RELIABILITY AND VALIDITY OF SCAPULAR MUSCLE STRENGTH TESTING IN PATIENTS WITH SHOULDER PAIN AND FUNCTIONAL LOSS

Abstract

PURPOSE Muscle strength testing is a commonly used outcome measure for patients with shoulder dysfunctions. The purpose of this study was to determine the test-retest reliability and validity of measurements of isometric scapular muscle strength, to thus determine the ability for use as an outcome measure. METHODS Patients (n=25) with shoulder pain and functional loss. A hand-held dynamometer (HHD) was used to measure the kilograms applied during four scapular manual muscle tests (MMT) for the lower trapezius(LT), upper trapezius(UT), middle trapezius(MT), and serratus anterior(SA). Simultaneously, surface electromyography (EMG) was collected on the same muscles. The amount of EMG muscle activity for each of the four muscles during each of the four muscles tests was calculated. The subjects returned 24–72 hours after the initial testing, for repeat of day 1 testing procedure, performed by the same tester. Each test was completed 3 times on both occasions, and the average used for data analysis. RESULTS Test-restest reliability coefficients [ICC (two-way random)] for HHD measures were 0.91 for the LT test, 0.93 for the SA test, 0.94 for the UT test, 0.94 for the MT test; and the standard error of measurement (90%CI) was 2.0kg, 2.6kg, 2.8kg, and 1.3kg respectively. Concurrent criterion validity was determined using the amount of EMG as the criterion measure, comparing the amount of EMG muscle activity of the designated muscle of the test to the other non-designated muscles during the test. The highest muscle activity during the LT test was the lower trapezius muscle, during the MT test was the middle and lower trapezius muscles, during the SA test was the serratus anterior muscle, and during the UT test was the upper trapezius and the serratus muscles. CONCLUSIONS The MMTs for scapular muscles using the HHD have excellent levels of test-retest reliability and error in patients with shoulder pain and functional loss. However, criterion validity results indicate that these four muscle tests do not isolate the muscle indicated in the description of the tests.