Measurement of an excursion of shoulder joint in view of an ergonomic cycle

Abstract

The data on shoulder joint rotation limits in anatomical literature were described in term of conventionalized terminologies, which define the joint movements : flexion, extension, abduction, adduction, etc. These medical terminology states several movements in a single plane or rotations about a single axis, despite the shoulder joint has 3 degrees of freedom with polycentric rotations. If 3-dimensional movement performed successively on single joint such as shoulder, the conjunct rotation was generated concurrently with an adjunct rotation. The problem is known as 'CODMAN'S PARADOX', and it is the need for triaxial joint at the root of a limb. In this study, we propose new convenient angle definition about upper arm axial rotation, considering the conjunct rotation and the ergonomic cycle. The definision enables an assessment of the shoulder joint angle limits interlligibly over 3-dimensional space under the assumption on the ergonomic cycle action. The method was used to measure internal and external upper arm axial rotation limits experimentally from 6 persons of 22-27 years old under the condition of restricted scapula. The result shows that the motion limits largely depends on the position of the upper arm, and the persons who experienced tennis or base ball pitcher have large external rotation range on the upper arm position abducted laterally.