The effect of muscle loading on the kinematics of in vitro glenohumeral abduction.

This in vitro study evaluated the effects of four different muscle-loading ratios on active glenohumeral joint abduction. Eight cadaveric shoulders were tested using a shoulder simulator designed to reproduce unconstrained abduction of the humerus via computer-controlled pneumatic actuation. Forces were applied to cables that were sutured to tendons or fixed to bone, to simulate loading of the supraspinatus, subscapularis, infraspinatus/teres minor, and anterior, middle, and posterior deltoid muscles. Four sets of muscle-loading ratios were employed, based on: (1) equal loads, (2) average physiological cross-sectional areas (pCSAs), (3) constant values of the product of electromyographic (EMG) data and pCSAs, and (4) variable ratios of the EMG and pCSA data which changed as a function of abduction angle. The investigator generated passive motions with no muscle loads simulated. Repeatability was quantified by five successive trials of the passive and simulated active motions. There was improved repeatability in the simulated active motions versus passive motions, significant for abduction angles less than 40 degrees (p=0.02). No difference was found in the repeatability of the four different muscle-loading ratios for simulated active motions (p0.067 for all angles). The improved repeatability of active over passive motion suggests simulated active motion should be employed for in vitro simulations of shoulder motion.