Abstract
We investigated the muscle activities of the infraspinatus and posterior deltoid,
as well as the ratio of the infraspinatus to posterior deltoid muscle activities,
in response to 3 resistance intensities during prone external rotation (PER) and sitting
external rotation (SITER) exercises. Fifteen healthy males participated; the subjects
performed two exercises randomly at three resistance intensity levels: 1) low intensity
(10–20%); 2) medium intensity (45–55%); 3) high intensity (60–70%). Surface electromyography
was used to measure the activities of the infraspinatus and posterior deltoid muscles.
The activities of the infraspinatus and posterior deltoid increased significantly
as the resistance intensity increased during both PER and SITER exercises (p<0.001). The infraspinatus-to-posterior deltoid activity ratio increased as the resistance
intensity decreased. Whereas the muscle activity ratio was highest under low and medium
intensity during PER and SITER, respectively, and the muscle activity ratio was significantly
increased at medium intensity compared with high intensity during both PER (p=0.023) and SITER (p=0.001). Our results suggest that low to medium intensity is the appropriate resistance
intensity for selective activation of the infraspinatus. In addition, our results
suggest that PER and SITER are effective for strengthening the infraspinatus.
Key words
infraspinatus - selective activation - resistance intensity - electromyography