The effects of shoulder abduction angle and elastic band resistance on shoulder muscle activity during external rotation exercises.
Level 4 - case-series / case-control
Cross-sectional repeated-measures laboratory biomechanical study in healthy volunteers
PubMed 41559205 · doi:10.1038/s41598-025-34767-w
What was done
Thirteen healthy young men (mean age 24.1 ± 2.9 years) performed standing shoulder external rotation exercises at three humeral abduction angles (0°, 45°, and 90°) against two elastic band resistance levels (low and moderate). Surface and fine-wire needle electromyography (EMG) recorded activity from seven shoulder and scapular muscles: posterior deltoid, upper trapezius (UT), lower trapezius (LT), serratus anterior (SA), supraspinatus (SSP), infraspinatus (ISP), and teres minor to assess activation and co-contraction indices (CCIs).
What was found
UT and SA activation increased significantly at 90° abduction compared to 0° and 45° (both p < 0.001). SSP activation was significantly higher at 45° (p = 0.007) and 90° (p = 0.015) than at 0°. Co-contraction indices for SA/UT (p = 0.036), LT/UT (p < 0.001), and SSP/ISP (p < 0.001) were significantly greater at 45° than at 0° abduction. Increasing resistance from low to moderate significantly increased activation in UT, LT, SSP, and ISP (all p ≤ 0.028) and elevated shoulder CCIs (p ≤ 0.012). Exact normalized EMG percentages were not provided in the abstract.
Why it matters
These findings show how varying abduction angle and band resistance alters muscle recruitment and scapular-rotator cuff co-activation, offering guidance for tailoring shoulder rehabilitation progressions.
Limits
The study evaluated a very small sample (n = 13) restricted entirely to young, healthy males, preventing direct generalization to females, older individuals, or patients with shoulder pathology or pain. Acute EMG measurements reflect neuromuscular activation patterns but do not evaluate long-term training adaptations, functional recovery, or clinical outcomes.
Cited by
- partial When attempting to externally rotate the shoulder, elevating the elbow away from the torso shifts the muscular workload from the rotator cuff to the deltoid.