Relative phase quantifies interjoint coordination.

This note illustrates by example how expression of joint movement on a phase plane can quantitatively describe multijoint coordination during complex actions. Automatic digitisation of high-speed video records was used to obtain lumbar vertebral, hip, knee and ankle joint angular kinematics in the sagittal plane of a subject performing a symmetric two-handed lifting movement. A consistent proximal-to-distal coordination was illustrated via angle-angle and relative phase angle presentations. During bending to pick up a load, the joints began their movement in the order proximal to distal while the reverse order of joint involvement occurred during extension. Phase angle relationships between joints may provide sufficiently sensitive measurements to identify changes in multijoint coordination induced by alterations in task variables such as (in the case of lifting) object mass, lifting height and load moment. Information regarding multijoint coordination is likely to be important in attempting to understand the respective roles and interaction between the bi and monarticular muscles which are involved in everyday complex actions like lifting.