 and 21(11). Moreover, a comparison of 7 studies by Singh et al. (11) all show a smaller average nerve conduction velocity than that of our subject, disproving the idea that it could be related to ethnic background. This is reinforced by a study performed on Northern Plain Indians, Mexican Americans and Caucasians by Lahoria et al. (12) that proved that ethnicity is not a major factor affecting nerve conduction velocity and that therefore reference values may be used for various ethnicities.

Conduction velocity median motor (m/s)



57.56 ± 5.70

BMI (kg/m2)


57.00 ± 5.36

Height (cm)


54.09 ± 5.85


55.97 ± 4.32


54.81 ± 5.67


55.68 ± 4.79


54.21 ± 5.26


53.89 ± 5.37


53.89 ± 6.42


50.95 ± 5.28


52.15 ± 6.09


55.05 ± 5.63

Figure 7. Conduction velocities median motor nerve grouped by age, BMI and height of 250 subjects (10).

Figure 8. Comparison of the average conduction velocity of the median nerve obtained by Singh et al. and other studies. (9)

In conclusion, due to our subject’s young age and appropriate health it is likely that his nerve conduction velocity would be higher than the total average of the population. However, the result is unexpectedly high when compared to other subjects with similar attributes. Which may be due to human error when performing the test. If the experiment were to be repeated we would suggest performing various independent measurements as well as using a stimulus of equal current in the distal and proximal points.

While the subject of our investigation didn’t suffer from any neurological pathologies the diagnosis and treatment of dysfunctional propagation of action potentials and synaptic transmission is the main clinical application of sEMG. One of the most common entrapment neuropathies, affecting 5.8% of women and 0.6% of men according to Atroshi et al. (13), is Carpal tunnel syndrome (14). It is caused by the entrapment of the median nerve at the wrist leading to pain, diminished sensation in the area supplied by the median nerve (Fig.8) and weakness of the APB muscle (15). CTS patients have lowered nerve conduction velocity, a study performed by Chang et al. (16) established a 36.2% (p<0.00001) decrease in wrist-palm motor nerve conduction velocity and a 4.43% (p<0.00001) decrease in median forearm motor conduction velocity between patients and controls of the same age. The mean wrist-palm motor conduction velocity of CTS patients was 33.26 ±6.74. The mean forearm motor conduction velocity (obtained following a similar protocol as the one described in this study) was 55.26±3.56, that is a 25.004% decrease with respect to the results of our subject (16). However, in no way must our result be normalised or used as a mean of comparison. The difference was just established in order to illustrate the decrease in motor nerve conduction velocity in CTS patients as well as the unlikeliness that the result obtained during this practical is representative of the actual nerve conduction velocity of the subject. In conclusion, sEMG is a powerful tool for the diagnosis of neuromuscular diseases however practitioners must not really solely on sEMG and should support sEMG results with more accurate techniques such as needle EMG before suggesting a treatment.

Figure 9. Diagram of the area of the hand innervated by the median nerve (15)


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