Rotator cuff strengthening dominates rehabilitation programs, but currently is not regionally specific. Resisted external rotation (ER) exercises are commonly included in rotator cuff rehabilitation and tend to recruit the entire cuff as indicated by EMG, with slight postural variations. However, regional muscle activation during ER exercises performed in different postures and at different intensities is unknown.
The aim of this research was to quantify regional muscular activations of the infraspinatus and supraspinatus across postures and intensities, to refine rehabilitation exercise prescription.
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.
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°, 30°, 90°) 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° of elevation (50.9% ± 5.7% MVIC) versus 30° of flexion (37.4% ± 3.9% MVIC) at maximal intensity. Inferior infraspinatus activity was higher in 90° of scaption (max = 59.8% ± 2.8% MVIC, submax = 29.4% ± 1.9% MVIC) versus 0° 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° of adbuction (max = 61.6% ± 3.1% MVIC; submax = 39.1% ± 3.8% MVIC) and lowest in 30° of flexion (max = 29.0% ± 3.4% MVIC, submax = 15.6% ± 1.7% MVIC) and 90° of flexion (max = 34.6% ± 2.4% MVIC, submax = 14.8% ± 1.9% MVIC).
Posterior suprasptinatus activity was lowest in 0° of elevation (34.2% ± 3.0% MVIC), 30° of flexion (33.0% ± 3.6% MVIC) and highest in 90° of abduction (56.2% ± 4.1% MVIC) and 90° 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° of arm elevation. Anterior and posterior supraspinatus activities were higher in the abduction and scaption planes, especially at 90° of elevation, as compared with the flexion plane.
Collectively, these results can help guide exercise prescription.
In general, maximal ER efforts elicit moderate to very high activity in all regions of the supraspinatus and infraspinatus. Currently, ER at 0° of elevation is recommended to isolate and strengthen the infraspinatus muscle, but is predicated on data from the middle infraspinatus region.
Current data suggest that this approach ineffectively engages the inferior infraspinatus region. Standing ER exertions at higher elevations (ie, 90°) elicit moderate to high or very high levels of activity in all 3 infraspinatus muscle regions.
Eliciting posterior supraspinatus activity while limiting anterior supraspinatus activity can be achieved through performing flexion plane exercises. These new regionally specific findings can be applied in concert to beneficially and selectively recruit muscle regions in exercises.
Clinical Relevance:
In shoulder rehabilitation of supraspinatus tendon injuries, ER exercises in the flexion plane challenge the whole infraspinatus muscle and require lower supraspinatus muscle activity.
Resource:
Author
Dr. Catherine Logan, MD, MBA
Catherine Logan, MD, MBA, MSPT, is a sports medicine orthopaedic surgeon specializing in complex knee and shoulder surgery at Colorado Sports Medicine and Orthopaedics (COSMO) in Denver, Colorado. As a former physical therapist & trainer, Dr. Logan provides a distinct expertise in sports injury, prevention, and surgical management.
