BACKGROUND: Electrical stimulation (ES) is considered to be effective on infraspinatus muscle with functional decline and atrophy. However, it is not clear which parameters of ES e.g., types of muscle contractions and shoulder joint positions have good effects for inducing hypertrophy and increasing muscular strength. The purpose of this study was to determine the acute effects of ES in different types of muscle contractions and shoulder joint positions on the infraspinatus muscle by measuring the muscle swelling after ES. METHODS: Forty subjects were randomly assigned to one of five groups: an isometric contraction with maximum internal rotation (IR), an isometric contraction with neutral position of rotation (NEUT), an isometric contraction with maximum external rotation (ER), a concentric contraction (CONCEN), and a control (CON) group. Subjects in all groups except for the CON group received ES for 20 min. The CON group did not receive intervention. The muscle thicknesses of the superior and inferior infraspinatus were measured using ultrasonography before and immediately after a single ES intervention. RESULTS: Percentage change in muscle thickness of the inferior infraspinatus was greater in the IR and NEUT groups than the muscle thickness of the CON group
however, the muscle thickness of the superior infraspinatus did not differ significantly among the groups. CONCLUSIONS: The results of this study indicate that applying ES to the inferior infraspinatus muscle in an isometric contraction with the muscle in a stretched position is an effective method to induce greater muscle swelling.