Regional Electromyography of the Infraspinatus and Supraspinatus Muscles During Standing Isometric External Rotation Exercises

Background: Whole-muscle electromyography (EMG) data of the rotator cuff support external rotation (ER) strengthening exercises during shoulder rehabilitation. However, distinct neuroanatomic regions in the supraspinatus and infraspinatus exist. Differences in regional muscle activity occur during rehabilitation exercises, but little information is available for ER exertions. Hypothesis: Regional infraspinatus and supraspinatus muscle activity during standing ER exertions will differ with posture and intensity. Study Design: Descriptive laboratory study. Methods: Twenty healthy individuals (12 men, 8 women) participated. Fine wire electrodes were inserted into 2 supraspinatus and 3 infraspinatus muscle regions. EMG data were recorded during standing isometric ER exertions at 2 intensities (maximal, submaximal) and in 7 postures defined by the angle (0 degrees, 30 degrees, 90 degrees) and plane (abduction, scaption, flexion) of arm elevation. EMG data were normalized to maximum voluntary isometric contraction (% MVIC) to examine the influences of posture, intensity and their interaction on muscle activity. Results: Superior infraspinatus activity was higher in 0 degrees of elevation (50.9% +/- 5.7% MVIC) versus 30 degrees of flexion (37.4% +/- 3.9% MVIC) at maximal intensity. Inferior infraspinatus activity was higher in 90 degrees of scaption (max = 59.8% +/- 2.8% MVIC, submax = 29.4% +/- 1.9% MVIC) versus 0 degrees of elevation (max = 42.3% +/- 4.5% MVIC, submax = 22.4% +/- 2.8% MVIC) (P = 0.02, P = 0.05, respectively). Anterior supraspinatus activity was highest in 90 degrees of adbuction (max = 61.6% +/- 3.1% MVIC; submax = 39.1% +/- 3.8% MVIC) and lowest in 30 degrees of flexion (max = 29.0% +/- 3.4% MVIC, submax = 15.6% +/- 1.7% MVIC) and 90 degrees of flexion (max = 34.6% +/- 2.4% MVIC, submax = 14.8% +/- 1.9% MVIC). Posterior suprasptinatus activity was lowest in 0 degrees of elevation (34.2% +/- 3.0% MVIC), 30 degrees of flexion (33.0% +/- 3.6% MVIC) and highest in 90 degrees of abduction (56.2% +/- 4.1% MVIC) and 90 degrees of scaption (46.7% +/- 2.8% MVIC) (all Ps < 0.04). Conclusion: Regional infraspinatus and supraspinatus muscle activity differed with posture and intensity. Superior and middle infraspinatus muscle activities were similar across postures, but inferior infraspinatus activity was highest in 90 degrees of arm elevation. Anterior and posterior supraspinatus activities were higher in the abduction and scaption planes, especially at 90 degrees of elevation, as compared with the flexion plane.

Automated summary

This study investigated the regional muscle activity of supraspinatus and infraspinatus during shoulder rehabilitation exercises involving external rotation. The results showed that posture and intensity had an impact on muscle activity in different regions of the supraspinatus and infraspinatus muscles, highlighting the importance of considering these factors in shoulder rehabilitation protocols.