Laboratory 1 Shoulder loading & Electromyography

Transcription

1 Objectives: Laboratory 1 Shoulder loading & Electromyography Determine the linearity between electrical activity and muscle loading Gain familiarity with shoulder anatomy Determine how loading in the deltoid and trapezoid muscles changes as the arm is abducted and elevated Prelab assignment 1. Review the bones and muscles of the shoulder and the elbow including their origins, 1 insertions and pathways. You may use any reference materials you like. Wikipedia has a nice picture of the major bones: You should know the location and general shape of the humerus, ulna, scapula, clavicle, and sternum. The UW radiology department has a good online resource for muscles, at Pay particular attention to the deltoid, trapezius, rhomboid, biceps brachii, triceps brachii, and serratus anterior. 2. Read Introduction to the Shoulder and Introduction to Electromyography in this handout. 3. Consider the Discuss with your group questions. Bring your ideas to class; you do not need to turn them in. 4. Read the Experimental Procedure and Data Analysis sections in this handout. In the laboratory 1. Form a group of 3 at one of the data acquisition stations, and log on. 2. Follow the instructions in the Using Logger Pro section to familiarize yourself with the data acquisition software. 3. Follow procedure 1 to analyze the relationship between the load that a muscle is carrying and the corresponding EMG signal. 4. Follow procedures 2-3, using the EMG to analyze the force generated in the trapezius and deltoid muscles (and others if appropriate) while weight is held in the hand and the arm is outstretched. Lab report 1. Follow the data analysis steps for each experiment. 2. Complete an abbreviated lab report (Methods and Results only) as specified at the end of this handout. 1 Italicized words may be found in the vocabulary list, unless they indicate the Latin names of species, or are simply italicized for emphasis. Let context be your guide. print date: 4/2/2010 1

2 Introduction to the Shoulder The shoulder is a ball-and-plate joint stabilized by complex musculature that gives it a nearly hemispherical range of motion. During this lab we will consider its motion in the frontal plane only, i.e. as the arm moves directly sideways and upward from the standing body. Motion away from the body is called abduction, and motion upwards past the horizontal is called elevation. Motion down toward the side of the body is called adduction. The angle between the torso and the arm is called the angle of abduction, θ ab. Each group will choose two experimental subjects, who will lift various weights by hand while standing and holding the elbow straight. In this situation, the deltoid and trapezoid muscles carry the majority of the load. Note that these muscles operate in series, not in parallel, so the movement is shared between the muscles but the load is often carried by both (not divided). Our measure of muscle activity will be the electromyogram. It is convenient and noninvasive, and effective for monitoring the relative activity of one muscle over time. Scapula Introduction to Electromyography Clavicle Figure 1. X-ray image of the bones of the shoulder, using inverted gray scale. Source: MS Office ClipArt. The relative movement of our various body parts can occur passively or actively. Passive motion is caused by external forces such as a joint therapy machine or by body forces arising from gravitational acceleration. Active motion is effected by the coordinated contraction of muscles against a skeletal structure. Although passive motion is important both as a potential source of trauma and a facilitator for healing, we will begin our study of biomechanics with the force of muscle contraction and its contribution to the load-carrying capacity of the human body. Direct measurement of the force generated by muscle contraction is quite difficult, especially when one does not wish to dissect down to the muscle in question to apply instrumentation. One can calculate the contractile force necessary for a given movement based on anatomical dimensions and known loading, but such calculation without some sort of experimental confirmation is unsatisfying. Therefore, we would like to find a non-invasive method to estimate muscle force. Our measure of muscle activity will be the electromyogram (EMG). An EMG is a record of electrical activity, not strength, and is not a reliable means of comparing muscle force among individuals or even among one person s muscles. However, it is convenient and non-invasive, and effective for monitoring the relative activity of one muscle over time. Humerus print date: 4/2/2010 2

3 The physiological basis of the EMG will be studied in detail in BIOEN 305. Briefly, ion currents within the neurons and muscle produce temporary voltage differences that can be read on the skin. The activity of a single motor group produces voltage spikes at frequencies of 6-30 Hz and amplitudes of 20 microvolts to 2 millivolts on the skin. We will amplify these signals and will therefore record higher voltages. Higher muscle loading will generally be visible as increases in both frequency and amplitude in the EMG. voltage time Figure 1. Electromyogram from the trapezius. The signal has been rectified (similar to absolute value) and covers eight seconds. Note that the highest peaks exceed the data acquisition range and have been truncated. Source: Kim Du, In this lab exercise, each group will choose one or more experimental subjects, who will lift various weights while the electrical activity in the muscles is recorded. The relationship between the observed electrical activity and the supported weight will then be plotted and analyzed. Discuss with your group Suppose that we are examining a small muscle composed mainly of fast-twitch fibers, each of which contracts in approximately 8 msec. If each motor group (collection of fibers stimulated by a single motor neuron) contracts 30 times per second, how fast should we sample for our surface electrode measurements? What are possible sources of noise in your measurements? In what order should the various weights be applied so that you minimize (or at least infer) the impact of muscle fatigue on the experiment? Would you expect a plot of applied load vs. EMG activity to pass through (0,0)? Consider a plot in which the x-axis is angle of abduction and the y-axis is muscle force. Sketch your guess of force in the trapezius and deltoid as θ ab increases from 0 to 160. How should you collect the data (and what other information should you record) so you can do the analysis for your report? Vocabulary Proximal: Closer to the head or spine. Distal: Farther from the head or spine. Origin: The proximal end of a muscle, or its attachment point at the bone. Insertion: The distal end of a muscle, or its point of attachment. Noise: Voltage fluctuations (fast or slow) that are in a signal but do not carry information about the variable you intend to measure. print date: 4/2/2010 3

4 Materials Alcohol prep pads or paper towels and soap Adhesive Ag-AgCl electrodes (10) Data acquisition system with two channels: Two Qubit Systems S210 EMG amplifiers or two Vernier EKG amplifiers LabPro A/D converter Logger Pro software Weights (dumbbells or pails of water) Optional: Spring scales Ruler and protractor Standard Setup Procedure Note: Participation as an experimental subject in all BIOEN 301 lab exercises is voluntary and has no impact on the course grade. 1. Allow the subject to exercise lightly before starting. 2. Select the locations where the electrodes will be placed. Two electrodes are used for each muscle, and an extra electrode should be placed on a bony place away from the shoulder (e.g., over the hip bone, forehead, or vertebra). Each pair of electrodes should be spaced about 5 cm apart, along the direction of contraction; at least one should be over the muscle and the other may be over muscle or tendon. 3. Clean these areas so that they are free from oils and extra epidermis. 4. Place the electrodes and connect them to the amplifiers. For the S210 EMG Sensor/amplifier box, white=positive, green=negative, and black=ground. For the EKG sensor, red=positive, green=negative, and black=ground. 5. Set the sampling rate to 250 Hz, and confirm that a signal can be acquired. print date: 4/2/2010 4

5 Experimental Procedure 1 The purpose of procedure 1 and its analysis is to determine the relationship between average electrical activity and the effort provided by a muscle. The deltoid or biceps muscles are appropriate candidates because neither one shared a significant part of its load with another muscle. 1. Follow the setup procedure using the biceps brachii as the muscle of interest. 2. Briefly investigate how much of the signal is due to effects other than muscle contraction, such as motion of the skin. 3. Observe and record the electrical activity that occurs when each of four different weights is held steady. 4. Repeat with additional subjects as time permits. Experimental Procedure 2 1. Prepare the trapezoid and deltoid muscles for measurement. 2. Using the shoulder that is not instrumented, determine the maximum weight that the subject can hold with the shoulder abducted to Using half of the weight determined above, record and save the deltoid and trapezoid signals while the arm is briefly held static at 30 intervals from 0 to 150. This data will be used to create a plot of EMG magnitudes vs. θ ab. Run this trial three times. 4. The previous test involved static equilibrium. To include a dynamic force due to acceleration, move a weight up and down such that the arm moves in the approximate range 80 < θ ab < 100. Try to establish a rhythm in which the arm seems weightless on the descent. Save one signal that shows how the magnitude of the spikes varies as the weight is moved up and down. Compare the results for the static and dynamic trials, especially at times when the arm is slowing down and reversing the motion of the weight. 5. Attempt to find a loading condition (that is, a particular way of holding, pushing, or pulling a weight) that isolates the action of the deltoideus and trapezius. Record a representative signal from both muscles for each case. 6. Record and save signals from the deltoid and trapezoid while the arm is held static at 90 for enough time to cause muscle fatigue. Experimental Procedure 3 1. Propose an experiment that measures the force of one or more muscles that share the load on a particular bone. Note that this is different from measuring the deltoid and trapezius, which act in series; the new experiment involves muscles (or parts of muscles) that act in parallel. Examples include the anterior and posteriolateral deltoid the ascending and descending trapezius print date: 4/2/2010 5

6 the ascending trapezius and serratus anterior. An alternative is to instrument two muscles that act in opposition, such as the biceps and triceps. 2. Collect signals from these two areas over an appropriate range of arm motion. Loading may be static or dynamic. Standard Data Analysis Before we can use the EMG signal to analyze force produced by the muscles, we need to convert the spiky EMG signal into a smoother signal (or sometimes just one number) that indicates the force generated during some short time interval. This number can be based on any of the following calculations: a) the maximum voltage during the interval b) the average voltage over the interval c) the maximum of a running average (every 7 points or so), or d) the root mean square of voltage during the interval. Use of a spreadsheet program or MATLAB is suggested. The lab web page provides an averaging example in Excel, and MATLAB code that calculates a time-varying root-meansquare sequence from the original signal. The force value that you calculate can be compared only to the force produced at other times by the same muscle with the same electrode placement and amplifier gain. For this reason it is effectively a normalized force, which is dimensionless (has no units of measure). Data Analysis for Procedure 1 Once you have chosen an EMG processing method, use it to create a single force value for each dumbbell weight. Plot the force data in Excel and perform a linear regression analysis. Data Analysis for Procedure 2 & 3 The purpose of this analysis is to compare the force in pairs of muscles as the angle of loading changes. A comparison of experiment to theory will be undertaken later. 1. Choose your favorite data processing method to convert the EMG signals to force data. 2. Process three sets of data: Signals from step 3, using the EMG output during the time when the arm was static at each particular angle of abduction. In the end you will have only two data points per angle, one for each muscle. Signals from step 4, to get values at peak effort when the arm was catching the falling weight. One pair of signals you collected from muscles that act in parallel or opposition. You may choose what information to extract from the signal. print date: 4/2/2010 6

7 3. Save your data for comparison to a theoretical analysis you will do later in the quarter. Report Each student should submit an individual report containing the following two sections: Methods: Describe the placement of electrodes, the limb positions used, weights, control measurements (if any) and data analysis methods. Any computer code that you wrote for the analysis may be included in an appendix. Data analysis may be done with Excel, in which case you should state the formulas that you used, in lieu of computer code. Results: Do not include the V vs. t trace for every trial you did. Do include a representative trace, plus the processed data. If you would like to compare results among different subjects, you are welcome to include data from your entire lab section. The total number of figures should be 3-5, plus one if you want to show the electrode placement with a picture. Reports are due at the beginning of next week s lab meeting. If you would like to get our comments sooner, please turn in your lab reports sooner. print date: 4/2/2010 7

8 Using Logger Pro 1) Set up the LabPro data acquisition module and the appropriate signal amplifiers. 2) Start LoggerPro, Programs>>Vernier software>>logger Pro 3) Establish the data acquisition settings, one of two ways: a) Open the.cmbl file in which the data acquisition settings are saved. b) Create a new setup i) Experiment Setup Sensors Show all interfaces ii) Select inputs for Channels 1 and 2. Use Voltage>>Differential voltage if you don t know a better input type. iii) Select Experiment >> Data Collection (or press ctrl-d) iv) Set the sampling rate and data collection time length. For most experiments you will use 250 Hz and the time window will vary from 5 sec to 60 sec. 2 v) Save your setup. 4) Optional: Zero the input channels before each data collection, using Experiment>>Zero 5) To start data collection, click the green COLLECT button or press <space>. You should see a trace for each input channel. 6) Adjust the gain on the amplifier(s) to ensure your signal is not clipped at high or low values. 7) To stop data collection, click the red STOP button or press <space>. 8) To save data: Use File>>Export As>>Text Remember to do this after each collection period in which you captured a good signal. To clear charts and erase unsaved data: ctrl-shift-k. Note : Adjust the sampling parameters and graph options as needed to get the data at the best resolution. Note : Select Options>>Graph Options to change axes or plots. Go to the Axis Options tab. This option allows you to manually set y-axis scale (as opposed to auto-scaling). Setting the scales equal in neighboring plots can facilitate comparisons between experimental measurements, because differences will be more readily apparent. Axes may also be adjusted interactively, by dragging the axis or by double clicking the highest or lowest values on the scale. 2 If you would like to use continuous data collection (which may be selected in the Experiment>>Data Collection window), it is best to let the data scroll past the window as data is collected. To do this, choose Insert>>Additional Graphs>>Strip Chart and set it up with the proper axes (e.g. Potential 1 vs. time). print date: 4/2/2010 8