Introduction: The diagnosis of cuff tendon pathology is usually based on physical examination, which has a limited predictive value. The ultimate cost effective diagnostic tool for this purpose should combine the simplicity and low cost of the physical examination with precision of the imaging scans. Since the pathological process involving the rotator cuff structures is usually intrinsic to the muscle and tendon tissue, one of its main expressions, apart of pain, will be weakness of the muscle involved. Measurements of muscle strength may potentially provide a valuable diagnostic tool for evaluation of integrity of a specific muscle or a group of muscles. The purpose of the study is to evaluate the normal patterns of the isometric strength curves of the rotator cuff muscles.

Methods: Isometric time-force distribution for suspraspinatus, infraspionatus and subscapularis muscles in 400 healthy volunteers was measured, i.e.. 50 healthy individuals of both sexes for every decade of age from twenty to sixty years of age were evaluated. Specially designed dynamometer with measurement rate of 5 Hz was used. The measurements were done in a standard body-arm positions in order to eliminate the influence of the synergistic contribution of other muscles. The force- time curves were presented as moment of force values normalized to the lean body mass of the examinees. The characteristics curves of the study groups were compared by non parametric statistical analysis, since not normal distribution of values was found.

Results: Isometric strength of each of the rotator cuff muscles was higher in dominant limbs, higher in men in every age group and gradually rises from second to fifth decade of life in both sexes. The significant drop in muscle force was evident only in the sixth decade of life in both sexes. The force-time curves were characteristic and different in the different studied groups.

Discussion: A data base of normal isometric strength values of rotator cuff muscles in healthy adult population was established. This provides an effective comparative tool for the further evaluation of force-time curves in patients with rotator cuff pathology. The unpredictable evidence of significant rise of rotator cuff muscles’ strength with increasing age challenges the present concepts of the understanding of rotator cuff degeneration pathophysiology and its treatment. The decrease in rotator cuff isometric strength in the sixth decade of life is consistent with the higher incidence of non symptomatic rotator cuff intrinsic pathology at this age