Download EMG Lab

HK 474 Advanced Biomechanics
EMG and Force Transducer Lab
Purpose:
 Demonstrate the correlation between muscle force and EMG.
 Demonstrate how limb position changes the contributions for muscles crossing a particular
joint. Specifically, brachioradialis and biceps brachii when producing an isometric elbow
flexor torque in pronation, supination, and neutral forearm orientations.
Resources:
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EMGlab.xls
Practicum on sEMG v1.5.pdf (pages 1-72 and page 102)
Deluca 1997 – Surface Electromyography: JAB
Delsys EMG system and host computer with EMGworks software
Force-transducer set-up (transducer, amplifier, conversion box)
Force plate computer with Datatranslations or AMTINetforce software
Handle-Chain system with force dynamometer
25 lb dumbbell
Prior to the lab, students must do an article/textbook search to determine the proper
location to affix an electrode for recording from the biceps brachii, brachioradialis, and the
lateral head of the triceps brachii.
Preliminary equipment check
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2.
Ensure the force transducer is calibrated and that the system is collecting data in an
expected manner.
a. Apply a known amount of tension through the transducer and determine if the
collected data is consistent.
Ensure the EMG system is properly collecting data.
a. Perform a 10 second contraction of increasing intensity. The RMS of the EMG
signal should steadily increase over the 10 second interval.
Dr. Sasho MacKenzie – Advanced Biomechanics
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Effect of forearm position on the contribution from the elbow flexors
Collect EMG data simultaneously from the biceps brachia and brachioradialis during a five
second isometric contraction while holding a 25 dumbbell in the hand under three conditions.
1) With the forearm in a pronated position.
2) With the forearm in a neutral position.
3) With the forearm in a supinated position.
Each condition will have the upper arm hanging vertically at the side with the forearm flexed at
90.
You will also need to capture a reference trial so that the EMG data can be normalized to a
known standard. This reference trial could be with the forearm in a neutral position with the
muscles activated to approximately 80% Maximum Voluntary Contraction (MVC). This can be
accomplished by using the force transducer to determine what 100% MVC is for elbow flexion.
Perform a simple calculation to determine how much force would be produced at 80% MVC.
Then collect EMG muscles while the subject continual generates 80% MVC.
After normalizing the data to the reference trial, use a bar graph to plot the peak RMS EMG for
each of the three conditions for both muscles. Write a brief discussion regarding your findings
and any limitations of the experiment.
Relationship between endpoint force (joint torque) and muscle EMG
Perform an isometric elbow flexion contraction in which the force continually increases over a 10
second interval from minimal activation to MVC. Collect both EMG and force transducer data
simultaneously. Ensure you take steps which allow you to time synchronize the force and EMG
signals following data collection.
Questions: For each answer include the source(s) if applicable (Deluca, 1997, p.7, line 18)
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Explain why the raw EMG signal contains both positive and negative spikes? (2)
What is a single differential electrode? What types of noise does it attenuate? What
types of noise does it not attenuate? (3)
What is a pre-amplified electrode and what purpose does it serve? (1)
What is the purpose of a reference electrode? (1)
For most biomechanical applications the raw EMG signal should be band-passed
filtered. What are the suggested lower and upper cut-off frequencies (Hz) for an EMG
filter and why are these values suggested. (2)
Explain the concept of “normalization” with regards to EMG and provide an example of
an experiment in which normalization is necessary. Explain two methods for
normalizing an EMG signal. (2)
Provide an example of an experiment in which normalization is not necessary. (1)
Dr. Sasho MacKenzie – Advanced Biomechanics
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During an isometric contraction, the force generated by a muscle is correlated with the
surface EMG measured from that muscle. Describe three main factors that confound this
relationship. (3)
Explain why surface EMG collected during a dynamic contraction is not necessarily
indicative of the force being generated by the muscle. (1)
You have a strain gauge force transducer connected to a 12 bit A/D card. If the output of
the force transducer is +/- 300 lbs, then what is the resolution of the system? In other
words, your system can measure to the nearest ___ lbs. (1)
Calculate the RMS of the Biceps EMG signal in the EMGlab.xls file. Use a window
length of 1 s and an overlap of 0.999 s. The RMS equation is shown below. The first
window will start at t = 0 s and end at t = 1 s, which means your first RSM value will be
in the middle of this window at 0.5 s. Likewise, your last RMS value will be at 9.5 s.
Here are some tips: (2)
a. First create a column which squares each raw EMG data point and multiplies that
squared value by the time interval.
b. Create a second column which sums the values from the column you created in a.,
divides the sum by the time length of the window, and then takes the square root.
c. If your window length is 1 s, then your first entry in the column created in b.
should occur at t = 0.5 s. You should be able to drag this equation down the
column until you reach the row equivalent to 9.5 s.
Plot the RMS EMG versus time on a clearly labeled graph. On the same graph, also
include the raw EMG signal. Be sure to modify the order of the time series so that the
RMS shows up on top of the raw signal (as opposed to behind). (2)
Plot the Cable Tension versus time on a clearly labeled graph. (1)
Plot the RMS EMG on the X-axis and Force on the Y axis. Be sure these signals are
synchronized in time. Use the “Add Trendline” feature in Excel to show the regression
line, regression equation and R2 value for the relationship between force and RMS EMG.
(2)
Comment on the results from the last question and explain what factors may prevent a
perfectly linear correspondence between force and RMS EMG. Be specific to this
experimental set-up. (2)
 1 t T
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RMS (t  1 2 T )    EMG 2  t 
T t

1
2
Where,
t
= time between data points in seconds
EMG = raw EMG signal
T
= the length of the RMS window in seconds
Dr. Sasho MacKenzie – Advanced Biomechanics
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