Biomechanical Elbow Model. Rochester Institute of Technology, Rochester, NY, USA

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1 Biomechanical Elbow Model 1 Amanda Cook, 1 Maria Romero, 1 Chris Harley, and 1 Shannon Keenan 1 Rochester Institute of Technology, Rochester, NY, USA avc9971@rit.edu INTRODUCTION The goal of the biomechanical elbow model is to provide second year biomedical engineering students with a better understanding of the function of the muscles in the arm during elbow flexion and wrist rotation. The current classroom model only includes one muscle, the biceps brachii, and one wrist position, supination. We want to improve upon the current classroom model by creating an elbow model with three 'muscles' that work as a system on the elbow joint and generates more realistic results. The three muscles and three bones included in the model are the biceps brachii, brachialis, brachioradialis, humerus, ulna and radius, respectively. The model will incorporate the ability to change elbow flexion angles and wrist positions, furthering student s understanding of the impact of joint position in biomechanics. METHODS The model is comprised of five main components: a truss structure, muscles, bones, force gauges, and a goniometer. The truss structure is built from PASCO structure sets and acts as the base to which the humerus bone and force gauges connect. In this elbow model, the three muscles mentioned above are represented by PASCO strings. The three bones were modeled in Solidworks. This allowed the choice of adding structures to the bones that allow for a better interface with other model components. For example, spacers were added to the humerus and ulna to create a better placement point for the goniometer. Holes were added to the humerus, ulna, and radius to allow for easier assembly of the muscles and muscle attachments. The CAD models of the bones were

2 3-D printed in the Construct with a 40% density infill using PLA filament. 3-D printing the bones was advantageous since it allowed the freedom to add necessary changes to the bones in a relatively short time. The force gauges are connected to the truss structure on one side, and the string is connected to the other side. The goniometer is used to measure the angle of elbow flexion, distance between the humerus and ulna. The different wrist positions are achieved using two strategically placed pin locks. One pin lock is located on the upper part of the radius (near the elbow) and the other is at the end of the radius (near the wrist). The pin locks both connect through the radius and ulna. The pin lock near the elbow only has one set position, however the pin lock near the wrist has three set placement positions. These three set positions correspond to an arm in supination, neutral, and pronation. The force gauges measure the force (tension) on each muscle (string) for the given wrist position and elbow flexion angle. During research, EMG data from literature was discovered and used for theoretical comparison. The EMG data gave a percentage of the force each of the three muscles should expect to experience when a weight is held in the hand. Our model was developed by matching the EMG data to the percentage of force our model s muscles experienced. However, matching the data was not as simple as it seems because the tension measurement of the strings is highly sensitive. A slight deviation off of the desired length can significantly alter the force readings. To combat this issue, the group used a combination of bow line and taught line knots. The bow line will not slip whatsoever and the taught line is free to move only when someone slides the knot. Using the taught line, all three strings were able to be adjusted at the same time until the desired tensions were obtained. The taught lines were then all glued in place, ensuring students would not be able to change the string lengths during experimentation. The goniometer measures

3 the elbow flexion angle, with full extension acting as the origin and flexion upwards is a positive, increasing angle. To complete the experiment, a weight is placed on the end of the radius and ulna and the force on each string is measured. The ratio of force is determined by the force on each muscle compared to the force of the weight placed on the model. The ratios are then compared to the theoretical data found in the literature mentioned above to ensure the model s accuracy. Figure 1: The truss structure with the muscles, bones, force gauges, and goniometer attached. Figure 2: The three positions of wrist rotation (left to right: supination, neutral, and pronation).

4 Figure 3: The interface of Capstone with the three force gauges and goniometer. RESULTS AND DISCUSSION Figure 4: Force on each muscle at each wrist position with 90 degrees elbow flexion. Results for each muscle at each wrist position were obtained for an elbow flexion of 90 degrees and can be viewed in the table above. The percentage of force on each muscle is significantly higher than the expected value. However, these forces are higher by about a factor of two. When the percentage force on each muscle was halved, the results were almost identical to the expected force (within a 10% range).

5 There are several sources of error that may have an influence on the percentage force experienced by each muscle. First, the weight of the arm in the model is not being taken into account. The 3-D printed bones, goniometer, pins, and hooks will all lead to a force and moment around the elbow which would explain a higher force reading than expected. In addition, the force gauges from PASCO each carry a measurement error of ± 1 N. This force gauge error is significant because the force reading from the brachioradialis is generally close to 0.5 N, resulting in an error possibly as high as 200%. However, the percent error the bicep and brachialis experience from measurement will generally be closer to 10-15%. Furthermore, our model only incorporates muscles and bones, not ligaments and tendons, which is not anatomically accurate. In human experimental data, some of the force may have been taken off of the muscles and loaded onto the tendons and ligaments. Therefore, the simplification of the anatomy of the arm in our model would lead to a higher reading on the muscles, since no stabilizing tendons and ligaments are present. Furthermore, the expected percentages are based off of human data, which carries its own error range. The EMG data used as a baseline for our model carried an error range of 10-33%. The reason the error is relatively high is due to anatomical differences between test subjects. Each person has a different bone length, muscle attachment locations and amount of muscle fibers, which all result is different forces on the test subjects. Between all the sources of error described above there is some overlap between our expected and actual results. However, it is suggested that the adjustment factor of two is used for students to be able to better compare the data. CONCLUSION AND RECOMMENDATIONS The model provides statistical data of the force of each muscle in two different elbow flexion angles, in addition to three different wrist positions. After experimentation, students will

6 be able to compare their results to accurately determine the most efficient arm positioning for lifting weight during elbow flexion. The customer has been provided with a manual to build their own models, since only one completed model was developed. The customer s final goal is to have 12 working models for students to experiment with during class. The most difficult part of recreating the model will be tying the strings and getting accurate string tension, but the manuals should help make that process much easier.

7 REFERENCES 1. Je-Hun Lee, Hye-Won Kim, Sun Im, Xiaochun An, Mi-Sun Lee, U-Young Lee and Seung-Ho Han. Localization of motor entry point and terminal intramuscular nerve endings of the musculocutaneos nerve to biceps and brachialis muscles, Surg Radiol Anat, ACKNOWLEDGMENTS Special thanks to Dr. Bailey in the Biomedical Engineering Department, our mentor, Charlie, and the RIT Athletic Trainers. FINAL PAPER CONTRINUTIONS Amanda Cook- Wrote the final paper- 5 Christopher Harley- Worked to build the elbow model and collect experimental data -2 Maria Romero- Worked to build the elbow model and collect experimental data -2 Shannon Keenan- Worked to build the elbow model and collect experimental data -2