The purpose of this study was to investigate the influence of rotator cuff tear size on muscle activity and onset time recorded from shoulder muscles in patients with rotator cuff tear.
Eighty-two patients (age: 59.7±12.1 years, height: 162.2±26.4 cm, weight: 69.6±12.2 kg) who planning to undergo arthroscopic rotator cuff repair in our hospital were participated. Sixty patients had small to medium rotator cuff tears and 22 patients had large to massive rotator cuff tears. An experienced physician made the diagnosis of rotator cuff tear based on magnetic resonance imaging and arthroscopic findings. The subjects performed shoulder flexion in the scapular plane from 0° to 120°. The movement task was performed along vertically placed aluminum poles. The angular velocity was set at 30°/sec, and the movement speed was kept constant in accordance with the sound of an electronic metronome. The movement task was performed three times. EMG signals from the infraspinatus (ISP), anterior deltoid (AD), middle deltoid (MD), upper trapezius (UT), middle trapezius (MT), lower trapezius (LT), and serratus anterior (SA) muscles were recorded using active surface electrodes. The shoulder joint angle was recorded using an electrogoniometer. The average rectified EMG values were calculated as a percentage of the movement task (%ARV). The onset of shoulder muscle EMG relative to the start time of shoulder flexion in the scapular plane was calculated using integrated profile methods. Friedman’s test was used to compare the %ARV values and the muscle onset time. The level of significance was set at 5%.
The large･massive ISP showed significantly lower activity than the unaffected ISP in the 90°-120° phase (large･massive 132.5±48.0, unaffected 149.3±33.9). The onset time of small･medium AD showed significantly earlier than that of unaffected AD (small･medium 0.7±0.8, unaffected 1.0±1.0). The onset time of large･massive MD showed significantly earlier than that of unaffected MD (large･massive 0.3±0.7, unaffected 0.6±0.8).
The current results revealed that the EMG activity of the ISP differed among the large･massive and unaffected sides. And it was shown that the onset time of AD and MD starts earlier than that of the unaffected side. Therefore, the present EMG results indicated that the ISP, AD, and MD might contribute to the execution of shoulder flexion to compensate for torn muscles. And each muscle of the affected side would have different EMG activity compared to that of the unaffected side during shoulder flexion depending on the size of the rotator cuff tear.
Rehabilitation of ISP, AD, and MD is important. And the data should be utilized when creating an exercise program in the case of a rotator cuff tear.
Shoulder joint
Rotator cuff tear
