A robot‐assisted study of intrinsic muscle regulation on proximal interphalangeal joint stiffness by varying metacarpophalangeal joint position

The tightness of intrinsic hand muscles is a common cause of finger joint stiffness. The purposes of this study were to develop a robot‐assisted methodology to obtain torque–angle data of a finger joint, and to investigate the regulation of the intrinsic muscles on finger joint stiffness. Our robot system features the integration of a low payload robot arm, a controller, and a force/torque transducer. The system provided highly reproducible torque–angle curves. Torque–angle data of the proximal interphalangeal joint with the metacarpophalangeal joint at 0 and 60 degrees were obtained from eight asymptomatic hands. The torque–angle curve shifted with the position of the metacarpophalangeal joint. As the metacarpophalangeal joint flexion angle changed from 60 to 0 degrees, the equilibrium of the proximal interphalangeal joint increased more than 20 degrees, and joint stiffness increased more than 50%. The dependence of the stiffness of the proximal interphalangeal joint on metacarpophalangeal joint position supports the regulatory role of the intrinsic muscles on finger joint mechanics. This regulatory mechanics is likely amplified in hands with intrinsic muscle tightness, justifying the commonly used Bunnell Intrinsic Tightness Test. © 2005 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 24:407–415, 2006