Supraspinatus activation precedes the infraspinatus muscle during the shoulder abduction in different levels of handgrip strengths

Purpose: It is unknown whether the effect of increased distal stabilization can trigger the onset of the supraspinatus and infraspinatus muscles, and if handgrip strength levels can elicit early proximal shoulder stabilization. Hence, we aimed to compare the electromyography activation of the Supraspinatus and Infraspinatus muscles during the abduction motion with handgrip strength in different levels (0%, 30%, and 60%) of maximal voluntary isometric contraction (MVIC). Methods: Twenty participants were submitted to abduction shoulder movement with three different handgrip strengths (0%, 30%, and 60%) using electromyography. The onset of Supraspinatus and Infraspinatus muscles was measured. A Friedman’s test was used to compare handgrip conditions and the onset between muscles. Then, multiple comparisons were performed. All alpha errors were set to 5%. Results: There was an anticipated onset for Supraspinatus muscle at 0% of the MVIC {− 0.554 [− 0.657 to − 0.497] ms vs. − 0.098 [− 0.264 to 0.108] ms, p < 0.001}, at 30% of the MVIC {− 0.560 [− 0.628 to − 0.521] ms vs. − 0.125 [− 0.243 to − 0.031] ms, p < 0.001), and at 60% of the MVIC {− 0.543 [− 0.573 to − 0.514] ms vs. − 0.215 [− 0.325 to − 0.017] ms, p = 0.001}. Conclusion: Shoulder abduction with handgrip triggers the onset of the Supraspinatus and Infraspinatus muscles. The Supraspinatus muscle elicits an anticipated onset. Two stabilizing strategies are suggested; the internal rotation instability capable be induced by the deltoid muscles (abduction movement) and from the wrist flexor-pronator muscles (handgrip). Both tasks are counteracted and anticipated by the action of the Supraspinatus and Infraspinatus muscles.