An apparatus and process for measuring the motion of internal joint structures during and for measuring the function of muscles involved with the motion of a joint of a subject are disclosed. The apparatus can be configured in three forms: a horizontal, vertical, or butterfly motion control device. Each configuration comprises a static member and a moving member, in which the moving member operates to move or be moved by the subject being studied. Also disclosed are processes for using each apparatus to measure of the relative motion of skeletal structures in a subject, in which the subject is positioned in the apparatus and commanded to move while diagnostic medical images are taken or captured. Additionally, processes for using each apparatus to specifically measure and collect data on the function of the subject's muscles are disclosed.
