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Shoulder stiffness: quantification of overuse injury in competitive swimmers
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Shoulder stiffness: quantification of overuse injury in competitive swimmers

Michael Reed Vennel
Master of Science (M.S.), Drexel University
Jun 2017
DOI:
https://doi.org/10.17918/etd-7347
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Abstract

Human mechanics Shoulder pain Biomedical Engineering
Overhead athletes, and swimmers in particular, are highly susceptible to shoulder injuries (Hibberd & Myers, 2013). Collegiate swimmers complete an average of between 10 and 14 kilometers every day they practice, which assuming 10 stroke cycles per 25-meter length, gives over 4000 revolutions forced upon the shoulders each day of practice (Tate et al., 2015; Hibberd & Myers, 2013). Furthermore, the shoulder transmits 90% of the force used to propel swimmers through the water, resulting in frequent overuse injury, broadly classified as "Swimmer's Shoulder." Estimates of the prevalence of shoulder pain amongst competitive swimmers range between 40% and 91% (Tate et al., 2012). Beyond being uncomfortable, this pain is associated with stiffness, reduced range of motion, and increased risk of serious injury, such as rotator cuff tears (Ma, 2015). The condition is thought to be caused by compensation for fatigue during long swim practices, and as a result treatment focuses on strengthening and stretching the affected tissues in an attempt to reduce the pain and the stiffness of the shoulder. However, stiffness is currently a qualitative assessment, based on the therapists perceived resistance to motion. Most often, clinical studies equate an increase or decrease in the range of motion, as a decrease or increase in stiffness. However, no device previously existed which was capable of recording the torque required to rotate the shoulder, allowing quantification of the rotational stiffness of the shoulder. OBJECTIVE: Previously a senior design group adapted a device developed for animal studies to measure shoulder stiffness in internal and external rotation in humans (Sarver, 2015). However, this device was incapable of capturing the full range of internal and external rotation; did not securely attach to the arm, and was excessively heavy. The objectives of this project, therefore, were to revise the shoulder stiffness device (STROM) and to use it to assess shoulder stiffness in swimmers before and after various forms of exercise. More specifically: Aim 1: Re-design STROM to suit the unique challenges of evaluating swimmers in a mobile testing set up. The following requirements were established to ensure the mobility of the device, comfort of the subject, and accuracy of the results: a) Testing must cause minimal discomfort and no pain b) Testing must take less than two minutes to occur c) Location of attachment site must be adjustable within minimum range of 18-32 cm. d) Device ROM must include up to 135o in either direction from vertical e) Device must be able to fit within a standard carryon bag, max linear dimensions 115 cm f) Maximum device weight is 10 kg g) Clamping mechanism must accommodate minimum of 6.4 cm thick table Aim 2: Use STROM to study shoulder stiffness in swimmers before and after swimming Given that swimming involves stretching of the tissues, and swimmers anecdotally report "loose" shoulders, it was hypothesized that: ROM, [theta]max, and [theta]break, would all increase. Furthermore, as shown in fatigue testing (Wren, Lindsey, Beaupré, & Carter, 2003), repeated loading is expected to reduce stiffness, therefore, rotational stiffness, as well as the torque required to move a given amount was expected to decrease. Aim 3: Use STROM to study shoulder stiffness in swimmers before and after functional warmup The functional warmup consisted of stretches, body weight exercises, and other low intensity movements. Consistent with the increase in rotation, and decrease in rotational stiffness of the ankle in response to stretching (Morse, 2008), it was hypothesized that range of motion, maximum angles, and break angles would all increase, while stiffnesses and break torques would decrease. RESULTS: Aim 1: Verification tests showed that a) No pain or discomfort was experienced by test subjects b) Testing could be completed within approximately 90 seconds per shoulder c) The attachment site could accommodate forearms up to 36 cm in length, with no minimum d) The device can accommodate angles between +170° and -185° e) The device comfortably fit in a 35x41x24 cm suitcase for transport, giving a total linear dimension of 100 cm f) Weight was decreased by 2 kg giving a final weight of 4.5 kg. g) The clamping mechanism could accommodate tables up to 12 cm thick. Aim 2: Two swim practices were used in the course of this study. In the combined analysis, the only statistically significant finding was a decrease in the internal break torque with swimming. Value Before (Mean ± Stdev) After (Mean ± Stdev) P Value [tau]break, ext 0.95 ± 0.62 N-m 0.75 ± 0.46 N-m 0.004 However, on the second day significant decreases in both internal and external stiffnesses were found in addition to the decrease in internal break torque. The discrepancy is possibly due to variations in individual workouts or may have been obscured on the first day due to the large variation between individuals. Value Before (Mean ± Stdev) After (Mean ± Stdev) P Value Krot, int 0.16 ± 0.05 N-m/° 0.14 ± 0.04 N-m/° 0.041 Krot, ext 0.18 ± 0.06 N-m/° 0.14 ± 0.04 N-m/° 0.009 [tau]break, ext 1.06 ± 0.70 N-m 0.73 ± 0.43 N-m 0.007. Aim 3: The functional warmup resulted in increases in both internal and external stiffnesses, increases in range of motion (biased towards additional internal motion), increases in the internal break and finally increases in the external break torque. Value Before (Mean ± Stdev) After (Mean ± Stdev) P Value ROM 185.0 ± 13.9 ° 190.6 ± 14.6 ° 0.003 [theta]max, ext 101.9 ± 11.3 ° 106.1 ± 11.7 ° 0.002 [theta]break, ext 82.6 ± 10.4 ° 86.7 ± 10.4 ° 0.004 Krot, ext 0.15 ± 0.04 N-m/° 0.17 ± 0.04 N-m/° 0.027 Krot, int 0.13 ± 0.04 N-m/° 0.15 ± 0.05 N-m/° 0.013 [tau]break, int 0.58 ± 0.33 N-m 0.91 ± 0.54 N-m <0.001. DISCUSSION: This was the first study to quantify changes in passive shoulder mechanics in swimmers following functional warmup and high volume swimming. The device met all design criteria, but the clamping mechanism would need to be revised to accommodate different types of exam tables. Also, further input from athletic trainers, an important stake holder, as well as feedback from subjects, should be evaluated to improve future versions. Both activities caused a significant change in rotational stiffness, with swimming decreasing it, and functional warmup increasing stiffness. Significant variability across individuals suggests the need for stricter inclusion criteria to allow clearer results, although different approaches to the data analysis are still being attempted. Future studies could examine more subjects, and examine changes in shoulder stiffness throughout a season. Finally, linking the stiffness data with medical history could begin to clarify the relationship between shoulder stiffness and should pain. Currently, we don't know if swimmers with stiff shoulders are more or less likely to suffer from an injury, or if there is any relation between stiffness and injury, however, this study indicates the device is sensitive enough so as to be able to ask such questions.

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