Changes in the electromyographic activities of the infraspinatus and posterior deltoid according to abduction angles of the shoulder joint during shoulder external rotation in closed kinetic chain exercise.
Daehee Lee, Sangyong Lee, Seulki Han
PMID 27821927WHAT IT FOUND
Infraspinatus activity was 8.3%, 7.7%, and 9.6% of maximum voluntary isometric contraction at 45°, 90°, and 135° and did not differ significantly; posterior deltoid was significantly lower at 90° and 135° than at 45° (13.7% and 12.1% versus 17.5%).
Key findings
01Infraspinatus activity was 8.3 ± 4.0% at 45° abduction, 7.7 ± 4.8% at 90° abduction, and 9.6 ± 6.7% at 135° abduction; the highest mean was at 135°, but the differences were not statistically significant (p>0.05).
02Posterior deltoid activity was 17.5 ± 6.1% at 45° abduction, 13.7 ± 3.4% at 90° abduction, and 12.1 ± 5.6% at 135° abduction; post hoc analysis showed significantly greater activation at 45° than at 90° or 135° (p<0.05).
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What it does not show
The study measured only 15 healthy young students, so it does not show what happens in patients with shoulder pain or functional problems. All participants were healthy males in their 20s, so results may not apply to females or older adults. Only shoulder abduction angles of 45°, 90°, and 135° were tested, and the authors noted that angles below 45° were not checked. Measurements were made in one standing wall-push position, so results may not apply to other exercise positions. The outcome was muscle electrical activity normalized to maximum voluntary isometric contraction, not a clinical outcome such as pain, strength, or function.
The easy way to misread this
Do not conclude that 90° or 135° shoulder external rotation strengthens the infraspinatus better than 45°. The infraspinatus activity did not differ significantly across angles, and the study measured muscle activity in healthy young students rather than clinical outcomes in patients.