In the present study, we examined the ratio of scapular muscles activities and analyzed whether the correlation with muscle strength is also observed in patients with shoulder disorders.
Thirteen patients who had shoulder joint disease for 3 months were recruited (Age, 69.5±11.1 years). Inclusion criteria were as follows: patients could elevate 90 degrees, had no pain, had sufficient cognitive function to perform the task, and had no history of central nervous system disease and shoulder trauma on the non-injured (contralateral) side. The participant flexed from 0° to 90° (0-90) and remained at 90° (90-isom). The arm then lowered to the drooped position (90-0). Muscle activities of the serratus anterior (SA), upper trapezius (UT), middle trapezius (MT), and lower trapezius (LT) were measured during the task. For normalization, the muscle activities during the maximum isometric contraction of the shoulder flexion were measured at 90° (reference contraction), and muscle strength was measured simultaneously with handheld dynamometer. The electromyographic integrated value obtained during the task was divided by the reference contraction value to obtain the relative electromyographic integrated value, and UT/SA, UT/MT, and UT/LT, which represent the ratio of activity of UT to each muscle, were calculated. Correlations with muscle strength of shoulder flexion and relative values to contralateral flexion muscle strength (contralateral flexion muscle strength ratio) were analyzed using Spearman's rank correlation coefficient.
In 0-90, UT/SA of the contralateral and injured side were 1.40 and 1.62, respectively, UT/MT were 2.55 and 3.16, and UT/LT were 1.10 and 1.62. A significant negative correlation was found between muscle strength of shoulder flexion and UT/SA in 0-90, as was the contralateral flexor strength ratio. In 90-isom and 90-0, significant negative correlations were found between contralateral flexion muscle strength ratio and UT/SA.
Compared to the values in young healthy subjects by previous study (UT/SA and UT/LT ranged from 0.5 to 1, and UT/MT ranged from 1 to 2), the values were large on the contralateral side, which is the non-injured side, and even larger on the injured side. It is clear that the ratio of muscle activity in upper trapezius muscle increases with age, and the ratio is even higher with injury. The ratio of scapular muscles activities must be influenced by many factors, including shoulder flexion strength. However, the large changes in muscle strength due to injury have a greater effect on scapular muscles activities.
The results of this study provide a benchmark for the need for exercises of scapular muscles and help to elucidate the causes of scapular dyskinesia.
muscle strength
electromyography
