The quantification of reciprocal shoulder strength relation in various working modes at different movement velocities

Muscular balance appears to be decisive for dynamic shoulder stability, since passive stabilizers do not take effect until the end of range of motion in order to not limit joint mobility. Changes in strength relationships are frequently viewed as one of the causes of shoulder pathologies. However, objective quantification of reciprocal strength relationships is difficult. In particular, the influence of movement velocity on the strength ratios during the test has not been clarified. Determination of the strength relationships of reciprocal muscle groups (quotient of maximum torques) was performed on a total of 166 untrained men (M) and 83 untrained women (F) for extension/flexion, abduction/adduction and rotation movements of the shoulder. Moreover, differentiation was made by various working modes and movement velocities (isometric at various angle positions, concentric at 60°/s and 180°/s, eccentric at 60°/s and 120°/s). The results show a decreasing order of strength relationships: Abduction/Adduction > Flexion/Extension > External/Internal rotation. In spite of marked differences in absolute values, no differences were noted among the contraction modes (isometric, concentric, eccentric) or movement velocities (p > 0.05). A slightly reduced ratio of rotational movements in concentric mode was the only conspicuous finding. Unlike isokinetically-determined absolute torque values, a relevant velocity-dependence must not be expected with respect to reciprocal strength relationships of the shoulder muscles. A different (neuro)-muscular structure of the muscle groups involved with a corresponding effect on the strength-performance ratio thus does not appear to be present or cannot be detected in isokinetic tests. However, further research is warranted regarding the effect of the various associated factors (e.g. stabilization) on the reliability and validity of isokinetic findings relating to this specific joint-muscles system.