1. The difference in strengthening of isometric and dynamic exercise routines on the external rotators of the shoulder in adolescents Lucas Spaeth Mentored by Susan Dumler, OT Coordinator; OTR/L, CHT, CLT-UE Introduction The Center for Disease Control states that almost twenty percent of adults strength train more than twice a week, but experts do not agree on which form of strength training increases muscle strength the most: isometric or dynamic. Isometric strength training programs use static force to train the muscle as the participant contracts muscles without changing the overall muscle length (Hoffman, 2002). Dynamic strength training requires the muscle to shorten and lengthen with both concentric and eccentric contractions, improving strength by moving joints across the full range of motion (Hoffman, 2002). The shoulder is one of the most frequently injured joints on the human body, because it is one of the most frequently used joints in many physical activities. As a result, it is important to prevent injury, by strengthening all of the muscles in the glenohumeral joint. Little is known about strength training two muscles, the infraspinatus and the teres minor, which perform external rotation as well as provide stabilization of the glenohumeral joint. Few, if any, studies have explored the differences between isometric and dynamic strength training in strengthening the external rotators of the shoulder. The purpose of this study is to obtain data on these differences in adolescents. Due to the popular use of dynamic lifts during strength training, it was hypothesized that subjects should gain more strength with a dynamic exercise. Methods (continued) A Manual Muscle Tester (MMT) was used to record strength gains during the five weeks of strengthening. To ensure accurate testing subjects were placed into a splint which kept the glenohumeral joint at ninety degrees of abduction, and the elbow at ninety degrees of flexion (Figure 3). Subjects were tested using the Kendall method with all subjects lying prone on a table with their right arm in the splint and their left arm at their side (Kendall, 1983). All force data was recorded after five weeks for each subject. The box plot shows a number of outliers, especially in the isometric group of subjects. Normality graphs (not shown) were also made which verified that the isometric group is fairly normal while the dynamic group is slightly skewed right. Due to subject number, data choice, and the outliers present in both exercises, a Mann-Whitney test was performed on the data. This test gave a significance level of 0.0921 for the null hypothesis of isometric strength gain and dynamic strength gain being equal. Results (continued) While it was originally hypothesized that a dynamic exercise would strengthen more than an isometric exercise routine because of its popularity among weight trainers, it makes sense that when the exercises are performed by people who have not weight trained, isometric would have a higher strength gain. This is due to the fact that isometric exercises are primarily used with strength training during early recovery from injury. However, this can only be said for the data when looking at the mean and median. When the statistical results of the Mann-Whitney test are analyzed, it can not be concluded as to whether dynamic or isometric strength training programs cause a greater increase in strength in a healthy adolescent population. With a p-value of 0.0921, greater than an alpha level of 0.05, there is not enough statistical evidence to reject the null hypothesis that isometric and dynamic strength gains are equal. Since people will gain equal strength from the exercises, both will help prevent shoulder injuries in adolescent athletes. Neither exercise caused a sufficient enough strength gain to be proven more effective. A larger subject pool and increased time for strengthening may support differences in the two exercises. Both exercises are viable ways to strengthen the external rotators of the shoulder to provide stability during rigorous training. Conclusions Hoffman, J. (2002). Physiological Aspects of Sport Training and Performance. Champaign, IL: Human Kinetics. Kendall, F. (1983). Muscles, testing and function. Baltimore, MD: Williams & Wilkins. References Thanks to the staff at the Union Memorial Sports Medicine Center especially Marian Mackenzie for her extensive help at the beginning of the project. Thanks to my faculty advisor Mrs. Gabriel and all of those who participated and helped in my study. Acknowledgements MMT Data, collected from February 2 nd to March 8 th, was compared between the isometric and dynamic groups. The percentage change in strength for subjects in each group, which was found by finding the ratio of the difference in force between the initial and final test and the initial tests, was plotted in box plots (Graph 1). Results Figure 3 (left): the strength testing procedure for the external rotators of the shoulder. The subject was laying prone on a level surface with their right arm in the splint off the table. A force that was resisted, was applied to the subject’s wrist. A hand was placed on the subject’s back to ensure that the subject only utilized the strength in their external rotators. Graph 1 (above): a box plot of the percentage of strength gained for each group. The isometric exercise had higher percentages of strength gain than the dynamic. Methods In order to compare these two exercises, twenty four subjects (twelve females and twelve males) between the ages of fourteen to eighteen were randomly placed into two groups. One group performed a dynamic exercise routine (Figure 1) and the other performed an isometric exercise routine (Figure 2).These routines were performed for approximately five minutes twice a week for five weeks. Each dynamic subject lifted seventy-five percent of their one repetition maximum which then determined the amount of sets and repetitions done during the exercise routine. Figure 1 (above): a dynamic exercise being performed. Subject was side-lying with the head supported. Figure 2 (left): an isometric exercise being performed; feet are placed shoulder width apart with the forearm against the wall. The subject pushes on the wall for five seconds, ten times, for two sets.