Ankle Joint Intrinsic Dynamics is More Complex than a Mass-Spring-Damper Model.

Area of Science:

• Biomechanics
• Human physiology
• Musculoskeletal system modeling

Background:

• The intrinsic mechanics of the ankle joint are crucial for understanding human locomotion and stability.
• Existing models, such as the second-order mass-spring-damper (IBK) model, are widely used but may not fully capture ankle dynamics.
• Accurate modeling of ankle joint mechanics is essential for clinical applications and research in movement science.

Purpose of the Study:

• To develop and validate a new parametric model for small-signal ankle joint intrinsic mechanics in healthy subjects.
• To compare the performance of the new model against the conventional IBK model across different ankle positions and tasks.
• To investigate the structural consistency of the proposed model with the human ankle's musculoskeletal anatomy.

Main Methods:

• Experimental data were collected from five healthy subjects performing tasks without voluntary muscle contraction across seven ankle positions.
• A multi-step identification procedure was employed, starting with a non-parametric model of intrinsic joint stiffness (input: ankle position, output: torque).
• The non-parametric model was converted into a continuous-time transfer function representing ankle compliance (inverse of stiffness), leading to a third-order model.

Main Results:

• Intrinsic ankle mechanics were identified as a third-order system, not adequately represented by the conventional second-order IBK model at all frequencies.
• The discrepancy between the new third-order model and the IBK model increased from dorsiflexion to plantarflexion.
• The proposed third-order model demonstrated structural consistency with the agonist-antagonist musculoskeletal arrangement of the human ankle.

Conclusions:

• A novel third-order parametric model provides a more accurate representation of intrinsic ankle joint mechanics compared to the traditional IBK model.
• The findings highlight the limitations of the second-order IBK model in capturing the full complexity of ankle dynamics, particularly across the range of motion.
• The developed third-order model offers improved structural validity and potential for more precise biomechanical analyses of the human ankle.