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Purpose This study aimed to investigate the time-course changes in the active stiffness of the supraspinatus muscle after arthroscopic rotator cuff repair. Methods Eight male patients (mean age 61.5 ± 9.4 years) who underwent arthroscopic rotator cuff repair for small to medium tears were recruited for this study. Movement tasks included 30° shoulder isometric abduction and maximal voluntary isometric contraction of shoulder abduction in the scapular plane. The stiffness of the supraspinatus (anterior superficial, anterior deep, posterior superficial, and posterior deep regions), upper trapezius, and middle deltoid muscles in bilateral shoulders was recorded using ultrasound shear wave elastography. For each subject, the measurement was performed preoperatively and 3, 6, and 12 months postoperatively. ResultsThe stiffness of the affected anterior superficial region of the supraspinatus muscle 12 months postoperatively was significantly higher than that measured preoperatively and 3 months postoperatively (p < 0.05); it was significantly higher at 6 months postoperatively than at 3 months postoperatively (p < 0.05). Further, the maximal voluntary isometric contraction had significantly improved 12 months postoperatively compared to that measured preoperatively and 3 months postoperatively (p < 0.05). The stiffness of the affected upper trapezius and middle deltoid muscles 12 months postoperatively was significantly lower than that preoperatively (p < 0.05). ConclusionThe maximal voluntary isometric contraction 12 months postoperatively possibly increased because of improvement in the active stiffness of the anterior superficial region. Active stiffness of the anterior superficial region may improve 6 months rather than 3 months postoperatively because of the different stages of muscle force, structural repair tendon strength, and remodeling.

This study aimed to compare the effect of the load of the upper limb on the stiffness of supraspinatus muscle regions during isometric shoulder abduction in the scapular plane in healthy individuals and patients with a rotator cuff tear. Thirteen male patients were scheduled for arthroscopic rotator cuff repair, and 13 healthy male individuals were recruited. The movement task involved 30 isometric shoulder abduction in the scapular plane. The tasks included passive abduction, abduction with half-weight of the upper limb (1/2-weight), and full weight of the upper limb (fullweight). The stiffness of the supraspinatus muscle (anterior superficial, anterior deep, posterior superficial, and posterior deep regions) was recorded using ultrasound shear-wave elastography. The stiffness of the anterior superficial region on the affected side was significantly lower than that on the control side for the 1/2-weight and full-weight tasks. The stiffness of the anterior deep, posterior superficial, and

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