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Objective This study investigated the location of the axis of rotation in sagittal plane movement of the spine in a free sitting condition to adjust the kinematics of a mobile seat for a dynamic chair. Background Dynamic office chairs are designed to avoid continuous isometric muscle activity, and to facilitate increased mobility of the back during sitting. However, these chairs incorporate increased upper body movement which could distract office workers from the performance of their tasks. A chair with an axis of rotation above the seat would facilitate a stable upper back during movements of the lower back. The selection of a natural kinematic pattern is of high importance in order to match the properties of the spine. Method Twenty-one participants performed four cycles of flexion and extension of the spine during an upper arm hang on parallel bars. The location of the axis of rotation relative to the seat was estimated using infrared cameras and reflective skin markers. Results The median axis of rotation across all participants was located 36 cm above the seat for the complete movement and 39 cm for both the flexion and extension phases, each with an interquartile range of 20 cm. Conclusion There was no significant effect of the movement direction on the location of the axis of rotation and only a weak, non-significant correlation between body height and the location of the axis of rotation. Individual movement patterns explained the majority of the variance. Application The axis of rotation for a spinal flexion/extension movement is located above the seat. The recommended radius for a guide rail of a mobile seat is between 36 cm and 39 cm.

Background The critical shoulder angle combines the acromion index and glenoid inclination and has potential to discriminate between shoulders at risk for rotator cuff tear or osteoarthritis and those that are asymptomatic. However, its biomechanics, and particularly the role of the glenoid inclination, are not yet fully understood. Methods A shoulder simulator was used to analyze the independent influence of glenoid inclination during abduction from 0 to 60°. Spindle motors transferred tension forces by a cable-pulley on human cadaveric humeri. A six-degree-of-freedom force transducer was mounted directly behind the polyethylene glenoid to measure shear and compressive joint reaction force and calculate the instability ratio (ratio of shear and compressive joint reaction force) with the different force ratios of the deltoid and supraspinatus muscles (2:1 and 1:1). A stepwise change in the inclination by 5° increments allowed simulation of a critical shoulder angle range of 20° to 45°. Findings Tilting the glenoid to cranial (increasing the critical shoulder angle) increases the shear joint reaction force and therefore the instability ratio. A balanced force ratio (1:1) between the deltoid and the supraspinatus allowed larger critical shoulder angles before cranial subluxation occurred than did the deltoid-dominant ratio (2:1). Interpretation Glenoid inclination-dependent changes of the critical shoulder angle have a significant impact on superior glenohumeral joint stability. The increased compensatory activity of the rotator cuff to keep the humeral head centered may lead to mechanical overload and could explain the clinically observed association between large angles and degenerative rotator cuff tears.