Incomplete functional recovery after rotator cuff surgery can be caused by rerupture or incomplete restoration of the contractile properties of the muscle-tendon-bone unit. We measured the passive tension generated in the supraspinatus musculotendinous unit at the time of repair of the supraspinatus tendon performed for the treatment of long-standing rupture in four patients and compared our results with the values of an intact musculotendinous unit. In stepwise elongation from 10 to 20 mm, passive tension increased by a factor of 2.2 +/- 0.4 in the study group. In the control case passive tension increased by a factor of only 1.3. Mean tension in 60 degrees of abduction was 14.25 +/- 3.4 N in the four long-standing ruptures and 10 N in the control case. If the arm was brought to the side, tension rose to 25 N in the control case, whereas mean tension increased to 59.25 +/- 12.7 N in long-standing rupture of the supraspinatus muscle. Our findings demonstrate that passive tension in the supraspinatus is increased after long-standing rupture of its tendon. This result suggests that active force generation by this muscle will be compromised after surgery and that the high strain after repair may expose the musculotendinous unit to further damage.