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Recently, supraspinatus muscle exercise has been reported to treat rotator cuff disease and to recover shoulder function. However, there have been no report on the direct measurement of strain on the supraspinatus tendon during simulated isometric gleno-humeral joint elevation.Ten fresh-frozen shoulder specimens with the rotator cuff complex left intact were used as experimental models. Isometric gleno-humeral joint elevation in a sitting position was reproduced with low angle of step-by-step elevation in the scapular plane and strain was measured on the surface layer of the supraspinatus tendon.In isometric conditions, applied tensile force of the supraspinatus tendon increased significantly with increases in adduction torque on the gleno-humeral joint. Significant increases in the strain on the layer were observed by increase in adduction torque, which were recorded in isometric elevation at − 10° and 0°, but little increase in the strain was observed at 10° or greater gleno-humeral elevation.Increased strain on the surface layer of the supraspinatus tendon was observed during isometric gleno-humeral elevation from − 10 to 0°. These findings demonstrate a potential risk of inducing overstretching of the supraspinatus tendon during supraspinatus muscle exercise.Strain in the supraspinatus tendon was measured directly in cadaver shoulder models.The shoulder elevation at − 10°, 0° increased strain of the supraspinatus tendon.The elevation at 10°, 20°, and 30° did not increased strain of the tendon.These strain values are inconsistent from those obtained from ultrasound or MRI.There is a potential risk during isometric exercise in low gleno-humeral elevation.