Shoulder muscle force and electromyography activity during make versus break tests

Background: Shoulder muscle force is commonly assessed during clinical examination using both an isometric "make" test against a fixed resistance or a "break" test where the examiner exerts enough force to break the isometric contraction. The purpose of this study was to explain the difference in force produced during these two forms of testing. Methods: Data were collected on 25 subjects. Both shoulder external rotation and elevation force were measured over three trials, isometrically, for approximately 3 s, after which the examiner exerted enough force to move the arm. Surface EMG was recorded for the infraspinatus for external rotation and middle deltoid for elevation. Peak isometric and break forces, and normalized, averaged EMG data at peak isometric and break forces were compared with paired t-tests. Findings: External rotation peak break force was 46.9% (SD33.6, range - 3% to 108.6%) greater than isometric force (p < 0.01). EMG for the infraspinatus was 17.0% (20.8) greater at break (p < 0.01). For elevation, peak break force was 63% (73.1, range - 3.5 to 238.16%) greater than isometric force (p < 0.01). EMG for the middle deltoid was 11.1% (30.8) greater during peak break force (not significant). Interpretations: There is a difference in both force and muscle activity between "make" and "break" tests. Clinicians should use a consistent method when measuring force and a break test will provide the maximum force. The cause of greater force produced during a break test is likely attributable to the brief eccentric contraction rather than greater recruitment. Future analysis will include examining the differences in make and break forces based on activity levels.