Defining relative joint stiffness in multi-articular muscle-tendon driven systems

Joint stiffness significantly affects system compliance during movement. Even for planar joints, such as the knee, stiffness is evaluated in multiple axes, presented in a 6 by 6 matrix that defines the relationship between stress and strain of bending movements in multiple directions. This method, combined with serial dissection, determines the contribution of different tissue types to joint stiffness. Empirically measuring joint stiffness in multi-articular muscle-tendon driven structures is even more complex because a joint's stiffness profile depends on all joints' configurations. Mammal tails are particularly challenging because muscles connect to tendons that span 5-30 multiaxial joints, and removing tendons causes joint dislocation. We seek to establish standardized methods to empirically characterize tail joint stiffness in a way that incorporates overall tail configuration, demonstrated on a Jaculus jaculus.

First, we built a customized rig to precisely control tail configuration while measuring joint angle as force is applied. In lieu of serial dissection, we measured individual joint stiffness with tendons loaded (connected to muscle) and unloaded (disconnected from muscle) to isolate passive connective tissue contributions. We introduce a new compliance matrix function that defines a joint’s stiffness profile dependending on the configuration of the whole tail. Future work aims to add this function-based definition of stiffness to the OpenSim modeling environment. These methods will facilitate more precise biomechanical examinations of tail performance and evolution.

@inproceedings{zhang2025defining,
  title={Defining relative joint stiffness in multi-articular muscle-tendon driven systems},
  author={Zhang, Margaret and Moore, Talia},
  booktitle={INTEGRATIVE AND COMPARATIVE BIOLOGY},
  volume={65},
  pages={S584--S584},
  year={2025},
  organization={OXFORD UNIV PRESS INC JOURNALS DEPT, 2001 EVANS RD, CARY, NC 27513 USA}
}