Physical Ergonomics is the science and study of using appropriate work postures to reduce the strain on workers which in return will maximize productivity. This grip strength experiment measures the maximum grip strength capability and EMG generated between the neutral and flexed wrist position as well as between genders.
After letting the subject know what data you intend to collect and how you will collect it, you need to make sure there are no previous wrist or hand injuries such as carpel tunnel. If so, let the patient know that they are not eligible for testing because administering the experiment could lead to injury. After verifying that your patient is fully aware of the procedure and that there are no previous condition that could alter the experiment, prepare the subject. Since this was a simple lab experiment, we did not clean or prepare the skin for subjects.
If this was a more in depth lab, it’s suggested to fully clean the skin of any contaminants and to prepare the skin by minor abrasion as to acquire to most accurate EMG readings. Set the subject in the testing chair and get ready to apply all the testing sensors. The first sensor is the external EMG electrode which is secured via double sided tape. Placement of the external EMG sensor should be ? the distance from the Humeral Medial epicondyle to the Styloid Process on the Radius side (see attached diagram) of the subject’s dominant arm.
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After marking the placement, we had the subject keep their finger on the location, prepare the EMG sensor. Place the double sided tape onto the EMG electrode and place at the marked location. The EMG electrode should be placed with the marked spot between the two sensors and be oriented parallel to the muscle fibers. The EMG electrode will collect electrical muscle activity at a rate of 1000 Hz/sec. Do not plug the EMG electrode into the DataLog reader just yet. Next on the list is to secure the R206 EMG ground.
This sensor should be placed on the Humeral Medial epicondyle from which you measured in the previous step. Make sure the sensor is secure and has a snug fit. The subject now has all of the appropriate and required sensors needed for the experiment/data collection. The EMG ground should remain unplugged for now, just like the EMG electrode. Open up your Biometrics Data Analysis Software. We will now zero out the DataLog to acquire the most accurate readings. With the EMG electrode and ground still unplugged, go to the ‘Set Up’ tab on your software. Select Analog Input.
The “Flexors” or EMG ld be collecting at 1000Hz/sec and the dynamometers should collect at 50Hz. Once you have verified these collection rates, click ‘Zero All’ and then OK. The DataLog is now zero’d out and you can plug in the EMG electrode and ground into their respective ports. The Dynamometers should be plugged in at this point at well. You are now ready to start collecting EMG and dynamometer data. Press the ‘S’ key to start collecting. Have the subject squeeze and release two or three times, posture doesn’t matter at this point, and then press the ‘S’ key again to stop collection.
This is just a test to see if everything’s reading correctly. The EMG readings, purple on the graph, should correlate with the force line, blue on the graph. If these appose each other, or one doesn’t show up, go back to the first step and re apply all the sensors while checking their placement; mistakes happen. Follow the steps just as before. If the graph checks out and everything looks correct, we can now start the actual data collection. We collected the neutral grip strength first, followed by the flexed grip force. Collect 2 samples for each position.
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Neutral Postures: For the neutral grip tests, the subject should sit in an upright position, feet flat on the floor in front of them, with the test arm at a 90degree angle parallel to the floor. Their wrist should be in the neutral position. Neutral Positions Flexed Positions Flexed Positions: The flexed grip test had the same seating posture and the only change was in the posture of the wrist. Pinch grip tests use the same seating positions and wrist postures as Grip, but with the grip dynamometer. See attached photos for a better look at wrist postures with each dynamometer.
Regardless of which Dynamometer you’re using, the method of collecting and recording data will be the same. The person running the software should let the subject know when to start, after the subject starts gripping/pinching, press the ‘S’ Key to start recording. The subject should keep gripping/pinching as hard as they can until the recorded tells them to release. After 3-4 seconds, the recorder should press the ‘ S’ key once more to stop recording and then tell the subject to release. Select the appropriate option for which dynamometer you are using and record the maximum force applied.
Repeat once more in the neutral position and record max force. Have the subject switch to the flexed position and repeat the process for recording max force. Depending on which dynamometer you used first, switch to the other and repeat the above steps for recording max force. The measurement of EMG we are looking for is the mean EMG emitted for each test. Work backwards through each trial to record the EMG. To display the appropriate EMG, you need to alter the graph to not show the negative EMG reading; otherwise the mean will just be zero.
First, make note of which EMG you are recording, the first will be the 8th or final test for each subject. Select ‘Rectify’ and then check the box next to manipulate the negative values into positive values. The graph will still be really “choppy” at this point and you need to smooth out the curve. To do this, Select ‘Average’ and select the check box next to it as well. You can now record the mean EMG reading for that test. After recording, close out the current graph and move to the previous one; repeating the above process of displaying the true EMG mean for each test and recording as you go.
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You are now done with the first subject. Remove the EMG ground and electrode from the subject and unplug them from the DataLog before moving onto the next subject. You will need to prepare and zero out the DataLog for each subject. Zeroing out the DataLog, Posture, force tests and collection, as well as EMG collection will follow the same procedure for each subject. After recording all your data for each subject, you are now ready to analyze our data. The methods we used to analyze the data were converting the EMG readings to percentages so that each of the four subject’s readings would be on the same scale.
For forces were also converted to percentages to be able to compare them to the EMG readings. Results: The mean and standard deviations of force and EMG are presented in Table 1. The most force was produced in the neutral power grip posture. The least force was produced in the flexed pinch grip posture. Respectively, maximum and minimum EMG readings were in the same grips/postures. Table 1. Mean (SD) Grip Force and EMG (%MVC) as a function of grip type and Posture Force: Efficiency: EMG:
The goal of this study is to show how posture affects force capability as well as corresponding muscle activity ,all of which affects worker performance. Our results showed that in each grip type, the neutral posture yielded the highest and most efficient forces and EMG. Shifting to the flexed posture for both grip types resulted in a significant decrease in force generated and muscle activity; some decreased by more than 50 percent. These findings are consistent with Mogk & Kier (2003) as well as Hallbeck and McMullins (1993) findings which showed that flexed positions reduced maximum force output.
It is important to know the relationship between muscle length, and cross sectional area is directly related to the force capabilities. When the wrist is in a flexed position, the flexor muscles are shortened which reduces their ability to generate force and withstand tension. The neutral position is optimal because the muscles are neither pre flexed or stretched, resulting in a higher ability to withstand tension and generate force. With these findings we learn that if possible, work tasks and tools should be designed to eliminate inefficient postures and reduce worker stress thereby increasing productivity and worker efficiency.
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Design should avoid flexed wrist postures whenever possible because it requires more work to do the same task’ increasing tension on the muscles. Repetitive motions and longer work durations when in the flexed postures increases the chances of muscles strain and quickens fatigue which may result in injuries and musculoskeletal disorders. Conclusion: Awkward and flexed postures decrease muscle forces able to be generated as well as muscle activity. These awkward postures also increase strain on the workers which leads to faster fatigue.
With this in mind, tasks and tools need to be designed around the worker rather than having the worker adapt to their environment; reducing the need for awkward postures whenever possible. By doing so, this will increase productivity and make worker output more consistent. Power grip should be preferred over pinch grip when applicable. If pinch grip isn’t avoidable, the task and tool should be designed in a way which puts the least amount of stress possible.
