Unintentional forces developed during isometric test of the shoulder

Objective. To study unintentional, spontaneous forces developed during isometric shoulder strength testing. Design. An experimental study. Background. During torque measurements, subjects may apply forces in directions other than requested, thereby obscuring the interpretation of the results. Methods. A shoulder strength test was performed with a strength test device permitting measurements of force in the requested direction as well as perpendicular to it. Nine fishermen and nine welders performed isometric abduction and elevation with the arm held in various angles. Moreover four rotational postures were investigated. Results. Spontaneous force deviations from requested direction were found in almost all posture tested, with errors––defined as 100 times the ratio of the magnitude of intentional torque to the magnitude of unintentional (erroneous) torque generated perpendicular to it––of more than 30% found for some postures. Abduction in 45° horizontal flexion was better controlled than abduction in 90° horizontal flexion. Detailed analysis indicated some groupwise differences between fishermen and welders. Also pairwise comparison of handiness indicated some differences. Conclusions. Considerable deviations from requested direction of action can arise during strength tests. Thus, one must either monitor these deviations or minimize them by a proper design of the examination to get relevant data. As the error can differ between certain subject categories depending on posture this indicate a new tool for discriminating between musculo-skeletal problems.