A commentary from the pioneers on the innovation of the relative motion concept: History, biologic considerations, and anatomic rationale.

The relative motion concept is simply recognition of the normal functional anatomic relationships that allow powerful extrinsic muscles, the extensor digitorum communis (EDC) and flexor digitorum profundus (FDP), to vary forces on individual finger joints and function in response to the relative position of adjacent metacarpophalangeal joints (MCPJs) in the hand, one to another. First identified as a cause of complications after surgery, a better understanding now allows us to harness these forces by way of differential metacarpophalangeal joint (MCPJ) positioning using an orthosis. This can reduce undesirable tension and allow immediate controlled active motion while permitting functional use of the hand. Tissue gliding with active motion prevents restrictive scarring, maintains joint mobility and avoids unnecessary limitations and stiffness on normal neighboring structures. The historical development of this concept is shared with explanation of the anatomic and biologic rationale for this approach. Acute and chronic hand conditions that may benefit from better understanding of relative motion are numerous and growing.