The dependence of muscle strength in the shoulder rotators on the shoulder abduction angle

: Background. The shoulder joint has a very complex structure. Normal shoulder function is dependent on joint stability and proper neuromuscular control. Given its anatomical structure and range of movement, this joint is highly susceptible to numerous disorders and injuries. Normal stability of the shoulder guarantees physiological function, and is also the most important element of injury prevention. Disturbances of the strength and reciprocal proportions of the inner and outer rotator muscles, as well as the shoulder abductors, can be a mechanism leading to shoulder instability. The goal of our study was to evaluate the muscle strength of the shoulder rotators in terms of the abduction angle in healthy individuals. Material and methods. We examined 100 healthy volunteers (200 shoulders), 50 men and 50 women. Each subject enrolled in the study performed 32 measurements of muscle strength in the shoulder rotators, for a grand total of 3200 measurements, performed in alternation (outward and inward rotation) at various angles of shoulder abduction. Results. The strength of the muscles responsible for inward and outward rotation in persons without shoulder injury rose along with shoulder abduction, from 0 degrees to 70 degrees . When the shoulder was abducted to 70 degrees , the strength of the inner rotators was the highest, significantly higher than that of the outer rotators. When the shoulder was abducted to 90 degrees , there was relative equilibrium of muscle strength (N) between the inner and outer rotators. Conclusions. 1. Muscle strength in the outer and inner shoulder rotators in healthy individuals was dependent on the abduction angle. 2. The overall strength was higher in the men than in the women, but there were no significant differences in the direction of changes in strength at various angles of abduction. 3. The results obtained here in persons without shoulder injury can serve as norms for evaluating progress in physiotherapy in 20-30-year-old patients with shoulder injuries.