Semi-rigid insoles serve as the essential mechanical foundation for Bowden cable-driven exoskeletons used in rehabilitation. They function primarily as the foot-side anchor points, transforming the raw tension from the cables into controlled ankle movement. By utilizing a specific level of stiffness, these insoles ensure that the force generated by the exoskeleton is effectively applied to the foot to correct gait abnormalities without compromising the patient's stability.
The core function of the semi-rigid insole is to act as a transmission medium; it converts the tensile force of the cables into effective ground reaction force, allowing for precise correction of foot drop and inversion while protecting the foot from excessive pressure.
The Mechanics of Force Transmission
Acting as a Critical Anchor Point
For a Bowden cable system to work, the cables must have a secure termination point. The semi-rigid insole provides this anchor, allowing the cables to exert significant pull without slipping or deforming the shoe structure.
Without this specific rigidity, the tension from the cables would be lost to material stretching rather than moving the joint.
Enabling Bi-Directional Movement
These insoles support force transmission in two distinct directions: dorsiflexion (lifting the foot) and plantarflexion (pushing the foot down).
Because the insole runs the length of the foot and maintains its shape, the exoskeleton can manipulate the angle of the ankle with precision in both phases of the gait cycle.
Converting Auxiliary Force
The ultimate goal of the exoskeleton is to generate Ground Reaction Force (GRF) to propel or stabilize the patient.
The insole ensures that the auxiliary force provided by the motors is successfully transferred through the foot structure and into the ground, rather than being dissipated by a soft, cushioning shoe insert.
Correcting Hemiparetic Gait Patterns
Addressing Foot Drop
Hemiparetic patients often suffer from foot drop, where the front of the foot drags. The insole's stiffness ensures that when the cable pulls up, the entire forefoot lifts evenly.
A softer insole would simply curl upward at the toe, failing to lift the actual weight of the foot.
Correcting Foot Inversion
Beyond simple lifting, these insoles help correct foot inversion (the rolling of the foot inward).
By providing a stable, semi-rigid platform, the system can apply torque to align the ankle correctly, preventing the lateral instability that leads to rolling.
Understanding the Trade-offs
The Balance of Stiffness vs. Comfort
The effectiveness of the system relies on the specific stiffness of the material.
If the insole is too rigid, it becomes uncomfortable for the wearer, potentially causing pressure points or limiting natural sensory feedback.
Efficiency vs. Energy Loss
If the insole is too flexible, the system loses efficiency. The cable pulls, but the energy is wasted bending the rubber insole rather than moving the ankle.
The "semi-rigid" designation represents the calculated compromise: stiff enough to transmit force, yet flexible enough to be wearable for extended periods.
Optimizing for Patient Outcomes
When selecting or designing the interface for a hemiparetic patient, the goal is to maximize force transfer while maintaining compliance.
- If your primary focus is Gait Correction: Prioritize a higher stiffness in the insole to ensure maximum torque is applied to correct severe foot drop or inversion.
- If your primary focus is Long-Duration Usage: Opt for an insole with slightly more compliance to reduce fatigue and pressure spots, accepting a slight reduction in force transmission efficiency.
Success lies in ensuring the insole acts as a seamless bridge between the robot's power and the patient's anatomy.
Summary Table:
| Feature | Role in Force Transmission | Benefit to Hemiparetic Patients |
|---|---|---|
| Anchor Point | Provides secure cable termination | Ensures force moves the joint, not the shoe |
| Material Stiffness | Minimizes energy dissipation | Effectively lifts the forefoot to fix foot drop |
| Stable Platform | Enables precise torque application | Corrects foot inversion and improves stability |
| Semi-Rigid Design | Balances efficiency and compliance | Enhances wearability for long-duration therapy |
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References
- Lizeth H. Sloot, Conor J. Walsh. Effects of a soft robotic exosuit on the quality and speed of overground walking depends on walking ability after stroke. DOI: 10.1186/s12984-023-01231-7
This article is also based on technical information from 3515 Knowledge Base .
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