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RESEARCH PAPERS

# An Apparatus and Protocol to Measure Shoulder Girdle Strength

[+] Author and Article Information
Brian A. Garner

Department of Mechanical Engineering, Baylor University, One Bear Place 97356, Waco, Texas 76798briaṉgarner@baylor.edu

Jaeho Shim

Department of Health, Human Performance, and Recreation, Baylor University, One Bear Place 97313, Waco, Texas 76798

Scott Wilson

Department of Mechanical Engineering, Baylor University, One Bear Place 97356, Waco, Texas 76798

J. Med. Devices 1(4), 246-253 (Aug 10, 2007) (8 pages) doi:10.1115/1.2796190 History: Received August 25, 2006; Revised August 10, 2007

## Abstract

Muscles actuating the shoulder girdle are important for stabilizing the scapula and coordinating phased kinematics of the shoulder complex. If these muscles become weak or imbalanced, joint instability and injury may result. Reliable measurement of shoulder strength is thus important for prevention, diagnosis, and rehabilitation of shoulder problems. To date, studies quantifying the strength of the shoulder girdle are limited. The purpose of this work was to design and evaluate a custom apparatus and corresponding protocol for measuring maximal, voluntary, isometric strength of the shoulder girdle during various forms of shrugging exercise. A custom apparatus was constructed as a rigid frame with a vertical post supporting a seat, seat back, and horizontal beam. The beam extends laterally on either side beyond and around the shoulders of a seated subject. A pair of arm extension members pivots on the beam about an axis aligned with the shoulder flexion-extension axis. These members can be locked in place at any angle. Between them is mounted a force-sensing grip assembly, which can be adjusted proximally or distally to accommodate varying shoulder girdle positions. Subjects grasp the grip assembly handles with extended elbows and push or pull as forcefully as possible. Nine female and ten male subjects participated in a protocol using the apparatus to measure maximum isometric force generated at three positions each for elevation, depression, protraction, and retraction of the shoulder girdle $(3positions×4modes=12tests)$. A video motion capture system was used to measure shoulder girdle angles. The reliability of the approach was evaluated based on the repeatability of measured shoulder elevation angle, protraction angle, and total force over three days of testing. The apparatus performed well during the tests, providing a stable, rigid, yet adjustable platform for measuring shoulder girdle strength. Repeatability of force measurements was interpreted as very good to excellent, with intraclass correlation coefficient (ICC) (2,1) values ranging from 0.83 to 0.95 for all tests except one $(ICC=0.79)$. Repeatability of angle measurements was interpreted as good to excellent. For tests measuring elevation and depression strength, the ICC of elevation angle ranged from 0.85 to 0.89. For tests measuring protraction and retraction strength, the ICC of protraction angle ranged from 0.68 to 0.88. This type of apparatus could be an effective clinical tool for measuring strength in the shoulder girdle muscles. Use of the video motion capture system is optional.

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## Figures

Figure 1

Subject demonstrating use of the custom apparatus in a test measuring shoulder elevation strength at a depressed position of the shoulder girdle. The subject performs the test with fully extended elbows by pulling upward on the force-sensing handles with maximal, voluntary effort. The apparatus is adjustable to accommodate tests measuring strength (pulling or pushing) at a range of shoulder flexion-extension angles, and a range of shoulder girdle “shrugging” positions.

Figure 2

Custom apparatus for measuring the isometric strength of shoulder shrugging exercises. The apparatus base consists of a four-footed frame, vertical post, attached seat back, adjustably attached seat support assembly, and laterally projecting horizontal beams with anterior projections. Hinged to the anterior projections are the arm extension members, which rotate about the hinge axis from vertically downward, to horizontally forward (as shown), to vertically upward. The hinge axis is aligned with the center of shoulder rotation to facilitate isometric tests at a range of shoulder flexion angles. Pins inserted through semicircular plates on the arm extensions arrest the rotation at a prescribed angle. Adjustably mounted between the arm extensions is a grip assembly containing force-sensing handles (inset) to measure pulling or pushing strength. Pins inserted through the grip assembly and arm extensions permit the grip handles to be locked in place at a range of positions from proximal to distal. The handles can also be adjusted mediolaterally to accommodate shoulders of different widths.

Figure 3

Graphs of sample data from three separate testing days showing total force (thick solid lines), elevation angle (thin solid lines), and protraction angle (thin dashed lines). These data were measured from one of the strongest male subjects during elevation trials in the neutral shoulder girdle position. The vertical dashed lines indicate the time instant, and corresponding data points, selected to represent the subject’s performance during each respective trial.

Figure 4

Illustration of elevation angle (A) and protraction angle (B) used to describe overall shoulder girdle position. The angles are defined with respect to three anatomical landmarks, the JN, the spinous process of the C7, and the AC, and the MP between C7 and JN. Elevation angle is the angle of the line from MP to AC superior to the horizontal plane. Protraction angle is the angle of the line from MP to AC anterior to the frontal plane. These definitions align angular displacements with the direction of force measured for elevation and protraction trials, respectively, and provide for neutral positions with angular values near zero.

Figure 5

Bar graphs of ICCs and 95% LOAs on elevation angle (from Table 1) and protraction angle (from Table 2), averaged over all tests (all three shoulder girdle positions) for each of the four modes of testing: elevation (Elev), depression (Dep), protraction (Prot), and retraction (Ret). The bar graphs highlight results indicating that elevation angle measurements were more repeatable than protraction angle measurements for elevation and depression tests, but that for protraction and retraction tests, the relative repeatability of the two angle measurements was fairly similar.

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