In vivo measurements of subacromial impingement: Substantial compression develops in abduction with large internal rotation

Subacromial impingement is a widely recognized mechanism of chronic shoulder pain. The magnitudes of the compressive forces that impinge the subacromial structures were often measured from cadaveric specimens, but it is questionable to use this data as a sole basis to determine the shoulder motions and/or shoulder configurations that induce impingement in live subjects performing active motion. The purpose of the present study was to determine in vivo the magnitude of the compressive force at selected shoulder configurations.