Rotator cuff (RC) muscle dysfunction impacts the ability to perform daily functional tasks, such as driving. It has been suggested that RC muscle fatigue can mimic rotator cuff tears (RCT) during sudden steering in terms of kinematics. It has also been found that two RC muscles (infraspinatus and supraspinatus) are highly active during driving. However, it is unknown whether fatigue of these muscles would change the kinematic strategy during driving. The aim of this research was to analyze changes in joint angle and electromyography (EMG) signals of the upper extremity in simulated driving to identify compensatory mechanism of rotator cuff muscles.

Mean, maximum, standard deviation, and range of motion (ROM) of joint angles for four degrees of freedom (shoulder plane, shoulder elevation, shoulder rotation, and elbow flexion) were examined for four steering patterns (straight, left, right, and complex) and compared between before and after fatigue. Along with kinematic analyses, EMG signals of four muscles (deltoid, supraspinatus, infraspinatus, and biceps) were measured to analyze the relationship between kinematics and muscle usage before and after fatigue.

In straight and left turns, usage of the right deltoid significantly increased (p≤0.05) in all three measurements (mean,standard deviation, and maximum) whereas in complex turn, the right bicep was used more (p≤0.05). However, kinematics in corresponding muscles did not show significant change, which indicates change in muscle usage did not impact driver’s kinematic strategy. The results suggest that in simple steering, the deltoid compensates for fatigue of RC muscles while in more dynamic steering, the biceps compensate for fatigue of RC muscles. However, the extent of this compensation was minimal as activation level of infraspinatus reached close to its maximum contraction (∼96.5% MVC) while non-RC muscles were generally below 30% MVC in all turns.