Effect of transient moment-oscillations on the reliability of isokinetic shoulder elevation in the scapular plane

Isokinetic assessment of the shoulder rotators: a study of optimal test position

BACKGROUND

  Isokinetic assessment of the shoulder rotator cuff is a common component of shoulder muscles assessment. Nevertheless, the extensive mobility of the shoulder poses great difficulty in finding a consensus protocol for evaluation.

OBJECTIVE

  To select an optimal protocol, among three, based on the best reproducibility and reliability of strength scores derived from internal and external rotator tests.

METHOD

  The dominant side external and internal rotator muscles of twelve healthy male subjects were evaluated based on a concentric protocol (60° s(-1) and 240° s(-1) ) in three different test positions: two in lying supine with the arm in either 90° or 45° abduction and one in the seated position with the arm moving in the scapular plane on the dominant shoulder. Subjects were tested twice over 10 days.

RESULTS

  The two lying installations were associated with the lowest coefficient of variation (7.1-11.8%) and smallest detectable difference (7-15.9 N.m) for peak moment and strength ratios. Consequently, the lying positions were chosen for rotators assessment with 90° of abduction associated with a higher reproducibility for the agonist/antagonist ratios or 45° if some pain was provoked. In any case, the coefficient of variation did not exceed 12%.

CONCLUSION

  Based on a reproducibility and reliability analysis, we recommend the testing of isokinetic strength of the shoulder rotators to be conducted in supine lying, with the arm at 90° or 45° abduction in the frontal plane.

An Apparatus and Protocol to Measure Shoulder Girdle Strength

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.