Electromyographic analysis of deltoid and rotator cuff function under varying loads and speeds

The purpose of this study was to compare the effect of increasing loads and doubling speed on the deltoid and rotator cuff muscles during isotonic scapular plane abduction (scaption) with neutral humeral rotation. These muscles were studied in 16 volunteers with asymptomatic shoulders with the use of fine wire electromyography. The addition of load to the arm during scaption caused an increase in electromyographic activity during the first 90° of motion. Furthermore electromyographic activity decreased during the final 30° of motion with each increase in load. Doubling the speed caused an increase in electromyographic activity during the first 60° of motion while causing a decrease in activity in the final 60°. This study demonstrates the response of the rotator cuff and deltoid muscles to varying loads and speeds during the most basic shoulder motion. With the data obtained in this study, rehabilitation exercises and experimental shoulder models can be refined to reflect this more physiologic situation. (J Shoulder Elbow Surg 2000;9:47-58.)