THE EFFECTS OF DIFFERENT DEGREES OF UNSTABLE SURFACES ON TRAPEZIUS MUSCLE ACTIVITY IN SUBJECTS WITH ROUNDED SHOULDER POSTURE DURING STRENGTH EXERCISES

Abstract

Lower trapezius muscle weakness can cause round shoulder posture (RSP). Understanding the use of instability devices is
beneficial to the enhancement of muscle activity. Thus, this study determined whether varying degrees of instability have
different effects on muscle activity. Using surface electromyography (EMG) on 23 subjects with RSP, muscle activities of the
upper, middle, and lower trapezius were recorded during three strengthening exercises on four different degrees of unstable
surfaces. The ratios of the middle and lower trapezius muscle activities to the upper trapezius muscle activity were compared.
Significant differences were found between the four surfaces and between the three exercises in all muscles and ratios (p <
.05). In comparing the surfaces, muscle activities were found to increase with higher instability in each exercise situation (p <
.008), and the two ratios decreased with higher instability only during the modified prone cobra exercise (p < .008). In
comparing the exercises regardless of the surface, the upper trapezius muscle activity was significantly higher in the prone Vraise exercise (p < .017), and the middle trapezius muscle activity was significantly higher in the prone L-raise exercise (p <
.017), the lower trapezius muscle activity was significantly lower in the modified prone cobra exercise (p < .017), and the two
ratios were significantly higher in the prone L-raise and modified prone cobra exercises (p < .017). The results suggest that
the prone L-raise exercise performed on a Swiss ball or a balance cone and the modified prone cobra exercise performed on a
foam pad or a BOSU ball can more effectively enhance the EMG activity of the lower trapezius muscle and alleviate the
imbalance between the three parts of the trapezius muscle to improve RSP.