Appropriate Resistance Intensity for Activation of Infraspinatus and Control of Anterior Translation of the Humeral Head

Abstract

Background In general, external rotation (ER) exercises are commonly used in rotator cuff retraining programs to improve stabilization ability. ER exercises at 90° of shoulder abduction are common and effective in strengthening the infraspinatus (ISP). However, if the ISP is weak during ER at 90° shoulder abduction, the posterior deltoid muscle (PD) is overactivated, which can lead to anterior translation of the humeral head. Therefore, the selective activation of ISP muscles is an important factor in rehabilitation. Objectives We investigated the muscle activities of the ISP, PD, and middle trapezius (MT), the ratio of the ISP to PD muscle activities, and the anterior translation of the humeral head in response to three resistance intensities during sitting external rotation (SITER) exercises. Methods A total of 12 healthy women recorded changes in the anterior translation of the humeral head using a load cell, the muscle activities of the ISP, PD, and MT, the ratio of the ISP to PD muscle activities using surface electromyography (EMG) during SITER under three different intensities [low (10-20%) medium (45-55%) high (60-70%)]. Results The ISP, PD, and MT amplitudes were highest at high intensity, followed by medium and low intensities during SITER (p < 0.001). The muscle activity ratio was significantly highest at medium intensity compared with low and high intensities (p = 0.031). The anterior translation of the humeral head was highest at high intensity, followed by medium and low intensities during SITER (p < 0.001). Conclusions Medium intensity (45-55%) was the most appropriate intensity during the SITER exercise. However, anterior gliding of the humeral head was the lowest at low intensity. Therefore, during ER exercises, we should think about how to control the anterior gliding of the humeral head.