Performance of Ankle Exoskeletons on Irregular Terrains: Key Design Principles and Benchmarking Tests.

Area of Science:

• Biomechanics
• Rehabilitation Engineering
• Robotics

Background:

• Exoskeletons are crucial for physical impairment assistance and rehabilitation.
• Mechanical design critically influences user kinematics and limb movement.
• Ankle exoskeletons require careful design to balance assistance with natural motion.

Purpose of the Study:

• To present the mechanical design of two novel ankle exoskeletons: S-RANK (single DoF) and M-RANK (multi-DoF).
• To evaluate the impact of these exoskeleton designs on lower limb kinematics and gait symmetry in healthy individuals across various terrains.
• To compare the kinematic effects and gait symmetry alterations between the single and multi-DoF ankle exoskeleton prototypes.

Main Methods:

• Two ankle exoskeleton prototypes (S-RANK and M-RANK) with differing degrees-of-freedom (DoF) and torque transmission were designed.
• Healthy subjects wore the exoskeletons on their right leg and walked on five distinct terrains.
• Human lower limb kinematics were recorded using inertial measurement units (IMUs), analyzed for trend symmetry (TS), and compared using statistical parametric mapping (SPM).

Main Results:

• Both S-RANK and M-RANK significantly influenced joint kinematics and gait symmetry.
• S-RANK showed higher overall difference (OD) at the ankle, especially on softer surfaces, and worsened gait symmetry on flat terrain.
• M-RANK had less impact on ankle kinematics but increased OD at the knee and hip on flat and softer terrains.

Conclusions:

• Exoskeleton mechanical design significantly affects user kinematics and gait symmetry.
• S-RANK provides stability with less proximal joint impact, while M-RANK offers better adaptability and natural gait maintenance due to its increased DoF.
• The choice of exoskeleton design (DoF and transmission) is critical for optimizing rehabilitation and assistance outcomes.